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
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
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
- 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:
- Disclosure: The authors have not received any financial
assistance or interest for the study.