Fractures are doubtlessly a disastrous health issue, but bone, as a living tissue, almost instantly begins a healing process. Bone is light and strong, adapts its structure to the functional demands and self-repairs. As so, it is often perceived as the ultimate biomaterial.
Two major kinds of osseous tissues exist: trabecular (cancellous) and cortical (compact).
Major kinds of osseous tissues.
Cortical bone forms the outer shell (cortex) of most bones and accounts for about 80% of the weight of a human skeleton. It is dense and thus, relatively hard, strong and stiff.
Compared to cortical bone, trabecular bone is less dense. It has a higher surface area to mass ratio and is thus softer, weaker and more flexible. Trabecular bone is typically found at the ends of long bones, proximal to joints and within the interior of vertebrae. The greater surface area makes trabecular bone highly vascular and suitable for metabolic activity. It frequently contains red bone marrow where the production of blood cells occurs.
Plastic is often used for implants even though it is the worst material for bone formation because bone doesn’t attach to it. Most of the other implants on the market are made of bulk metal, thus are stiff and don’t deform.
The design freedom of additive manufacturing allows redistributing the material through a truss structure, it’s possible to get a significant strain with a strong metal such as titanium. Bone attach to titanium. If the titanium structure is elastic enough to deform significantly, attached cells deform with it, which triggers an osteogenic response, meaning bone formation.