Tibiofemoral and patellofemoral mechanics are altered at small knee flexion angles in people with patellofemoral pain.

Objectives: To test the hypothesis that, at 0° and 20° of knee flexion, patellofemoral contact area would be lower, while tibiofemoral rotation and patellofemoral malalignment would be higher in participants with patellofemoral pain (PFP) compared to pain-free participants. We hypothesized that no differences would be detected at 40° due to increasing patellar stability.

Design: Cross-sectional, descriptive study.

Methods: Twenty-seven people with PFP and 29 pain-free people participated. Participants underwent magnetic resonance imaging at 0°, 20°, and 40° knee flexion with the limb in a simulated weight-bearing position. Patellofemoral contact area, tibiofemoral rotation angle, patellofemoral alignment (bisect offset index and patellar tilt angle) were quantified and compared between groups at each angle using Student's t-tests. An a-posteriori comparison was made between the pain-free group and a subgroup of 15 participants with patellofemoral pain who demonstrated a faulty lower limb movement pattern ("medial collapse").

Results: In the patellofemoral pain group, contact area was lower at 0° (203.8±45.5 mm² vs. 224.1±46.6 mm², p=0.05) and 20° (276.8±56.2 mm² vs. 316.7±82.8 mm², p=0.02), bisect offset index (BOS) and patellar tilt angle (PTA) were higher at 0° (bisect offset index: 0.69±0.13 vs. 0.64±0.09, p=0.04; patellar tilt angle: 12.5±7.6° vs. 9.2±5.8°, p=0.04). In the patellofemoral pain subgroup, tibiofemoral rotation was higher at 0° compared to pain-free participants (6.4±5.9° vs. 4.0±4.6°, p=0.07).

Conclusions: While contact area and patellofemoral alignment were altered in people with patellofemoral pain, tibiofemoral rotation was altered in a subgroup of people who demonstrated medial collapse. Subgroup classification may help identify mechanisms of pain and assist in developing targeted interventions.