Titanium and titanium alloys. Suitable materials for medical applications

Interest in titanium alloys used in the medical field for bone replacement and reconstruction has grown considerably in recent decades. Their fields of application are vast: maxillofacial surgery, ENT, orthopedics, implantology... These alloys represent a considerable economic and social challenge. As life expectancy rises, these materials have to meet increasingly demanding specifications in terms of both chemical and mechanical biocompatibility, in order to last in the human body. The success of the operation depends to a large extent on the complex osseointegration mechanisms involving both mechanical and biochemical aspects occurring at the implant/surrounding tissue interface.

Mechanical osseointegration, which conditions a large part of bone remodeling (i.e. the healing response of the bone), has long been ignored in the selection of biomaterials for prostheses and implants. For a long time, the criteria sought were essentially limited to good corrosion resistance, high mechanical strength and ductility to facilitate shaping. However, the presence of an implant in the bone leads to a redistribution of mechanical stresses (a phenomenon known as "stress-shielding"). Too great a difference in rigidity between bone and implant leads to stress concentration zones and unloaded zones, resulting in necrosis or osteolysis and compromising the implant's durability. The mechanical properties of implants/prostheses, and in particular the modulus of elasticity, must therefore be carefully adapted, as they determine the quality of stress transfer at the implant/bone interface.

Pure titanium and the TA6V alloy offer the best compromise in terms of combination of properties, in particular because of their elastic modulus (E = 100 GPa) (half that of steels), high strength in the 900 to 1,000 MPa range and recognized biocompatibility in the case of titanium. However, the modulus of elasticity of these alloys is not perfectly matched to that of cortical bone, which is estimated at 20 to 30 GPa. What's more, the presence of elements such as vanadium and aluminum entails toxicity risks, which health professionals cite as one of the causes of implant failure.

Research has therefore recently focused on the development of new titanium alloys, with biomimicry taking into account the adaptation of implant mechanical properties to the bone matrix. Titanium alloys, containing only non-toxic elements, represent an excellent alternative to the materials mentioned above. In particular, β-metastable titanium alloys are of great interest for biomedical applications. Due to their metallurgical characteristics, these alloys have a very wide range of properties that can be modulated by heat and/or mechanical treatments, in order to obtain a better compromise between...