title: Differential fracture healing resulting from fixation stiffness variability: a mouse model.
abstract: BACKGROUND
  The mechanisms underlying the interaction between the local mechanical environment and fracture healing are not known. We developed a mouse femoral fracture model with implants of different stiffness, and hypothesized that differential fracture healing would result.
  
  
  METHODS
  Femoral shaft fractures were created in 70 mice, and were treated with an intramedullary nail made of either tungsten (Young's modulus = 410 GPa) or aluminium (Young's modulus = 70 GPa). Mice were then sacrificed at 2 or 5 weeks. Fracture calluses were analyzed using standard microCT, histological, and biomechanical methods.
  
  
  RESULTS
  At 2 weeks, callus volume was significantly greater in the aluminium group than in the tungsten group (61.2 vs. 40.5 mm(3), p = 0.016), yet bone volume within the calluses was no different between the groups (13.2 vs. 12.3 mm(3)). Calluses from the tungsten group were stiffer on mechanical testing (18.7 vs. 9.7 N/mm, p = 0.01). The percent cartilage in the callus was 31.6% in the aluminium group and 22.9% in the tungsten group (p = 0.40). At 5 weeks, there were no differences between any of the healed femora.
  
  
  CONCLUSIONS
  In this study, fracture implants of different stiffness led to different fracture healing in this mouse fracture model. Fractures treated with a stiffer implant had more advanced healing at 2 weeks, but still healed by callus formation. Although this concept has been well documented previously, this particular model could be a valuable research tool to study the healing consequences of altered fixation stiffness, which may provide insight into the pathogenesis and ideal treatment of fractures and non-unions.
authors: Department of Orthopaedic Surgery, Washington University School of Medicine, 660 S. Euclid Ave, Campus Box 8233, St. Louis, MO 63110, USA. gardnerm@wudosis.wustl.edu
authors: Michael J
authors: MJ
authors: Gardner
authors: Sara M
authors: SM
authors: Putnam
authors: Ambrose
authors: A
authors: Wong
authors: Philipp N
authors: PN
authors: Streubel
authors: Akhilesh
authors: A
authors: Kotiya
authors: Matthew J
authors: MJ
authors: Silva
mesh_terms: D000535:Aluminum; D000818:Animals; D001696:Biomechanical Phenomena; D001858:Bone Nails; D004195:Disease Models, Animal; D005264:Femoral Fractures / Q000000981:diagnostic imaging / Q000601:surgery*; D005594:Fracture Fixation, Intramedullary / Q000295:instrumentation*; D017102:Fracture Healing; D008297:Male; D008422:Materials Testing; D051379:Mice; D008810:Mice, Inbred C57BL; D011859:Radiography; D014414:Tungsten
chemical_list: D000535:Aluminum; D014414:Tungsten