Fluoroscopic analyses of cruciate-retaining and medial pivot knee implants.

Contemporary posterior cruciate-retaining total knee designs have provided pain relief and improved knee function but have failed to reproduce the kinematics and stability of the normal nonarthritic knee. The Medial Pivot total knee design features a near constant radius of curvature of the femoral component. The tibial surface is highly congruent and asymmetric, permitting a medial pivot motion during knee flexion. The purpose of the current study was to analyze and compare the gait kinematics of the Sigma posterior cruciate-retaining total knee implant, the Advance Traditional posterior cruciate-retaining total knee implant, and the Advance Medial Pivot knee implant using fluoroscopic analysis. In vivo kinematics were determined for 15 clinically successful total knee arthroplasties. Five knee implants were evaluated from each group. The authors analyzed the kinematics of knee motion during the stance phase of gait for each patient. On average, subjects with the Medial Pivot knee implant had a medial pivot motion. Both posterior cruciateretaining designs had a paradoxical roll forward of the tibia on femur during knee flexion and had greater excursion of both condyles during knee flexion than the medial pivot design. Nine of 10 of the posterior cruciate-retaining designs had condylar lift-off averaging 1.7 mm whereas only one Medial Pivot knee implant had condylar lift-off measuring 1.1 mm.