A bone is subjected to various kinds of forces in our day to day life. Some try to compress it, some bend while others rotate. Depending on their action, the forces on bone could be classified as compression, tension, and shear forces.
Compressive Forces or Compression
A compressive force acts along the long axis of a bone so as to compress the bone material [see image below]. Compressive forces result in impacted fractures or compression fractures. A force of compression causes metaphyseal impaction fractures, crushing injury to epiphysis, the collapse of vertebrae etc.
For compressive force to result in fracture, one end of a bone must be in a fixed position while the other end is forced toward the fixed end. An example is a man jumping on the ground from a height on his feet and getting a compression fracture of the vertebra as a result.
A force that pulls apart both the axial ends is an example of tensile force. For tensile force to result in fracture, one end of a bone must be in a fixed position while the other end is forced away from the fixed end.
This will create a separation or avulsion fracture.
Common areas for this to occur include the tibial tuberosity, greater trochanter, and olecranon.
This kind of force will be a force that tries to twist the bone along its long axis. This usually is a result of one end of a bone being placed in a fixed position while the other end of the bone is forced to rotate. Torsional forces generally result in short or long spiral fractures.
Shearing forces are unaligned forces pushing one part of a body in one direction, and another part the body in the opposite direction.
A shearing force will produce a fracture parallel to the direction of the applied force. Bone is weakest under shear stress making fractures of this nature common even with minimal trauma. The following image is an example of shear forces on bone.
These are a major classification of types of forces that a bone has to withstand. But actually, a bone has to resist the combination of these forces. For example, a bone may undergo a force called bending force, [ imagine bending a stick]. It would amount to compressive forces on one surface and tensile forces on other leading to two kinds of forces acting simultaneously.
Why it is important to understand forces acting on the bone. Bone, as discussed, function to support the skeleton and has to withstand many kinds of forces, some of which may result in fracture.
An understanding of a type of force that caused injury is important to
- Classify fracture pattern
- Devise treatment methods and gadgets
- Formulate measures for injury prevention
Bottom image credit: cal.vet.upenn.edu