Fixed- vs. Mobile-Bearing Implants in UKA: Is One Better than the Other?

    A systematic review and meta-analysis suggests that neither bearing design is clearly superior.

    Two design concepts are currently in use for unicondylar knee arthroplasty (UKA) prostheses: fixed-bearing and mobile-bearing implants. Mobile-bearing prostheses have become increasingly popular due to their theoretical advantages over the fixed-bearing design:

    • Mobile-bear prostheses have a congruent bearing that is thought to reduce contact stresses and polyethylene wear.
    • Mobile-bearing prostheses are thought to more closely recreate native knee kinematics.

    However, it is not clear whether the mobile-bearing design truly is superior to that of the fixed-bearing prostheses for UKA.

    Dr. Geert Peersman, from ZNA Stuivenberg,Schilde Antwerp, Belgium, and colleagues, recently conducted a systematic review and meta-analysis on outcomes for fixed- versus mobile-bearing UKA. They presented their findings at the recent ICJR East meeting in New York.

    The purpose of the study presented at ICJR East, Dr. Peersman said, was to “examine survivorship differences and differences in failure modes between fixed- and mobile-bearing designs. We also wanted to explore possible differences among age groups.”

    This is significant for Dr. Peersman’s own practice. “About 30% of the knee arthroplasties in my practice are UKAs, and I use both fixed- and mobile-bearing implants,” he said. “I prefer resurfacing fixed-bearing unicompartmental knee arthroplasties in young patients, but I have no preference in patients older than 65 years.”

    Dr. Peersman and his colleagues found 44 studies involving 9,894 knees that met the criteria for eligibility: randomized clinical studies, cohort studies, and case series reporting clinical outcomes for medial and/or lateral UKA.

    The outcomes studied included knee function, survivorship, and the reasons for (and incidence of, revision for fixed- and mobile-bearing prostheses. The revision rate was expressed as number of revisions per 100 component years and was compared between prosthesis designs.

    Mean follow-up was 8.5 years for fixed-bearing and 5.2 years for mobile-bearing prostheses. There were no other relevant differences in baseline characteristics.

    The overall crude revision rate for fixed-bearing and for mobile-bearing prostheses was 1.01 and 1.24 per 100 component years, respectively.

    Cause-specific revision rates were not reported or were incomplete in 57% of studies. In the remaining studies, aseptic loosening accounted for 1.27 and 0.40 revisions per 100 component years for fixed-bearing and mobile-bearing prostheses, respectively. The mean time to revision for aseptic loosening was 7.7 years in the fixed-bearing group and 3.8 years in the mobile-bearing group.

    In the mobile-bearing group, insert dislocation accounted for 0.30 revisions per 100 component years at a mean time to revision of 2.0 years.

    Adjusting for follow-up time resulted in a comparable overall revision rate for fixed-bearing and mobile-bearing prostheses of 1.26 and 1.21 revisions per 100 component years, respectively. In the adjusted analysis, aseptic loosening accounted for 0.34 and 0.35 revisions per 100 component years, respectively.

    The meta-analysis suggests a survival advantage for fixed-bearing prostheses in UKA. However, adjusting for follow-up time confirmed the equivalence of the two UKA designs. The risk of loosening decreases over time. And while insert dislocation is an additional cause of revision in mobile-bearing knees, it does not increase the overall risk of failure. Inferences should be drawn with caution, however, as the meta-analysis is based on observational data with large variations in reporting standards.

    Dr. Peersman has not changed his practice as a result of the study. “My practice supports the results of the study,” he said, “although I intuitively expected the results to favor the mobile-bearing implants.”

    The takeaway message for orthopaedic surgeons, Dr. Peersman said, is that “there are no major differences in survival rate between fixed-bearing and mobile-bearing implants. The revision rate for mobile-bearing seems to be higher in younger patients. The overall risk of aseptic loosening in UKA decreases over time, which confirms that most UKAs fail the first years after surgery, and that early failure is about surgical error.”