Why is "Titanium" the Preferred Metal for Human Implants?

In the medical field, when implants such as artificial joints, bone plates, screws, or cardiovascular stents are required, titanium and its alloys (like Ti-6Al-4V) are undoubtedly the preferred choice among metallic materials. This is no coincidence but stems from titanium's unique combination of properties that allow it to coexist in high harmony with the human body. Its primary advantages lie in the following areas:

Biocompatibility is the foremost requirement for implant materials, referring to the material's ability to coexist peacefully with human tissues, blood, and bone without causing toxic side effects or rejection.

• Stable Passive Layer: Titanium instantly forms an extremely dense, stable, and self-repairing oxide layer on its surface. This inert film effectively prevents the underlying titanium metal from being corroded by the complex environment of bodily fluids, significantly reducing the release of metal ions. In contrast, some other metals (like stainless steel or cobalt-chromium alloys) may release trace amounts of ions (e.g., nickel, chromium, cobalt) over long-term implantation, potentially triggering allergies, inflammation, or other adverse reactions. This characteristic makes titanium one of the most 'quiet' and 'harmless' metals inside the human body.

An implant must not only be compatible but its mechanical properties must also match those of human bone, a crucial concept known as 'mechanical compatibility'.

• High Strength-to-Weight Ratio and Low Modulus: Titanium alloys have a very high strength-to-weight ratio, meaning implants can be made lighter while ensuring sufficient mechanical strength. More importantly, their elastic modulus (a measure of stiffness) is much lower than that of stainless steel and cobalt-chromium alloys, making it closer to that of human bone. If an implant (like a bone plate) is too stiff, it bears most of the stress, leading to the underlying bone becoming osteoporotic and atrophying due to 'stress shielding'. The moderate stiffness of titanium alloys helps maintain a more normal stress distribution in the bone, promoting healing and long-term bone health.

This is the core, often irreplaceable, advantage of titanium in orthopedics and dentistry.

Osseointegration refers to the direct structural and functional connection between living bone and the surface of a load-bearing implant, without intervening fibrous connective tissue. The surface characteristics of titanium, especially after roughening or porous processing, provide an excellent scaffold for bone cells to attach, migrate, and grow. New bone tissue can grow into the micro-pores of the titanium, creating a firm 'biological lock' rather than just a mechanical fixation. This strong integration ensures the long-term stability of the implant, significantly reducing the risk of loosening and failure.

In summary, titanium is the premier choice for human implants due to its unparalleled biocompatibility, well-matched mechanical properties, and unique ability to osseointegrate. It is not only chemically stable and non-toxic but also works in synergy with the human body on both mechanical and biological levels, supporting reconstruction and repair. It represents a milestone discovery in modern medical materials science.