Bone Design: Selected Examples

Figure 128 sketches how the femur is loaded when standing on one leg. The abductor muscles play an important role here, preventing the upper body supported on the ball of the hip joint from tipping inwards (Fig. 128A). Because of the unfavourable lever relationships, the neck of the femur must transmit about 2.5 to 6 times body weight as axial loading (Fig. 128B). These enormous loads can only be coped with by an outstandingly well-adapted design. In the zone near the joint the femur is filled with trabecular bone, also called spongiform bone. This is a micro-framework of very fine small struts of bone which fill the whole head and neck of the femur. Further down they run into the bone-free marrow cavity in the middle, and laterally into the compact bone wall (cortical bone). In the lower region, the femur, which is almost exclusively loaded in bending, is a simple tube with a non-circular cross-section. This is certainly sensible, for we have the highest bending stresses at the edge, and in the middle, i.e. in the marrow cavity where there is no bone, the stresses are correspondingly equal to zero. (A hollow tree is therefore not so bad, for it too has no bending loads in the middle!)