As we age, we sometimes outlive our joints. The aging of the largest segment of our population (the boomers) will surely increase the number of joint failures. When joints fail, the solution is often to replace the problem area with an artificial joint. Before that can happen, however, a number of thermal processes must take place because most of these replacement joints – prostheses – are made from a cast metallic alloy. A prosthesis is simply defined as “a device designed to replace a missing part of the body or to make a part of the body work better.”
The use of alloys in surgical implants dates back about a century. Glass was used in very early hip-replacement trials for its biocompatibility, but it could not endure the wear and tear. Medical scientists experimented with plastic and stainless steel in the 1930s and 1940s. The development of 316 stainless and cobalt-chromium alloys in the 1920s and 1930s resulted in materials with superior characteristics to steel for many prosthetic applications. Titanium – a relative newcomer in the mid-20th century – was found to have excellent biocompatibility, and alloys of titanium were developed to optimize the mechanical properties.
In order to fully optimize the properties of titanium alloys, a heat treatment is necessary. This proprietary thermal process is designed to create mechanical properties in the implant material that mimic the bone it is replacing. Advancements continue to occur in the heat-treatment arena. Coating technology and hot isostatic pressing are two other areas of current development.
Titanium and cobalt-chromium alloys are the preferred materials for medical implants. The good strength-to-weight ratio of the titanium alloy Ti6Al4V, as well as its fatigue resistance and elastic properties similar to that of bone, make it an ideal alloy for bone implantation. Unfortunately, its wear resistance is not as good as cobalt chromium, which is often used for wear surfaces. For instance, most hip stems are made of titanium or cobalt chromium, while the ball portion is either cobalt chromium or ceramic. Often, the implants require coatings to meet the necessary surface requirements. A ceramic coating is used to simulate the bone’s structure and surface.
The investment-casting process is most common because it can best reproduce the details of complex patterns and hold tight tolerances. Certain sizes and geometries of implants are manufactured using the powder-metal process. These parts are often used for applications such as dental prostheses. The very small size makes investment casting a less attractive option. Parts made from powder metals require pressing and sintering as production requirements.
Many developments – some involving thermal processing – continue to occur as research increases our knowledge. Surface-modification technologies, such as nitriding, are being expanded to improve biocompatibility, enhance bone bonding and reduce wear and corrosion. A new alloy – NiTi, or nitinol – has gained keen interest due to its shape-memory and superelastic properties. Its elasticity is closest to bone as compared with any other implant material.
New technological developments improve the success of these procedures, and the increasing numbers of joint replacements are driving these advancements. As of the middle of this decade, almost 500,000 knee replacements and 250,000 hip replacements were being performed annually.
New developments in prosthetic-materials’ technology have resulted in remarkable replacement limbs and joints. Did you know that the current world record for the 100-meter sprint by an amputee athlete is only about one second slower than the fastest Olympic sprinters? Material development for artificial limbs includes the use of carbon-fiber materials. Additionally, electronics holds the key to improved mobility. Thanks to implanted devices that read and harness subtle control signals in the brain, some day users of artificial hands will be able to play the piano. Can you say bionic?IH
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