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.