Bone Biomechanics – Stress and Strain

Bone is a very tough structure. The primary objective of the skeleton is to support the body load. Every day 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 the biomechanics of the materials and in orthopedics, are important from the aspect of bone strength and fracture mechanics.

We understand them in the following short notes.

Strain

Strain is defined as the 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

– The force that is applied to bone

– The area on which the stress is applied

If the same force is applied to a smaller area, it would result in greater stress than if it is applied to a widespread area.