In vivo patellar tracking: clinical motions and patellofemoral indices.

Patellar tracking during in vivo weightbearing knee function is not well understood. This study investigated patellar tracking of eight subjects during a full range of weightbearing flexion using magnetic resonance imaging and dual orthogonal fluoroscopy. The data were reported using a clinical description based on patellar and femoral joint coordinate systems and using patellar indices based on geometrical features of the femur and patella. The mean patellar shift was within 3 mm over the entire range of flexion. The patella tilted laterally from 0 degrees to 75 degrees, and then tilted medially beyond 75 degrees of flexion. The mean tilt was within 6 degrees. Similarly, the mean patellar rotation was small at early flexion, and the mean total excursion of patellar rotation was about 8 degrees. The patellofemoral indices showed that the mean sulcus angle and congruence angle varied within 8 degrees over the entire flexion range. The mean lateral patellar displacement was within 6 mm. A consistent decrease in lateral patellar tilt and an increase in lateral patellofemoral angle were observed with knee flexion. In conclusion, patellar motion is relatively small with respect to the femur during in vivo weightbearing knee flexion. These data may provide baseline knowledge for understanding normal patellar tracking.