In vivo kinematics of high-flex mobile-bearing total knee arthroplasty, with a new post-cam design, in deep knee bending motion

Abstract

Purpose: The objective of this study was to evaluate the in vivo knee kinematics to assess the available functional motion of the characteristic mobile-bearing prosthesis design and to examine whether the artificial joint would work in vivo according to its design concept.

Methods: We studied 14 knees (11 patients) implanted with the Vanguard RP Hi-Flex prosthesis. This prosthesis has a highly original form of post-cam called a PS saddle design with high compatibility, and with a rotating plate mobile-bearing mechanism. The cylinder-type post-cam is designed to enable contact in early flexion ranges, and to prevent paradoxical anterior femoral component movement. Each patient performed weight-bearing deep knee bending under fluoroscopic surveillance. Motion between each component including the polyethylene insert was analyzed using the 2D/3D registration technique.

Results: The mean range of motion was 122.0°. The mean femoral component rotation for the tibial tray was 5.0°. No paradoxical anterior movement of the nearest point was confirmed between the femoral component and the tibial tray in the early flexion ranges. Initial contact of the post-cam was confirmed at a knee flexion angle of 33.8°. Subsequently, the wide contact of the post-cam was maintained until flexion reached 120° in all knees, but disengagement of the post-cam was observed in two knees when flexion was ≥130°.

Conclusions: The results of this study demonstrated that the prosthesis design generally works in vivo as intended by its design concept. The present kinematic data may provide useful information for improvement of high-flex type prostheses.

Fig. 1

Vanguard RP Hi-Flex (Biomet Europe, Bridgend, UK) is a prosthesis with a highly original form of post-cam called a PS saddle design with high compatibility, and with a rotating plate mobile-bearing mechanism

Fig. 2

a 2D/3D registration technique uses computer-assisted design models to reproduce the spatial position of femoral and tibial components from single-view fluoroscopic images. b Rotation of the insert was determined using a 3D model of the insert containing five tantalum beads positioned just before insertion during the operation

Fig. 3

a Amount of femoral component axial rotation for the tibial tray from maximum extension to maximum flexion. The horizontal scale shows the flexion angle between the component, and the vertical scale shows the amount of axial rotation. Positive values represent external rotation. b Absolute value for axial rotation of the femoral component for the insert. The mean absolute value beyond flexion of 120° tended to be larger than that at flexion of 110°

Fig. 4

a Anteroposterior translation of the bilateral nearest point between the femoral component and the insert. b Anteroposterior translation of the bilateral nearest point between the femoral component and the tibial tray. The difference was significant in AP translation between medial and lateral (*P = 0.003)

Fig. 5

a–f Dynamic deep knee flexion was shown under weight-bearing conditions. The red area denoted a quasi-contact area that was defined by the closest distances between the cam of the femoral component and the post of the insert. b In this patient, initial contact of the post-cam was confirmed at flexion of 30°. f The post-cam disengagement was observed at flexion of 130°