Age-Related Changes in the Tensile Properties of Cortical Bone

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. UItimate 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 *No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding source was Action Research for the Crippled Child. tDepartment of Orthopaedic Surgery and Trauma, Royal Adelaide Hospital. North Terrace, Adelaide 5000. South Australia. Please address requests for reprints to Dr. McCalden. Univcrsity of Edinburgh, Frogston Road West. Edinburgh EHIO 7ED, Scotland, United Kingdom. §Department (.)f Orthopaedic Surgery. University of Toronto, 94 Glenmore Road. ibrolito. Ontario M4L 3M3. Canada. 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. Age-related changes in the structure and composition of bone tissue are becoming increasingly important as people continue to live longer. These changes influence the mechanical behavior of bone and may be partially responsible for the increased prevalence of fractures in the elderly2. A better appreciation of the effects of aging on the mechanical properties of bone is critical to an understanding of the risks and mechanics of fractures and is desirable for the design of more effective forms of skeletal and endoprosthetic implants in the elderly. Several previous studies have examined the changes in the mechanical behavior of cortical bone associated with increasing age’6’6 243235 . However, the lack of uniformity in the materials and methods used makes it difficult to draw any general conclusions from these early studies. To our knowledge, Burstein et al.5 reported the first study of aging cortical bone in which the elastic-plastic characteristics of the bone were measured. They described the changes in tensile, compressive, and shear properties of cortical bone, although they did not do an analysis of the bone material to explain their findings. Currey and Butler’4 related changes in bending strength and stiffness to the mineral content in the bones of children. Currey” also studied the relationship between impact energy and the age, mineral content, density, and porosity of bone. This work provides useful, although limited, information about the mechanical events and properties that lead to a fracture. We know of no comprehensive examination of the age-related changes in the tensile properties of cortical bone in which the important associated changes in the bone tissue have been evaluated. The current study describes the effects of age-related changes in porosity, mineralization, and microstructure on the mechanical properties of human bone.