The design and performance of an experimental external fixator with variable axial stiffness and a compressive force transducer.

A unilateral external fixator has been designed for controlled experiments into the effects of micromovement on fracture healing. The experimental model used is based on a diaphyseal osteotomy of the right tibia of the sheep. The main bar has linear bearings, which allows free axial movement. This is then controlled by a spring whose stiffness can be varied. The resulting axial micromovement can be calculated from the measured compressive force and the known axial stiffness of the fixator. The transducer has limits of error of +13.3 N and -44.5 N. Preliminary measurements showed maximum micromovement at the fracture site of 0.48 mm during slow walking.