title: Temporal variation in fixation stiffness affects healing by differential cartilage formation in a rat osteotomy model.
abstract: BACKGROUND
  Dynamization involves a reduction in fixation construct stiffness during bone healing, allowing increased interfragmentary movement of the fracture through physiologic weightbearing and muscle contraction. Within some optimal range, interfragmentary movement stimulates healing, but this range likely varies across stages of bone healing.
  
  
  OBJECTIVE
  How does the time of dynamization affect the cartilage formation, bony bridging, and bone resorption in a rat fracture-healing model?
  
  
  METHODS
  Unilateral external fixators, stabilizing a 1-mm gap, were dynamized at 1 (D1 group, n = 10), 3 (D3 group, n = 11), or 4 (D4 group, n = 11) weeks postoperatively. Continuously 5 weeks stiff (S group, n = 10) and flexible (F group, n = 11) fixation were included for comparison. After 5 weeks, healing was evaluated by histomorphometric methods.
  
  
  RESULTS
  Advanced healing, indicated by less cartilage and a greater rate of bony bridging, was observed in the S group compared to the D1 or F group. In contrast, the D3 and D4 groups had less cartilage and more bridging compared to the S group. Also, the S group had less cortical resorption than the F and D1 groups.
  
  
  CONCLUSIONS
  These data suggest late dynamization at the onset of bony bridging led to enhanced healing, whereas dynamization at the early stage of cartilage differentiation delayed healing.
  
  
  CONCLUSIONS
  Although our observations from this small-animal study cannot be directly transferred to humans, these data suggest, once bony bridging begins, dynamization may stimulate bone healing and accelerate remodeling.
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, University of Ulm, Helmholtzstr 14, 89081 Ulm, Germany.
authors.forename: Bettina M
authors.initials: BM
authors.lastname: Willie
authors.forename: Robert
authors.initials: R
authors.lastname: Blakytny
authors.forename: Melanie
authors.initials: M
authors.lastname: Glöckelmann
authors.forename: Anita
authors.initials: A
authors.lastname: Ignatius
authors.forename: Lutz
authors.initials: L
authors.lastname: Claes
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000502:physiology; D016723:Bone Remodeling / Q000502:physiology; D002358:Cartilage, Articular / Q000254:growth & development*; D020219:Chondrogenesis / Q000502:physiology*; D004195:Disease Models, Animal; D004548:Elasticity / Q000502:physiology*; D016267:External Fixators; D005264:Femoral Fractures / Q000503:physiopathology* / Q000601:surgery; D017102:Fracture Healing / Q000502:physiology*; D008297:Male; D010027:Osteotomy; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors; D016474:Weight-Bearing / Q000502:physiology