There are a large number of hip implant devices on the market. Each manufacturer has different models but each style falls into one of four basic material categories:
- metal on plastic (polyethylene or UHMWPE)
- metal on metal (MoM)
- ceramic on plastic (UHMWPE)
- ceramic on ceramic (CoC)
These category names reference the materials used for the implant bearings. The stem and ball fit into and articulate against the cup or acetabulum. Each component can be made of one of several materials.
There is no consensus in the orthopedic community regarding the single “best” bearing or material. The choice generally comes down to your surgeon’s preference. Each surgeon has innumerable reasons for choosing one device over another, including his or her personal experience as well as tool and implantation method preferences.
Metal-on-plastic (polyethylene) is the longest tried and tested bearing. The convex femoral stem is constructed of metal (usually a cobalt chrome alloy) and the concave cup liner is made of a plastic called polyethylene.
This combination has been in use in various forms since some of the earliest hip replacements back in 1960 (when it was called the Low Friction Arthroplasty (LFA)). Some years later, the make-up of the polyethylene was improved. The current plastic used in hip replacement implants is referred to as Ultra Highly Cross-Linked PolyEthylene (UHXLPE) or Ultra High Molecular Weight PolyEthylene (UHMWPE), a very stable and reliable plastic material with greatly reduced risk for wear.
Because of its durability and performance, Metal-on-Polyethylene has been the leading artificial hip component material chosen by surgeons since hip replacement surgeries were first been performed. It is also the least expensive bearing.
All implants shed debris as they wear. Over time, the body may see polyethylene wear particles as invaders or a source of infection. As the body starts to attack them, this leads to osteolysis, a “dissolving of the bone”, which may result in having to replace the implant (known as revision).
As noted above, technological advances have reduced the risk of wear in Metal-on-Polyethylene implants. They wear at a rate of about 0.1 millimeters each year. The other materials, metal and ceramic, being more modern developments, already have high wear resistance built in.
Metal-on-Metal hip implants have been used even longer than metal-on-plastic implants. MoM bearings (cobalt chromium alloy, titanium alloy or sometimes stainless steel) were in use from as far back as 1955 though they were not approved for use in the U.S. by the FDA until 1999. They offer the potential for greatly reduced wear, with less inflammation and less bone loss. Some device recalls have brought negative attention to MoM.
Metal bearings are available in many sizes (28 mm to 60 mm); there are also several neck lengths available. Only metal-on-metal components allow the largest heads throughout the entire range of implant sizes. Large ball heads provide increased range of motion and greater stability, which can significantly reduce the risk of hip dislocation, a crucial factor in the long term success of an implant.
Because the human femoral (ball) head is naturally large, it makes sense to implant a large, anatomic replacement. This was not possible in the past because traditional design parameters made smaller femoral heads necessary. However, with the introduction of metal-on-metal implant components, liners may be eliminated, allowing surgeons to use large femoral heads.
MoM implants have a potential wear rate of about 0.01 millimeters each year. Although wear is reduced with Metal-on-Metal implants, the wear products (sub-microscopic particulates, soluble metal ions) are distributed throughout the body. This has raised concerns about long-term bio-compatibility. At present these are only concerns, for there have been no definitive clinical findings that these wear products are harmful. It should also be noted that this issue arises fairly rarely.
Recalls: In 2011, the FDA issued a communication regarding metal-on-metal implants, indicating some concern and need for further study or review. Some metal-on-metal hip implants have been recalled by their manufacturers. A medical device recall does not always mean that you must have the device removed. A recall sometimes means that the medical device just needs to be checked, adjusted, or fixed. Patients with recalled implants should immediately discuss the situation with their surgeons to learn more about the recall and the risk of removing the device compared to the risk of leaving it in place. More about hip implant recalls >>
Ceramic is the 21st century answer to hip replacement as it is both hard and durable, it wears minimally and the material is widely deemed to have no toxic or side effects in the human body. Hip implants can be constructed as ceramic-on-UHMPE or ceramic-on-ceramic.
If you are a very active individual or a relatively young patient, your surgeon may prescribe an all-ceramic hip joint. Ceramic-on-Ceramic is a good combination with longevity and reliability.
In these hip joints, the traditional metal ball and polyethylene liner are replaced by a high-strength ceramic bearing that has the reputation for ultra low wear performance. Clinical studies, monitored by the FDA and begun in 1998, have demonstrated excellent performance although it should be noted that ceramic has been used in hip replacements for many years prior to that.
All-ceramic hip joints have been used in Europe since the 1980s but have only more recently received the FDA’s approval for marketing in the United States.
There was a history of two issues with ceramic hips: catastrophic shattering and squeaking. Shattering was more of an issue in the 1980’s and 1990’s but the product has been substantially improved since then, essentially eradicating the shattering problem. Squeaking, however, remains a bit of a problem for a few patients. Often the noises abate over time but sometimes they don’t. If the squeaking is intolerable, a revision may be necessary.
Ceramic is the hardest implant material used in the body, and has the lowest wear rate of all, to almost immeasurable amounts (1000 times less than Metal-on-polyethylene, about 0.0001 millimeters each year). Consequently, there is usually no inflammation or bone loss, nor systemic distribution of wear products in the body. New ceramics offer improved strength and more versatile sizing options.
Ceramic-on-Plastic (or UHMWPE)
Ceramic-on-UHMWPE (Ultra High Molecular Weight PolyEthylene) is a good combination of two very reliable materials. Ceramic heads are harder than metal and are the most scratch-resistant implant material. The hard, ultra-smooth surface can greatly reduce the wear rate on the polyethylene bearing. The potential wear rate for this type of implant is less than Metal-on-Polyethylene.
(Zimmer Biomet M/L Taper Hip Prosthesis with CeramTec BIOLOX® delta head)
Ceramic-on-Polyethylene is more expensive than Metal-on-Polyethylene, but less than Ceramic-on-Ceramic. In the past, there had been incidents of fractures in ceramic components, but newer, stronger ceramics have resulted in considerable reduction of fracture rates (0.01%) compared to the original, more brittle ceramics.
Some ceramic-on-polyethylene implants utilize a vitamin E-stabilized, highly crosslinked polyethylene bearing material. Vitamin E, a natural antioxidant, is expected to improve the longevity of the implant bearings used in total joint replacements. In laboratory testing, these liners have demonstrated 95-99% less wear than some other highly crosslinked polyethylene liners.
Ceramic-on-Polyethylene implants have a potential wear at a rate of about 0.05 millimeters each year, i.e. 50% less than Metal-on-Polyethylene. The newer, highly crosslinked polyethylene liners have shown potential wear rates as little as 0.01 millimeters each year.
Only a qualified orthopedic surgeon can determine which implant system is best for your unique situation. There are many factors your surgeon uses when recommending hip implants. Be sure to ask questions and share any concerns you might have regarding specific implants with your surgeon early in the process.