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The Size of Intramedullary Fixation Affects Endochondral Mediated Angiogenesis During Fracture Repair.

OBJECTIVES
To explore the effect of intramedullary pin size on the biology of a healing fracture, specifically endochondral angiogenesis. We hypothesized that fracture fixation with a smaller pin would permit greater interfragmentary strain resulting in increased total amount of VEGF within the callus and greater angiogenesis compared to fixation with a larger pin.


METHODS
Transverse mid-shaft femur fractures in 8-week old mice were fixed with either a 23 gauge (G) or 30G pin. Differences in interfragmentary strain at the fracture site were estimated between cohorts. A combination of histology, gene expression, serial radiography, and micro-computed tomography with and without vascular contrast agent were utilized to assess fracture healing and vascularity for each cohort.


RESULTS
Larger soft tissue callus formation, increased VEGF-A expression, and a corresponding increase in vascular volume was observed in the higher strain, 30G cohort. Radiographic analysis demonstrated earlier hard callus formation with greater initial interfragmentary strain, similar rates of union between pin size cohorts, yet delayed callus remodeling in mice with the larger pin size.


CONCLUSIONS
These findings suggest that the stability conferred by an intramedullary nail influences endochondral angiogenesis at the fracture.


LEVELS OF EVIDENCE
Prospective basic science report with appropriate randomization and controls.