• P Augat, J Merk, S Wolf, and L Claes.
• Institute of Orthopedic Research and Biomechanics, University of Ulm, Germany. peter.augat@medizin.uni-ulm.de
• J Orthop Trauma. 2001 Jan 1; 15 (1): 54-60.

ObjectiveTo determine whether an externally induced interfragmentary movement enhances the healing process of a fracture under flexible fixation.DesignRandomized, prospective in vivo animal study with control group. Twenty-four skeletally mature Merino sheep were randomly assigned to six groups of four animals, which received cyclic interfragmentary movements of 0.2 and 0.8 millimeters and stimulation frequencies of 1, 5, and 10 Hertz, respectively. Twelve animals did not receive any externally applied stimulation and served as a control group.SettingUnrestricted stall activity with weight bearing reduced by tenotomy of the Achilles tendon.InterventionsOsteotomy of the tibial diaphysis with three-millimeter gap width fixed with a six-pin, monolateral, double-bar external fixator. Interfragmentary movement of the osteotomy gap was externally induced by a motor-driven actuator unit. Five hundred cycles inducing nonuniform tensile strains within the gap were performed each day.Main Outcome MeasurementsNine weeks after surgery, the animals were killed, and bone mineral density and callus cross-sectional area were measured with quantitative computed tomography. Callus projectional area was assessed by radiographs, and mechanical stability was determined with a three-point bending test.ResultsExternal stimulation with nonuniform cyclic tensile strains did slightly affect but not significantly enhance the fracture healing process. Varying the stimulation frequency had no influence on the healing process. The stimulation with 0.8 millimeter displacement magnitude resulted in a larger periosteal callus, but a decreased bone mineral density compared with the 0.2-millimeter displacement magnitude. The stimulation had no significant influence on the mechanical properties of the healing bone.ConclusionsInduced cyclic tensile strains did not produce a relevant enhancement of bone healing under flexible fixation.

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