Compression or tension? The stress distribution in the proximal femur

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Compression or tension? The stress distribution in the proximal femur

Kate E Rudman, M Aspden and Jude R Meakin

BioMedical Engineering OnLine 2006, Published 20 February 2006

http://www.biomedical-engineering-online.com/content/5/1/12/abstract

Background

Questions regarding the distribution of stress in the proximal human

femur have never been adequately resolved. Traditionally, by

considering the femur in isolation, it has been believed that the

effect of body weight on the projecting neck and head places the

superior aspect of the neck in tension. A minority view has proposed

that this region is in compression because of muscular forces pulling

the femur into the pelvis. Little has been done to study stress

distributions in the proximal femur. We hypothesise that under

physiological loading the majority of the proximal femur is in

compression and that the internal trabecular structure functions as

an arch, transferring compressive stresses to the femoral shaft.

Methods

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. Two-legged and one-legged stances, the

latter including an abductor force, were investigated.

Results

The inclusion of ligamentous forces in two-legged stance generated

compressive stresses in the proximal femur. The increased modulus in

areas of greater structural density focuses the stresses through the

arch-like internal structure. Including an abductor muscle force in

simulated one-legged stance also produced compression, but with a

different distribution.

Conclusions

This 2D model shows, in principle, that including ligamentous and

muscular 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. The greater strength of

bone in compression than in tension is then used to advantage. These

results support the hypothesis presented. If correct, 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.