**1. Introduction**

The orthopaedic implant market is expected to grow from its current \$30 billion value due to the rising demands for orthopaedic implant procedures in a universally aging civilisation. A plethora of synthetic materials capable of encouraging bone growth are available in the market, referred to as bioactive materials. The clinical success rates of bioactive orthopaedic implant validate the concerted research undertaken to enhance their abilities, underpinned by their propensity to alleviate pain, expedite recovery, and ameliorate quality of life for the patient. Applicable artificial implants can be in the form of plates, rods, screws, or scaffolds (a porous structure used to substitute missing osseous tissue). Indeed, artificial implants can be fabricat‐ ed from metals, ceramics, polymers, and composites; however, due to the complexity of the human skeleton, no one class of materials is suited for all applications. Moreover, bioactive materials are not without their drawbacks. The aim of this chapter is to provide an overview of howmaterial science andengineering techniques are employedtomaximise theirpotential and thus ensuring long-term efficiency.

**Figure 1.** Schematic to show how implants are used for load-bearing applications.

#### **1.1. Background**

Bone resides in a perpetual resorption-regeneration state dictated by osseous cells and, like the skin, has a natural tendency to heal when fractured over time. There are instances when the healing cannot be accomplished, such as non-union fractures, which leads to medical intervention. Bone grafting is considered a strong candidate in such cases. A graft can either be natural or synthetic in its form but serves the purpose of encouraging the bone to grow. Bone grafts can be retrieved from the patients' own skeleton (autograft) or from a donor (allograft); however, concerns including but not limited to histocompatibility, disease transfer, and lack of availability necessitate the use of synthetic materials—of which metals and ceramics have been extensively researched.

Synthetic materials can exert several responses within physiological environment. If no adverse reaction occurs, then the material is said to be biologically compatible, or "biocom‐ patible." This can further be subdivided into two groups: bioinert and bioactive, where the former is used to refer to a material that does not interact with the surrounding tissues. A bioactive implant can elicit an efficacious reaction that induces a phenomenon where a bonelike layer is formed around the implant providing an initial rapid and robust bond between the bone and implant that can culminate in complete integration. This type of response is technically referred to as *osseointegration*. Materials can recruit pre-existing bone cells to lay the groundwork for the integration, which is referred to as *osteoconduction*. Others stimulate undifferentiated cells into bone cells are referred to as *osteoinduction.* Implants eliciting such a response are associated with high success rates in clinical settings. Such exceptional attributes are inherent in some materials, and others need additional processing to implement the trait. An assortment of bioactive materials is capable of dissolving gradually within the human body, under physiological environment. The concept of a synthetic material inducing bone growth and vanishing, so to speak, when a new bone is remodelled is very attractive as it can avoid added patient inconvenience and healthcare costs. Materials that can dissolve or degrade under the physiological conditions are referred to as "biodegradable" or "bioresorbable." The aim for bioresorbable materials in load-bearing applications is for the implant to bear the majority of the load when implanted, and as the bone heals and more bone tissue is formed, the load is shared between the implant and healing tissues. As the scaffold is resorbed and consequently weakened, the healing bone sustains the majority of the load until the scaffold is completely resorbed and bone is fully restored. Preferably, if the graft resorption occurs in tandem with bone regeneration, structural weakness can be mitigated and minimising premature graft failure. A porous structure is also favoured because opportunities for bone to grow within the implant (as opposed to solely on the surface) can be achieved that leads to enhanced osseointegration and early implant stabilisation. Therefore, designing an artificial implant should incorporate as many of the aforementioned attributes.

**Figure 2.** Figure depicting the hierarchy of biocompatible materials.

**1. Introduction**

196 Advanced Techniques in Bone Regeneration

thus ensuring long-term efficiency.

**1.1. Background**

**Figure 1.** Schematic to show how implants are used for load-bearing applications.

ceramics have been extensively researched.

Bone resides in a perpetual resorption-regeneration state dictated by osseous cells and, like the skin, has a natural tendency to heal when fractured over time. There are instances when the healing cannot be accomplished, such as non-union fractures, which leads to medical intervention. Bone grafting is considered a strong candidate in such cases. A graft can either be natural or synthetic in its form but serves the purpose of encouraging the bone to grow. Bone grafts can be retrieved from the patients' own skeleton (autograft) or from a donor (allograft); however, concerns including but not limited to histocompatibility, disease transfer, and lack of availability necessitate the use of synthetic materials—of which metals and

The orthopaedic implant market is expected to grow from its current \$30 billion value due to the rising demands for orthopaedic implant procedures in a universally aging civilisation. A plethora of synthetic materials capable of encouraging bone growth are available in the market, referred to as bioactive materials. The clinical success rates of bioactive orthopaedic implant validate the concerted research undertaken to enhance their abilities, underpinned by their propensity to alleviate pain, expedite recovery, and ameliorate quality of life for the patient. Applicable artificial implants can be in the form of plates, rods, screws, or scaffolds (a porous structure used to substitute missing osseous tissue). Indeed, artificial implants can be fabricat‐ ed from metals, ceramics, polymers, and composites; however, due to the complexity of the human skeleton, no one class of materials is suited for all applications. Moreover, bioactive materials are not without their drawbacks. The aim of this chapter is to provide an overview of howmaterial science andengineering techniques are employedtomaximise theirpotential and A material's characteristics (e.g. its resorbability in the body, and at what rate) is ultimately determined by their composition and the fabrication process employed. The overall process involves multiple steps that determine the structure and properties of the final product, ranging from structural modifications at the atomic level through to the gross level visible to the eye, such as colour and surface roughness. All materials have their atoms arranged in some manner, which can be altered through, for example, heat treatment. If the arrangement is homogenous throughout the material's microstructure, then it is referred to as homogenous, or single phase. However, if two or more discrete zones are evident within the microstructure, then the additional zones will be referred to as secondary, tertiary, etc., phases. Alternatively, a complete change in atomic orientation can occur, resulting in a transition of phases. Greek letters are used to denote between different phases of a material, e.g. α-titanium, β-tricalcium phosphate, etc.

**Figure 3.** Figure illustrating examples for composition, processing, microstructure, and properties.

Metals are a popular choice for synthetic implants and their strength lies in the various processing routes available—owing to their mechanical properties. Their mechanical proper‐ ties are either comparable or exceed that of bone. Contrarily, their chemical makeup is a limiting factor for both load and non-load-bearing orthopaedic implants. Ceramics, on the other hand, offer far more options with respect to their excellent bioactivity, but the manufac‐ turing processes are a limiting factor that prevents them from producing the mandatory physical and mechanical properties for load-bearing applications. Therefore, the scientific interest varies for metals and ceramics. The chapter herein draws from contemporary research to provide examples of how material engineering is capitalised to tackle the challenges faced.
