Purpose

To describe the native length changes of the intact medial patellofemoral ligament (MPFL) in knees of patients with contralateral patellar instability.

Methods

3D digital knee models were created from dynamic computed tomography images of the asymptomatic knee in patients with unilateral patellar instability. Knees were grouped into 10? intervals of knee flexion and assessed from 0? to 50?. At each flexion angle, MPFL length was calculated between the known anatomic attachment points on the patella and femur. Straight line representation of the fibers was allowed to wrap around the femoral condyle to ensure accurate length calculations. Changes in MPFL length were assessed relative to knee flexion angle and correlated with morphologic risk factors for patellar instability.

Results

11 knees were included in this study. MPFL length varied from to 74.0+/-7.8mm at 0 degrees to 64.2+/-4.7mm from 50 degrees flexion, with a 15.5+/-11.7%, or 9.8+/-7.4mm change in length (p<0.001). Stepwise multiple regression analysis showed an independent relationship between increasing MPFL anisometry and TTTG distance (R=0.62, R2 0.392, p=0.039). The number of morphologic risk factors present in each knee demonstrated a strong relationship with % change in MPFL length (R=0.81, R2=0.66, p=0.002).

Conclusion

In asymptomatic knees of patients with contralateral patellar instability, calculated MPFL length was up to 22.6mm greater at 0 degrees than 50 degrees flexion, increasing with greater number of morphological risk factors present. This suggests that the isometric function of the intact MPFL in these patients may not reflect previously described findings in anatomically normal patients. Further studies are needed to understand the pathoanatomy related to these changes, as well as the implications for graft placement and assessment of isometry in MPFL reconstruction techniques.