Skeletal system: biomechanical concepts and relationships to normal and abnormal conditions

The human skeleton is a remarkable organ that is uniquely designed to provide structural support and to house the body's hematopoietic system and mineral reservoirs. Seven concepts that will assist the clinician in understanding skeletal function are (1) material properties of bone, (2) stress and strain, (3) bending moments and torsional loads, (4) area moments of inertia, (5) fatigue and catastrophic failure, (6) Wolff's law, and (7) stress risers and open section effect. For example, as the modulus of a bone, a measure of stiffness decreases as in Padget's disease or fibrous dysplasia and the same levels of stress will cause greater deformations. The sum of these principles also explains the torus fracture (ductility), fracture of the olecranon by contracting tricep muscle (tensile loading), osteoporotic compression fracture of the spine, and the other biomechanical lesions that are encountered. Understanding these basic biomechanical principles can help physicians comprehend neoplastic processes and fractures that are the metabolic responses of the skeleton to stress and that appear on the radionuclide bone scan.