Background:

Patellofemoral instability is a major cause of anterior knee pain. The aim of this study was to examine how the medial and lateral stability of the patellofemoral joint in the normal knee changes with knee flexion and measure its relationship to differences in femoral trochlear geometry.

Methods:

Twelve fresh-frozen cadaveric knees were used. Five components of the quadriceps and the iliotibial band were loaded physiologically with 175 N and 30 N, respectively. The force required to displace the patella 10 mm laterally and medially at 0°, 20°, 30°, 60° and 90° knee flexion was measured. Patellofemoral contact points at these knee flexion angles were marked. The trochlea cartilage geometry at these flexion angles was visualized by Computed Tomography imaging of the femora in air with no overlying tissue. The sulcus, medial and lateral facet angles were measured. The facet angles were measured relative to the posterior condylar datum.
Results:

The lateral facet slope decreased progressively with flexion from 23° ± 3° (mean ± S.D.) at 0° to 17 ± 5° at 90°. While the medial facet angle increased progressively from 8° ± 8° to 36° ± 9° between 0° and 90°. Patellar lateral stability varied from 96 ± 22 N at 0°, to 77 ± 23 N at 20°, then to 101 ± 27 N at 90° knee flexion. Medial stability varied from 74 ± 20 N at 0° to 170 ± 21 N at 90°. There were significant correlations between the sulcus angle and the medial facet angle with medial stability (r = 0.78, p < 0.0001).
Conclusions:

These results provide objective evidence relating the changes of femoral profile geometry with knee flexion to patellofemoral stability.