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.

The low contact stress and self-aligning properties of mobile bearing total knee replacements (TKR) make them an increasingly popular implant choice worldwide. Two variations on the mobile bearing knee concept have been commonly adopted: systems that retain the posterior cruciate ligament (PCL) and provide free rotation and translation (RT) of the mobile bearing, and systems that sacrifice the PCL and provide for rotation only (RO) motion of the mobile bearing. The purpose of this study was to evaluate the in vivo kinematics of these two types of mobile bearing TKR during gait, stair, and two deep knee flexion activities.

Twelve patients (6 RT, 6 RO) with unilateral mobile bearing knee arthroplasty and excellent functional outcomes at least one year after TKR were studied. Fluoroscopic images of the knee were acquired as patients walked on a treadmill, ascended a step, performed a deep knee bend, and knelt to maximum flexion. Knee kinematics were derived from CAD model based shape matching techniques.

The RT knees exhibited greater posterior translation of the femur on the tibia during early stance in gait (RT: 5mm vs. RO: 2mm) and during knee extension during stair ascent (RT: 5mm vs. RO: 1.5mm). There were no differences between the two groups in the flexion angles achieved during deep knee bend or kneeling.

Although there were no significant clinical or functional differences in these patients, the RO knees exhibited smaller tibio-femoral translations and less intersubject variability in knee kinematics during dynamic weight-bearing activities.

Aims: The purpose of this study was to determine the intra-subject repeatability of the motion pattern of the PE inlay in a mobile-bearing total knee replacement (TKR) with respect to the post-op time. Methods: 75 mobile-bearing TKRs in 73 patients were included in this prospective study. Sagittal radiographs at 0°, 30°, 60° and maximum flexion were taken 3, 12, and 24 months post-op. On each X-ray, the AP position and the rotation of the PE inlay with respect to the tibial baseplate were determined based on a 2D algorithm. The accuracy of the method was ± 0.2 mm for the AP position and ± 1.7° for the angle of rotation. To classify the repeatability, the mean AP and rotation motion with respect to the flexion angle for each patient was computed and the overall standard deviation (STD) of all measurements with respect to the mean curves was calculated. The repeatability was defined as ‘excellent’ if the STD in the AP direction was less than 0.5 mm and the STD in rotation was less than 2.5°. It was defined as ‘good’ if the STD in the AP direction was less than 1 mm and the STD in rotation was less than 5°. Results: 19 of 75 knees (25%) showed an excellent repeatability and 33 of 75 knees (44%) displayed a good repeatability of the PE motion. Motion patterns were more repeatable between the 12 and 24 month results than between the 3 and 12 month results. Conclusions: The majority of the mobile-bearing knees exhibited a repeatable, patient specific motion pattern of the PE inlay. The fact that the repeatability was higher between 12 and 24 months may be attributed to a more stable state after rehabilitation. Mobile-bearing knees support patient specific motion in contrary to constrained fixed bearing knees.