Abstract
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.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
References:
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Hartford J.M. et al. (2001). J Bone Joint Surg Am., 83-A, 229–234. Google Scholar
Lemaire R. (1998). Cah Enseign SOFCOT, 17–34. Google Scholar
Stiehl J.B. et al. (1997). Clin Orthop., 345, 60–66. Google Scholar
