- Metal Ball and Polyethylene Liners
- Ceramic Ball and Polyethylene Liner
- Metal Ball and Metal Liner
- Ceramic Ball and Ceramic Liner
Metal-on-Metal bearings (cobalt chromium alloy and 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 continue to offer the potential for greatly reduced wear, with less inflammation and less bone loss. 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.
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.
Metal-on-Metal implants have a potential wear rate of about 0.01 millimeters each year.
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. Use of a larger ball head has been shown to increase the range of motion an individual may experience as well as decrease the possibility of dislocation.