Weight-bearing MRI of patellofemoral joint cartilage contact area.

PURPOSE To measure contact area of cartilage in the patellofemoral joint during weight bearing using an open MRI scanner. MATERIALS AND METHODS We developed an MR-compatible back support that allows three-dimensional imaging of the patellofemoral cartilage under physiologic weight-bearing conditions with negligible motion artifact in an open MRI scanner. To measure contact areas, we trained observers using a phantom of known area and tested intra- and interobserver variability. We measured in vivo contact areas between the patella and femoral cartilage with the knee in 30 degrees of flexion, loaded and unloaded, in six volunteers. RESULTS We were able to measure the contact area of the patellofemoral cartilage with small interobserver (CV 7.0%) and intraobserver (CV 3.0%) variation. At 30 degrees of knee flexion, mean contact area increased from 400 mm2 (unloaded) to 522 mm2(loaded to 0.45 times body weight per leg). CONCLUSION Using an open magnet and specially designed apparatus, it is possible to image the patellar cartilage during physiologic loading. Knowledge of patellar cartilage contact area is needed to assess patellofemoral stress, which may be increased in patients with patellofemoral pain syndrome.