Tensile testing to failure was done on 235 cortical specimens that had been machined from forty-seven femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the porosity, mineralization, and microstructure were determined.
Linear regression analysis showed that the mechanical properties deteriorated markedly with age. Ultimate stress, ultimate strain, and energy absorption decreased by 5, 9, and 12 per cent per decade, respectively. The porosity of bone increased significantly with age, while the mineral content was not affected. Microstructural analysis demonstrated that the amount of haversian bone increased with age.
Both bivariate and multivariate analyses demonstrated the importance of age-related changes in porosity to the decline in mechanical properties. Changes in porosity accounted for 76 per cent of the reduction in strength. Microstructural changes were highly correlated with porosity and therefore had little independent effect. Mineral content did not play a major role. Thus, the quantitative changes in aging bone tissue, rather than the qualitative changes, influence the mechanical competence of the bone.
Clinical Relevance: This study provides information concerning the difference in properties of cortical bone tissue as a function of donor age. The importance of changes in porosity, reflected by changes in the density of bone, will allow investigators to use non-invasive means to estimate the mechanical properties of cortical bone in vivo. Thus, it may be possible to predict the risk of fractures and further explain some aspects of the mechanics of fractures in the elderly. In addition, this study provides information that is useful in the design of fracture and endoprosthetic implants, the success of which is dependent on the implant-bone interface. Future designs must take into account the increase in porosity, which plays the greatest role in the mechanical decline of elderly bone.