Summary

• Bone is a biomaterial with a complex hierarchical structure, which gives it some impressive material properties.
• It is primarily composed of a bioceramic (similar to hydroxyapatite) and collagen, a fibrous protein.
• At a microscopic level, it can be seen that the collagen-bone mineral composite forms concentric lamellar structures known as osteons, which are the main structural element of bone. The osteons are densely packed together in cortical bone and their long axes tend to run parallel to the long axis of the bone.
• In common with many biomaterials, bone is anisotropic: its mechanical properties differ depending on the orientation of the sample being tested.
• Hip replacements are a very common surgical procedure, particularly among older people, as bones can become more brittle with age.
• There are three main parts to a total hip replacement: the femoral stem, femoral head, and acetabular components.
• The main challenge with hip replacements is to find a material that has mechanical properties similar to that of bone, is capable of operating for many years in the biological conditions in the human body, and causes minimal adverse host response from the body.
• The femoral stem is the main load-bearing component. Increasingly the titanium alloy Ti-6Al-4V with 40% porosity is used, as it combines excellent material properties with the advantage of being, to a large extent, biologically inert.
• The main mechanical requirement of the femoral head and acetabular cup is to minimise wear. The femoral head is generally a highly polished metal or ceramic, and the acetabular cup is usually made of UHMWPE - a dense, crystalline polyethylene.

You should note that no one material or design has emerged as the definitive hip replacement; each has its own advantages and disadvantages, which must be considered for each individual patient taking into account their age, general health and lifestyle. Also, as is often the case, cost is a very important factor for biomaterials companies, with superior mechanical properties sometimes being sacrificed for ease of production.