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.