Abstract
The relationships between the tensile properties of cortical bone and the microstructural variations at the level of the osteon are investigated. With controlled testing conditions the tensile strength, elastic modulus and % elongation are evaluated for longitudinally orientated test specimens machined from the mid-diaphyses of bovine and human femoral shafts. The pre-testing storage conditions are shown to have no significant effect on the subsequent mechanical properties. A comprehensive structural analysis is made of the total cross-section at the actual site of fracture and at random transverse sections along the gauge length. This is achieved by microradicgraphic and photometric techniques. Coefficients of correlation and associated regression curves are computed for all combinations of mechanical properties and structural components. Where applicable, the results of the preliminary work involving bovine bone are in close agreement with the published literature. In particular, the generally accepted positive correlation between the primary bone area and the tensile strength is supported. The subsequent work on human bone modifies this concept by showing it to be only valid when the area occupied by the cavities in the structure is decreasing. Certain structural/mechanical correlations indicate the capacity of both cavities and cementing lines to act as sources of weakness - the cavities being particularly effective in this role at the clinically significant high rates of strain. In addition, crack arrest mechanisms are observed to be operative in the area of partial osteons and this is thought to be attributable to their abundance of cementing lines. The probable mechanics of crack initiation and arrest in these components, together with the manner in which their sensitivities are modified by stress relaxation and plastic flow are discussed. The factors influencing the elastic deformation of cortical bone are the least understood. It is suggested that little progress will result in this respect until the effects of regional variations in the microconstituents on the localized elastic moduli and time-dependent characteristics are examined. The design and development of an apparatus for the measurement of the gross porosity is described and a technique for the measurement of the absolute mineral density and distribution is discussed. During the course of the work several topics requiring further examination become apparent. These, together with related subjects, whose study would extensively broaden the scope of the present investigation, are considered in the final chapter.