Mechanical stimulation by external application of cyclic tensile strains does not effectively enhance bone healing

Objective: To determine whether an externally induced interfragmentary move ment enhances the healing process of a fracture under flexible fixation. Design: Randomized, prospective in vivo animal study with control group. Tw enty-four skeletally mature Merino sheep were randomly assigned to six grou ps of four animals, which received cyclic interfragmentary movements of 0.2 and 0.8 millimeters and stimulation frequencies of 1, 5, and 10 Hertz, res pectively. Twelve animals did not receive any externally applied stimulatio n and served as a control group. Setting: Unrestricted stall activity with weight bearing reduced by tenotom y of the Achilles tendon. Interventions: Osteotomy of the tibial diaphysis with three-millimeter gap width fixed with a six-pin, monolateral, double-bar external fixator. Inter fragmentary movement of the osteotomy gap was externally induced by a motor -driven actuator unit. Five hundred cycles inducing nonuniform tensile stra ins within the gap were performed each day. Main Outcome Measurements: Nine weeks after surgery, the animals were kille d, and bone mineral density and callus cross-sectional area were measured w ith quantitative computed tomography. Callus projectional area was assessed by radiographs, and mechanical stability was determined with a three-point bending test Results: External stimulation with nonuniform cyclic tensile strains did sl ightly affect but not significantly enhance the fracture healing process. V arying the stimulation frequency had no influence on the healing process. T he stimulation with 0.8 millimeter displacement magnitude resulted in a lar ger periosteal callus, but a decreased bone mineral density compared with t he 0.2-millimeter displacement magnitude. The stimulation had no significan t influence on the mechanical properties of the healing bone. Conclusions: Induced cyclic tensile strains did not produce a relevant enha ncement of bone healing under flexible fixation.