Bone is a very tough structure. Primary objective of the skeleton is to support the body load. Everyday the bones undergo repeated loading as we perform our daily chores.
Bone is a very strong material which can withstand extremely high loads and by virtue of its constant repair, it maintains its strength without undergoing fatigue.
Stress and strain are two important terms in biomechanics of the materials and in orthopedics, are important from aspect of bone strength and fracture mechanics.
We understand them in following short notes.
Strain
Strain is defined as percentage of change in length of the material in relation to original length. When a force is applied to any material, such as bone, it undergoes deformation. The amount of deformation in the material relative to its original length, is the strain.
When a material is pushed together, the material shortens (compressive strain),when pulled, it gets longer (tensile strain).
In terms of mathematical units, the strain can be expressed as a percentage.
Strain = (change in length/original length) x100
When a muscle contracts, the tendon can strain as much as 5% in tension during intense activities. Compressive strains in bone during peak activities only rise to about 0.3%.
The bone begins to fail at 0.7% strain or 7000 microstrain.
Stress
Stress is related to force applied and is defined as force per unit area. Newtons per square meter, or Pascals is the measurement unit of stress.
Thus the stress is dependent on two factors
– Force that is applied to bone
– Area on which the stress is applied
If the same force is applied on a smaller area, it would result in greater stress than if it is applied on a widespread area.
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