Fig. 4. Lateral trochlear inclination angle, trochlear facet asymmetry, and trochlear depth in a patient with patellofemoral instability.

The lateral inclination is measured by the angle between the two lines. The first line is placed on the posterior aspect of the femoral condyle (red line). The second line is placed on the lateral facet of the trochlea (red line). In this case, the lateral inclination angle was 10 degrees, which is abnormal (< 11 degrees). Trochlear facet asymmetry is measured in the same axial plane as that used for the lateral inclination angle. The length of the medial facet (yellow M) is divided by that of the lateral facet (yellow L). Trochlear facet asymmetry in this case was 33% (9.6/28.6 × 100) and abnormal (< 40%). The trochlear depth was measured using the same image. A line tangential to the posterior aspect of the femoral condyle was used as the reference line. The largest AP distance between the line and the medial (A) and lateral (C) facets, and the deepest portion of the sulcus (B) were measured. The trochlear depth is calculated using the formula ([A + C]/2) - B. Trochlear depth in this case is 2 mm ([56 mm + 58 mm]/2) - 55 mm and abnormal (< 3 mm).