Cancellous bone mechanical properties

A representative stress-strain curve of one of the PDMS-CaO-Si02 nano-hybrids is shown in Figure 11.7, in comparison with that reported for human cancellous bone [29]. Unlike the usual brittle ceramics, the nano-hybrid was deformable and showed mechanical properties analogous to those of human cancellous bone. 

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. 

Typically, bone has a solid outer portion called cortical bone and a porous inner part called cancellous bone. The amounts of each vary with location in the body. The cortical bone is a ceramic containing calcium compounds and viscous liquids, a protein called collagen , and an organic polymer. In addition to HAP, bone consists of calcium carbonate and calcium phosphate. HAP is 69 wt.% of total calcium phosphate compounds [4]. Part of the Ca in these compounds is substituted by Na, K, Mg, and Sr. Hydroxyl ions in the HAP are also substituted by F, CO3, or Cl, which makes the apatite a fluoroapatite, dahllite or chloroapatite, respectively. These substitutions are considered to play significant roles in the structure and mechanical properties of bones. 

Cancellous bone is a very porous material, with an average density of 1.3gcm, implying a porosity of nearly 35%. In practice, the density lies between 5 and 95% varying gradually between cortical and cancellous regions. The pore size distribution is bimodal. The pores are elongated and filled with soft tissues that include bone marrow, blood vessels, and various bone-related cells. It is the overall porosity and the pore size distribution that mostly control the mechanical properties of bone. 

The mechanical properties of the implant are clearly very important. Figure 35.4 compares T of various implant materials to that of cortical and cancellous bone. 

There are two t5 es of bone tissue in the human oiganism. Cortical bone thanks to the presence of Haversian channels shows good osteoconductive properties. Thanks to its mechanical properties it can be used in cases when recreation of tridimensional cavities within the facial part of the skeleton is required. As opposed to cortical bone, cancellous bone is extremely rich in osteogenic cells. Living osteoblasts of cancellous bone may survive even for a few hours from the time of harvesting of the tissue early revascularization in closed cavities takes place after 48 hr. The disadvantage of the cancellous bone grafts is their small mechanical endurance. It is also connected with the lack of possibility to use them in case of tridimensional reconstructions [3]. 

Cortical bone is mechanically stronger than cancellous bone because it is denser than cancellous bone. The relative density and some mechanical properties of bone are shown in Table 2. However, these properties changes with sex, age, dietary history, health status, and anatomical location. Generally, lower density and weaker mechanical properties are observed for diseased bmie. 

Beta-tricalcium phosphate ( 3-TCP) is a highly porous ceramic with chemical and physical structures that mimic bone [66,67]. While it is osteoconductive and porous, the compressive mechanical properties of p-TCP are considerably lower than those of HA [66-69] and it degrades more rapidly than HA [70]. p-TCP has been incorporated in both injectable and implantable polyurethane composites to capitalize on its osteoconductive properties and overcome weaker mechanical strength [39]. Addition of as little as 10wt% p-TCP increased the modulus and strength of the composites, and the composites approach the strength of cancellous bone with incorporation of 70wt%... 

Aging tends to influence the post-yield properties of whole bone, especially the toughness, which is particularly related to the reduction in strength of the collagen network. Hydroxyproline analysis showed a 50% decrease in total collagen concentration in cancellous bone with age, for both males and females. The study also revealed a steady reduction in mechanical properties of the cancellous bone with... 

In microsphere sintering, pre-synthesized polymer microspheres or polymer/ceramic/bioactive addiction composites are sintered to produce a 3-D porous scaffold (56). Bioactive scaffolds can be fabricated through this technique, and they are demonstrated to be supportive to human osteoblast-like cells adhesion, growth, and mineralization (57). Scaffolds fabricated through this technique can have graded porosity structures. Mechanical properties close to cancellous bone also become possible when the microspheres are sintered into... 

The feasibility of additive manufactured poly(caprolactone) (PCL) silanized tricalcium phosphate scaffolds coated with carbonated hydroxyapatite-gelatin composite for bone tissue engineering has been tested (4). In order to reinforce the scaffolds to match the mechanical properties of cancellous bone, tricalcium phosphate has been modified with y-glycidoxypropyltrimethoxysilane and incorporated into PCL to synthesize a PCL/silanized tricalcium phosphate composite. y-GlycidoxypropyltrimethoxysUane is shown in Figure 3.1. 

The mechanical properties of bone are related to its complex hierarchical structure. Several levels of structural organization, from macro- to subnanostructure, can be identified (a) the macrostructure (cancellous and cortical bone) (b) the microstructure (from 10 to 500 mm Haversian systems, osteons, single trabeculae) (c) the submicrostructure (1-10 mm lamellae) (d) the nanostructure (from a few hundred nanometers to 1 mm fibrillar Col and embedded mineral) and (e) the subnanostructure (below a few hundred nanometers the molecular structure of constituent elements, such as mineral, collagen, and non-collagenous organic proteins). The hierarchical structural organization of bone is shown in Fig. 1 [12]. 

Strain rates. This behaviour is similar to that of bone, with mechanical characteristics closer to bone than current non-foamed polymer systems. Viscoelasticity and anisotropy, both in morphology and in mechanical properties, are promising characteristics for bone replacement. Moreover, gas foaming proved to be a flexible technique that enabled scaffolds to be processed with various macrostructures suitable for replacing different types of cancellous bones [148]. 

Modification of inorganic component CaO-Si02 with organic component PDMS gives a hybrid material with apatite-forming ability and mechanical properties analogous to those of human cancellous bone. 

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