Millions of people around the world enjoy healthier lives due to the use of implants (dental implants, hip replacements etc.). Small metallic implants, such as those used in dentistry, craniofacial applications, bone-anchored hearing aids and some orthopaedic applications (spinal/finger joints) are particularly challenging as miniaturisation reduces the surface area available for bone ingrowth. Small devices are associated with stress concentration and a susceptibility to chronic local inflammation which can lead to progressive bone loss in up to 56% of devices. This can cause pain, and the implant may loosen and fail. The increased strength of metallic glasses would be ideally suited to such applications if they can be produced with suitable corrosion resistance.
Ti is protected from corrosion by a thin film of TiO2 and conventional “biocompatibility” testing focuses on the interactions between this surface and human tissue. However, most Ti implant failures occur through “mechanically-assisted crevice corrosion” (MACC), which takes place when a Ti implant is in tight contact with another solid surface such as metal, cement or bone. Slight vibrations abrade the oxide film, leading to Ti dissolution generating a highly acidic solution in the gap that can leach into the adjacent tissue causing pain and tissue damage. Solid corrosion product particles and metal fragments can also be released into the tissue, causing adverse physiological reactions. It is therefore clear that the improved mechanical properties and corrosion resistance of Ti metallic glasses offers great promise for small implants. However, their corrosion products are completely unknown, and they have never been subjected to MACC tests.
The main objective was to evaluate the local corrosion behaviour and MACC of new Ti-based bulk metallic glasses with focus on characterisation of corrosion products with methods including in situ synchrotron X-rays. Another objective was to determine the biological compatibility of the new alloys including corrosion products simulants. Finally another objective was to work with an industrial partner to identify the feasibility of translating one or more metallic glasses into clinical practice.
A series of Ti-based bulk metallic glasses were found to have good corrosion resistance in simulated body conditions. Using in situ synchrotron X-ray diffraction it was found that a Ti-based bulk metallic glass generates a series of corrosion products including metallic nanoparticles. In vitro biological testing with structural and immune cell lines revealed that the different corrosion products elicit significantly different cellular behaviours highlighting the need to identify accurately the nature of the corrosion products as part of routine biocompatibility testing. Initial MACC experiments indicate that certain Ti-based bulk metallic glasses might have significantly higher MACC resistance than current implant Ti alloys. A certain Ti-based bulk metallic glass was found to have regulatory and manufacturing feasibility and further detailed characterisation is under way to determine the best indication for its use.
