DAMAGE TYPE AND STRAIN MODE ASSOCIATIONS IN HUMAN COMPACT-BONE BENDING FATIGUE

When compact bone is subjected to fatigue loading, it develops matrix microdamage, which reduces the tissue's ability to resist fracture. Th e relative influence of different strain modes on damage and strength in compact bone has not been characterized, to our knowledge. In this study, the nonuniform strain field produced by four-point bending was used to introduce fatigue damage into tibial bending beam specimens fr om men 40-49 years old. The specimens were then bulk-stained with basi c fuchsin to mark damage surfaces and were examined histologically and with confocal microscopy to describe damage morphologies and position relative to tension and compression-strained regions of the specimen. Histomorphometric methods were used to quantify the amounts of differ ent types of bone microdamage. Three major types were observed. In reg ions subjected to tensile strains, the bone had focal regions of diffu sely increased basic fuchsin staining (i.e., diffuse microdamage). Con focal microscopy of these regions showed them to be composed of extens ive networks of fine, ultrastructural-level cracks. In compressive str ain regions, the tissue developed linear microcracks in interstitial a reas similar to those originally described by Frost. Fine, tearing-typ e (wispy-appearing) cracks were observed near and in the plane of the neutral axis. The paths of these fine cracks were not influenced by mi crostructural boundaries. Other minor damage morphologies (sector-stai ned osteons, delamination of regions of lamellae, and intraosteonal cr acking) were observed, but their distribution was unrelated to local s train field. Thus, in fatigue of human compact bane, the principal mec hanisms of matrix failure (i.e., linear microcrack, diffuse damage foc i, and tearing-type damage) are strongly dependent on local strain typ e.