Study finds extensor mechanism problems may be more prevalent in rotating platform TKA devices
Ormonde M. Mahoney
Although we could identify no functional difference between the two devices, there was a trend toward more extensor mechanism complaints in the rotating platform knees, said lead investigator Ormonde M. Mahoney, MD, president of Athens Orthopedic Clinic in Athens, Ga. Therefore, there is room to be somewhat concerned about the possibility of encountering extensor mechanism problems at a higher rate in this type of mobile-bearing device.
Mahoney previously served as principal investigator of a 14-center FDA Investigational Device Exemption study sponsored by Stryker Orthopedics, which assessed the companys single-radius, posterior-stabilized, mobile-bearing total knee arthroplasty (TKA) device. The prospective, randomized study compared that mobile-bearing device to a single-radius, posterior-stabilized, fixed-bearing device that was otherwise similar.
That study showed there was no functional difference between the two devices, Mahoney, who presented results of a secondary review of the initial studies at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons, told Orthopedics Today.
The follow-up evaluation consisted of 507 primary TKAs in 416 patients who were randomized to receive fixed-bearing or mobile-bearing devices between November 2001 and August 2007. Follow-up ranged from 2 to 7.5 years for any device failure, revisions or complications.
You would expect to see less anterior knee pain, or extensor mechanism problems or complaints in a rotating platform knee, Mahoney said. But we saw the opposite. There was certainly no less frequent anterior knee pain or extensor mechanism problem in these rotating platform knees. In fact, there was a trend toward higher levels of problems and pain, which is slightly counterintuitive.
Extensor mechanism-related complications were reported in 23 of the 252 mobile-bearing (MB) knees, compared with 12 of the 255 fixed-bearing (FB) knees. Most common were tendonitis (six MB, two FB); patellar subluxation (four MB, two FB); patellar clunk (four MB, three FB); and anterior knee pain (four MB, two FB).
Because the study was underpowered, we were unable to show a statistically significant difference between the two groups; however, just a difference of a few patients would have given us a statistically significant result, Mahoney said. Complications appeared to be related to the mobile-bearing tibial component design.
Mahoney said mobile-bearing devices may actually increase the stress across the patellar femoral joint rather than reducing it as some have suggested. In fact, as the knee flexes and the tibia rotates internally, the mobile bearing insert will rotate externally relative to the bony tibia, which results in forward displacement of the medial femuro-tibial contact point. This paradoxical shift in the contact point anteriorly, he said, can increase stresses across the patello-femoral joint, potentially leading to the extensor mechanism seen here and in other series.
Furthermore, the development of improved wear-resistant plastics may make the theoretic wear advantage of rotating platform devices relatively obsolete, he said.
Nonetheless, each individual physician needs to weigh the risks and benefits of a mobile-bearing device vs. a fixed-bearing device, according to Mahoney.
That varies from one design to another. There are fixed-bearing devices that are as rotationally free as a mobile-bearing device, he said. by Bob Kronemyer
- Mahoney OM, Kinsey T. Increased rate of extensor mechanism problems in rotating platform total knees. Paper #587. Presented at the 2011 Annual Meeting of the American Academy of Orthopaedic Surgeons. Feb. 15-19. San Diego.
- Ormonde M. Mahoney, MD, can be reached at Athens Orthopedic Clinic, 1765 Old West Broad St., Athens, GA 30606; 706-549-1663; email: firstname.lastname@example.org.
- Disclosure: Mahoney is a consultant for Stryker.