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Biomechanical Comparison of Hamstring Tendon Fixation Devices for Anterior Cruciate Ligament Reconstruction: Part 2. Four Tibial Devices
Of the procedures performed by surgeons specializing in sports medicine and by general orthopedists, anterior cruciate ligament (ACL) reconstruction remains one of the most common.1 Recent years have seen a trend toward replacing the “gold standard” of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.2 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, it is important to determine the strength of different methods of graft fixation.
Rigid fixation of hamstring grafts is recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand early rehabilitation forces as high as 500 N.2 There is therefore much concern about the strength of tibial fixation, given the lower bone density of the tibial metaphysis versus the femoral metaphysis. In addition, stability is more a concern in the tibia, as the forces are directly in line with the tibial tunnel.3,4
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed. There is much interest in determining which devices have the most fixation strength,4-9 but so far several products have not been compared with one another.
We conducted a study to determine if tibial hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Forty porcine tibias were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 4 different tibial fixation devices (Figure 1): Delta screw and Retroscrew (Arthrex, Naples, Florida), WasherLoc (Arthrotek, Warsaw, Indiana), and Intrafix (Depuy Mitek, Raynham, Massachusetts). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the tibias using the 4 tibial fixation devices. All fixations were done according to manufacturer specifications. All interference screws were placed eccentrically. The testing apparatus and procedure are described in an article by Kousa and colleagues.6 The specimens were mounted on the mechanical testing apparatus by threaded bars and custom clamps to secure fixation (Figure 2). Constant tension was maintained on all 4 strands of the hamstring grafts to equalize the tendons. After the looped end of the hamstring graft was secured by clamps, 25 mm of graft was left between the clamp and the intra-articular tunnel.
In the cyclic loading test, the load was applied parallel to the long axis of the tibial tunnel. A 50-N preload was initially applied to each specimen for 10 seconds. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 120 seconds were performed. Standard force-displacement curves were then generated. Each tibial fixation device underwent 10 cyclic loading tests. Specimens surviving the cyclic loading then underwent a single-cycle load-to-failure (LTF) test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF (yield load) data were generated from the single-cycle LTF test; ultimate LTF was defined as the load at the point where the slope of the load displacement curve initially decreases.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices were similar. In all 10 tests, Intrafix was pulled through the tunnel with the hamstring allografts. WasherLoc failed in each test, with the tendons eventually being pulled through the washer and thus out through the tunnel. Delta screw and Retroscrew both failed with slippage of the fixation device and the tendons pulled out through the tunnel.
For the cyclic loading tests, 8 of the 10 Delta screws and only 2 of the 10 Retroscrews completed the 1500-cycle loading test before failure. The 2 Delta screws that did not complete the testing failed after about 500 cycles, and the 8 Retroscrews that did not complete the testing failed after about 250 cycles. All 10 WasherLoc and Intrafix devices completed the testing.
Residual displacement data were calculated from the cyclic loading tests (Table). Mean (SS) residual displacement was lowest for Intrafix at 2.9 (1.2) mm, followed by WasherLoc at 5.6 (2.2) mm and Delta at 6.4 (3.3) mm. Retroscrew at 25.5 (11.0) mm had the highest residual displacement, though only 2 completed the cyclic tests. Intrafix, WasherLoc, and Delta were not statistically different, but there was a statistical difference between Retroscrew and the other devices (P < .001).
Stiffness data were calculated from the LTF tests (Table). Mean (SD) stiffness was highest for Intrafix at 129 (32.7) N/mm, followed by WasherLoc at 97 (11.6) N/mm, Delta at 93 (9.5) N/mm, and Retroscrew at 80.2 (8.8) N/mm. Intrafix had statistically higher stiffness compared with WasherLoc (P < .05), Delta (P < .01), and Retroscrew (P < .05). There were no significant differences in stiffness among WasherLoc, Delta, and Retroscrew.
Mean (SD) ultimate LTF was highest for Intrafix at 656 (182.6) N, followed by WasherLoc at 630 (129.3) N, Delta at 430 (90.0) N, and Retroscrew at 285 (33.8) N (Table). There were significant differences between Intrafix and Delta (P < .05) and Retroscrew (P < .05). WasherLoc failed at a significantly higher load compared with Delta (P < .05) and Retroscrew (P < .05). There were no significant differences in mean LTF between Intrafix and WasherLoc.
Discussion
In this biomechanical comparison of 4 different tibial fixation devices, Intrafix had results superior to those of the other implants. Intrafix failed at higher LTF and lower residual displacement and had higher stiffness. WasherLoc performed well and had LTF similar to that of Intrafix. The interference screws performed poorly with respect to LTF, residual displacement, and stiffness, and a large proportion of them failed early into cyclic loading.
Intrafix is a central fixation device that uses a 4-quadrant sleeve and a screw to establish tensioning across all 4 hamstring graft strands. The theory is this configuration increases the contact area between graft and bone for proper integration of graft into bone. Intrafix has performed well in other biomechanical studies. Using a study design similar to ours, Kousa and colleagues7 found the performance of Intrafix to be superior to that of other devices, including interference screws and WasherLoc. Starch and colleagues10 reported that, compared with a standard interference screw, Intrafix required significantly higher load to cause a millimeter of graft laxity. They concluded that this demonstrates superior fixation strength and reduced laxity of the graft after cyclic loading. Coleridge and Amis4 found that, compared with WasherLoc and various interference screws, Intrafix had the lower residual displacement. However, they also found that, compared with Intrafix and interference screws, WasherLoc had the highest ultimate tensile strength. Their findings may be difficult to compare with ours, as they tested fixation of calf extensor tendons, and we tested human hamstring grafts.
An important concern in the present study was the poor performance of the interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.11 Delta screws and Retroscrews have not been specifically evaluated, and their fixation strengths have not been directly compared with those of other devices. In the present study, Delta screws and Retroscrews consistently performed the poorest with respect to ultimate LTF, residual displacement, and stiffness. Twenty percent of the Delta screws and 80% of the Retroscrews did not complete 1500 cycles. The poor performance of the interference screws was echoed in studies by Magen and colleagues12 and Kousa and colleagues,7 in which the only complete failures were in the cyclic loading of the interference screws.
Three possible confounding factors may have affected the performance of the interference screws: bone density of porcine tibia, length of interference screw, and location of screw placement. In addition, in clinical practice these screws may be used with other modes of graft fixation. Combined fixation (interference screws, other devices) was not evaluated in this study.
Porcine models have been used in many biomechanical graft fixation studies.4,6,7,12,13 Some authors have found porcine tibia to be a poor substitute for human cadaver tibia because the volumetric density of porcine bone is higher than that of human bone.14,15 Other authors have demonstrated fairly similar bone density between human and porcine tibia.16 The concern is that interference screw fixation strength correlates with the density of the bone in which screws are fixed.17 Therefore, one limitation of our study is that we did not determine the bone density of the porcine tibias for comparison with that of young human tibias.
Another important variable that could have affected the performance of the interference screws is screw length. One study found no significant difference in screw strength between various lengths, and longer screws failed to protect against graft slippage.18 However, Selby and colleagues19 found that, compared with 28-mm screws, 35-mm bioabsorbable interference screws failed at higher LTF. This is in part why we selected 35-mm Delta screws for our study. Both 35-mm Delta screws and 20-mm Retroscrews performed poorly. However, we could not determine if the poorer performance of Retroscrews was related to their length.
We used an eccentric placement for our interference screws. Although some studies have suggested concentric placement might improve fixation strength by increasing bone–tendon contact,20 Simonian and colleagues21 found no difference in graft slippage or ultimate LTF between eccentrically and concentrically placed screws. Although they were not biomechanically tested in our study, a few grafts were fixed with concentrically placed screws, and these tendons appeared to be more clinically damaged than the eccentrically placed screws.
Combined tibial fixation techniques may be used in clinical practice, but we did not evaluate them in our study. Yoo and colleagues9 compared interference screw, interference screw plus cortical screw and spiked washer, and cortical screw and spiked washer alone. They found that stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with interference screw plus cortical screw and spiked washer. In a similar study, Walsh and colleagues13 demonstrated improved stiffness and LTF in cyclic testing with the combination of retrograde interference screw and suture button over interference screw alone. Further study may include direct comparisons of additional tibial fixation techniques using more than one device. Cost analysis of use of additional fixation devices would be beneficial as well.
Study results have clearly demonstrated that tibial fixation is the weak point in ACL reconstruction3,17 and that early aggressive rehabilitation can help restore range of motion, strength, and function.22,23 Implants that can withstand early loads during rehabilitation periods are therefore of utmost importance.
Conclusion
Intrafix demonstrated superior strength in the fixation of hamstring grafts in the tibia, followed closely by WasherLoc. When used as the sole tibial fixation device, interference screws had low LTF, decreased stiffness, and high residual displacement, which may have clinical implications for early rehabilitation after ACL reconstruction.
1. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
2. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
3. Brand J Jr, Weiler A, Caborn DN, Brown CH Jr, Johnson DL. Graft fixation in cruciate ligament reconstruction. Am J Sports Med. 2000;28(5):761-774.
4. Coleridge SD, Amis AA. A comparison of five tibial-fixation systems in hamstring-graft anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2004;12(5):391-397.
5. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
6. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
7. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
8. Weiler A, Hoffmann RF, Stähelin AC, Bail HJ, Siepe CJ, Südkamp NP. Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone. Arthroscopy. 1998;14(1):29-37.
9. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
10. Starch DW, Alexander JW, Noble PC, Reddy S, Lintner DM. Multistranded hamstring tendon graft fixation with a central four-quadrant or a standard tibial interference screw for anterior cruciate ligament reconstruction. Am J Sports Med. 2003;31(3):338-344.
11. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
12. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
13. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
14. Nurmi JT, Järvinen TL, Kannus P, Sievänen H, Toukosalo J, Järvinen M. Compaction versus extraction drilling for fixation of the hamstring tendon graft in anterior cruciate ligament reconstruction. Am J Sports Med. 2002;30(2):167-173.
15. Nurmi JT, Sievänen H, Kannus P, Järvinen M, Järvinen TL. Porcine tibia is a poor substitute for human cadaver tibia for evaluating interference screw fixation. Am J Sports Med. 2004;32(3):765-771.
16. Nagarkatti DG, McKeon BP, Donahue BS, Fulkerson JP. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am J Sports Med. 2001;29(1):67-71.
17. Brand JC Jr, Pienkowski D, Steenlage E, Hamilton D, Johnson DL, Caborn DN. Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med. 2000;28(5):705-710.
18. Stadelmaier DM, Lowe WR, Ilahi OA, Noble PC, Kohl HW 3rd. Cyclic pull-out strength of hamstring tendon graft fixation with soft tissue interference screws. Influence of screw length. Am J Sports Med. 1999;27(6):778-783.
19. Selby JB, Johnson DL, Hester P, Caborn DN. Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel. Am J Sports Med. 2001;29(5):614-619.
20. Shino K, Pflaster DS. Comparison of eccentric and concentric screw placement for hamstring graft fixation in the tibial tunnel. Knee Surg Sports Traumatol Arthrosc. 2000;8(2):73-75.
21. Simonian PT, Sussmann PS, Baldini TH, Crockett HC, Wickiewicz TL. Interference screw position and hamstring graft location for anterior cruciate ligament reconstruction. Arthroscopy. 1998;14(5):459-464.
22. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med. 1990;18(3):292-299.
23. Shelbourne KD, Wilckens JH. Current concepts in anterior cruciate ligament rehabilitation. Orthop Rev. 1990;19(11):957-964.
Of the procedures performed by surgeons specializing in sports medicine and by general orthopedists, anterior cruciate ligament (ACL) reconstruction remains one of the most common.1 Recent years have seen a trend toward replacing the “gold standard” of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.2 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, it is important to determine the strength of different methods of graft fixation.
Rigid fixation of hamstring grafts is recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand early rehabilitation forces as high as 500 N.2 There is therefore much concern about the strength of tibial fixation, given the lower bone density of the tibial metaphysis versus the femoral metaphysis. In addition, stability is more a concern in the tibia, as the forces are directly in line with the tibial tunnel.3,4
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed. There is much interest in determining which devices have the most fixation strength,4-9 but so far several products have not been compared with one another.
We conducted a study to determine if tibial hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Forty porcine tibias were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 4 different tibial fixation devices (Figure 1): Delta screw and Retroscrew (Arthrex, Naples, Florida), WasherLoc (Arthrotek, Warsaw, Indiana), and Intrafix (Depuy Mitek, Raynham, Massachusetts). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the tibias using the 4 tibial fixation devices. All fixations were done according to manufacturer specifications. All interference screws were placed eccentrically. The testing apparatus and procedure are described in an article by Kousa and colleagues.6 The specimens were mounted on the mechanical testing apparatus by threaded bars and custom clamps to secure fixation (Figure 2). Constant tension was maintained on all 4 strands of the hamstring grafts to equalize the tendons. After the looped end of the hamstring graft was secured by clamps, 25 mm of graft was left between the clamp and the intra-articular tunnel.
In the cyclic loading test, the load was applied parallel to the long axis of the tibial tunnel. A 50-N preload was initially applied to each specimen for 10 seconds. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 120 seconds were performed. Standard force-displacement curves were then generated. Each tibial fixation device underwent 10 cyclic loading tests. Specimens surviving the cyclic loading then underwent a single-cycle load-to-failure (LTF) test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF (yield load) data were generated from the single-cycle LTF test; ultimate LTF was defined as the load at the point where the slope of the load displacement curve initially decreases.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices were similar. In all 10 tests, Intrafix was pulled through the tunnel with the hamstring allografts. WasherLoc failed in each test, with the tendons eventually being pulled through the washer and thus out through the tunnel. Delta screw and Retroscrew both failed with slippage of the fixation device and the tendons pulled out through the tunnel.
For the cyclic loading tests, 8 of the 10 Delta screws and only 2 of the 10 Retroscrews completed the 1500-cycle loading test before failure. The 2 Delta screws that did not complete the testing failed after about 500 cycles, and the 8 Retroscrews that did not complete the testing failed after about 250 cycles. All 10 WasherLoc and Intrafix devices completed the testing.
Residual displacement data were calculated from the cyclic loading tests (Table). Mean (SS) residual displacement was lowest for Intrafix at 2.9 (1.2) mm, followed by WasherLoc at 5.6 (2.2) mm and Delta at 6.4 (3.3) mm. Retroscrew at 25.5 (11.0) mm had the highest residual displacement, though only 2 completed the cyclic tests. Intrafix, WasherLoc, and Delta were not statistically different, but there was a statistical difference between Retroscrew and the other devices (P < .001).
Stiffness data were calculated from the LTF tests (Table). Mean (SD) stiffness was highest for Intrafix at 129 (32.7) N/mm, followed by WasherLoc at 97 (11.6) N/mm, Delta at 93 (9.5) N/mm, and Retroscrew at 80.2 (8.8) N/mm. Intrafix had statistically higher stiffness compared with WasherLoc (P < .05), Delta (P < .01), and Retroscrew (P < .05). There were no significant differences in stiffness among WasherLoc, Delta, and Retroscrew.
Mean (SD) ultimate LTF was highest for Intrafix at 656 (182.6) N, followed by WasherLoc at 630 (129.3) N, Delta at 430 (90.0) N, and Retroscrew at 285 (33.8) N (Table). There were significant differences between Intrafix and Delta (P < .05) and Retroscrew (P < .05). WasherLoc failed at a significantly higher load compared with Delta (P < .05) and Retroscrew (P < .05). There were no significant differences in mean LTF between Intrafix and WasherLoc.
Discussion
In this biomechanical comparison of 4 different tibial fixation devices, Intrafix had results superior to those of the other implants. Intrafix failed at higher LTF and lower residual displacement and had higher stiffness. WasherLoc performed well and had LTF similar to that of Intrafix. The interference screws performed poorly with respect to LTF, residual displacement, and stiffness, and a large proportion of them failed early into cyclic loading.
Intrafix is a central fixation device that uses a 4-quadrant sleeve and a screw to establish tensioning across all 4 hamstring graft strands. The theory is this configuration increases the contact area between graft and bone for proper integration of graft into bone. Intrafix has performed well in other biomechanical studies. Using a study design similar to ours, Kousa and colleagues7 found the performance of Intrafix to be superior to that of other devices, including interference screws and WasherLoc. Starch and colleagues10 reported that, compared with a standard interference screw, Intrafix required significantly higher load to cause a millimeter of graft laxity. They concluded that this demonstrates superior fixation strength and reduced laxity of the graft after cyclic loading. Coleridge and Amis4 found that, compared with WasherLoc and various interference screws, Intrafix had the lower residual displacement. However, they also found that, compared with Intrafix and interference screws, WasherLoc had the highest ultimate tensile strength. Their findings may be difficult to compare with ours, as they tested fixation of calf extensor tendons, and we tested human hamstring grafts.
An important concern in the present study was the poor performance of the interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.11 Delta screws and Retroscrews have not been specifically evaluated, and their fixation strengths have not been directly compared with those of other devices. In the present study, Delta screws and Retroscrews consistently performed the poorest with respect to ultimate LTF, residual displacement, and stiffness. Twenty percent of the Delta screws and 80% of the Retroscrews did not complete 1500 cycles. The poor performance of the interference screws was echoed in studies by Magen and colleagues12 and Kousa and colleagues,7 in which the only complete failures were in the cyclic loading of the interference screws.
Three possible confounding factors may have affected the performance of the interference screws: bone density of porcine tibia, length of interference screw, and location of screw placement. In addition, in clinical practice these screws may be used with other modes of graft fixation. Combined fixation (interference screws, other devices) was not evaluated in this study.
Porcine models have been used in many biomechanical graft fixation studies.4,6,7,12,13 Some authors have found porcine tibia to be a poor substitute for human cadaver tibia because the volumetric density of porcine bone is higher than that of human bone.14,15 Other authors have demonstrated fairly similar bone density between human and porcine tibia.16 The concern is that interference screw fixation strength correlates with the density of the bone in which screws are fixed.17 Therefore, one limitation of our study is that we did not determine the bone density of the porcine tibias for comparison with that of young human tibias.
Another important variable that could have affected the performance of the interference screws is screw length. One study found no significant difference in screw strength between various lengths, and longer screws failed to protect against graft slippage.18 However, Selby and colleagues19 found that, compared with 28-mm screws, 35-mm bioabsorbable interference screws failed at higher LTF. This is in part why we selected 35-mm Delta screws for our study. Both 35-mm Delta screws and 20-mm Retroscrews performed poorly. However, we could not determine if the poorer performance of Retroscrews was related to their length.
We used an eccentric placement for our interference screws. Although some studies have suggested concentric placement might improve fixation strength by increasing bone–tendon contact,20 Simonian and colleagues21 found no difference in graft slippage or ultimate LTF between eccentrically and concentrically placed screws. Although they were not biomechanically tested in our study, a few grafts were fixed with concentrically placed screws, and these tendons appeared to be more clinically damaged than the eccentrically placed screws.
Combined tibial fixation techniques may be used in clinical practice, but we did not evaluate them in our study. Yoo and colleagues9 compared interference screw, interference screw plus cortical screw and spiked washer, and cortical screw and spiked washer alone. They found that stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with interference screw plus cortical screw and spiked washer. In a similar study, Walsh and colleagues13 demonstrated improved stiffness and LTF in cyclic testing with the combination of retrograde interference screw and suture button over interference screw alone. Further study may include direct comparisons of additional tibial fixation techniques using more than one device. Cost analysis of use of additional fixation devices would be beneficial as well.
Study results have clearly demonstrated that tibial fixation is the weak point in ACL reconstruction3,17 and that early aggressive rehabilitation can help restore range of motion, strength, and function.22,23 Implants that can withstand early loads during rehabilitation periods are therefore of utmost importance.
Conclusion
Intrafix demonstrated superior strength in the fixation of hamstring grafts in the tibia, followed closely by WasherLoc. When used as the sole tibial fixation device, interference screws had low LTF, decreased stiffness, and high residual displacement, which may have clinical implications for early rehabilitation after ACL reconstruction.
Of the procedures performed by surgeons specializing in sports medicine and by general orthopedists, anterior cruciate ligament (ACL) reconstruction remains one of the most common.1 Recent years have seen a trend toward replacing the “gold standard” of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.2 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, it is important to determine the strength of different methods of graft fixation.
Rigid fixation of hamstring grafts is recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand early rehabilitation forces as high as 500 N.2 There is therefore much concern about the strength of tibial fixation, given the lower bone density of the tibial metaphysis versus the femoral metaphysis. In addition, stability is more a concern in the tibia, as the forces are directly in line with the tibial tunnel.3,4
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed. There is much interest in determining which devices have the most fixation strength,4-9 but so far several products have not been compared with one another.
We conducted a study to determine if tibial hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Forty porcine tibias were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 4 different tibial fixation devices (Figure 1): Delta screw and Retroscrew (Arthrex, Naples, Florida), WasherLoc (Arthrotek, Warsaw, Indiana), and Intrafix (Depuy Mitek, Raynham, Massachusetts). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the tibias using the 4 tibial fixation devices. All fixations were done according to manufacturer specifications. All interference screws were placed eccentrically. The testing apparatus and procedure are described in an article by Kousa and colleagues.6 The specimens were mounted on the mechanical testing apparatus by threaded bars and custom clamps to secure fixation (Figure 2). Constant tension was maintained on all 4 strands of the hamstring grafts to equalize the tendons. After the looped end of the hamstring graft was secured by clamps, 25 mm of graft was left between the clamp and the intra-articular tunnel.
In the cyclic loading test, the load was applied parallel to the long axis of the tibial tunnel. A 50-N preload was initially applied to each specimen for 10 seconds. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 120 seconds were performed. Standard force-displacement curves were then generated. Each tibial fixation device underwent 10 cyclic loading tests. Specimens surviving the cyclic loading then underwent a single-cycle load-to-failure (LTF) test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF (yield load) data were generated from the single-cycle LTF test; ultimate LTF was defined as the load at the point where the slope of the load displacement curve initially decreases.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices were similar. In all 10 tests, Intrafix was pulled through the tunnel with the hamstring allografts. WasherLoc failed in each test, with the tendons eventually being pulled through the washer and thus out through the tunnel. Delta screw and Retroscrew both failed with slippage of the fixation device and the tendons pulled out through the tunnel.
For the cyclic loading tests, 8 of the 10 Delta screws and only 2 of the 10 Retroscrews completed the 1500-cycle loading test before failure. The 2 Delta screws that did not complete the testing failed after about 500 cycles, and the 8 Retroscrews that did not complete the testing failed after about 250 cycles. All 10 WasherLoc and Intrafix devices completed the testing.
Residual displacement data were calculated from the cyclic loading tests (Table). Mean (SS) residual displacement was lowest for Intrafix at 2.9 (1.2) mm, followed by WasherLoc at 5.6 (2.2) mm and Delta at 6.4 (3.3) mm. Retroscrew at 25.5 (11.0) mm had the highest residual displacement, though only 2 completed the cyclic tests. Intrafix, WasherLoc, and Delta were not statistically different, but there was a statistical difference between Retroscrew and the other devices (P < .001).
Stiffness data were calculated from the LTF tests (Table). Mean (SD) stiffness was highest for Intrafix at 129 (32.7) N/mm, followed by WasherLoc at 97 (11.6) N/mm, Delta at 93 (9.5) N/mm, and Retroscrew at 80.2 (8.8) N/mm. Intrafix had statistically higher stiffness compared with WasherLoc (P < .05), Delta (P < .01), and Retroscrew (P < .05). There were no significant differences in stiffness among WasherLoc, Delta, and Retroscrew.
Mean (SD) ultimate LTF was highest for Intrafix at 656 (182.6) N, followed by WasherLoc at 630 (129.3) N, Delta at 430 (90.0) N, and Retroscrew at 285 (33.8) N (Table). There were significant differences between Intrafix and Delta (P < .05) and Retroscrew (P < .05). WasherLoc failed at a significantly higher load compared with Delta (P < .05) and Retroscrew (P < .05). There were no significant differences in mean LTF between Intrafix and WasherLoc.
Discussion
In this biomechanical comparison of 4 different tibial fixation devices, Intrafix had results superior to those of the other implants. Intrafix failed at higher LTF and lower residual displacement and had higher stiffness. WasherLoc performed well and had LTF similar to that of Intrafix. The interference screws performed poorly with respect to LTF, residual displacement, and stiffness, and a large proportion of them failed early into cyclic loading.
Intrafix is a central fixation device that uses a 4-quadrant sleeve and a screw to establish tensioning across all 4 hamstring graft strands. The theory is this configuration increases the contact area between graft and bone for proper integration of graft into bone. Intrafix has performed well in other biomechanical studies. Using a study design similar to ours, Kousa and colleagues7 found the performance of Intrafix to be superior to that of other devices, including interference screws and WasherLoc. Starch and colleagues10 reported that, compared with a standard interference screw, Intrafix required significantly higher load to cause a millimeter of graft laxity. They concluded that this demonstrates superior fixation strength and reduced laxity of the graft after cyclic loading. Coleridge and Amis4 found that, compared with WasherLoc and various interference screws, Intrafix had the lower residual displacement. However, they also found that, compared with Intrafix and interference screws, WasherLoc had the highest ultimate tensile strength. Their findings may be difficult to compare with ours, as they tested fixation of calf extensor tendons, and we tested human hamstring grafts.
An important concern in the present study was the poor performance of the interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.11 Delta screws and Retroscrews have not been specifically evaluated, and their fixation strengths have not been directly compared with those of other devices. In the present study, Delta screws and Retroscrews consistently performed the poorest with respect to ultimate LTF, residual displacement, and stiffness. Twenty percent of the Delta screws and 80% of the Retroscrews did not complete 1500 cycles. The poor performance of the interference screws was echoed in studies by Magen and colleagues12 and Kousa and colleagues,7 in which the only complete failures were in the cyclic loading of the interference screws.
Three possible confounding factors may have affected the performance of the interference screws: bone density of porcine tibia, length of interference screw, and location of screw placement. In addition, in clinical practice these screws may be used with other modes of graft fixation. Combined fixation (interference screws, other devices) was not evaluated in this study.
Porcine models have been used in many biomechanical graft fixation studies.4,6,7,12,13 Some authors have found porcine tibia to be a poor substitute for human cadaver tibia because the volumetric density of porcine bone is higher than that of human bone.14,15 Other authors have demonstrated fairly similar bone density between human and porcine tibia.16 The concern is that interference screw fixation strength correlates with the density of the bone in which screws are fixed.17 Therefore, one limitation of our study is that we did not determine the bone density of the porcine tibias for comparison with that of young human tibias.
Another important variable that could have affected the performance of the interference screws is screw length. One study found no significant difference in screw strength between various lengths, and longer screws failed to protect against graft slippage.18 However, Selby and colleagues19 found that, compared with 28-mm screws, 35-mm bioabsorbable interference screws failed at higher LTF. This is in part why we selected 35-mm Delta screws for our study. Both 35-mm Delta screws and 20-mm Retroscrews performed poorly. However, we could not determine if the poorer performance of Retroscrews was related to their length.
We used an eccentric placement for our interference screws. Although some studies have suggested concentric placement might improve fixation strength by increasing bone–tendon contact,20 Simonian and colleagues21 found no difference in graft slippage or ultimate LTF between eccentrically and concentrically placed screws. Although they were not biomechanically tested in our study, a few grafts were fixed with concentrically placed screws, and these tendons appeared to be more clinically damaged than the eccentrically placed screws.
Combined tibial fixation techniques may be used in clinical practice, but we did not evaluate them in our study. Yoo and colleagues9 compared interference screw, interference screw plus cortical screw and spiked washer, and cortical screw and spiked washer alone. They found that stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with interference screw plus cortical screw and spiked washer. In a similar study, Walsh and colleagues13 demonstrated improved stiffness and LTF in cyclic testing with the combination of retrograde interference screw and suture button over interference screw alone. Further study may include direct comparisons of additional tibial fixation techniques using more than one device. Cost analysis of use of additional fixation devices would be beneficial as well.
Study results have clearly demonstrated that tibial fixation is the weak point in ACL reconstruction3,17 and that early aggressive rehabilitation can help restore range of motion, strength, and function.22,23 Implants that can withstand early loads during rehabilitation periods are therefore of utmost importance.
Conclusion
Intrafix demonstrated superior strength in the fixation of hamstring grafts in the tibia, followed closely by WasherLoc. When used as the sole tibial fixation device, interference screws had low LTF, decreased stiffness, and high residual displacement, which may have clinical implications for early rehabilitation after ACL reconstruction.
1. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
2. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
3. Brand J Jr, Weiler A, Caborn DN, Brown CH Jr, Johnson DL. Graft fixation in cruciate ligament reconstruction. Am J Sports Med. 2000;28(5):761-774.
4. Coleridge SD, Amis AA. A comparison of five tibial-fixation systems in hamstring-graft anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2004;12(5):391-397.
5. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
6. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
7. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
8. Weiler A, Hoffmann RF, Stähelin AC, Bail HJ, Siepe CJ, Südkamp NP. Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone. Arthroscopy. 1998;14(1):29-37.
9. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
10. Starch DW, Alexander JW, Noble PC, Reddy S, Lintner DM. Multistranded hamstring tendon graft fixation with a central four-quadrant or a standard tibial interference screw for anterior cruciate ligament reconstruction. Am J Sports Med. 2003;31(3):338-344.
11. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
12. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
13. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
14. Nurmi JT, Järvinen TL, Kannus P, Sievänen H, Toukosalo J, Järvinen M. Compaction versus extraction drilling for fixation of the hamstring tendon graft in anterior cruciate ligament reconstruction. Am J Sports Med. 2002;30(2):167-173.
15. Nurmi JT, Sievänen H, Kannus P, Järvinen M, Järvinen TL. Porcine tibia is a poor substitute for human cadaver tibia for evaluating interference screw fixation. Am J Sports Med. 2004;32(3):765-771.
16. Nagarkatti DG, McKeon BP, Donahue BS, Fulkerson JP. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am J Sports Med. 2001;29(1):67-71.
17. Brand JC Jr, Pienkowski D, Steenlage E, Hamilton D, Johnson DL, Caborn DN. Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med. 2000;28(5):705-710.
18. Stadelmaier DM, Lowe WR, Ilahi OA, Noble PC, Kohl HW 3rd. Cyclic pull-out strength of hamstring tendon graft fixation with soft tissue interference screws. Influence of screw length. Am J Sports Med. 1999;27(6):778-783.
19. Selby JB, Johnson DL, Hester P, Caborn DN. Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel. Am J Sports Med. 2001;29(5):614-619.
20. Shino K, Pflaster DS. Comparison of eccentric and concentric screw placement for hamstring graft fixation in the tibial tunnel. Knee Surg Sports Traumatol Arthrosc. 2000;8(2):73-75.
21. Simonian PT, Sussmann PS, Baldini TH, Crockett HC, Wickiewicz TL. Interference screw position and hamstring graft location for anterior cruciate ligament reconstruction. Arthroscopy. 1998;14(5):459-464.
22. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med. 1990;18(3):292-299.
23. Shelbourne KD, Wilckens JH. Current concepts in anterior cruciate ligament rehabilitation. Orthop Rev. 1990;19(11):957-964.
1. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
2. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
3. Brand J Jr, Weiler A, Caborn DN, Brown CH Jr, Johnson DL. Graft fixation in cruciate ligament reconstruction. Am J Sports Med. 2000;28(5):761-774.
4. Coleridge SD, Amis AA. A comparison of five tibial-fixation systems in hamstring-graft anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2004;12(5):391-397.
5. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
6. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
7. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
8. Weiler A, Hoffmann RF, Stähelin AC, Bail HJ, Siepe CJ, Südkamp NP. Hamstring tendon fixation using interference screws: a biomechanical study in calf tibial bone. Arthroscopy. 1998;14(1):29-37.
9. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
10. Starch DW, Alexander JW, Noble PC, Reddy S, Lintner DM. Multistranded hamstring tendon graft fixation with a central four-quadrant or a standard tibial interference screw for anterior cruciate ligament reconstruction. Am J Sports Med. 2003;31(3):338-344.
11. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
12. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
13. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
14. Nurmi JT, Järvinen TL, Kannus P, Sievänen H, Toukosalo J, Järvinen M. Compaction versus extraction drilling for fixation of the hamstring tendon graft in anterior cruciate ligament reconstruction. Am J Sports Med. 2002;30(2):167-173.
15. Nurmi JT, Sievänen H, Kannus P, Järvinen M, Järvinen TL. Porcine tibia is a poor substitute for human cadaver tibia for evaluating interference screw fixation. Am J Sports Med. 2004;32(3):765-771.
16. Nagarkatti DG, McKeon BP, Donahue BS, Fulkerson JP. Mechanical evaluation of a soft tissue interference screw in free tendon anterior cruciate ligament graft fixation. Am J Sports Med. 2001;29(1):67-71.
17. Brand JC Jr, Pienkowski D, Steenlage E, Hamilton D, Johnson DL, Caborn DN. Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque. Am J Sports Med. 2000;28(5):705-710.
18. Stadelmaier DM, Lowe WR, Ilahi OA, Noble PC, Kohl HW 3rd. Cyclic pull-out strength of hamstring tendon graft fixation with soft tissue interference screws. Influence of screw length. Am J Sports Med. 1999;27(6):778-783.
19. Selby JB, Johnson DL, Hester P, Caborn DN. Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel. Am J Sports Med. 2001;29(5):614-619.
20. Shino K, Pflaster DS. Comparison of eccentric and concentric screw placement for hamstring graft fixation in the tibial tunnel. Knee Surg Sports Traumatol Arthrosc. 2000;8(2):73-75.
21. Simonian PT, Sussmann PS, Baldini TH, Crockett HC, Wickiewicz TL. Interference screw position and hamstring graft location for anterior cruciate ligament reconstruction. Arthroscopy. 1998;14(5):459-464.
22. Shelbourne KD, Nitz P. Accelerated rehabilitation after anterior cruciate ligament reconstruction. Am J Sports Med. 1990;18(3):292-299.
23. Shelbourne KD, Wilckens JH. Current concepts in anterior cruciate ligament rehabilitation. Orthop Rev. 1990;19(11):957-964.
Biomechanical Comparison of Hamstring Tendon Fixation Devices for Anterior Cruciate Ligament Reconstruction: Part 1. Five Femoral Devices
Anterior cruciate ligament (ACL) reconstruction remains one of the most common orthopedic procedures; almost 100,000 are performed in the United States each year, and they are among the procedures more commonly performed by surgeons specializing in sports medicine and by general orthopedists.1,2 Recent years have seen a trend toward replacing the gold standard of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.3 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, graft fixation strength has become a priority in attempts to optimize recovery and rehabilitation.4
Rigid fixation of hamstring grafts is now recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand both early rehabilitation forces as high as 500 N5 and stresses to the native ACL during healing, which may take up to 12 weeks for soft-tissue incorporation.6
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed, and there is controversy regarding which provides the best stability and strength.7 Several studies have tested various fixation devices,8-16 but so far several devices have not been compared with one another.
We conducted a study to determine if femoral hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Fifty porcine femurs were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 5 different femoral fixation devices (Figure 1): Delta screw and Bio-TransFix (Arthrex, Naples, Florida) and Bone Mulch screw, EZLoc, and Zip Loop (Arthrotek, Warsaw, Indiana). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the femurs using the 5 femoral fixation devices. All fixations were done to manufacturer specifications.
Cyclic loading was followed by testing with the load-to-failure (LTF) protocol described by Kousa and colleagues.13 Specimens were tested in a custom load fixture (Figure 2). The base fixture used an adjustable angle vise mounted on a free rotary stage and a free x-y translation stage. This system allowed the load axis to be oriented to and aligned with the graft tunnel in the porcine femur, preventing off-axis or torsional loading of the grafts.
Pneumatic grips equipped with a custom pincer attachment allowed the graft to be grasped under a constant grip force during testing, regardless of graft thinning under tensile loads. Graft specimens were initially looped over a 3.8-mm horizontal metal shaft, and the 2 strands were double-looped at the graft insertion site. The 2 free strands were then drawn up around the metal shaft, and the shaft was placed above the serrated jaws. The metal shaft with enveloping tendon strands rested on a flat shelf at the top of the grip serrations. This configuration prevented the metal shaft and tendon strands from being pulled through the serrations when compressive force was applied to the jaws.
Before the study, the grip design was tested. There was no detectable relative motion of the strands at the grip end during graft testing to failure. The pincer attachment allowed close approach of the grips to the specimen at all femoral condyle orientations, so that a 25-mm length of exposed graft could be obtained for each specimen under initial conditions.
In the cyclic loading test, the load was applied parallel to the long axis of the femoral tunnel. A 50-N preload was initially applied to each specimen for 10 seconds, and the length of the exposed graft between grips and graft insertion was recorded. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 2 seconds (0.5 Hz) were performed. Standard force-displacement curves were then generated.
Specimens surviving the cyclic loading then underwent a single-cycle LTF test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF data were generated from the single-cycle LTF test; ultimate LTF was defined as the maximum load sustained by the specimen during a constant-displacement-rate tensile test for graft pullout.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices differed slightly (Table). Bone Mulch screw failed with a fracture through the femoral condyle extending to the bone tunnel. Zip Loop and EZLoc failed by pulling through their cortical attachment on the lateral femoral condyle. Bio-TransFix broke in the tunnel during LTF. Delta screw failed with slippage of the fixation device, and the tendons pulled out through the tunnel.
For the cyclic loading tests, only 2 of the 10 Delta screws completed the 1500-cycle loading test before failure. Of the 8 Delta screws that did not complete this testing, the majority failed after about 100 cycles. All 10 tests of Bone Mulch, Zip Loop, EZLoc, and Bio-TransFix completed the 1500-cycle loading test.
Residual displacement data were calculated from cyclic loading tests (Table). Mean (SD) residual displacement was lowest for Bio-TransFix at 4.1 (0.4) mm, followed by Bone Mulch at 5.2 (1.0) mm, EZLoc at 6.4 (1.1) mm, and Zip Loop at 6.8 (1.3) mm. Delta screws at 8.2 (1.4) mm had the highest residual displacement, though only 2 completed the cyclic tests. Bio-TransFix had significantly (P < .001) less residual displacement compared with EZLoc, Zip Loop, and Delta. Bone Mulch had significantly less residual displacement compared with Zip Loop (P < .05) and Delta (P < .01).
Stiffness data were calculated from LTF tests (Table). Mean (SD) stiffness was highest for Bone Mulch at 218 (25.9) N/mm, followed by Bio-TransFix at 171 (24.2) N/mm, EZLoc at 122 (24.1) N/mm, Zip Loop at 105 (18.9) N/mm, and Delta at 84 (16.4) N/mm. Bone Mulch had significantly (P < .001) higher stiffness compared with Bio-TransFix, EZLoc, Zip Loop, and Delta. Bio-TransFix had significantly (P < .001) higher stiffness compared with EZLoc, Zip Loop, and Delta.
Mean (SD) ultimate LTF was highest for Bone Mulch at 867 (164) N, followed by Zip Loop at 615 (72.3) N, Bio-TransFix at 552 (141) N, EZLoc at 476 (89.7) N, and Delta at 410 (65.3) N (Table). Bone Mulch failed at a statistically significantly (P < .001) higher load compared with Zip Loop, Bio-TransFix, EZLoc, and Delta. There were no significant differences in mean LTF among Zip Loop, Bio-TransFix, EZLoc, and Delta.
Discussion
In this biomechanical comparison of 5 different femoral fixation devices, the Bone Mulch screw had results superior to those of the other implants. Bone Mulch failed at higher LTF and higher stiffness. Bio-TransFix performed well and had residual displacement similar to that of Bone Mulch, but significantly lower LTF. Overall, EZLoc and Zip Loop were similar to each other in performance. The Delta (interference) screw performed poorly with respect to LTF, residual displacement, and stiffness; a large proportion of these screws failed early into cyclic loading.
Bone Mulch and Bio-TransFix overall outperformed the other fixation devices. These 2 devices are cortical-cancellous suspension devices, which provide transcondylar fixation and resist tensile forces perpendicular to the pullout force. Multiple biomechanical studies have found superior performance for these types of devices compared with various implants.10,13,15,16
Our results were similar to those of Kousa and colleagues,13 who found the Bone Mulch screw to provide highest LTF, highest stiffness, and lowest residual displacement. Another study found significantly higher stiffness for the Bone Mulch screw than for the Endobutton, a cortical suspensory fixation device.14 Bone Mulch failure modes differed, however. In the study by Kousa and colleagues,13 3 specimens failed with bending of the screw tip, and 7 failed with rupture of the tendon loop. All specimens in our study failed with fractures through the condyle. It is unclear why the failure modes differed, as we followed similar manufacturer protocols for inserting the device. It is possible the bone mass density of the porcine femurs differed between studies. This was not reported by Kousa and colleagues,13 and we did not perform testing either. However, all the porcine femurs were about the same age for testing of each device in this study.
Bio-TransFix has also been compared with various implants, but not in the same study. Brown and colleagues8 found the TransFix device significantly stiffer than the Endobutton CL. Shen and colleagues16 determined that TransFix had significantly lower residual displacement compared with Endobutton CL. Milano and colleagues15 compared multiple cortical suspensory fixation devices, including Endobutton CL, with TransFix and Bio-TransFix, and concluded the cortical-cancellous devices (TransFix, Bio-TransFix) offered the best and most predictable results in terms of elongation, fixation strength, and stiffness. TransFix has also been shown to be superior to interference screw fixation in biomechanical studies.10,15
Clinical outcomes of studies using TransFix for femoral fixation have been favorable, with improved Lysholm scores and improved laxity according to the KT-1000 test.17 However, multiple prospective studies have found no clinical difference in knee laxity between interference screw and Endobutton at 1- to 2-year follow-up18-20 and no difference in clinical outcome scores, such as the International Knee Documentation Committee score.11,18-20
Although these studies have shown no major clinical differences at short-term follow-up, the early aggressive rehabilitation period is the larger concern. Our study clearly demonstrated the biomechanical strength of transcondylar devices over other devices. The concern with transcondylar devices (vs other devices) is the increased difficulty that inexperienced surgeons have inserting them. In addition, when removed, transcondylar devices leave a large bone void.
In the present study, an important concern with femoral graft fixation is the poor performance of interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.7 In the present study, Delta screws consistently performed poorest with respect to ultimate LTF, residual displacement, and stiffness. Only 20% of these screws completed 1500 cycles. Poor performance of interference screws has also been seen in other studies in tibial graft fixation21,22 and femoral graft fixation.13-15 Given their poor biomechanical properties, as seen in our study and these other studies, we think use of an interference screw alone is a poor choice for fixation.
Combined fixation techniques—interference screw plus other device(s)—may be used in clinical practice, but the present study did not evaluate any. In a biomechanical study, Yoo and colleagues23 compared an interference screw; an interference screw plus a cortical screw and a spiked washer; and a cortical screw and a spiked washer used alone in the tibia. Stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with the interference screw plus the cortical screw and the spiked washer. In a similar study involving femoral fixation, Oh and colleagues24 demonstrated improved stiffness, residual displacement, and LTF in cyclic testing with the combination of interference screw and Endobutton CL, compared with Endobutton CL alone. Further studies may include direct comparisons of additional femoral fixation techniques using more than 1 device.
The Zip Loop, or Toggle Loc with Zip Loop technology, is a suspensory cortical fixation device. It was initially designed for use in ACL fixation but has also been used in other surgeries, including distal biceps repair25 and ulnar collateral ligament reconstruction.26 The device itself is easy to use; more important, it allows for adjustment of graft length within the bone tunnel after deployment of the cortical fixation. Few biomechanical studies have been conducted with Zip Loop.9,12 The present study is the first to compare Zip Loop with devices other than suspensory cortical fixation devices. Zip Loop performed very well in LTF testing but had lower stiffness and higher residual displacement compared with the transcondylar fixation devices. Despite these findings, we have continued to use this device for femoral fixation in ACL reconstruction because of its ease of insertion, the ability to adjust graft tension within the bone tunnel, and the difficulties encountered inserting and removing transcondylar fixation.
We recognize the limitations in our study design with respect to how axial and cyclical loading compares with the physiologic orientation of the ACL during ambulation and running activities. This biomechanical study was not able to replicate these types of activities. However, it did provide good data supporting early rehabilitation with various fixation devices, though concern with use of interference screws remains.
Conclusion
Superior strength in fixation of hamstring grafts in the femur was demonstrated by Bone Mulch screws, followed closely by Bio-TransFix. Delta screws demonstrated poor displacement, stiffness, and LTF. When used as the sole femoral fixation device, a device with low LTF, decreased stiffness, and high residual displacement should be used cautiously in patients undergoing aggressive rehabilitation.
1. Dooley PJ, Chan DS, Dainty KN, Mohtadi NGH, Whelan DB. Patellar tendon versus hamstring autograft for anterior cruciate ligament rupture in adults. Cochrane Database Syst Rev. 2006;(2):CD005960.
2. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
3. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
4. Hapa O, Barber FA. ACL fixation devices. Sports Med Arthrosc. 2009;17(4):217-223.
5. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
6. Rodeo SA, Arnoczky SP, Torzilli PA, Hidaka C, Warren RF. Tendon-healing in a bone tunnel. A biomechanical and histological study in the dog. J Bone Joint Surg Am. 1993;75(12):1795-1803.
7. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
8. Brown CH Jr, Wilson DR, Hecker AT, Ferragamo M. Graft-bone motion and tensile properties of hamstring and patellar tendon anterior cruciate ligament femoral graft fixation under cyclic loading. Arthroscopy. 2004;20(9):922-935.
9. Conner CS, Perez BA, Morris RP, Buckner JW, Buford WL Jr, Ivey FM. Three femoral fixation devices for anterior cruciate ligament reconstruction: comparison of fixation on the lateral cortex versus the anterior cortex. Arthroscopy. 2010;26(6):796-807.
10. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
11. Harilainen A, Sandelin J, Jansson KA. Cross-pin femoral fixation versus metal interference screw fixation in anterior cruciate ligament reconstruction with hamstring tendons: results of a controlled prospective randomized study with 2-year follow-up. Arthroscopy. 2005;21(1):25-33.
12. Kamelger FS, Onder U, Schmoelz W, Tecklenburg K, Arora R, Fink C. Suspensory fixation of grafts in anterior cruciate ligament reconstruction: a biomechanical comparison of 3 implants. Arthroscopy. 2009;25(7):767-776.
13. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
14. Kudo T, Tohyama H, Minami A, Yasuda K. The effect of cyclic loading on the biomechanical characteristics of the femur–graft–tibia complex after anterior cruciate ligament reconstruction using Bone Mulch screw/WasherLoc fixation. Clin Biomech. 2005;20(4):414-420.
15. Milano G, Mulas PD, Ziranu F, Piras S, Manunta A, Fabbriciani C. Comparison between different femoral fixation devices for ACL reconstruction with doubled hamstring tendon graft: a biomechanical analysis. Arthroscopy. 2006;22(6):660-668.
16. Shen HC, Chang JH, Lee CH, et al. Biomechanical comparison of cross-pin and Endobutton-CL femoral fixation of a flexor tendon graft for anterior cruciate ligament reconstruction—a porcine femur–graft–tibia complex study. J Surg Res. 2010;161(2):282-287.
17. Asik M, Sen C, Tuncay I, Erdil M, Avci C, Taser OF. The mid- to long-term results of the anterior cruciate ligament reconstruction with hamstring tendons using Transfix technique. Knee Surg Sports Traumatol Arthrosc. 2007;15(8):965-972.
18. Capuano L, Hardy P, Longo UG, Denaro V, Maffulli N. No difference in clinical results between femoral transfixation and bio-interference screw fixation in hamstring tendon ACL reconstruction. A preliminary study. Knee. 2008;15(3):174-179.
19. Price R, Stoney J, Brown G. Prospective randomized comparison of Endobutton versus cross-pin femoral fixation in hamstring anterior cruciate ligament reconstruction with 2-year follow-up. ANZ J Surg. 2010;80(3):162-165.
20. Rose T, Hepp P, Venus J, Stockmar C, Josten C, Lill H. Prospective randomized clinical comparison of femoral transfixation versus bioscrew fixation in hamstring tendon ACL reconstruction—a preliminary report. Knee Surg Sports Traumatol Arthrosc. 2006;14(8):730-738.
21. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
22. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
23. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
24. Oh YH, Namkoong S, Strauss EJ, et al. Hybrid femoral fixation of soft-tissue grafts in anterior cruciate ligament reconstruction using the Endobutton CL and bioabsorbable interference screws: a biomechanical study. Arthroscopy. 2006;22(11):1218-1224.
25. DiRaimo MJ Jr, Maney MD, Deitch JR. Distal biceps tendon repair using the Toggle Loc with Zip Loop. Orthopedics. 2008;31(12). doi: 10.3928/01477447-20081201-05.
26. Morgan RJ, Starman JS, Habet NA, et al. A biomechanical evaluation of ulnar collateral ligament reconstruction using a novel technique for ulnar-sided fixation. Am J Sports Med. 2010;38(7):1448-1455.
Anterior cruciate ligament (ACL) reconstruction remains one of the most common orthopedic procedures; almost 100,000 are performed in the United States each year, and they are among the procedures more commonly performed by surgeons specializing in sports medicine and by general orthopedists.1,2 Recent years have seen a trend toward replacing the gold standard of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.3 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, graft fixation strength has become a priority in attempts to optimize recovery and rehabilitation.4
Rigid fixation of hamstring grafts is now recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand both early rehabilitation forces as high as 500 N5 and stresses to the native ACL during healing, which may take up to 12 weeks for soft-tissue incorporation.6
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed, and there is controversy regarding which provides the best stability and strength.7 Several studies have tested various fixation devices,8-16 but so far several devices have not been compared with one another.
We conducted a study to determine if femoral hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Fifty porcine femurs were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 5 different femoral fixation devices (Figure 1): Delta screw and Bio-TransFix (Arthrex, Naples, Florida) and Bone Mulch screw, EZLoc, and Zip Loop (Arthrotek, Warsaw, Indiana). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the femurs using the 5 femoral fixation devices. All fixations were done to manufacturer specifications.
Cyclic loading was followed by testing with the load-to-failure (LTF) protocol described by Kousa and colleagues.13 Specimens were tested in a custom load fixture (Figure 2). The base fixture used an adjustable angle vise mounted on a free rotary stage and a free x-y translation stage. This system allowed the load axis to be oriented to and aligned with the graft tunnel in the porcine femur, preventing off-axis or torsional loading of the grafts.
Pneumatic grips equipped with a custom pincer attachment allowed the graft to be grasped under a constant grip force during testing, regardless of graft thinning under tensile loads. Graft specimens were initially looped over a 3.8-mm horizontal metal shaft, and the 2 strands were double-looped at the graft insertion site. The 2 free strands were then drawn up around the metal shaft, and the shaft was placed above the serrated jaws. The metal shaft with enveloping tendon strands rested on a flat shelf at the top of the grip serrations. This configuration prevented the metal shaft and tendon strands from being pulled through the serrations when compressive force was applied to the jaws.
Before the study, the grip design was tested. There was no detectable relative motion of the strands at the grip end during graft testing to failure. The pincer attachment allowed close approach of the grips to the specimen at all femoral condyle orientations, so that a 25-mm length of exposed graft could be obtained for each specimen under initial conditions.
In the cyclic loading test, the load was applied parallel to the long axis of the femoral tunnel. A 50-N preload was initially applied to each specimen for 10 seconds, and the length of the exposed graft between grips and graft insertion was recorded. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 2 seconds (0.5 Hz) were performed. Standard force-displacement curves were then generated.
Specimens surviving the cyclic loading then underwent a single-cycle LTF test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF data were generated from the single-cycle LTF test; ultimate LTF was defined as the maximum load sustained by the specimen during a constant-displacement-rate tensile test for graft pullout.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices differed slightly (Table). Bone Mulch screw failed with a fracture through the femoral condyle extending to the bone tunnel. Zip Loop and EZLoc failed by pulling through their cortical attachment on the lateral femoral condyle. Bio-TransFix broke in the tunnel during LTF. Delta screw failed with slippage of the fixation device, and the tendons pulled out through the tunnel.
For the cyclic loading tests, only 2 of the 10 Delta screws completed the 1500-cycle loading test before failure. Of the 8 Delta screws that did not complete this testing, the majority failed after about 100 cycles. All 10 tests of Bone Mulch, Zip Loop, EZLoc, and Bio-TransFix completed the 1500-cycle loading test.
Residual displacement data were calculated from cyclic loading tests (Table). Mean (SD) residual displacement was lowest for Bio-TransFix at 4.1 (0.4) mm, followed by Bone Mulch at 5.2 (1.0) mm, EZLoc at 6.4 (1.1) mm, and Zip Loop at 6.8 (1.3) mm. Delta screws at 8.2 (1.4) mm had the highest residual displacement, though only 2 completed the cyclic tests. Bio-TransFix had significantly (P < .001) less residual displacement compared with EZLoc, Zip Loop, and Delta. Bone Mulch had significantly less residual displacement compared with Zip Loop (P < .05) and Delta (P < .01).
Stiffness data were calculated from LTF tests (Table). Mean (SD) stiffness was highest for Bone Mulch at 218 (25.9) N/mm, followed by Bio-TransFix at 171 (24.2) N/mm, EZLoc at 122 (24.1) N/mm, Zip Loop at 105 (18.9) N/mm, and Delta at 84 (16.4) N/mm. Bone Mulch had significantly (P < .001) higher stiffness compared with Bio-TransFix, EZLoc, Zip Loop, and Delta. Bio-TransFix had significantly (P < .001) higher stiffness compared with EZLoc, Zip Loop, and Delta.
Mean (SD) ultimate LTF was highest for Bone Mulch at 867 (164) N, followed by Zip Loop at 615 (72.3) N, Bio-TransFix at 552 (141) N, EZLoc at 476 (89.7) N, and Delta at 410 (65.3) N (Table). Bone Mulch failed at a statistically significantly (P < .001) higher load compared with Zip Loop, Bio-TransFix, EZLoc, and Delta. There were no significant differences in mean LTF among Zip Loop, Bio-TransFix, EZLoc, and Delta.
Discussion
In this biomechanical comparison of 5 different femoral fixation devices, the Bone Mulch screw had results superior to those of the other implants. Bone Mulch failed at higher LTF and higher stiffness. Bio-TransFix performed well and had residual displacement similar to that of Bone Mulch, but significantly lower LTF. Overall, EZLoc and Zip Loop were similar to each other in performance. The Delta (interference) screw performed poorly with respect to LTF, residual displacement, and stiffness; a large proportion of these screws failed early into cyclic loading.
Bone Mulch and Bio-TransFix overall outperformed the other fixation devices. These 2 devices are cortical-cancellous suspension devices, which provide transcondylar fixation and resist tensile forces perpendicular to the pullout force. Multiple biomechanical studies have found superior performance for these types of devices compared with various implants.10,13,15,16
Our results were similar to those of Kousa and colleagues,13 who found the Bone Mulch screw to provide highest LTF, highest stiffness, and lowest residual displacement. Another study found significantly higher stiffness for the Bone Mulch screw than for the Endobutton, a cortical suspensory fixation device.14 Bone Mulch failure modes differed, however. In the study by Kousa and colleagues,13 3 specimens failed with bending of the screw tip, and 7 failed with rupture of the tendon loop. All specimens in our study failed with fractures through the condyle. It is unclear why the failure modes differed, as we followed similar manufacturer protocols for inserting the device. It is possible the bone mass density of the porcine femurs differed between studies. This was not reported by Kousa and colleagues,13 and we did not perform testing either. However, all the porcine femurs were about the same age for testing of each device in this study.
Bio-TransFix has also been compared with various implants, but not in the same study. Brown and colleagues8 found the TransFix device significantly stiffer than the Endobutton CL. Shen and colleagues16 determined that TransFix had significantly lower residual displacement compared with Endobutton CL. Milano and colleagues15 compared multiple cortical suspensory fixation devices, including Endobutton CL, with TransFix and Bio-TransFix, and concluded the cortical-cancellous devices (TransFix, Bio-TransFix) offered the best and most predictable results in terms of elongation, fixation strength, and stiffness. TransFix has also been shown to be superior to interference screw fixation in biomechanical studies.10,15
Clinical outcomes of studies using TransFix for femoral fixation have been favorable, with improved Lysholm scores and improved laxity according to the KT-1000 test.17 However, multiple prospective studies have found no clinical difference in knee laxity between interference screw and Endobutton at 1- to 2-year follow-up18-20 and no difference in clinical outcome scores, such as the International Knee Documentation Committee score.11,18-20
Although these studies have shown no major clinical differences at short-term follow-up, the early aggressive rehabilitation period is the larger concern. Our study clearly demonstrated the biomechanical strength of transcondylar devices over other devices. The concern with transcondylar devices (vs other devices) is the increased difficulty that inexperienced surgeons have inserting them. In addition, when removed, transcondylar devices leave a large bone void.
In the present study, an important concern with femoral graft fixation is the poor performance of interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.7 In the present study, Delta screws consistently performed poorest with respect to ultimate LTF, residual displacement, and stiffness. Only 20% of these screws completed 1500 cycles. Poor performance of interference screws has also been seen in other studies in tibial graft fixation21,22 and femoral graft fixation.13-15 Given their poor biomechanical properties, as seen in our study and these other studies, we think use of an interference screw alone is a poor choice for fixation.
Combined fixation techniques—interference screw plus other device(s)—may be used in clinical practice, but the present study did not evaluate any. In a biomechanical study, Yoo and colleagues23 compared an interference screw; an interference screw plus a cortical screw and a spiked washer; and a cortical screw and a spiked washer used alone in the tibia. Stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with the interference screw plus the cortical screw and the spiked washer. In a similar study involving femoral fixation, Oh and colleagues24 demonstrated improved stiffness, residual displacement, and LTF in cyclic testing with the combination of interference screw and Endobutton CL, compared with Endobutton CL alone. Further studies may include direct comparisons of additional femoral fixation techniques using more than 1 device.
The Zip Loop, or Toggle Loc with Zip Loop technology, is a suspensory cortical fixation device. It was initially designed for use in ACL fixation but has also been used in other surgeries, including distal biceps repair25 and ulnar collateral ligament reconstruction.26 The device itself is easy to use; more important, it allows for adjustment of graft length within the bone tunnel after deployment of the cortical fixation. Few biomechanical studies have been conducted with Zip Loop.9,12 The present study is the first to compare Zip Loop with devices other than suspensory cortical fixation devices. Zip Loop performed very well in LTF testing but had lower stiffness and higher residual displacement compared with the transcondylar fixation devices. Despite these findings, we have continued to use this device for femoral fixation in ACL reconstruction because of its ease of insertion, the ability to adjust graft tension within the bone tunnel, and the difficulties encountered inserting and removing transcondylar fixation.
We recognize the limitations in our study design with respect to how axial and cyclical loading compares with the physiologic orientation of the ACL during ambulation and running activities. This biomechanical study was not able to replicate these types of activities. However, it did provide good data supporting early rehabilitation with various fixation devices, though concern with use of interference screws remains.
Conclusion
Superior strength in fixation of hamstring grafts in the femur was demonstrated by Bone Mulch screws, followed closely by Bio-TransFix. Delta screws demonstrated poor displacement, stiffness, and LTF. When used as the sole femoral fixation device, a device with low LTF, decreased stiffness, and high residual displacement should be used cautiously in patients undergoing aggressive rehabilitation.
Anterior cruciate ligament (ACL) reconstruction remains one of the most common orthopedic procedures; almost 100,000 are performed in the United States each year, and they are among the procedures more commonly performed by surgeons specializing in sports medicine and by general orthopedists.1,2 Recent years have seen a trend toward replacing the gold standard of bone–patellar tendon–bone autograft with autograft or allograft hamstring tendon in ACL reconstruction.3 This shift is being made to try to avoid the donor-site morbidity of patellar tendon autografts and decrease the incidence of postoperative anterior knee pain. With increased use of hamstring grafts in ACL reconstruction, graft fixation strength has become a priority in attempts to optimize recovery and rehabilitation.4
Rigid fixation of hamstring grafts is now recognized as a crucial factor in the long-term success of ACL reconstruction. Grafts must withstand both early rehabilitation forces as high as 500 N5 and stresses to the native ACL during healing, which may take up to 12 weeks for soft-tissue incorporation.6
The challenge has been to engineer devices that provide stable, rigid graft fixation that allows expeditious tendon-to-bone healing and increased construct stiffness. Many new fixation devices are being marketed, and there is controversy regarding which provides the best stability and strength.7 Several studies have tested various fixation devices,8-16 but so far several devices have not been compared with one another.
We conducted a study to determine if femoral hamstring fixation devices used in ACL reconstruction differ in fixation strength. We hypothesized we would find no differences.
Materials and Methods
Fifty porcine femurs were harvested after the animals had been euthanized for other studies at our institution. Our study was approved by the institutional animal care and use committee. Specimens were stored at –25°C and, on day of testing, thawed to room temperature. Gracilis and semitendinosus tendon grafts were donated by a tissue bank (LifeNet Health, Virginia Beach, Virginia). The grafts were stored at –25°C; on day of testing, tendons were thawed to room temperature.
We evaluated 5 different femoral fixation devices (Figure 1): Delta screw and Bio-TransFix (Arthrex, Naples, Florida) and Bone Mulch screw, EZLoc, and Zip Loop (Arthrotek, Warsaw, Indiana). For each device, 10 ACL fixation constructs were tested.
Quadrupled human semitendinosus–gracilis tendon grafts were fixed into the femurs using the 5 femoral fixation devices. All fixations were done to manufacturer specifications.
Cyclic loading was followed by testing with the load-to-failure (LTF) protocol described by Kousa and colleagues.13 Specimens were tested in a custom load fixture (Figure 2). The base fixture used an adjustable angle vise mounted on a free rotary stage and a free x-y translation stage. This system allowed the load axis to be oriented to and aligned with the graft tunnel in the porcine femur, preventing off-axis or torsional loading of the grafts.
Pneumatic grips equipped with a custom pincer attachment allowed the graft to be grasped under a constant grip force during testing, regardless of graft thinning under tensile loads. Graft specimens were initially looped over a 3.8-mm horizontal metal shaft, and the 2 strands were double-looped at the graft insertion site. The 2 free strands were then drawn up around the metal shaft, and the shaft was placed above the serrated jaws. The metal shaft with enveloping tendon strands rested on a flat shelf at the top of the grip serrations. This configuration prevented the metal shaft and tendon strands from being pulled through the serrations when compressive force was applied to the jaws.
Before the study, the grip design was tested. There was no detectable relative motion of the strands at the grip end during graft testing to failure. The pincer attachment allowed close approach of the grips to the specimen at all femoral condyle orientations, so that a 25-mm length of exposed graft could be obtained for each specimen under initial conditions.
In the cyclic loading test, the load was applied parallel to the long axis of the femoral tunnel. A 50-N preload was initially applied to each specimen for 10 seconds, and the length of the exposed graft between grips and graft insertion was recorded. Subsequently, 1500 loading cycles between 50 N and 200 N at a rate of 1 cycle per 2 seconds (0.5 Hz) were performed. Standard force-displacement curves were then generated.
Specimens surviving the cyclic loading then underwent a single-cycle LTF test in which the load was applied parallel to the long axis of the drill hole at a rate of 50 mm per minute.
Residual displacement, stiffness, and ultimate LTF data were recorded from the force-displacement curves. Residual displacement data were generated from the cyclic loading test; residual displacement was determined by subtracting preload displacement from displacement at 1, 10, 50, 100, 250, 500, 1000, and 1500 cycles. Stiffness data were generated from the single-cycle LTF test; stiffness was defined as the linear region slope of the force-displacement curve corresponding to the steepest straight-line tangent to the loading curve. Ultimate LTF data were generated from the single-cycle LTF test; ultimate LTF was defined as the maximum load sustained by the specimen during a constant-displacement-rate tensile test for graft pullout.
Statistical analysis generated standard descriptive statistics: means, standard deviations, and proportions. One-way analysis of variance (ANOVA) was used to determine any statistically significant differences in stiffness, yield load, and residual displacement between the different fixation devices. Differences in force (load) between the single cycle and the cyclic loading test were determined by ANOVA. P < .05 was considered statistically significant for all tests.
Results
The modes of failure for the devices differed slightly (Table). Bone Mulch screw failed with a fracture through the femoral condyle extending to the bone tunnel. Zip Loop and EZLoc failed by pulling through their cortical attachment on the lateral femoral condyle. Bio-TransFix broke in the tunnel during LTF. Delta screw failed with slippage of the fixation device, and the tendons pulled out through the tunnel.
For the cyclic loading tests, only 2 of the 10 Delta screws completed the 1500-cycle loading test before failure. Of the 8 Delta screws that did not complete this testing, the majority failed after about 100 cycles. All 10 tests of Bone Mulch, Zip Loop, EZLoc, and Bio-TransFix completed the 1500-cycle loading test.
Residual displacement data were calculated from cyclic loading tests (Table). Mean (SD) residual displacement was lowest for Bio-TransFix at 4.1 (0.4) mm, followed by Bone Mulch at 5.2 (1.0) mm, EZLoc at 6.4 (1.1) mm, and Zip Loop at 6.8 (1.3) mm. Delta screws at 8.2 (1.4) mm had the highest residual displacement, though only 2 completed the cyclic tests. Bio-TransFix had significantly (P < .001) less residual displacement compared with EZLoc, Zip Loop, and Delta. Bone Mulch had significantly less residual displacement compared with Zip Loop (P < .05) and Delta (P < .01).
Stiffness data were calculated from LTF tests (Table). Mean (SD) stiffness was highest for Bone Mulch at 218 (25.9) N/mm, followed by Bio-TransFix at 171 (24.2) N/mm, EZLoc at 122 (24.1) N/mm, Zip Loop at 105 (18.9) N/mm, and Delta at 84 (16.4) N/mm. Bone Mulch had significantly (P < .001) higher stiffness compared with Bio-TransFix, EZLoc, Zip Loop, and Delta. Bio-TransFix had significantly (P < .001) higher stiffness compared with EZLoc, Zip Loop, and Delta.
Mean (SD) ultimate LTF was highest for Bone Mulch at 867 (164) N, followed by Zip Loop at 615 (72.3) N, Bio-TransFix at 552 (141) N, EZLoc at 476 (89.7) N, and Delta at 410 (65.3) N (Table). Bone Mulch failed at a statistically significantly (P < .001) higher load compared with Zip Loop, Bio-TransFix, EZLoc, and Delta. There were no significant differences in mean LTF among Zip Loop, Bio-TransFix, EZLoc, and Delta.
Discussion
In this biomechanical comparison of 5 different femoral fixation devices, the Bone Mulch screw had results superior to those of the other implants. Bone Mulch failed at higher LTF and higher stiffness. Bio-TransFix performed well and had residual displacement similar to that of Bone Mulch, but significantly lower LTF. Overall, EZLoc and Zip Loop were similar to each other in performance. The Delta (interference) screw performed poorly with respect to LTF, residual displacement, and stiffness; a large proportion of these screws failed early into cyclic loading.
Bone Mulch and Bio-TransFix overall outperformed the other fixation devices. These 2 devices are cortical-cancellous suspension devices, which provide transcondylar fixation and resist tensile forces perpendicular to the pullout force. Multiple biomechanical studies have found superior performance for these types of devices compared with various implants.10,13,15,16
Our results were similar to those of Kousa and colleagues,13 who found the Bone Mulch screw to provide highest LTF, highest stiffness, and lowest residual displacement. Another study found significantly higher stiffness for the Bone Mulch screw than for the Endobutton, a cortical suspensory fixation device.14 Bone Mulch failure modes differed, however. In the study by Kousa and colleagues,13 3 specimens failed with bending of the screw tip, and 7 failed with rupture of the tendon loop. All specimens in our study failed with fractures through the condyle. It is unclear why the failure modes differed, as we followed similar manufacturer protocols for inserting the device. It is possible the bone mass density of the porcine femurs differed between studies. This was not reported by Kousa and colleagues,13 and we did not perform testing either. However, all the porcine femurs were about the same age for testing of each device in this study.
Bio-TransFix has also been compared with various implants, but not in the same study. Brown and colleagues8 found the TransFix device significantly stiffer than the Endobutton CL. Shen and colleagues16 determined that TransFix had significantly lower residual displacement compared with Endobutton CL. Milano and colleagues15 compared multiple cortical suspensory fixation devices, including Endobutton CL, with TransFix and Bio-TransFix, and concluded the cortical-cancellous devices (TransFix, Bio-TransFix) offered the best and most predictable results in terms of elongation, fixation strength, and stiffness. TransFix has also been shown to be superior to interference screw fixation in biomechanical studies.10,15
Clinical outcomes of studies using TransFix for femoral fixation have been favorable, with improved Lysholm scores and improved laxity according to the KT-1000 test.17 However, multiple prospective studies have found no clinical difference in knee laxity between interference screw and Endobutton at 1- to 2-year follow-up18-20 and no difference in clinical outcome scores, such as the International Knee Documentation Committee score.11,18-20
Although these studies have shown no major clinical differences at short-term follow-up, the early aggressive rehabilitation period is the larger concern. Our study clearly demonstrated the biomechanical strength of transcondylar devices over other devices. The concern with transcondylar devices (vs other devices) is the increased difficulty that inexperienced surgeons have inserting them. In addition, when removed, transcondylar devices leave a large bone void.
In the present study, an important concern with femoral graft fixation is the poor performance of interference screws. Other authors recently expressed concern with using interference screws in soft-tissue ACL grafts—based on biomechanical study results of increased slippage, bone tunnel widening, and less strength.7 In the present study, Delta screws consistently performed poorest with respect to ultimate LTF, residual displacement, and stiffness. Only 20% of these screws completed 1500 cycles. Poor performance of interference screws has also been seen in other studies in tibial graft fixation21,22 and femoral graft fixation.13-15 Given their poor biomechanical properties, as seen in our study and these other studies, we think use of an interference screw alone is a poor choice for fixation.
Combined fixation techniques—interference screw plus other device(s)—may be used in clinical practice, but the present study did not evaluate any. In a biomechanical study, Yoo and colleagues23 compared an interference screw; an interference screw plus a cortical screw and a spiked washer; and a cortical screw and a spiked washer used alone in the tibia. Stiffness nearly doubled, residual displacement was less, and ultimate LTF was significantly higher in the group with the interference screw plus the cortical screw and the spiked washer. In a similar study involving femoral fixation, Oh and colleagues24 demonstrated improved stiffness, residual displacement, and LTF in cyclic testing with the combination of interference screw and Endobutton CL, compared with Endobutton CL alone. Further studies may include direct comparisons of additional femoral fixation techniques using more than 1 device.
The Zip Loop, or Toggle Loc with Zip Loop technology, is a suspensory cortical fixation device. It was initially designed for use in ACL fixation but has also been used in other surgeries, including distal biceps repair25 and ulnar collateral ligament reconstruction.26 The device itself is easy to use; more important, it allows for adjustment of graft length within the bone tunnel after deployment of the cortical fixation. Few biomechanical studies have been conducted with Zip Loop.9,12 The present study is the first to compare Zip Loop with devices other than suspensory cortical fixation devices. Zip Loop performed very well in LTF testing but had lower stiffness and higher residual displacement compared with the transcondylar fixation devices. Despite these findings, we have continued to use this device for femoral fixation in ACL reconstruction because of its ease of insertion, the ability to adjust graft tension within the bone tunnel, and the difficulties encountered inserting and removing transcondylar fixation.
We recognize the limitations in our study design with respect to how axial and cyclical loading compares with the physiologic orientation of the ACL during ambulation and running activities. This biomechanical study was not able to replicate these types of activities. However, it did provide good data supporting early rehabilitation with various fixation devices, though concern with use of interference screws remains.
Conclusion
Superior strength in fixation of hamstring grafts in the femur was demonstrated by Bone Mulch screws, followed closely by Bio-TransFix. Delta screws demonstrated poor displacement, stiffness, and LTF. When used as the sole femoral fixation device, a device with low LTF, decreased stiffness, and high residual displacement should be used cautiously in patients undergoing aggressive rehabilitation.
1. Dooley PJ, Chan DS, Dainty KN, Mohtadi NGH, Whelan DB. Patellar tendon versus hamstring autograft for anterior cruciate ligament rupture in adults. Cochrane Database Syst Rev. 2006;(2):CD005960.
2. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
3. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
4. Hapa O, Barber FA. ACL fixation devices. Sports Med Arthrosc. 2009;17(4):217-223.
5. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
6. Rodeo SA, Arnoczky SP, Torzilli PA, Hidaka C, Warren RF. Tendon-healing in a bone tunnel. A biomechanical and histological study in the dog. J Bone Joint Surg Am. 1993;75(12):1795-1803.
7. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
8. Brown CH Jr, Wilson DR, Hecker AT, Ferragamo M. Graft-bone motion and tensile properties of hamstring and patellar tendon anterior cruciate ligament femoral graft fixation under cyclic loading. Arthroscopy. 2004;20(9):922-935.
9. Conner CS, Perez BA, Morris RP, Buckner JW, Buford WL Jr, Ivey FM. Three femoral fixation devices for anterior cruciate ligament reconstruction: comparison of fixation on the lateral cortex versus the anterior cortex. Arthroscopy. 2010;26(6):796-807.
10. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
11. Harilainen A, Sandelin J, Jansson KA. Cross-pin femoral fixation versus metal interference screw fixation in anterior cruciate ligament reconstruction with hamstring tendons: results of a controlled prospective randomized study with 2-year follow-up. Arthroscopy. 2005;21(1):25-33.
12. Kamelger FS, Onder U, Schmoelz W, Tecklenburg K, Arora R, Fink C. Suspensory fixation of grafts in anterior cruciate ligament reconstruction: a biomechanical comparison of 3 implants. Arthroscopy. 2009;25(7):767-776.
13. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
14. Kudo T, Tohyama H, Minami A, Yasuda K. The effect of cyclic loading on the biomechanical characteristics of the femur–graft–tibia complex after anterior cruciate ligament reconstruction using Bone Mulch screw/WasherLoc fixation. Clin Biomech. 2005;20(4):414-420.
15. Milano G, Mulas PD, Ziranu F, Piras S, Manunta A, Fabbriciani C. Comparison between different femoral fixation devices for ACL reconstruction with doubled hamstring tendon graft: a biomechanical analysis. Arthroscopy. 2006;22(6):660-668.
16. Shen HC, Chang JH, Lee CH, et al. Biomechanical comparison of cross-pin and Endobutton-CL femoral fixation of a flexor tendon graft for anterior cruciate ligament reconstruction—a porcine femur–graft–tibia complex study. J Surg Res. 2010;161(2):282-287.
17. Asik M, Sen C, Tuncay I, Erdil M, Avci C, Taser OF. The mid- to long-term results of the anterior cruciate ligament reconstruction with hamstring tendons using Transfix technique. Knee Surg Sports Traumatol Arthrosc. 2007;15(8):965-972.
18. Capuano L, Hardy P, Longo UG, Denaro V, Maffulli N. No difference in clinical results between femoral transfixation and bio-interference screw fixation in hamstring tendon ACL reconstruction. A preliminary study. Knee. 2008;15(3):174-179.
19. Price R, Stoney J, Brown G. Prospective randomized comparison of Endobutton versus cross-pin femoral fixation in hamstring anterior cruciate ligament reconstruction with 2-year follow-up. ANZ J Surg. 2010;80(3):162-165.
20. Rose T, Hepp P, Venus J, Stockmar C, Josten C, Lill H. Prospective randomized clinical comparison of femoral transfixation versus bioscrew fixation in hamstring tendon ACL reconstruction—a preliminary report. Knee Surg Sports Traumatol Arthrosc. 2006;14(8):730-738.
21. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
22. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
23. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
24. Oh YH, Namkoong S, Strauss EJ, et al. Hybrid femoral fixation of soft-tissue grafts in anterior cruciate ligament reconstruction using the Endobutton CL and bioabsorbable interference screws: a biomechanical study. Arthroscopy. 2006;22(11):1218-1224.
25. DiRaimo MJ Jr, Maney MD, Deitch JR. Distal biceps tendon repair using the Toggle Loc with Zip Loop. Orthopedics. 2008;31(12). doi: 10.3928/01477447-20081201-05.
26. Morgan RJ, Starman JS, Habet NA, et al. A biomechanical evaluation of ulnar collateral ligament reconstruction using a novel technique for ulnar-sided fixation. Am J Sports Med. 2010;38(7):1448-1455.
1. Dooley PJ, Chan DS, Dainty KN, Mohtadi NGH, Whelan DB. Patellar tendon versus hamstring autograft for anterior cruciate ligament rupture in adults. Cochrane Database Syst Rev. 2006;(2):CD005960.
2. Garrett WE Jr, Swiontkowski MF, Weinsten JN, et al. American Board of Orthopaedic Surgery Practice of the Orthopaedic Surgeon: part-II, certification examination case mix. J Bone Joint Surg Am. 2006;88(3):660-667.
3. West RV, Harner CD. Graft selection in anterior cruciate ligament reconstruction. J Am Acad Orthop Surg. 2005;13(3):197-207.
4. Hapa O, Barber FA. ACL fixation devices. Sports Med Arthrosc. 2009;17(4):217-223.
5. Walsh MP, Wijdicks CA, Parker JB, Hapa O, LaPrade RF. A comparison between a retrograde interference screw, suture button, and combined fixation on the tibial side in an all-inside anterior cruciate ligament reconstruction: a biomechanical study in a porcine model. Am J Sports Med. 2009;37(1):160-167.
6. Rodeo SA, Arnoczky SP, Torzilli PA, Hidaka C, Warren RF. Tendon-healing in a bone tunnel. A biomechanical and histological study in the dog. J Bone Joint Surg Am. 1993;75(12):1795-1803.
7. Prodromos CC, Fu FH, Howell SM, Johnson DH, Lawhorn K. Controversies in soft-tissue anterior cruciate ligament reconstruction: grafts, bundles, tunnels, fixation, and harvest. J Am Acad Orthop Surg. 2008;16(7):376-384.
8. Brown CH Jr, Wilson DR, Hecker AT, Ferragamo M. Graft-bone motion and tensile properties of hamstring and patellar tendon anterior cruciate ligament femoral graft fixation under cyclic loading. Arthroscopy. 2004;20(9):922-935.
9. Conner CS, Perez BA, Morris RP, Buckner JW, Buford WL Jr, Ivey FM. Three femoral fixation devices for anterior cruciate ligament reconstruction: comparison of fixation on the lateral cortex versus the anterior cortex. Arthroscopy. 2010;26(6):796-807.
10. Fabbriciani C, Mulas PD, Ziranu F, Deriu L, Zarelli D, Milano G. Mechanical analysis of fixation methods for anterior cruciate ligament reconstruction with hamstring tendon graft. An experimental study in sheep knees. Knee. 2005;12(2):135-138.
11. Harilainen A, Sandelin J, Jansson KA. Cross-pin femoral fixation versus metal interference screw fixation in anterior cruciate ligament reconstruction with hamstring tendons: results of a controlled prospective randomized study with 2-year follow-up. Arthroscopy. 2005;21(1):25-33.
12. Kamelger FS, Onder U, Schmoelz W, Tecklenburg K, Arora R, Fink C. Suspensory fixation of grafts in anterior cruciate ligament reconstruction: a biomechanical comparison of 3 implants. Arthroscopy. 2009;25(7):767-776.
13. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part I: femoral site. Am J Sports Med. 2003;31(2):174-181.
14. Kudo T, Tohyama H, Minami A, Yasuda K. The effect of cyclic loading on the biomechanical characteristics of the femur–graft–tibia complex after anterior cruciate ligament reconstruction using Bone Mulch screw/WasherLoc fixation. Clin Biomech. 2005;20(4):414-420.
15. Milano G, Mulas PD, Ziranu F, Piras S, Manunta A, Fabbriciani C. Comparison between different femoral fixation devices for ACL reconstruction with doubled hamstring tendon graft: a biomechanical analysis. Arthroscopy. 2006;22(6):660-668.
16. Shen HC, Chang JH, Lee CH, et al. Biomechanical comparison of cross-pin and Endobutton-CL femoral fixation of a flexor tendon graft for anterior cruciate ligament reconstruction—a porcine femur–graft–tibia complex study. J Surg Res. 2010;161(2):282-287.
17. Asik M, Sen C, Tuncay I, Erdil M, Avci C, Taser OF. The mid- to long-term results of the anterior cruciate ligament reconstruction with hamstring tendons using Transfix technique. Knee Surg Sports Traumatol Arthrosc. 2007;15(8):965-972.
18. Capuano L, Hardy P, Longo UG, Denaro V, Maffulli N. No difference in clinical results between femoral transfixation and bio-interference screw fixation in hamstring tendon ACL reconstruction. A preliminary study. Knee. 2008;15(3):174-179.
19. Price R, Stoney J, Brown G. Prospective randomized comparison of Endobutton versus cross-pin femoral fixation in hamstring anterior cruciate ligament reconstruction with 2-year follow-up. ANZ J Surg. 2010;80(3):162-165.
20. Rose T, Hepp P, Venus J, Stockmar C, Josten C, Lill H. Prospective randomized clinical comparison of femoral transfixation versus bioscrew fixation in hamstring tendon ACL reconstruction—a preliminary report. Knee Surg Sports Traumatol Arthrosc. 2006;14(8):730-738.
21. Kousa P, Järvinen TL, Vihavainen M, Kannus P, Järvinen M. The fixation strength of six hamstring tendon graft fixation devices in anterior cruciate ligament reconstruction. Part II: tibial site. Am J Sports Med. 2003;31(2):182-188.
22. Magen HE, Howell SM, Hull ML. Structural properties of six tibial fixation methods for anterior cruciate ligament soft tissue grafts. Am J Sports Med. 1999;27(1):35-43.
23. Yoo JC, Ahn JH, Kim JH, et al. Biomechanical testing of hybrid hamstring graft tibial fixation in anterior cruciate ligament reconstruction. Knee. 2006;13(6):455-459.
24. Oh YH, Namkoong S, Strauss EJ, et al. Hybrid femoral fixation of soft-tissue grafts in anterior cruciate ligament reconstruction using the Endobutton CL and bioabsorbable interference screws: a biomechanical study. Arthroscopy. 2006;22(11):1218-1224.
25. DiRaimo MJ Jr, Maney MD, Deitch JR. Distal biceps tendon repair using the Toggle Loc with Zip Loop. Orthopedics. 2008;31(12). doi: 10.3928/01477447-20081201-05.
26. Morgan RJ, Starman JS, Habet NA, et al. A biomechanical evaluation of ulnar collateral ligament reconstruction using a novel technique for ulnar-sided fixation. Am J Sports Med. 2010;38(7):1448-1455.
