BIOMECHANICS AND BIOLOGY OF FRACTURE-HEALING WITH EXTERNAL SKELETAL FIXATION

The morphology of fracture healing depends on the mechanical environme nt afforded by external fixation and ranges between the extremes of di rect bone healing and spontaneous hone healing. The mechanical environ ment at a fracture is affected by the stiffness of the fixator and the degree of load sharing of the fixator with the bone. As the stability of the fracture increases because of increased fixator rigidity, anat omic fracture reduction, or a combination of these factors, callus for mation decreases. However, optimal rigidity may consist of early rigid immobilization followed by destabilization of the fixator to allow co ntrolled loading of the fracture site. The biological Environment at t he fracture also affects fracture healing with external fixation. Surg ical techniques and implants, which compromise the surrounding soft ti ssue and interfere with the vasculature to the injured bone, will dela y bone union. Fixation techniques for severely comminuted fractures us ing closed reduction and external skeletal fixation allow presentation of soft tissue. These biologically oriented techniques have been term ed biological fracture fixation and feature realignment of the limb (i .e., joint surfaces) rather than anatomic reconstruction of the fractu re; minimal to no sort tissue dissection; and optimal stability, resul ting in moderate strain levels and allowing the progression of bone fo rmation. Fractures treated in this manner seem to heal with endosteal bone formation and bone bridging between fragments rather than with pe riosteal callus.