β -tricalcium phosphate has the chemical formula Ca₃(PO₄)₂. It is characterized by a Ca / P ratio of 1.5. It is rhombohedral structure (R3c group) and has the lattice parameters a = b = 1.0439 nm and c = 3.7375 (for multiple hexagonal mesh).

Its preparation is made by sintering at high temperature deficient apatite calcium Ca/P ratio of 1.5 or by solid-solid reaction at high temperatures. Sintering temperatures are usually between 1000 and 1250 ° C [49].

There is another way of implementing the TCP wet. The principle is to obtain a precipitate poorly crystallized using a phosphate source and a source of calcium while controlling the temperature and pH. The whole is then calcined at high temperature.

In order to fabricate bioceramics in progressively complex shapes, scientists are investigating the use of both old and new manufacturing techniques. These techniques range from an adaptation of the age-old pottery techniques to the newest manufacturing methods for high-temperature ceramic parts for airplane engines. Namely, reverse engineering [50, 51] and rapid prototyping [52–54] technologies have revolutionized a generation of physical models, allowing the engineers to efficiently and accurately produce physical models and customized implants with high levels of geometric intricacy. Combined with the computer-aided design and manufacturing (CAD/CAM), complex physical objects of the anatomical structure can be fabricated in a variety of shapes and sizes. In a typical application, an image of a bone defect in a patient can be taken and used to develop a three-dimensional (3D) CAD computer model [55–58]. Then a computer can reduce the model to slices or layers. Afterwards, 3D objects and coatings are constructed layer-by-layer using rapid prototyping techniques. A custom-made implant of actual dimensions would reduce the time it takes to perform the medical implantation procedure and subsequently lower the risk to the patient. Another advantage of a pre-fabricated, exact-fitting implant is that it can be used more effectively and applied directly to the damaged site rather than a replacement, which is formulated during surgery from a paste or granular material (Fig. 15.1).

In addition to the aforementioned modern techniques, classical forming and shaping approaches are still widely used. The selection of the desired technique depends greatly on the ultimate application of the bioceramic device, e.g., whether it is for a hard-tissue replacement or an integration of the device within the surrounding tissues. In general, three types of the processing technologies might be used:

A sintering (or firing ) procedure appears to be of a great importance to manufacture bulk bioceramics with the required mechanical properties. Usually, this stage is carried out according to controlled temperature programs of electric furnaces in air. The firing step can include temporary holds at intermediate temperatures to burn out organic binders [59]. The heating rate, sintering temperature and holding time depend on the starting materials. For example, in the case of HA, these values are in the ranges of 0.5–3 °C/min, 1000–1250 °C and 2–5 h, respectively [60]. In the majority cases, sintering allows a structure to retain its shape. However, this process might be accompanied by a considerable degree of shrinkage, which must be accommodated in the fabrication process.

In general, sintering occurs only when the driving force is sufficiently high, while the latter relates to the decrease in surface and interfacial energies of the system by matter (molecules, atoms or ions) transport, which can proceed by solid, liquid or gaseous phase diffusion. Namely, when solids are heated to high temperatures, their constituents are driven to move to fill up pores and open channels between the grains of powders, as well as to compensate for the surface energy differences among their convex and concave surfaces (matter moves from convex to concave). At the initial stages, bottlenecks are formed and grow among the particles. Existing vacancies tend to flow away from the surfaces of sharply curved necks; this is an equivalent of a material flow towards the necks, which grow as the voids shrink. Small contact areas among the particles expand and, at the same time, a density of the compact increases and the total void volume decreases. As the pores and open channels are closed during a heat treatment, the particles become tightly bonded together and density, strength and fatigue resistance of the sintered object improve greatly. Grain-boundary diffusion was identified as the dominant mechanism for densification. Furthermore, strong chemical bonds are formed among the particles and loosely compacted green bodies are hardened to denser materials.

In the case of calcium orthophosphates, the earliest paper on their sintering was published in 1971 [61]. Since then, numerous papers on this subject were published and several specific processes were found to occur during calcium orthophosphate sintering. Firstly, moisture, carbonates and all other volatile chemicals remaining from the synthesis stage, such as ammonia, nitrates and any organic compounds, are removed as gaseous products. Secondly, unless powders are sintered, the removal of these gases facilitates production of denser ceramics with subsequent shrinkage of the samples. Thirdly, all chemical changes are accompanied by a concurrent increase in crystal size and a decrease in the specific surface area. Fourthly, a chemical decomposition of all acidic orthophosphates and their transformation into other phosphates (e.g., 2HPO₄ ²⁻ → P₂O₇ ⁴⁻ + H₂O↑) takes place. Besides, sintering causes toughening, densification, partial dehydroxylation (in the case of HA) [32], grain growth, as well as it increases the mechanical strength.

For their use in clinical, bone substitutes must meet certain criteria. These characteristics define the efficiency of filling material.