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Mobile bearing unicompartmental knees show high revision rates vs. fixed knees
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A study using the same unicompartmental knee replacement prosthesis found revision rates of 37% at 6.5 years with a mobile bearing variation and 14% with a fixed bearing component.
“This design of [unicompartmental knee replacement] UKR with the mobile bearing has an unacceptably high revision rate, and patients with this knee design should be closely monitored,” Brendan Joss, PhD, James Roche and colleagues wrote in their study. The work was presented on behalf of the authors by Odei Shannak.
Joss and colleagues randomized 40 patients undergoing UKR to receive the same femoral component and to be implanted with either a fixed or mobile bearing tibial component. The investigators defined failure as persistent pain requiring revision to total knee replacement. They compared their results to that of the Australian Orthopaedic Association National Joint Replacement Registry, which showed a 24.2% revision rate at 6.5 years for the mobile bearing.
The investigators postulated that overcorrection of the preoperative deformity may lead to excessive loading of the remaining compartments and cause failure, this was not the case in their sample. Fixed bearing UKRs are flat and have an incongruent surface lending itself to reduced constraint of the prosthesis. Conversely, mobile bearings have polyethylene coating that is congruent, contributing to a constraining surface. When the investigators studied the explanted mobile bearings of the Preservation UKR (DePuy), they found 83% had backside wear.
“Unlike the Oxford mobile-bearing UKR, [the] Preservation mobile bearings run in a captive convex dovetail track. This is designed to allow fore and aft movement but no lateral movement,” Joss told Orthopedics Today. “We therefore postulate that the poly insert stopped moving freely and was not constrained by the tract. Therefore, it acted as a constrained, relatively fixed bearing in conjunction with a high level of congruency; this increased stress of the implant bone interface causing loosening.”– by Renee Blisard
Reference:
- Roche J, Joss B, DeSteiger R, Miller L, et al. Fixed or mobile bearing unicompartmental knee replacement – A randomized controlled trial. Presented at the British Orthopaedic Association and the Irish Orthopaedic Association Combined Meeting 2011. Sept. 13-16. Dublin.
- Brendan Joss, PhD, can be reached at the University of Western Australia, Perth, Australia; 61-8 9386 9961; email: brendan.joss@hfrc.com.
- Disclosure: The authors have not received any financial assistance or interest for the study.