## 2 Natural Bone, Spinal Anatomy and Spinal Fusion  
The two types of tissue that comprise bones are compact (cortical) and cancellous (trabecular or spongy) bones. Compact bone, which forms the outer layer of bone, is a dense and durable tissue that makes up around 80% of adult bone mass. It forms the dense outer shell of the vertebrae and has a porosity of between 5 and 15%. Cancellous bone, on the other hand, makes up the remaining 20% of bone and is characterized by its lighter and less-dense nature, as well as its network of rod-like structures known as trabeculae [13]. When designing scaffolds, it is critical to consider the hierarchical structure of bones, their ability to bear mechanical stress, and factors like cell proliferation, adhesion, and osteogenic differentiation. Table 1 below shows the mechanical properties of human bones. The spine serves as the central support structure for the human body, linking various components of the musculoskeletal system. The spine enables fundamental movements, such as sitting, standing, walking, twisting, and bending. However, conditions such as spinal cord disorders, back injuries, and other issues can harm the spine and result in back pain [16; 17]. Spinal fusion surgery is a major surgical process, and the decision to undergo it is typically considered only after conservative treatment, including medications and therapy, have been explored, considering the patient's overall health. A typical spinal fusion procedure involves the joining of two or more vertebrae together so that there is no movement between them, and over time they heal to form a solid unit. The process involves the bones between two adjacent vertebrae being roughened, and then a graft is placed between them [18]. Plastic or metal spacers may be used to replace the disc materials. Until fusion or the bone regrows, rods and screws are placed to create an internal cast that holds and supports the vertebrae [18]. The primary concern associated with any spinal fusion surgery is the inability to alleviate back pain, a scenario observed in at least 20% of spinal fusion surgeries conducted in both the UK and the USA [18; 19]. Another postoperative risk involves the potential failure of the vertebrae to successfully fuse via the bone graft, potentially leading to pseudoarthrosis. From approximately 5 to 10% of spinal fusion \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Elastic Modulus (GPa)** & **Compressive Strength (MPa)** & **Tensile Strength (MPa)** \\ \hline \multirow{2}{*}{Compact bone [14]} & 17–30 (L) & 130–224 (L) & 78–151 (L) \\  & 6–13 (T) & 106–131 (T) & 50–66 (T) \\ \hline \multirow{2}{*}{Trabecular Bone [14; 15]} & 0.05–0.5 & 4–12 & - \\ \hline \hline \end{tabular} (L): longitudinal; (T): transverse.  
\end{table} 
Table 1: Mechanical properties of compact and trabecular bones. surgery patients experience suboptimal outcomes despite the application of contemporary techniques [18; 19]. The biomaterials used in spinal surgeries must meet several criteria, including sufficient strength, excellent corrosion resistance, bioadhesion, biofunctionality, biocompatibility, high wear resistance, and low friction. Some of the biomaterials which have been developed do not satisfy all the requirements mentioned above due to the nature of the materials. Corrosion and wear are two major issues that have been reported as the primary reasons for failure. Metallic implants, such as titanium (Ti), stainless steel, and cobalt (Co)-chromium (Cr) (Co-Cr) alloys, are commonly used to make fixation devices for spinal fusion surgery due to their excellent mechanical properties. The properties of biomaterials compared with those of natural bone are detailed in Table 2.  
Complications such as stress shielding were observed due to materials such as titanium being stiffer than natural bone. Metal debris circulates in the body and accumulates in the organs, which can be fatal [21].