Fractures of bones - mechanism, treatment. Bone healing
Biophysics of bone and mechanisms of fractures
The strength of bones is usually expressed in terms of the stress-strain relationship. Stress is the amount of force applied per unit area and strain is the degree of deformation in response to specific stress. Elastic deformation is the component of the stress-strain relationship in which the material deforms as load is applied yet returns to its original shape when the load is removed, this elastic modulus is known as Young’s modulus and is a measure of stiffness (the stiffer the material the sharper the slope). When bone is being put under stress within the range where it can return to its original shape, this is the elastic range. When bone is under stress in the range where it is no longer able to return to its original shape but it does not fail it is in the plastic range. Plastic deformation sees micro-cracks and disruption of collagen fibrils and trabecular architecture.
The material properties of bone are different for the outer cortex and the trabecular bone (found inside the cortex). The outer cortex is the part of the bone that predominantly is responsible for withstanding strain.
Bones are Anisotropic. This means that they have different values of strength in different directions. Longitudinally, bones are significantly stronger, compared to the transverse stress. This means that bone can withstand higher axial loads (200 Nm/m2 compared to 14 Nm/m2.
The toughness of bone is determined by bone density (which is a product of aging, disease, use and disuse). Bone density is shorthand for the amount of hydroxyapatite (the main mineral component and collagen (type I). The collagen provides the elasticity, while the hydroxyapatite provides the hardness. The optimal mineral component is about 65%.
Bone can also be considered Viscoelastic. This is the phenomena of time-dependent strain, effectively, its resistance and elasticity changes depending on the time over which force has been applied. What occurs in practice is that bone will fracture sooner if load is applied slower. This is the model that allows for stress fractures (I think).
So we’ve discussed a bit about the physics of bones, now lets talk about the mechanisms of fractures. Fractures occur in four stages:
- Instability phase
- Descent phase
- Impact phase
- Post impact phase