Strength cortical bone

The process of aging reduces bone size and strength. Thinning and resorption occur in the canceUous bone. Also, cortical bone resorbs and bone shrinks in diameter and thickness. The older the person, the more fragile the bone. 

The geometry and structure of a bone consist of a mineralised tissue populated with cells. This bone tissue has two distinct structural forms dense cortical and lattice-like cancellous bone, see Figure 7.2(a). Cortical bone is a nearly transversely isotropic material, made up of osteons, longitudinal cylinders of bone centred around blood vessels. Cancellous bone is an orthotropic material, with a porous architecture formed by individual struts or trabeculae. This high surface area structure represents only 20 per cent of the skeletal mass but has 50 per cent of the metabolic activity. The density of cancellous bone varies significantly, and its mechanical behaviour is influenced by density and architecture. The elastic modulus and strength of both tissue structures are functions of the apparent density. 

The human skeleton consists of both cortical and trabecular bone. Cortical bone is dense and compact and is responsible for much of bone strength. It is the most common type of... 

The mechanical properties of cancellous bone are dependent upon the bone density and porosity, and the strength and modulus are therefore much lower than those for cortical bone. The axial and compressive strength are proportional to the square of the bone density, and moduli can range from 1 to 3 GPa. 

The mechanical properties of cortical bone (specifically, the femur, tibia, humerus, and radius) of various species (specifically, horse, cattle, pig, and human) in tension, compression, and torsion are listed in Table I. It should be noted, for example, that human femur tensile strength (namely, 124 MPa) (Yamada, 1970) is in the same order of magnitude to that of cast iron (170 MPa) (Beer and Johnston, 1981) but, surprisingly, low in weight (Kaplan et al., 1994 Fung, 1993). These unique properties of bone are a direct consequence of the synergy of its molecular, cellular, and tissue arrangement. 

Cortical bone, also called compact or lamellar bone, is remodelled from woven bone by means of vascular channels that invade the embryonic bone from its periosteal and endosteal surfaces. It forms the internal and external tables of flat bones and the external surfaces of long bones. The primary structural unit is an osteon, also known as a Haversian system, a cylindrical shaped lamellar bone surrounding longitudinally oriented vascular channels (the Haversian canals). Horizontally oriented canals (Volkmann canals) connect adjacent osteons. The mechanical strength of cortical bone results from the tight packing of the osteons. 

Carboxyethylgermsesquioxane can inhibit bone resorption by osteoclasts in a concentration-dependent manner . The therapeutic effect of 2-carboxyethylgermanium sesquioxane (Ge-132) for experimental osteoporosis has been studied using ovariectomized rats maintained on a low calcium containing diet. The Ge-132 decreased the bone strength, and affected the femur cortical bone index and bone mineral mass caused by osteoporosis . ... 

Sintered HAp particles are remodeled by the host tissue when implanted into bone. Implantation into the cortical bone of the femur of sheep has revealed that stoichio-metrically pure HAp resorbs by several microns after 18 months with dissolution occurring mainly at the grain boundaries (Benhayoune et al. 2000). The tensile strength of bone from the tibia of a rabbit onto a HAp cylinder is 0.85 MPa after 3 months (Edwards et al. 1997). 

P. Zioupos, M. Gresle, K. Winwood, Fatigue strength of human cortical bone age, physical, and material hetert eneity effects, J. Biomed. Mater. Res. A 86 (2008) 627-636. 

FIGURE 8.4 Typical stress-strain behavior for human cortical bone. The bone is stiffer in the longitudinal direction, indicative of its elastic anisotropy. It is also stronger in compression than in tension, indicative of its strength asymmetry (modulus is the same in tension and compression). From Ref. 9.)... 

While it is often appropriate to assume average properties for cortical bone, as shown in Tables 8.1 and 8.2, it may be necessary in some cases to account for the heterogeneity that can arise from variations in microstructural parameters such as porosity and percentage mineralization. Both modulus and ultimate stress can halve when porosity is increased from 5 to 30 percent - (Fig. 8.6a). Small increases in percentage mineralization cause large increases in both modulus and strength (see Fig. 8.6b), and while this parameter does not vary much in adult humans, it can vary substantially across species. ... 

FIGURE 8.7 Reductions of human cortical bone mechanical properties with age. (a) Modulus is not reduced much, if at all, whereas strength is reduced more, at a rate of about 2 percent per decade. (From Ref. 25.) (b) Ultimate strain decreases markedly with age, at a rate of about 10 percent of its young value per decade. (From Ref. 10.)... 

Zioupos, P., and Currey, J. D. (1998), Changes in the stiffness, strength, and toughness of human cortical bone with age. Bone 22(l) 57-66. 

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