Measurements of patellofemoral congruence described by Merchant et al. F, Facet; L, lateral condyle; M, medial condyle; P, patellar ridge; S, sulcus. Angle MSL is sulcus angle (average, 137 degrees; standard deviation, 6 degrees). Line SO is zero reference line bisecting sulcus angle. Angle PSO is congruence angle (average, −8 degrees; standard deviation, 6 degrees). Line PF (lateral facet) and line ML form patellofemoral angle that should diverge laterally.