The effects of function in fracture healing and stability.

A mechanically based classification system and hypothesis for fracture healing with peripheral callus has been proposed based on many years of clinical experience and numerous laboratory research studies. This hypothesis describes the interactions and influences on fracture healing from the vascular, mechanical, electric, chemical, and thermal environments in specific callus regions surrounding the fracture site. The central theme to this phenomenon is motion at the fracture site. The single factor distinguishing the effects of closed functional bracing of fractures from all other treatment modalities is motion between the fragments which results from early functional activity. Perhaps the stimulus to a prolonged inflammatory response is provided by friction between moving fracture fragments and the surrounding tissues. An increased vascular response results, and environmental factors are stimulated to form an abundant peripheral callus. Three distinct callus zones exist which provide overlapping stages of healing based on structure and optimized physiologic conditions. Clinical signs and symptoms, such as fracture stability as well as motion and pain at the fracture site, combine to provide the optimum feedback mechanism to control functional activity, which in turn governs environmental factors. Roentgenographic appearance can be better judged for mechanical significance if these three callus regions are appreciated. In animals as well as in humans, if function has correctly influenced the environment during callus formation, refracture strength is often greater than that of the original bone.