15. CHARACTERIZATION OF RAT AND MOUSE FORELIMB COMPRESSION MODELS FOR STUDIES OF WOVEN BONE REPAIR IN RESPONSE TO FATIGUE DAMAGE

Abstract

Purpose: Bone fatigue damage can lead to stress fractures and may play a role in fragility fractures. The rat forelimb compression model has been used to examine biological responses and gene expression associated with woven bone repair after fatigue damage. Development a similar mouse model would enable the use of genetically modified mice to study molecular mechanisms associated with bone repair.

Method: Following approval from our Central Animal Facility, forelimbs of male retired breeder C57BL/6 mice and Sprague Dawley rats (n=31 each) were loaded in axial compression across the carpus and olecranon. First, both forelimbs (postmortem, n=6 each) were monotonically loaded to determine failure load. Next, both forelimbs of animals (postmortem, n=5 each) were loaded cyclically to sub-fracture load (67% of monotonic load for mice, 55% for rats) until fatigue failure. Following analysis of fatigue displacement histories, right forelimbs (post-mortem, n=10 each) were loaded cyclically to a set displacement short of the expected failure displacement (mice–30%; rats–55%). Non-loaded left forelimbs served as controls. Three-point bending tests were performed on the ulnae; mechanical properties were compared between fatigued and non-loaded limbs. Finally, right forelimbs (n=10 each) were cyclically loaded in anaesthetised (2.5% isofluorane) animals to 30% (mice) and 55% (rats) of failure displacement. Animals recovered for seven days; microCT imaging and three-point bend tests were performed on the ulnae.

Results: Ultimate forelimb failure loads were 5.63 ± 0.47 N (mouse) and 57.1 ± 5.8 N (rat). Measured from the 10th cycle, fatigue failure occurred at displacements of 1.68 ± 0.21 mm (mouse) and 2.96 ± 0.22 mm (rat). In three-point bending, fatigue damaged ulnae failed at significantly lower loads versus control (mouse −51.6%; rat −32.1%). After seven days healing, bone cross-sectional area was significantly greater (microCT) and mechanical properties partially recovered (−13.8% versus control).

Conclusion: Rat and mouse forelimb fatigue loading models have been developed to induce repeatable bone damage. Observed differences in fatigue behaviour necessitated different loading parameters between models. Following seven days of healing, recovery of mechanical strength accompanied woven bone formation (demonstrated by microCT). Further work will compare the biological, woven bone, response between the mouse and rat forelimb models.