The role of fixator frame stiffness in the control of fracture healing. An experimental study.

External skeletal fixation is used widely in the management of fractures. Frame configuration is known to affect frame stiffness and, thereby, the local mechanical environment at the fracture site. In previous investigations of the influence of mechanical conditions upon fracture healing, the frames have always been applied so that they influence the biological environment in different ways. As a result, the influence of stiffness, per se, could not be studied as a single variable, and its effect on the repair process remains unclear. In this study, using a standard osteotomy of the ovine tibia, stabilised by an external skeletal fixator, the local mechanical environment was altered solely by increasing the 'offset' distance between the bone and the fixator frame. The biological conditions at the fracture remained identical in both groups. Increasing the frame stiffness by 40%, brought about by reducing the offset distance of the fixator bar by 10 mm, caused a significant reduction in the rate of healing. In addition, the frame stiffness influenced the ground reaction force with greater weight-bearing in the initial stages in the more rigid group, but despite this, the resultant interfragmentary displacement in this group appeared to be insufficient to stimulate fracture healing. This work emphasises the importance of the local mechanical environment on the process of fracture healing. It also demonstrates the value of in vivo assessment of fracture stiffness as a means of monitoring mechanical events during fracture healing.