Introduction: In contrast to fixed-bearing total knee replacements (TKRs), mobile-bearing TKRs allow for unconstrained kinematics while providing a high congruency between the femoral component and polyethylene inlay. The concept of a mobile-bearing TKR is based on the mobility of the inlay. It has been suggested that inlay mobility may decrease due to the in-growth of fibrous tissue (Lemaire 1998). Previous studies report a loss of inlay mobility between 0% and 50% at an average follow-up time between 1.5 and 6 years postop (Bradley et al. 1987, Stiehl et al. 1997, Hartfort et al. 2001). However these studies are retrospective and do not define a threshold for inlay mobility.

Methods: In this prospective study, 61 mobile-bearing TKRs (SAL, Zimmer) in 60 patients were followed up 3, 12, 24 and 60 months post-op. The implant design allows for 6–9 mm (size dependent) inlay translation in the anteroposterior direction while the rotation is not constrained. A complete 60 month follow-up of 40 patients was available. At each follow-up the knees were X-rayed at 0, 30, 60 and maximal flexion. Using the 4 X-rays from each follow-up, digital image analysis was performed to compute inlay translation and rotation. An inlay was classified as mobile, if it translated more than 1 mm or rotated more than 5. Group means where compared using one-way ANOVA with a significance level of 5%.

Results: No significant change in average inlay translation and rotation with time was found. The average inlay translation was 2.5 mm (s.d. 1.8 mm) at 3 months postop, 3.0 mm (s.d. 1.8 mm) at 1 year post-op, 3.1 mm (s.d. 1.9 mm) at 2 years post-op, and 3.1 mm (s.d. 2.0 mm) at 5 years post-op. The average inlay rotation was 6.6 (s.d. 3.4) at 3 months post-op, 6.7 (s.d. 4.0) at 1 year post-op, 7.9 (s.d. 3.9) at 2 years post-op, and 8.3 (s.d. 4.1) at 5 years post-op. At all follow-ups, the inlay was classified as mobile in at least 90% of the cases. The patterns of inlay motion were observed to be repeatable for the individual knee joints but varied substantially between subjects.

Conclusion: In 40 mobile-bearing TKRs that were prospectively examined 3, 12, 24, and 60 months post-op, no significant change in average inlay motion or percentage of mobile inlays was found. The results do not support the hypothesis that inlay mobility is reduced due to in-growth of fibrous tissue. Mobile-bearing TKRs allow the kinematics to follow the knee specific soft tissue constraints.