Patellofemoral joint contact area increases with knee flexion and weight-bearing

Patellofemoral pain is a common and debilitating disorder. Elevated cartilage stress of the patellofemoral joint is hypothesized to play a role in the onset of pain. Estimating cartilage stress requires accurate measurements of contact area. The purpose of this study was to estimate patellofemoral joint contact areas in a group of healthy, pain-free subjects during upright, weight-bearing conditions. Sixteen subjects (8 female, 8 male) were scanned in a GE Signa SP open configuration MRI scanner, which allowed subjects to stand or squat while reclining 25° from vertical with the knee positioned at 0°, 30°, or 60° of flexion. A custom-built backrest enabled subjects to be scanned without motion artifact in both weight-bearing (0.45 body weight per leg) and reduced loading conditions ('unloaded' at 0.15 body weight) at each knee flexion posture. Male subjects displayed mean unloaded patellofemoral joint contact areas of 210, 414, and 520 mm2 at 0°, 30° and 60° of knee flexion, respectively. Female subjects' unloaded contact areas were similar at full extension (0°), but significantly smaller at 30° and 60° (p < 0.01), with mean values of 269 and 396 mm2, respectively. When normalized by patellar dimensions (height × width), contact areas were not different between genders. Under weight-bearing conditions, contact areas increased by an average of 24% (p < 0.05). This study highlights the differences in patellofemoral joint contact area between gender, knee flexion postures, and physiologic loading conditions.