Figure 3.

Explanation on PTA, PS and F which is defined as an indicator for the fitting depth of the patella into the femoral trochlea and expressed by the following equation: F (%) = (D/T) × 100. (a) PTA is the angle (°) between the transverse axis of the patella (L _p) and the anterior intercondylar line (L ₁), considered positive when the patella tilted laterally. (b) PS is the distance (mm) between the central ridge of the patella and the deepest point of the femoral trochlea (asterisk), considered positive when the patella tilted laterally. (c) F is low when the fitting is shallow (LCS; F = 18%). (d) F is large when the fitting is deep (ROCC: F = 34%). D, partial patellar thickness located below L ₁; L ₁, anterior intercondylar line; L ₂, line parallel to the L ₁ and passing through the central ridge of the patella; L _p, transverse axis of the patella; LCS, low contact stress; PS, patella shift; PTA, patella tilting angle; ROCC, rotating concave–convex; T, total patellar thickness.