Implant design affects knee arthroplasty kinematics during stair-stepping.

Knee implant motions have a direct influence on patient function and implant longevity. The purpose of this study was to determine if there were consistent differences in knee motions among three groups of knee implants. Two hundred thirteen knees in 173 patients, with 25 implant designs, were studied using fluoroscopy during stair-stepping. All knee implants were assigned to one of three groups based on the design: fixed-bearing posterior-stabilized, fixed-bearing posterior cruciate-retaining, and mobile-bearing. All types of implants had the same pattern of internal/external rotations, but different designs had different anteroposterior translations. Seventy-five percent of posterior-stabilized knee implants had a medial center of rotation, indicating posterior femoral translation with flexion. Sixty-three percent of cruciate-retaining fixed-bearing knee implants had a lateral center of rotation. Eighty-six percent of mobile-bearing knee implants had a lateral center of rotation, indicating anterior femoral translation with flexion. Knee motion in patients with successful total knee arthroplasties is related directly to the constraints of the implant design.