The replacement knee joint is comprised of a flat metal plate and stem implanted in your tibia, a polyethylene bearing surface and a contoured metal implant fit around the end of the femur. The use of components made from metals and polyethylene allow for optimum articulation (or joint mobility) between the joint surfaces with little wear. Because the knee implant has a flatter bearing, wear is less of a problem than in a hip implant which has a very deep bearing.
Materials which can be used in knee implants are:
Due to limited ability to withstand corrosion in the human body in the long term, stainless steel is not often used in knee replacement implants. It is more suited to being used as temporary implants such as fracture plates and screws.
Cobalt-chromium alloys are hard, tough, corrosion resistant, bio-compatible metals. Along with titanium, cobalt chrome is one of the most widely used metals in knee implants. There is no consensus as to which material is better and more suitable.
Although the percentage of patients having allergic reactions related to the use of cobalt-chromium alloys to is very low, one area of concern is the issue of tiny particles (metal ions) that may be released into the body as a result of joint movement. These particles can sometimes cause reactions in the human body, especially in case of those patients who have allergy to special metals like nickel.
Titanium and Titanium Alloys
Pure titanium is generally used in implants where high strength is not necessary. For example, pure titanium is sometimes used to create fiber metal, a layer of metal fibers bonded to the surface of an implant which allows bone to grow into the implant or allows cement to better bond to the implant for stronger fixation.
Titanium alloys are bio-compatible in nature. They commonly contain amounts of vanadium and aluminum in addition to titanium. The most used titanium alloy in knee implants is Ti6Al4V. Titanium and titanium alloys have great corrosion resistance, making them inert biomaterial (which means they will not change after being implanted in the body).
Titanium and its alloys have a lower density compared to other metals used in knee implants. Additionally, the elastic nature of titanium and titanium alloys is lower than that of the other metals used in knee implants. Because of this, the titanium implant acts more like the natural joint, and as a result, the risk of some complications like bone resorption and atrophy are reduced.
Knee implants may be “cemented” or “cementless” depending on the type of fixation used to hold the implant in place. The majority of knee replacements are generally cemented into place. There are also implants designed to attach directly to the bone without the use of cement.
These cementless designs rely on bone growth into the surface of the implant for fixation. Most implant surfaces are textured or coated so that the new bone actually grows into the surface of the implant.
For this, surface of the titanium is modified by coating the implant with hydroxyapatite, a bioactive surfacing agent that will ultimately bond as the bone grows into it.
Tantalum is a type of pure metal, which has excellent biological and physical properties, namely flexibility, corrosion resistant, and biocompatibility. Recently, a new porous substance has been made of tantalum named Trabecular Metal. It contains pores, the size of which makes this material very good for bone in-growth. In addition, Trabecular Metal has an elastic nature which aids bone remodeling.
The tibial and patellar components in knee replacements are made of polyethylene. Though standard polyethylene surfaces traditionally suffered from wear in hip implants, wear is less of a problem in knee implants as the bearing surfaces are flatter and do not result in the same kind of wear. The use of Ultra Highly Cross Linked PolyEthylene (UHXLPE) or Ultra High Molecular Weight PolyEthylene (UHMWPE) reduces even the minimal wear enabling the knee implants to last for a much longer time.
Zirconium alloy and all plastic tibial component:
Zirconium alloy is used in a new ceramic knee implant. The zirconium alloy is combined with an all-plastic tibial component, replacing the metal tray and plastic insert used in other knee replacements. It is believed that this new knee could last for 20-25 years, substantially more than the 15-20 years that cobalt chromium alloy and polyethylene implants are effective.
The new combination can be lubricated, which results in a smoother and easier articulation through plastic.
Another important characteristic of this material is that it is biocompatible, meaning that people who have nickel allergies and cannot have knee implants made of cobalt chromium alloy (because nickel is an ingredient of cobalt chromium alloy). Zirconium alloy implants eliminate the risk to nickel-allergic patients because this new material contains no nickel.
Oxinium oxidized zirconium:
Oxinium oxidized zirconium is a new material used in knee implants since 2001. It is basically a transformed metal alloy that has a ceramic bearing surface. It contains zirconium and niobium alloy that was oxidized to convert the surface of the material into zirconia ceramic. The advantage of this metal is that just the surface has been changed, so the rest of the implant component is a high tensile metal. Although it is twice as hard as cobalt chromium alloys, it provides half the friction thus performs with higher quality and lasts for a longer time.
Ultimately, your knee replacement surgeon will recommend using whichever implant or implants he or she feels is right for your situation and whichever product he or she has previous success with. You should use this information to have informative pre-op conversations with your surgeon and to ask appropriate questions when investigating surgeons and surgery options.