In vivo kinematic analysis of a mobile bearing total knee prosthesis.

Ten normal subjects and 10 patients with a posterior cruciate retaining mobile bearing total knee replacement performed successive deep knee bends under fluoroscopy to determine tibiofemoral contact positions. At full extension the average initial contact position for the normal and mobile total knee replacement was 6.2 mm (range, 4.8 to 12 mm) anterior, and -4.4 mm (range, 3.9 to 11 mm) posterior to the sagittal tibial midplane, respectively. At 60 degrees flexion, the normal knee rolled back to -5.8 mm (range, -2.5 to -13.2 mm), whereas the mobile bearing total knee replacement rolled back to -9.2 mm (range, -4 to -17 mm). From 60 degrees to 90 degrees, normal knees rolled back to -7.8 mm (range, -5.8 to -13.8 mm), but the mobile bearing total knee replacement slid anteriorly to -5 mm (range, 2 to -12 mm). All mobile bearing total knee replacements had some form of roll back, but some slid anterior more than others. Five of 10 mobile bearing total knee replacements had some movement of the bearings while the others remained fixed. Patellar kinematics was similar to normal but reflected tibiofemoral abnormalities.