The use of fractography to supplement analysis of bone mechanical properties in different strains of mice

Abstract

Fractography has not been fully developed as a useful technique in assessing failure mechanisms of bone. While fracture surfaces of osteonal bone have been explored, this may not apply to conventional mechanical testing of mouse bone. Thus, the focus of this work was to develop and evaluate the efficacy of a fractography protocol for use in supplementing the interpretation of failure mechanisms in mouse bone. Micro-computed tomography and three-point bending were performed on femora of two groups of 6-month-old mice (C57BL/6 and a mixed strain background of 129SV/C57BL6). SEM images of fracture surfaces were collected, and areas of “tension”, “compression” and “transition” were identified. Percent areas of roughness were identified and estimated within areas of “tension” and “compression” and subsequently compared to surface roughness measurements generated from an optical profiler. Porosity parameters were determined on the tensile side. Linear regression analysis was performed to evaluate correlations between certain parameters. Results show that 129 mice exhibit significantly increased bone mineral density (BMD), number of “large” pores, failure strength, elastic modulus and energy to failure compared to B6 mice ( p < 0.001). Both 129 and B6 mice exhibit significantly ( p < 0.01) more percent areas of tension (49 ± 1%, 42 ± 2%; respectively) compared to compression (26 ± 2%, 31 ± 1%; respectively). In terms of “roughness”, B6 mice exhibit significantly less “rough” areas (30 ± 4%) compared to “smooth” areas (70 ± 4%) on the tensile side only ( p < 0.001). Qualitatively, 129 mice demonstrate more evidence of bone toughening through fiber bridging and loosely connected fiber bundles. The number of large pores is positively correlated with failure strength ( p = 0.004), elastic modulus ( p = 0.002) and energy to failure ( p = 0.041). Percent area of tensile surfaces is positively correlated with failure strength ( p < 0.001), elastic modulus ( p = 0.016) and BMD ( p = 0.037). Percent area of rough compressive surfaces is positively correlated with energy to failure ( p = 0.039). Evaluation of fracture surfaces has helped to explain why 129 mice have increased mechanical properties compared to B6 mice, namely via toughening mechanisms on the compressive side of failure. Several correlations exist between fractography parameters and mechanical behavior, supporting the utility of fractography with skeletal mouse models.