Surgeons Wax Skeptical on Mobile-Bearing Knee Implants

WASHINGTON — Mobile-bearing knee implants are hyped in advertisements and demanded by patients, but the jury is still out on whether the devices deliver what's promised.

During a panel discussion on “controversial issues and hot topics” in primary total knee replacement at the annual meeting of the American Academy of Orthopaedic Surgeons, several panelists objected to the idea that rotating platform knee implants are superior in many ways to fixed-bearing designs.

“There's certainly some skepticism here about whether mobile-bearing designs are really more forgiving [of rotational misalignment of the femoral and tibial components] and whether there truly is less wear,” said William J. Maloney, M.D., professor of orthopedic surgery at Stanford (Calif.) University, who moderated the discussion.

Rotating platform, or mobile-bearing, knee replacements are designed for potentially longer performance with less wear to parts of the prosthesis. The devices have been marketed and are recommended by some physicians, particularly for younger, active, or overweight patients.

The mobile-bearing knees use three components—just like fixed-bearing replacements—but have a different bearing surface. The metallic femoral component and the metallic tibial tray both move across a mobile polyethylene insert. The insert creates a dual-surface articulation, absorbing force across a greater contact surface and ensuring congruent contact between the femoral and tibial components.

These implants, said panelist Douglas A. Dennis, M.D., “allow increased conformity in both planes without dramatically increasing fixation stresses and the risk of component loosening.”

This, he said, reduces polyethylene wear—which should be the focus of “any total knee design.” Polyethylene wear has been the major mode of total knee replacement failure, said Dr. Dennis, of the Rocky Mountain Musculoskeletal Research Laboratory in Denver.

“We have seen in our laboratory better kinematics in gait with mobile bearings. They're more tolerant of condylar lift-off, which should reduce the potential for polyethylene wear, and I think they're more forgiving of component rotational mal-alignment—the bearing has the potential to self-correct,” he said.

In a 10-year study of total knee replacements, Dr. Dennis and his colleagues found that mobile-bearing knees allow for a wider range of axial rotation without creating excessive polyethylene stresses. “A fairly large number [of mobile-bearing knees] rotated greater than 20 degrees, which is beyond the rotational boundaries of most fixed-bearing designs,” he said.

Arlen D. Hanssen, M.D., argued that several studies have shown no difference in motion and no difference in patello-femoral mechanics between fixed and mobile-bearing knees. Early dislocation and instability continue to be a problem with the rotating-platform knee, and recently there have been reports of late dislocation.

“Late dislocation occurs in this knee because of advanced wear,” said Dr. Hanssen, of the Mayo Clinic in Rochester, Minn.

“One of the reasons to use the rotating-platform knee has been to avoid osteolysis wear … but osteolysis seems to be significantly higher [in patients with the mobile-bearing knee],” he said.

The rigid tibial trays that are required in the rotating platform design also contribute to stress shielding of the proximal tibia, he said.

“Why would you take a knee that [has only been studied] in the elderly, has no better motion, no better patello-femoral mechanics, has the unique complication of dislocation and instability, and now appears to have some wear and osteolysis problems and stress shielding problems?” Dr. Hanssen asked. “My answer is no thanks.”

Other panelists agreed. “The advertisements say [the mobile-bearing knee] is the best thing, that it's going to give us 20 years,” said Merrill A. Ritter, M.D., of the Center for Hip and Knee Surgery in Mooresville, Ind. “There [are] no data to support this, and there are too many things that do work.”

Leo A. Whiteside, M.D., of the Missouri Bone and Joint Center in St. Louis, said that theoretically, the mobile-bearing design should perform better. “What worries me [are] the multiple reports of higher wear,” he said.

Bearing surface is just one of several choices surgeons make when performing total knee arthroplasty. The type of fixation, the modularity of implants, and surgical technique are also controversial, Dr. Maloney added.

WASHINGTON — Mobile-bearing knee implants are hyped in advertisements and demanded by patients, but the jury is still out on whether the devices deliver what's promised.

During a panel discussion on “controversial issues and hot topics” in primary total knee replacement at the annual meeting of the American Academy of Orthopaedic Surgeons, several panelists objected to the idea that rotating platform knee implants are superior in many ways to fixed-bearing designs.

“There's certainly some skepticism here about whether mobile-bearing designs are really more forgiving [of rotational misalignment of the femoral and tibial components] and whether there truly is less wear,” said William J. Maloney, M.D., professor of orthopedic surgery at Stanford (Calif.) University, who moderated the discussion.

Rotating platform, or mobile-bearing, knee replacements are designed for potentially longer performance with less wear to parts of the prosthesis. The devices have been marketed and are recommended by some physicians, particularly for younger, active, or overweight patients.

The mobile-bearing knees use three components—just like fixed-bearing replacements—but have a different bearing surface. The metallic femoral component and the metallic tibial tray both move across a mobile polyethylene insert. The insert creates a dual-surface articulation, absorbing force across a greater contact surface and ensuring congruent contact between the femoral and tibial components.

These implants, said panelist Douglas A. Dennis, M.D., “allow increased conformity in both planes without dramatically increasing fixation stresses and the risk of component loosening.”

This, he said, reduces polyethylene wear—which should be the focus of “any total knee design.” Polyethylene wear has been the major mode of total knee replacement failure, said Dr. Dennis, of the Rocky Mountain Musculoskeletal Research Laboratory in Denver.

“We have seen in our laboratory better kinematics in gait with mobile bearings. They're more tolerant of condylar lift-off, which should reduce the potential for polyethylene wear, and I think they're more forgiving of component rotational mal-alignment—the bearing has the potential to self-correct,” he said.

In a 10-year study of total knee replacements, Dr. Dennis and his colleagues found that mobile-bearing knees allow for a wider range of axial rotation without creating excessive polyethylene stresses. “A fairly large number [of mobile-bearing knees] rotated greater than 20 degrees, which is beyond the rotational boundaries of most fixed-bearing designs,” he said.

Arlen D. Hanssen, M.D., argued that several studies have shown no difference in motion and no difference in patello-femoral mechanics between fixed and mobile-bearing knees. Early dislocation and instability continue to be a problem with the rotating-platform knee, and recently there have been reports of late dislocation.

“Late dislocation occurs in this knee because of advanced wear,” said Dr. Hanssen, of the Mayo Clinic in Rochester, Minn.

“One of the reasons to use the rotating-platform knee has been to avoid osteolysis wear … but osteolysis seems to be significantly higher [in patients with the mobile-bearing knee],” he said.

The rigid tibial trays that are required in the rotating platform design also contribute to stress shielding of the proximal tibia, he said.

“Why would you take a knee that [has only been studied] in the elderly, has no better motion, no better patello-femoral mechanics, has the unique complication of dislocation and instability, and now appears to have some wear and osteolysis problems and stress shielding problems?” Dr. Hanssen asked. “My answer is no thanks.”

Other panelists agreed. “The advertisements say [the mobile-bearing knee] is the best thing, that it's going to give us 20 years,” said Merrill A. Ritter, M.D., of the Center for Hip and Knee Surgery in Mooresville, Ind. “There [are] no data to support this, and there are too many things that do work.”

Leo A. Whiteside, M.D., of the Missouri Bone and Joint Center in St. Louis, said that theoretically, the mobile-bearing design should perform better. “What worries me [are] the multiple reports of higher wear,” he said.

Bearing surface is just one of several choices surgeons make when performing total knee arthroplasty. The type of fixation, the modularity of implants, and surgical technique are also controversial, Dr. Maloney added.

WASHINGTON — Mobile-bearing knee implants are hyped in advertisements and demanded by patients, but the jury is still out on whether the devices deliver what's promised.

During a panel discussion on “controversial issues and hot topics” in primary total knee replacement at the annual meeting of the American Academy of Orthopaedic Surgeons, several panelists objected to the idea that rotating platform knee implants are superior in many ways to fixed-bearing designs.

“There's certainly some skepticism here about whether mobile-bearing designs are really more forgiving [of rotational misalignment of the femoral and tibial components] and whether there truly is less wear,” said William J. Maloney, M.D., professor of orthopedic surgery at Stanford (Calif.) University, who moderated the discussion.

Rotating platform, or mobile-bearing, knee replacements are designed for potentially longer performance with less wear to parts of the prosthesis. The devices have been marketed and are recommended by some physicians, particularly for younger, active, or overweight patients.

The mobile-bearing knees use three components—just like fixed-bearing replacements—but have a different bearing surface. The metallic femoral component and the metallic tibial tray both move across a mobile polyethylene insert. The insert creates a dual-surface articulation, absorbing force across a greater contact surface and ensuring congruent contact between the femoral and tibial components.

These implants, said panelist Douglas A. Dennis, M.D., “allow increased conformity in both planes without dramatically increasing fixation stresses and the risk of component loosening.”

This, he said, reduces polyethylene wear—which should be the focus of “any total knee design.” Polyethylene wear has been the major mode of total knee replacement failure, said Dr. Dennis, of the Rocky Mountain Musculoskeletal Research Laboratory in Denver.

“We have seen in our laboratory better kinematics in gait with mobile bearings. They're more tolerant of condylar lift-off, which should reduce the potential for polyethylene wear, and I think they're more forgiving of component rotational mal-alignment—the bearing has the potential to self-correct,” he said.

In a 10-year study of total knee replacements, Dr. Dennis and his colleagues found that mobile-bearing knees allow for a wider range of axial rotation without creating excessive polyethylene stresses. “A fairly large number [of mobile-bearing knees] rotated greater than 20 degrees, which is beyond the rotational boundaries of most fixed-bearing designs,” he said.

Arlen D. Hanssen, M.D., argued that several studies have shown no difference in motion and no difference in patello-femoral mechanics between fixed and mobile-bearing knees. Early dislocation and instability continue to be a problem with the rotating-platform knee, and recently there have been reports of late dislocation.

“Late dislocation occurs in this knee because of advanced wear,” said Dr. Hanssen, of the Mayo Clinic in Rochester, Minn.

“One of the reasons to use the rotating-platform knee has been to avoid osteolysis wear … but osteolysis seems to be significantly higher [in patients with the mobile-bearing knee],” he said.

The rigid tibial trays that are required in the rotating platform design also contribute to stress shielding of the proximal tibia, he said.

“Why would you take a knee that [has only been studied] in the elderly, has no better motion, no better patello-femoral mechanics, has the unique complication of dislocation and instability, and now appears to have some wear and osteolysis problems and stress shielding problems?” Dr. Hanssen asked. “My answer is no thanks.”

Other panelists agreed. “The advertisements say [the mobile-bearing knee] is the best thing, that it's going to give us 20 years,” said Merrill A. Ritter, M.D., of the Center for Hip and Knee Surgery in Mooresville, Ind. “There [are] no data to support this, and there are too many things that do work.”

Leo A. Whiteside, M.D., of the Missouri Bone and Joint Center in St. Louis, said that theoretically, the mobile-bearing design should perform better. “What worries me [are] the multiple reports of higher wear,” he said.

Bearing surface is just one of several choices surgeons make when performing total knee arthroplasty. The type of fixation, the modularity of implants, and surgical technique are also controversial, Dr. Maloney added.