## 4 Bone Grafts and Substitutes for Bone Repair
### Synthetic Bone Substitute Materials
#### 4.2.3 Composite Bone Substitute Materials  
Composites are one of the advanced materials used in various kinds of bioapplications. By definition, they represent a mixture between one continuous component, named \"matrix\", and one or more components (continuous or discontinuous), named \"reinforcement\"(s). The symbol of a composite is generally represented by the formula: matrix material/reinforcement material/reinforcing content (wt.%). For instance, HA/HDPE/27 means a composite material made of hydroxyapatite as a matrix, which is reinforced by high-density polyethylene of 27% wt. The choice to select a biocomposite or a biomaterial for a specific application, i.e., bone substitute materials, is made according to functional, technological and (not the least) economic reasons. The properties provided by the conventional biomaterials (metallic or ceramic or polymers) are generated during their processing (by physical-chemical reactions between the chemical elements) but especially by post-processing operations (such as  
Figure 4: Synthetic biphasic ceramic (hydroxyapatite HA and beta tricalcium phosphate \(\beta\)-TCP) granules (**a**) vs. wedge (**b**) inserted into the high tibial osteotomy gap. The structural graft provides greater stability and strength to the construct. mechanical, thermal, biofunctional, etc.) involving extra-time and energy consumption costs, respectively. Graphically, the recommendations to use the composites for BS may be expressed as functions of the bone density and the driven mechanical properties governing the alloplastic composite selection, respectively (Figure 5). On the other hand, in order to obtain a particular property for a bone substitute material, the matrix and reinforcements are optimally selected from different points of view (structural, functional, market availability, biocompatible and environmentally friendly) to fulfil the most efficient ratio between the expected performance and necessary processing costs. The most important feature in designing a bicomposite is to create a working interface between the matrix and reinforcements, i.e., one able to efficiently transfer the loads (mechanical, thermal, etc.) during its functioning without any components' physical and chemical degradation. As far as the bone substitutes are concerned, biocomposites provide a wide range of possibilities to significantly improve the properties of the above-mentioned conventional solutions (autologous grafts, allografts and xenografts). Different types of biocomposites are further presented--But also, their selection criteria for a specific bone substitute. The mechanical properties of HA may be significantly improved by its reinforcing with carbon nanotubes (CNTs, 1-3 wt.%). Most of such applications concern dentistry applications [88]. The obtained properties were superior vs. the pure HA, such as the flexural strength, 83 MPa (1.6 times higher); and the fracture toughness, 2.4 MPa m\({}^{1/2}\) (2 times higher), because of the special interface geometry between the matrix and the reinforcing element, schematically represented in Figure 6. The higher is the CNT roughness, the more efficient is the interlocking effect towards the HA particles whose properties are provided by the processing technology. Figure 5: Graphical representation on the general prediction concerning the relationship between the structural features of the bone tissues and the governing mechanical properties for a specific grafting application (Courtesy of Prof. [PERSON], University of Craiova, Faculty of Mechanics). The processing of such interface morphology was released by means of the double in situ CNTs synthesis within the HA matrix by chemical vapor deposition (CVD) using Fe catalysts [89]. An alternative approach to reinforce the matrix of a bioccomposite is to use precursors, leading to in situ synthesis of the reinforcements. Among the processing technologies for bone substitutes, powder metallurgy (PM) is one of the most versatile.