Microcracks and the fatigue behaviour of compact bone.

Specimens from fresh bovine tibiae were machined into typical, waisted type of circular cross section and fatigue tested in cyclic compression at 3Hz. They were initially stained with alizarin, to label pre-existing damage, and other agents applied in sequence during the test. Specimens were then sectioned and examined using UV epifluorescence microscopy. Microcracks accumulated early in a specimen's life but the rate then levelled off before a second increase in the period prior to failure. Bone microstructure greatly influenced microcrack growth with the 75 - 95% of microcracks found in interstitial bone between Haversian systems. Microcracks grew in length primarily in the longitudinal direction, parallel to Haversian systems and the longitudinal axis of bone. These results support the concept of a microstructural barrier effect in bone.

Human ribs were stained with basic fuchsin to label in vivo microcracks and viewed in the longitudinal direction and in three dimensions using two different computer-based methods of reconstruction: (i) laser scanning confocal microscopy of sections stained in basic fuchsin followed by reconstruction of microcracks into a 3D image and (ii) serial sectioning of methyl-methacrylate embedded sections with identification of microcracks using UV epifluorescence followed by computerised reconstruction individual cracks. Both LSCM and serial sectioning showed similar results regarding the shape and size of microcracks in bone. Microcracks were elliptical in shape with a ratio of longitudinal length: transverse width of 4.6: 1, consistent with theoretical predictions.