Scandinavian Orthopedic Association

The influence of varying rate, and timing of application of strain to healing fractures were investigated in a sheep tibial osteotomy with a 3 mm gap to simulate unfavourable conditions. Methods:Using this model in the sheep tibia, cyclical mechanical stimuli were imposed across the fracture through a miniature servo-hydraulic actuator attached to the external fixation frame.The stimulus was applied for short periods (17 minutes) each day.The influence of different controlled regimes of mechanical stimulation was assessed in groups of six animals.The rate of mineralisation of the osteotomy was measured using dual photo-absorption densitometry; the rate of increase in fracture stiffness was measured in vivo from strain gauges.Results: Stress protection inhibition of healing seen with the rigid frame configuration, was significantly reversed in terms of the rate of mineralisation of the gap and restoration of fracture stiffness when using a stimulus regime with an initial displacement of 1 mm, a force of 200 N and a strain rate of 400 m per sec.The stimulus was most effective if applied in the first six weeks. Conclusions:In this controlled fracture model a close relationship was seen between mechanical loading and bone healing; early application of very short periods of appropriate loads of applied strain magnitude, and strain rate at high physiological levels maintained healing. Bone healing responses to axial loading under external fixation