There are currently two differing perspectives on the biocompatibility of Ti6Al4V alloy:
First, although titanium alloys possess excellent biocompatibility and corrosion resistance, if titanium is released into tissues through passive dissolution or wear, it may cause varying tissue reactions. Mild reactions may include discoloration of surrounding tissues, while severe reactions can lead to inflammation, pain, and even joint loosening due to osteolysis.
Second, if titanium alloy particles are present in tissues, the aluminum and vanadium they contain will also be present. Still, their physiological properties differ significantly from those of titanium. While some have suggested that improving other components in titanium alloys may improve corrosion resistance and biocompatibility, studies of several titanium alloys have not yielded sufficient evidence to support this view.
Under normal physiological conditions, regardless of other factors such as wear, titanium alloys are not degraded. However, even in these conditions, some material may dissolve into tissues through passive dissolution. The subsequent mixture of metal ions and proteins may cause tissue lesions. Therefore, the passive corrosion rate of a material is as important as its corrosion behavior. Researchers have conducted extensive research on the corrosion behavior of titanium alloys, using limiting potentials below 2000 mV and electrolytes without air flow. Even under these conditions, titanium alloys can still be damaged. Studies have found that wear is not the primary cause of short-term implant failure, but rather infection, joint loosening, and fracture. However, wear particles can induce tissue inflammation, leading to aseptic loosening of joints, making wear a primary cause of long-term prosthetic failure.
M.A. Khan et al. investigated the corrosion resistance of Ti6Al4V alloy and two new titanium alloys—Ti6Al17Nb and Ti13Nb13Zr. The experiments employed a higher limiting potential, ranging from 0 to 5000 mV. Oxygen was added to the electrolyte to simulate a physiological environment. Phosphate buffer solutions with pH values of 5, 7.4, and 9 were used. To account for the impact of corrosion on wear, sliding wear was employed in the experiments. Research has found that while pure titanium, near-p-phase Ti13Nb13Zr, and p-phase Ti15Mo alloys offer the best corrosion resistance, a+-phase Ti6Al4V alloys and Ti6Al17Nb offer the best combination of corrosion and wear resistance.
After implantation into the human body, metal materials bond to bone tissue in three ways:
The first is morphological fixation, which involves the mechanical interlocking of a bioinert material with bone tissue, resulting in discontinuous stress transfer.
The second is biological fixation, which involves mechanical interlocking and surface crosslinking of a bioinert porous material with bone tissue, also resulting in discontinuous stress transfer.
The third is bone bonding, also known as bioactive fixation, which involves direct contact between a bioactive material and bone tissue, without the intermediary of soft tissue, at the optical microscopic level. This allows for continuous stress transfer and is the optimal bonding mode for implants intended to remain in the body for extended periods.
To improve the various properties of medical titanium alloys, two approaches can be taken:
First, address the material itself, developing various new titanium alloys with superior properties, as described above;
Second, address the surface of the material, using various surface treatment methods to modify titanium alloys to make them more suitable for medical applications.
Surface modification of Ti6Al4V alloy titanium plates and titanium alloys not only maintains the qualities of the titanium alloy as a base material but also significantly improves its overall performance. Therefore, it has become a research hotspot in the field of medical titanium alloys in recent years. With the development of technologies such as ion implantation, plasma spraying, electroless plating, ion plating, PVD, CVD, micro-arc oxidation, and laser cladding, wear-resistant and corrosion-resistant ceramic coatings can be formed on titanium alloy surfaces, improving their wear and corrosion resistance. Bioactive coatings such as HA and BG can also be formed on the surface, preventing the release of V and Al ions in titanium alloys into physiological environments, further improving the biocompatibility of the materials. Therefore, studying the surface modification technology of titanium alloys, preparing metal ceramics with wear resistance and corrosion resistance, and studying their biotribological properties under physiological environments are of great significance for developing high-performance artificial joints, improving the service life of titanium alloy artificial joints, revealing their lubrication mechanism, and further improving the stability and reliability of artificial joint replacements.
