Dislocation of Rotating Hinge Knee Prostheses

he rotating hinge knee mechanism was designed to provide a stable total knee reconstruction when the intrinsic stability of the knee has been lost as a result of severe soft-tissue compromise1-5. It is not recommended for routine total knee arthroplasty because it has been associ-ated with high rates of revision2,3. Rotating hinge knee de-signs have a transversely (horizontally) oriented hinge axis for flexion-extension motion and a vertically oriented post-in-channel axis for internal and external rotation (Fig. 1). The post-in-channel design also allows distraction up to the limits imposed by soft-tissue tension. As a result, component dislocation due to distraction disengagement is prevented only by the restraint of the soft-tissue envelope. Once distrac-tion occurs, the degree of tilting of the central rotational stem within the channel directly reflects the intrinsic stability of the component design. The amount of distraction required for implant dislocation is directly related to the length, de-gree of taper, and tolerance of the stem in the cylinder (Fig. 1). To our knowledge, only one design (Link America, Pine Brook, New Jersey) possesses an antisubluxation feature that T Fig. 1 Line diagrams illustrating the minimal varus-valgus toggle and hence the excellent stability pro-vided by designs that employ a long (7-cm) vertical, cylindrical rotational stem within a cylindrical channel compared with the excessive toggle under similar conditions (2.5 cm) of joint distrac-tion, permitted in designs that have a substantial taper of the post-in-cylinder design (7.5°)