title: Mechanical stimulation by external application of cyclic tensile strains does not effectively enhance bone healing.
abstract: OBJECTIVE
  To determine whether an externally induced interfragmentary movement enhances the healing process of a fracture under flexible fixation.
  
  
  METHODS
  Randomized, 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.
  
  
  METHODS
  Unrestricted stall activity with weight bearing reduced by tenotomy of the Achilles tendon.
  
  
  METHODS
  Osteotomy 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.
  
  
  METHODS
  Nine 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.
  
  
  RESULTS
  External 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.
  
  
  CONCLUSIONS
  Induced cyclic tensile strains did not produce a relevant enhancement of bone healing under flexible fixation.
authors.affiliation: Institute of Orthopedic Research and Biomechanics, University of Ulm, Germany. peter.augat@medizin.uni-ulm.de
authors.forename: P
authors.initials: P
authors.lastname: Augat
authors.forename: J
authors.initials: J
authors.lastname: Merk
authors.forename: S
authors.initials: S
authors.lastname: Wolf
authors.forename: L
authors.initials: L
authors.lastname: Claes
mesh_terms: D000818:Animals; D002146:Bony Callus / Q000473:pathology*; D004195:Disease Models, Animal; D016267:External Fixators; D005592:Fracture Fixation / Q000379:methods*; D017102:Fracture Healing / Q000502:physiology; D008297:Male; D011446:Prospective Studies; D011897:Random Allocation; D012016:Reference Values; D012756:Sheep; D013314:Stress, Mechanical; D013978:Tibial Fractures / Q000601:surgery*