Cortical bone elastic moduli

The properties were given to TF components based on literature review [1]. Bones (tibia and femur) were defined cortical bone, elastic and linear material with Young modulus of llGPa and Poisson s ratio of 0.3. Femoral and tibial cartilages were considered linear and elastic material with a Young s modulus of 5 MPa and a Poisson s ratio of 0.46. Menisci were also considered elastic with a Young s modulus of 59 MPa and a Poisson s ratio of 0.49. 

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 stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

There are two basic structural types of bone cancellous (trabecular, spongy) and cortical (dense) bones. Cancellous bone matter is less dense than that of cortical bone and is found across the ends of the long bones. Owing to its lower density, cancellous bone has also a much lower modulus of elasticity but higher strain-to-failure rate compared to cortical bone (Table 3.1). Bone has higher moduli of elasticity than soft connective tissues, such as tendons and ligaments. The difference in stiffness (elastic modulus) between the various types of connective tissues ensures a smooth gradient in mechanical stress across a bone, between bones and between muscles and bones (Hench, 2014). 

PEEK can be coated with diamond-like carbon by plasma inunersion ion implantation and deposition to enhance its surface properties [89]. The elastic modulus of diamond-like carbon is closer to that of cortical bone than PEEK. Therefore, the combination of PEEK and diamond-like carbon has been proposed to enhance the stability and surface properties of PEEK in bone replacements. 

Zysset, P. K., Edward Guo, X., Edward Hoffler, C., Moore, K. E. Goldstein, S. A. Elastic modulus and hardness of cortical and trabecular bone lamellae measured by nanoindentation in the human femur. Journal of Biomechanics 32, 1005-1012, doi 10.1016/s0021-9290(99)00111-6(1999). 

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.)... 

Choi, K., Kuhn, J. L., Ciarelli, M. J., and Goldstein, S. A. (1990), The elastic moduli of human subchondral, trabecular, and cortical bone tissue and the size-dependency of cortical bone modulus, J. Biomech. 23(11) 1103-1113. 

Scaffolds made of PCL and HAp were a topic of investigations of Causa and coworkers [164]. They found that the mechanical properties of the composites are close to those of human bone only after the addition of 20 vol% of HAp. In particular, the elastic modulus is within the range of values for human cortical bone. Moreover, with the use of primary human osteoblasts, a high proUferation rate and a moderate increase of alkaline phosphatase activity were found, mainly on the surface of PCL-based composites with 13 and 20 vol% of HAp, though at the last time point (4 weeks) all the HAp-added polymers were covered by confluent layers of cells. It was concluded that the structure of a scaffold along with its surface physicochemical characteristics affect cell behaviour, but, on the other hand, mechanical properties are also crucial for implant performance. 

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