Content area
Abstract
Laser Scanning Confocal Microscopy was used to investigate microcracks in cortical bone. Confocal microscopy was able to provide detail on the morphology of very small, diffuse cracks which are not observable using conventional light microscopy methods. Microcrack arrays were found to be associated with blood vessels, remodeling cavities and osteocyte lacunae. There also seemed to be a relationship between collagen fibril orientation and microcrack orientation. Cortical bone, beam specimens were loaded several times to increasing stress, or in low strain fatigue, and examined intermittently between loading cycles. These experiments showed that microcrack arrays grow on increasing stress and increasing numbers of cycles, and that there is a non-linear relationship between loss of stiffness and appearance of microcracks. They also showed that microcrack arrays can become very dense before cracks coalesce to form large, potentially dangerous cracks.
The effect of these types of microcracks on bovine bone impact strength was examined. Damage created in tension, which caused a reduction in stiffness, of up to 20%, and clearly visible microcracking, did not reduce bone impact strength, compared to undamaged controls. However, damage created in compression did severely reduce impact strength.
The metacarpal bone of horses that had undergone an intensive training regime was compared with that of horses that had undergone little exercise, in order to investigate the effects of in vivo loading on mechanical properties and microcracking ability. There was no difference in Young's modulus or bending strength between the less exercised and the well exercised horses. However energy absorption properties did differ: exercised horses metacarpal bone absorbed more energy to failure in four-point bending, and substantially more energy in impact. On examining specimens for microcracks, it was found that those specimens with higher impact strengths had undergone more microcracking during failure.