Use of the rat forelimb compression model to create discrete levels of bone damage in vivo

Skeletal responses to damage are significant for understanding the etiology of stress fractures and possibly osteoporotic fractures. We refined the rat forelimb-loading model to produce a range of sub-fracture damage levels during in vivo cyclic loading. A total of 98 right forelimbs of anesthetized, male, 5-month old Fischer rats were loaded cyclically (2 Hz) in axial compression. Rats were killed immediately after loading. In the first experiment, forelimbs were loaded to fracture, which occurred after an increase in peak displacement of 2.0+/-0.2 mm, independent of peak force or cycle number. In the next experiment, we loaded forelimbs at a constant peak force until the peak displacement increased by 0.6-1.8 mm (30-90% of fracture displacement). Mechanical properties of the loaded (right) and contralateral control (left) ulnae were determined ex vivo using three-point bending, and cracks were analyzed using micro-computed tomography. Results demonstrated a dose-response between increased forelimb displacement and increased ulnar damage, with four discrete damage levels. "Low" damage was produced by cyclic loading to 30% of fracture displacement, with no visible cracks and a 10% strength loss. "Mild" damage was produced by loading to 45% of fracture displacement, with variable linear cracks and 20% strength loss. "Moderate" damage was produced by loading to 60-75% of fracture displacement, with consistent linear cracks and 40% strength loss. "High" damage was produced by loading to 85-90% of fracture displacement, with branching cracks and 60% strength loss. This loading model will be useful for examining biological responses to a range of sub-fracture damage levels in future experiments.