An in vivo determination of patellofemoral contact positions.

OBJECTIVE: To determine patellofemoral contact patterns in two-dimensions for normal and implanted patients.

DESIGN: An in vivo, weightbearing fluoroscopy analysis of 14 subjects with normal knees, 12 with anterior cruciate ligament deficient knees, 14 with a posterior cruciate retaining implant, and 25 with a posterior cruciate substituting implant.

BACKGROUND: Most previous experimental studies involving the knee joint have been either in vitro or under nonweightbearing conditions.

METHODS: Subjects were studied under fluoroscopic surveillance performing deep knee bends to maximum flexion. Video images were analyzed on a computer with a two-dimensional technique of digitizing discrete points on the patella, femur, and tibia.

RESULTS: The contact position, measured from the patella mass center, was inferior on the patella at extension and moved superior during flexion. Average contact positions of the implanted knee groups were more superior than the normal knee group throughout the flexion cycle. Analysis of patellar tilt angle demonstrated a flexed posture of the patella relative to the tibia. Increase in patellar tilt angle with increasing femorotibial flexion was substantially greater in implanted knees versus normal knees. Separation of the patella from the femur in full extension was absent in normal knees, but present in 86% and 44% of posterior cruciate retaining and posterior cruciate substituting total knee arthroplasties, respectively.

CONCLUSIONS: The patellofemoral kinematics of the total knee arthroplasties analyzed in the study was statistically different than the normal and anterior cruciate ligament-deficient knees. The kinematic variations observed between normal and implanted knees may be related to disturbed femorotibial kinematics previously observed to occur following total knee arthroplasty.

RELEVANCE: Patellofemoral complications, including polyethylene wear, are a major concern in total knee arthroplasty. Since the causes of polyethylene wear are multi-factorial, abnormal patellofemoral kinematics may play a role in patellar failure.