Common types of medical implants are ar-throplastical prostheses, or joint replacements, which serve to take over joint functions for patients suffering from musculoskeletal dysfunction or severe pains in e.g. the knee, hip (see Figure 1), or shoulder. Joint replacements generally consist of multiple parts made of different materials. The common hip joint re-placement prosthesis shown in Figure 2 consists of a metal ball, socket and stem and a plastic spacer. The metals used are mostly CoCr, austenitic stainless steels, Co–Cr–Mo alloys and Ti or Ti alloys, for their hardness, corrosion resistance, fracture toughness, low modulus and good machinability [1, 2]. The plastic spacer is typically made of polyethylene. The prosthesis is secured to the host body through either press-fitting or cementing .
Biocompatibility of a material refers to “(i) an appropriate response of the host tissue and living system to the material, and (ii) the response of the material due to contact with living matter” . A lack of biocompatibility can have an undesirable effect on the functionality of prostheses due to e.g. metal toxicity or sensitivity and osteolysis.
Figure 1. X-ray image of a pelvis with total hip joint replacement .
Osteolysis is the loss of bone due to polyethylene wear debris. Small parts of polyethylene from the prosthesis lead to inflammation , which in turn may cause loosening of the implant–tissue interface . Loosening may also occur due to wear debris or fatigue of the cement or metal, leading to painful implant movement inside the bone.
Figure 2. Typical metal-and-plastic ball-and-socket hip joint replacement prosthesis
Corrosion of metals under physiological conditions leads to the release of metal ions into the host tissue, sometimes leading to their accumulation in the body. High doses of metal ions are potentially dangerous and corrosion should therefore be avoided . Furthermore, contact between tissue and metals may lead to metal sensitivity, in turn causing immune reactions such as eczema or skin hives.
Another critical aspect that has to be taken in mind is bone ingrowth — the implant material must have a relatively high level of porosity (over 50%), a high coefficient of friction (close to 1) and a low modulus of elasticity in order to promote bone adhesion and prevent loosening of the implant . Currently used materials are inferior in promoting bone ingrowth with respect to natural tissue and further research on materials that satisfy these criteria is therefore required.
Ti and Ti alloys are the most widely used metals in biomedical devices. They are corrosion resistant under physiological conditions, have a good biocompatibility, and Ti allergies or hypersensitivity towards Ti or Ti alloys rarely occur. Furthermore, cell growth is decent on Ti–tissue interfaces. Smooth surfaces (both of thin films or polished bulk) are...