Biomechanics of fracture healing.

Internal fixation of fractures alters the physical environment of living bone. The authors have studied the reaction of living bone to force and motion, mainly in sheep. Cortical bone does not undergo pressure necrosis when compression is applied in internal fixation. Compressing the fragments increases the stability of the reduction and leads to uneventful healing without resorption. Small areas of plastic bone deformation owing to mechanical overload are nor removed by surface resorption but by internal remodelling. Interfragmentary motion produces callus and resorption of the contact surfaces. The static compression applied to cortical bone does not induce a change in the rate of internal remodelling. The static forces must exceed the dynamic load to maintain close coaptation at the contact surface.