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Biomechanics Overview   Torsion   Bending   Compression   Fracture  

Determining the physical behaviour of bone is important to the study of its mechanical function. This information is also used to assess the changes in the physical properties of bone as a result of age or disease as well as to determine the effectiveness of drug therapies to counteract these changes.

stress diagram.gif (9333 bytes)stress strain.gif (14556 bytes)When a force is applied to a bone, the bone will deform according to the specific material and structural relationships of that particular bone. The force-deformation relationship of bone (shown on the right) has certain characteristics. First, it can be divided into two sections: (i) elastic and (ii) plastic regions. In the elastic region, the deformations produced are reversible, so once the force is removed the bone returns to its original shape. Within this region, there is typically a linear relationship between force and deformation. The slope of the elastic curve represents the extrinsic stiffness or elastic rigidity of bone. If deformation continues beyond the yield point, the bone will behave plastically and a portion of the applied deformation becomes permanent. If deformation continues, the bone will eventually fail. The energy required to break the bone is called the toughness, which can determined from the area under the force-deformation curve.

It is import to remember that the force-deformation relationship of a bone reflects both its material behaviour and geometry. For example, a larger bone will break at higher forces than a smaller bone made of the same material. Thus, to determine the material properties of bone, the parameters determined from the force-deformation relationship are normalized to remove the geometric effects on its observed behaviour. For example, the rigidity of the bone reflects the stiffness of the bone material as well as its geometry whereas the modulus of elasticity (the normalized equivalent of rigidity) only reflects the stiffness of the bone material.

The direction of force application also is very important since a material can behave differently when subjected to different loading conditions. For example, bone is much weaker in tension than compression. For this reason specific experiments are performed to determine the mechanical properties of bone under different loading conditions, such as: torsion, bending and compression (shown on the left).