The four biomechanical stages of fracture repair.

Based on analysis of the torque-angle curves and roentgenographic findings in fifty-three healing tibial fractures in rabbits tested in torsion to failure, four biomechanical stages of fracture healing were defined, as follows: Stage I--failure through original fracture site, with low stiffness; Stage II--failure through original fracture site, with high stiffness; Stage III--failure partially through original fracture site and partially through intact bone, with high stiffness; and Stage IV--failure entirely through intact bone, with high stiffness. These stages correlated with the progressive increases in the average torque and energy absorption to failure as healing progressed and also with the average times since the original experimental fracture. It is hoped that this system of staging will provide both a standard by which important variables related to ultimate strength of healing fractures can be correlated and an objective way to predict delayed unions and non-unions and to determine the level of activity that is safe for patients with a healing fracture.