Bone as a composite material: the role of osteons as barriers to crack growth in compact bone.

This article summarises a number of studies in the area of bone microdamage which were carried out in our laboratory over the past 5 years. A technique was developed to label microcracks during mechanical testing. Fluorescent chelating agents were applied at intervals to bone specimens fatigue tested in cyclic compression until failure occurred. Microcrack densities were measured and microcrack length at the time of encountering the cement line surrounding an osteon was also recorded. Microcracks were shown to develop rapidly during the first stage of testing but then further accumulation of cracks did not occur until the period just before failure. The majority of microcracks were found in interstitial bone and did not penetrate cement lines. Only microcracks greater than 300 μm in length were found to be capable of penetrating osteons. This work provides experimental data to support the hypothesis that secondary osteons act as barriers to crack propagation in compact bone.