Strength osteons

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 structure of this material is generally similar to that of reinforced concrete, which contains steel rods embedded in concrete. Researchers are currently attempting to calculate the predicted strength of the osteon-mucopolysaccharide composite and to compare it with observed values. 

Mechanical properties of cancellous (spongy) bone are dependent on the bone density and porosity, and therefore the strength and stiffness of spongy bone is much lower than that of cortical or osteonic bone. The axial compressive strength is related to the square of the bone density. 

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. 

The same nomenclature applies as for moduli of osteons (Table A1.5). Table A1.8 Strength of Osteons... 

Strength (MPa) Longitudinal Osteons Alternate Osteons Source... 

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