[Influence of the quadriceps muscles on the patellofemoral contact in patients with low flexion patellofemoral instability after MPFL reconstruction].

Introduction: MPFL reconstruction represents one of the most important surgical treatment options for recurrent patellar dislocations at low flexion angles associated with low flexion patellofemoral instability. Nevertheless, the role of quadriceps muscles in patients with patellofemoral instability before and after patellofemoral stabilization using MPFL reconstruction has not been fully elucidated. The present study investigates the influence of quadriceps muscles on the patellofemoral contact in patients with low flexion patellofemoral instability (PFI) before and after surgical patellofemoral stabilization using MPFL reconstruction using 3 T MRI datasets in early degrees of flexion (0-30°).

Methods: In this prospective cohort study, 15 patients with low flexion PFI before and after MPFL reconstruction and 15 subjects with healthy knee joints were studied using dynamic MRI scans. MRI scans were performed in a custom-made pneumatic knee loading device to determine the patellofemoral cartilage contact area (CCA) with and without quadriceps activation (50 N). Comparative measurements were performed using 3D cartilage and bone meshes in 0-30° knee flexion in the patients with patellofemoral instability preoperatively and postoperatively.

Results: The preoperative patellofemoral CCA of patients with low flexion PFI was 67.3 ± 47.3 mm in 0° flexion, 118.9 ± 56.6 mm in 15° flexion, and 267.6 ± 96.1 mm in 30° flexion. With activated quadriceps muscles (50 N), the contact area was 72.4 ± 45.9 mm in extension, 112.5 ± 54.9 mm in 15° flexion, and 286.1 ± 92.7 mm in 30° flexion without statistical significance. Postoperatively determined CCA revealed 159.3 ± 51.4 mm , 189.6 ± 62.2 mm and 347.3 ± 52.1 mm in 0°, 15° and 30° flexion. Quadriceps activation with 50 N showed a contact area in extension of 141.0 ± 63.8 mm, 206.6 ± 67.7 mm in 15° flexion, and 353.5 ± 64.6 mm in 30° flexion, also without statistical difference compared with unloaded CCAs. Subjects with healthy knee joints showed an increase of 10.3% in CCA at 30° of flexion (p = 0.003).

Conclusion: Although patellofemoral CCA increases significantly after isolated MPFL reconstruction in patients with low flexion patellofemoral instability, there is no significant influence of quadriceps muscles either preoperatively or postoperatively.