Evaluation of continuous axial stress on tibial fracture healing in canine models using custom external fixator

To investigate the effects of physiologically relevant continuous axial stress on tibial fracture healing in canines using a custom external fixator.

A custom external fixator was developed to apply quantifiable stress. First, maximum axial stress derived from muscle contraction was measured using a film sensor. Subsequently, canine tibial fracture models were established and assigned to non-stress, half-maximum stress, or maximum stress groups. Healing was evaluated via radiography, micro-computed tomography (Micro-CT), biomechanical testing, Western Blot, and Real-time Polymerase Chain Reaction (PCR).

The maximum physiological axial stress was measured at 460 g. Compared with maximum stress and non-stress groups, the half-maximum stress group exhibited superior fracture healing, evidenced by increased callus formation and bone volume ratio (BV/TV) in X-ray and micro-CT analyses. Biomechanical testing demonstrated significantly higher ultimate strength in the half-maximum group. Molecular analyses further supported these observations, showing sustained upregulation of Runt-related transcription factor 2 (RUNX2) and Type I Collagen (COL-1) expression in the half-maximum group.

Muscle contraction generates a maximum axial stress of approximately 460 g at canine tibial fracture sites. Continuous application of half the maximum axial stress significantly enhances fracture healing compared to both maximum stress and non-stress conditions.