The development of microcracking and failure in bone depends on the loading mode to which it is adapted.

During locomotion, the anterior cortex of the equine radius is loaded predominantly in tension, the posterior predominantly in compression. The anterior cortex is relatively strong in tension, the posterior in compression. We investigated the pattern of failure of specimens from the two cortices using laser scanning confocal microscopy. All specimens were loaded in four-point bending to increasingly higher loads. We quantified the amount of diffuse microcracking on the tensile side of these specimens by observing the amount of light emitted under laser illumination. The amount of light emitted agreed well with subjective estimates of the amount of microcracking. Tensile microcracks first appeared at a strain of approximately 0.004, and all specimens showed considerable growth in microcrack density once the tensile strain had passed approximately 0.008. In specimens from the posterior cortex, there was little compressive microcracking, and such cracks as were present were small and diffuse. These specimens failed on the tensile side first. In specimens from the anterior cortex, compression cracks were more numerous, longer and less diffuse, and specimens failed initially in compression. The patterns of failure in the bone tissues of the two cortices are what would be expected assuming they were adapted to the mode of loading to which they are usually subjected.