Late Dynamization by Reduced Fixation Stiffness Enhances Fracture Healing in a Rat Femoral Osteotomy Model

Objectives: 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.