In vivo kinematics of total knee arthroplasty: Flat compared with concave tibial joint surface

This study evaluated the influence of the geometric configuration of the ti bial joint area on the kinematics of the knee. Twenty-two patients with non inflammatory arthritis and minor preoperative deformity were studied. They each received an AMK total knee replacement with retention of the posterior cruciate ligament. Eleven patients without any knee abnormalities were use d as controls. The patients were stratified to either the flat (terminology of the manufacturer: standard) or concave (terminology of the manufacturer : constrained) polyethylene insert (n = 11 in each group). Knee kinematics were assessed 1 year after the operation by having the patient ascend a pla tform corresponding to an extension of the knee from 50 to 70 degrees of fl exion. During this motion, two film-exchangers simultaneously exposed six t o 13 pairs of serial stereoradiographs. The concave geometric configuration of the tibial insert resulted paradoxically in increased anterior-posterio r translations compared with the flat insert but no significant change of r otations and translations in the other directions. Compared with normal kne es, the most obvious abnormality was increased anterior-posterior translati ons (p < 0.004). At 50<degrees> of flexion, the implants with the flat tibi al polyethylene insert had displaced 2 times and the concave ones had displ aced 2.5 times more posteriorly than the normal knees (p less than or equal to 0.001). Less internal tibial rotation was also recorded in the flexed p ositions for both types of inserts compared with the normal knees (p < 0.02 ). Four knees in four patients, who reported symptoms of instability and ab normal knee function, showed significantly increased proximal displacement of the center of the tibial plateau in the flexed position. The findings su ggest that current prosthetic designs and surgical technique do not restore normal knee kinematics and indicate that design improvements should rely o n in vivo kinematic studies.