Early and Predictable Restoration of Motion Using a "Kinematic Retaining" Total Knee Replacement: A Prospective Dynamic Fluoroscopic Study.

New total knee replacement designs aim to improve patient outcomes through restoration of normal knee joint movements. This study uses in vivo fluoroscopic analysis to quantify the kinematic characteristics of the Physica KR system. Twenty-one patients underwent kinematic fluoroscopic analysis 1 year following implantation of the Physica KR knee using three defined activities (step-up, kneel, and lunge). Assessments were made of initial rollback, maximum flexion, axial rotation, anteroposterior (AP) translation, and the presence or absence of condylar lift-off. A mean maximum passive flexion of 115.8 degrees (standard deviation ± 10.8) was achieved. All joints were congruent throughout range of movement in all three activities. During the step-up activity, the medial femoral condyle exhibited some initial rollback, the lateral condyle moved slightly posteriorly during initial flexion, then back toward its initial alignment with increased flexion. The combined effect produced tibial internal rotation of approximately 5 degrees during the first 90 degrees of flexion with a smooth progression toward slight varus alignment in maximal flexion, with minimal condylar lift-off. During maximal kneeling, both the medial and lateral femoral condyle contact points were just posterior to the AP midline and lift-off of both condyles was noted. During maximal lunge, the medial contact point was slightly anterior to the AP midpoint, with the lateral contact point slightly posterior, resulting in tibial internal rotation. There was no appreciable lift-off of the lateral condyle, although medial condylar lift-off increased from mid-flexion. These in vivo data demonstrate congruent kinematics throughout range of movement, with some initial femoral rollback during early flexion. Our study has shown that the Physica KR knee implant behaved similarly to other established cruciate-retaining implants.