The traditional view of load-bearing by the proximal femur is that the femoral neck is bent by loads applied to the femoral head. The tension this is supposed to generate has led to the group 1 trabeculae in Figure 1 being called the " principal tensile system" and the group 2 trabeculae the " principal compressive system" . This approach only sees the femur as an isolated bone and has dominated thinking about its structure and function for more than a century. Yet it clearly takes no account of the position of the femur in the body; strongly bound into the acetabulum by substantial ligaments and surrounded by some of the most powerful muscles in the body. Over this same period, however, there is a discernible thread of dissenting views suggesting that this makes little sense and proposing that in real life, the proximal femur is actually in a state of compressive stress.

Figure1. Radiograph of the proximal femur showing the trabecular architecture and marking the main regions where trabeculae are more densely arranged. The similarity of the group1 trabeculae to a flying buttress can be seen.

Surprisingly little has been done to study stress distributions in the natural proximal femur, though there has been a lot of interest in calculating forces in the shaft and the function and failure of implants. We have hypothesised that under physiological loading the majority of the proximal femur is in compression and, in addition, that the internal trabecular structure functions as an arch, transferring compressive stresses to the femoral shaft. This is based on the similarities between the organisation of the trabeculae and the engineering structure of a flying buttress (Figure 2).

To demonstrate the principle, we have developed a 2D finite element model of the femur in which body weight, a representation of the pelvis, and ligamentous forces were included. The regions of higher trabecular bone density in the proximal femur (the principal trabecular systems) were assigned a higher modulus than the surrounding trabecular bone.

The inclusion of ligamentous forces in two-legged stance generated compressive stresses in the proximal femur (Figure 3). The increased modulus in areas of greater structural density focuses the stresses through the arch-like internal structure. Though working in 2 dimensions severely restricts the introduction of muscles, this model shows that including ligamentous forces has the effect of generating compressive stresses across most of the proximal femur. The arch-like trabecular structure transmits the compressive loads to the shaft. That bone is roughly twice as strong in compression than in tension is then used to advantage. These results support our hypothesis and encourage us to go ahead with a detailed 3D model to include the pelvis and the muscles. A better understanding of the stress distribution in the proximal femur may lead to improvements in prosthetic devices and an appreciation of the effects of various surgical procedures affecting load transmission across the hip.