Relative motion of a mobile bearing inlay after total knee arthroplasty – dynamic in vitro study

Objective. The purpose of this study was to measure the in vitro range of motion of a mobile bearing inlay knee prosthesis under dynamic isokinetic loading conditions. Additionally, the effect on the range of motion of rotational malalignment of the tibia baseplate was determined. Design. Specimens with implanted knee prostheses were mounted onto a custom built knee simulator. 3-D inlay movement was measured by an ultrasonic tracking system. Background. More recent knee prostheses include mobile bearing inlays type designs. These systems are intended to allow higher conformity of the tibiofemoral joint and thereby decrease contact stress without decreasing the kneeʹs range of motion. Methods. Dynamic testing in the knee simulator mimicked both the speed and resulting moment of a knee isokinetic extension test. The tibia baseplate was first implanted with no rotational malalignment, followed by sequential internal and external rotation of upto 15°. Results. Correctly aligned, the inlay center moved 3.5 mm (SD, 1.5 mm) posterior during extension. With the tibia baseplate externally rotated more than 10° the movement pattern changed. Conclusion. At up to 10° of rotational malalignment the primary motion pattern of the mobile bearing is maintained. However, beyond 10° unintented motion may occur.