title: Late dynamization by reduced fixation stiffness enhances fracture healing in a rat femoral osteotomy model.
abstract: OBJECTIVE
  This study evaluated the effect of late dynamization on callus stiffness and size in a rat diaphyseal femoral osteotomy model. It was hypothesized that late dynamization, after bony bridging, would enhance healing.
  
  
  METHODS
  The external unilateral fixator was dynamized by removal of the inner fixator bar at either 3 weeks (D3 group) or 4 weeks (D4 group) postoperation. After 5 weeks, the rats were euthanized and healing was evaluated by biomechanical and densitometric methods. Published data of control groups, constant rigid (R group), and flexible fixation (F group) were included for comparison.
  
  
  RESULTS
  Enhanced healing was observed in both the 3- and 4-week dynamized groups compared with the constant flexible and constant rigid fixation. Late dynamization after both 3 and 4 weeks postoperation led to a stiffer callus (percent flexural rigidity relative to the intact side and elastic modulus) with a smaller callus bone volume compared with the F group. Both late dynamized groups (D3 and D4) had a similar flexural rigidity and bone mineral density as that of the R group. However, the D4 group had a significantly greater elastic modulus and significantly smaller callus bone volume compared with the R group suggesting increased remodeling occurred in the D4 group indicative of more advanced healing.
  
  
  CONCLUSIONS
  Late dynamization enhanced fracture healing in this animal model compared with a constant rigid or constant flexible fixation. Although results of the small animal study cannot directly be transferred to humans, these results indicate that once bony bridging has occurred, dynamization may accelerate bone remodeling processes.
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, University of Ulm, Germany. lutz.claes@uni-ulm.de
authors.forename: Lutz
authors.initials: L
authors.lastname: Claes
authors.forename: Robert
authors.initials: R
authors.lastname: Blakytny
authors.forename: John
authors.initials: J
authors.lastname: Besse
authors.forename: Christine
authors.initials: C
authors.lastname: Bausewein
authors.forename: Anita
authors.initials: A
authors.lastname: Ignatius
authors.forename: Bettina
authors.initials: B
authors.lastname: Willie
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D015519:Bone Density; D016723:Bone Remodeling / Q000502:physiology; D002146:Bony Callus / Q000502:physiology; D003720:Densitometry; D004195:Disease Models, Animal; D004867:Equipment Design; D005264:Femoral Fractures / Q000503:physiopathology* / Q000601:surgery; D005269:Femur; D005592:Fracture Fixation / Q000379:methods*; D017102:Fracture Healing / Q000502:physiology*; D008297:Male; D009068:Movement / Q000502:physiology*; D010027:Osteotomy; D018583:Pliability; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors