Unicompartmental Knee Arthroplasty:
Mobile versus Fixed-Bearing

Richard Kendall, M.D., FRCSC
Clinical Instructor, University of British Columbia
Richmond, BC

Unicompartmental knee arthroplasty (UKA) for arthritis is becoming a more popular procedure in the treatment of isolated single compartment disease (Figure 1). However, when first introduced in the 1970s unicompartmental arthroplasty was associated with high failure rates1, 2 and, for a time, fell out of favour. With better understanding of patient selection (single compartment, non-inflammatory disease, and an intact ACL), implant design (thicker polyethylene, cementing and avoidance of over correction) and minimally invasive techniques, there has been renewed interest in this technique.

Unicompartmental arthroplasty has been shown to be clinically superior to high tibial osteotomy3, 4, 5. Recent studies of UKA versus total knee arthroplasty have shown quicker recovery, better range of motion and greater patient satisfaction in the UKA population6, 7, 8.

Unicompartmental knee replacements are available in two main design types. These are 1) minimally congruent fixed-bearings and 2) fully congruent mobile-bearing, the Oxford Knee (Biomet Inc). To date, the most effective design has not been determined.

Design Considerations, Congruence and Kinematics
The Oxford Knee (Figure 2) was developed by Goodfellow and OConnor9 in 1974 as a fully congruent replacement. Their rationale was to design an implant which ensured maximal contact area between the femoral and tibial components thus minimizing polyethylene wear. The unconstrained bearing provides normal kinematics without excessive stresses at the bone-cement interface. The bearing has a contact area of 5.7cm2 and is available in multiple sizes10. Retrieval studies10 and more recently 10-year roentgenstereophotogametric analysis of in-vivo wear has shown a mean linear wear of 0.02mm/year11. Minimal wear has allowed the successful use of bearings of only 3mm thickness, thus allowing bone-sparing tibial bone cuts.

Fixed-bearing designs must overcome the kinematics of a knee with intact ligaments. To avoid overloading and subsequent high rates of loosening13, most fixed-bearing UKAs have been designed with minimal constraints between the articulating surfaces. Flat-on-flat or Round-on-Flat designs have lead to delamination in the polyethylene due to point loading of the metal-polyethylene interface and abrasion during sliding and roll back14. Weale15 has suggested that progressive lateral compartment degeneration may be due to polyethylene debris.

Figure 1
Medial Gonarthrosis

To overcome polyethylene creep in fixed-bearings some designs use metal-backed tibial components. Technically, this may necessitate a more aggressive tibial resection in order to accommodate the metal backing and adequate polyethylene thickness. This may increase the risk of fracture or failure and make subsequent revision more difficult.

Congruence may improve knee kinematics. Li et al16 have shown better kinematics with the Oxford Knee when compared to the Miller-Gallante fixed-bearing unicompartmental replacement. At two years, the fixed-bearings had a 37% incidence of radiolucency at the bone-cement interface compared to 8% for the mobile-bearing knee (p<0.05).

Clinical Results
Clinical results of unicompartmental replacement in the short to mid-term have been favourable regardless of bearing design. Svard17 reported 95% survival rate of a mobile-bearing device at ten years in medial UKA. The Oxford Group reported a 98% survival rate at ten years in their entire series18 and 97.3% survival rate in their MIS group at seven years19. Even in the hands of a community orthopaedic surgeon, the ten-year survival rate of the Oxford implant has been shown to be 94%20.

In comparison, Naudie et al. have demonstrated a 90% ten-year survival using a Miller-Galante unicompartmental implant21. Berger, using the same prosthesis, has reported 98% survival at ten years22. Other implants have had similar success; Cartier has reported 93% survivorship at ten years using the Marmor device23.

Figure 2
The Oxford Mobile-Bearing hemiarthroplasty

Long-term survivorship studies have also yielded encouraging results. The Oxford Group reported a 92.3% survival rate at 20 years24. Squire25,, using the Marmor Knee, showed an 87.5% survival rate at 15 years while ORourke showed an 86% survival rate in a similar knee after 21 years26.

Head-to-head comparisons of mobile versus fixed-bearing implants have been performed. At 11 years Emerson et al. have shown a 99% survival for the Oxford UKA compared to 93% for the fixed bearing Brigham UKA27. Gleeson, at two years, showed similar functional outcome results between the Oxford knee and the fixed-bearing St. Georg sled28. However, in this series, both groups had failure rates (9% failure for the Oxford and 5% for the fixed-bearing).

Technical Considerations
Technically, unicompartmental replacement is a more difficult operation than total knee arthroplasty. Both fixed and the mobile-bearing implants have unique technical considerations. Fixed-bearing implants are purported to be easier to implant than the mobile-bearing but these designs can lead to over or under correction of alignment. The former has been shown to lead to premature lateral compartment wear while the latter can lead to premature and excessive polyethylene wear29.

The mobile-bearing also has some technical considerations. Early Phase one results with first bearing designs demonstrated a dislocation rate of 3%. This lead to an improved (Phase 2) bearing design with a more acceptable dislocation rate of 0.4%12. An acceptable lateral compartment mobile-bearing is still under development.

Unicompartmental knee replacement is again growing in popularity due to the advances in minimally invasive techniques, rapid recovery, and excellent clinical results. Mobile-bearing implants may offer better kinematics of the knee when compared to fixed-bearing devices. Polyethylene wear rates with the Oxford mobile-bearing design has shown outstanding results compared to unconstrained unicompartmental replacements. Beyond the second decade mobile-bearing survivorship may prove to be superior to fixed devices. Both devices are, however, technically demanding and require appropriate training and attention to operative detail.


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16. Li et al. Mobile vs Fixed-Bearing Unicondylar Knee Arthroplasty: A Randomized Study on the Short-Term Clinical Outcomes and Knee Kinematics. Knee. 2006, June 21 (in print)

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28. Gleeson R. et al. Fixed or Mobile-Bearing Unicompartmental Knee Replacement? A Comparative Cohort Study. Knee. 2004; 11(5): 379-84.

29. Hernigou P. and Deschamps G. Alignment Influences Wear in the Knee After Medial Unicompartmental Arthroplasty. Clin Orthop 2004. 423: 161-5.

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