What do studies show about mobile-bearing knee designs?

The interest in mobile-bearing knee designs for both total knee arthroplasty (TKA) and medial unicompartmental knee arthroplasty (UKA), has grown in the past few years. Although U.S. regulatory restrictions have limited the growth of mobile-bearing TKAs compared to sales in European and Pacific Rim countries, they now represent approximately 10 percent to 12 percent of the TKA market, and 55 percent of the UKA market in the United States.

The first mobile-bearing UKA design was cleared by the U.S. Food and Drug Administration (FDA) for use in the United States in April 2004, with a physician training requirement prior to use. In June 2004, the FDA Ortho­paedic Advisory Panel recommended the reclassification of mobile-bearing knee systems for general use. To date, however, the FDA has not accepted this recommendation. Mobile-bearing revision TKAs have been used for some time in hinge-type implants with acceptable mid-term follow-up.

Design characteristics
Classic design characteristics of mobile-bearing knee implants include a dual articulation between a polyethylene insert and a metallic femoral and tibial component. Examples of this dual articulation can be seen in rotating platform designs (Fig. 1), constrained meniscal bearing designs (Fig. 2), and unconstrained meniscal-bearing UKA designs (Fig. 3).

Finite element modeling predicts reduction in contact stresses through distribution of the forces over a larger surface area, which may decrease polyethylene wear through increased articular conformity (Fig. 4a, b). A long-term clinical series of mobile-bearing TKAs has demonstrated acceptable survivorship into the second decade with uncemented fixation; occasional low rates of osteolysis have been reported. Recent retrieval studies have demonstrated that mobile-bearing TKA designs remain vulnerable to polyethylene wear damage at the superior surface and introduce an independent inferior surface that is vulnerable to wear.

For mobile-bearing UKA designs, 10-year in vivo linear wear measurement has been reported to be 0.02 mm/year. Revisions for wear remain uncommon in long-term studies of mobile-bearing UKA. Impingement of the bearing on retained bone or cement with an unconstrained mobile-bearing UKA, however, has been associated with an increase in polyethylene wear and emphasizes the importance of meticulous surgical technique when using mobile-bearing designs.

Clinical outcomes
Early and mid-term follow-up comparative studies for fixed-bearing TKA and mobile-bearing TKA have shown equivalent clinical outcomes; revisions for stiffness, implant-related issues, and bearing dislocation are slightly higher in mobile-bearing cohorts.

Despite their proposed biomechanical advantages, mobile-bearing knee designs do not correct for either significant component malalignment or flexion/extension gap imbalance (Fig. 5). Some authors have noted less axial rotation (mean less than 5o) than expected with both posterior cruciate ligament (PCL)-retaining and PCL-substituting mobile-bearing total knee designs. The role of PCL preservation or substitution with mobile-bearing knee designs continues to be debated.

Surgical technique remains an important factor for long-term performance of any TKA, independent of the implant design. Long-term comparative studies are required to document complications, clinical performance, and implant survivorship of mobile-bearing designs compared to their fixed-bearing counterparts. Michael E. Berend, MD, and A. Seth Greenwald, DPhil (Oxon) are members of the AAOS Biomedical Engineering Committee.

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