selected to be preferably less than the radius used to define the segment s2 of the prior art femoral component described above referring to fig7 8 and 11 12 the femoral component 200 is formed for articular bearing engagement with a bearing insert 300 having a superior bearing surface 302 shaped to articulate with the segment s2 of the femoral component 200 specifically the shape of the superior bearing surface 302 is defined by the plane curve 34 x 34 being rotated about an axis m4 at a radius w4 over an angle u03b84 to achieve congruent articulation between the bearing surface 302 and the segment s2 of the femoral component 200 the radius w4 is equal to the radius w2 in other respects the generation of the shape of the superior bearing surface 302 is disclosed in u s pat no 4 470 158 with respect to the primary load bearing surface 140 of the tibial bearing component 139 and reference is made thereto the segment s2 is the primary load bearing segment of the femoral component 200 since the segment s2 is in articular bearing engagement with the superior bearing surface 302 over a range of flexion which corresponds to the majority of normal knee movement such as that during walking standing running and sitting specifically with the femoral component 200 being in articular bearing engagement with the bearing insert 300 the femoral component 200 provides a wearer of the component 200 a range of motion extending from full extension 0 u00b0 of flexion to at least 45 u00b0 of flexion and up to approximately 105 u00b0 of flexion where the substantially.