Fig. 2

Definition of the three cross-sections of the femoral trochlea in a three-dimensional model of the knee. a, b To help define the geometrical parameters of interest, a cylinder was established in the sagittal plane, with its axis represented by the axis of the trochlear groove, and its radius adjusted to allow the cylindrical surface to closely fit the trochlear groove of the prosthesis (a) or that of the bone (b). The fit was first performed by eye, provided that the trochlear groove was clearly visible in both the model of the prosthesis (a) and in that of the native knee (b). c Cutting planes were defined at 0, 45 and 90° starting form a plane parallel to the transverse plane, and rotating in 45° increments towards the distal end of the trochlear groove. The cutting planes were applied to the cylinder whose surface best fit the trochlear groove (a, b). The axis of the best-fit cylinder for the native knee (green circle) corresponds to the groove axis (b). The axis of the best-fit cylinder for the prosthetic knee (red circle) may not coincide with that of the best-fit cylinder for the native knee (green), as the radius was adjusted to allow the cylindrical surface to closely fit the trochlear groove of the prosthesis, leading to variations in sulcus height for the three cross-sections analyzed