|One of most common problems with replacement of hips is the wear and tear that takes place through normal use. 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. New generation PE is being used in total hip and knee replacements due to excellent wear resistance. The second-generation, highly cross linked PE has better wear resistance compared to the first generation of highly cross linked ultra-high-molecular-weight (UHMW) PE introduced a decade ago, many of which were irradiated and melted.
Choices of Hip Replacement Implant Material
* Metal Ball and Polyethylene Liners
* Ceramic Ball and Polyethylene Liner
* Metal Ball and Metal Liner
* Anatomic Size Metal-on-Metal Ball Heads
* Ceramic Ball and Ceramic Liner
Metal Ball and Polyethylene Liners:
Because of its durability and performance, Metal-on-Polyethylene has been the leading artificial hip component material chosen by surgeons since FDA approval 30 years ago. The metal ball is cobalt chrome molybdenum alloy and the liner is polyethylene. Polyethylene is the most understood and used of all the liner materials, offering the surgeon a range of options to obtain stability in the body while the operation is underway. This ability to adapt and customize during the surgical procedure is an important attribute of polyethylene. It is also the least expensive bearing. All implants shed wear debris. The most active patients shed the most debris, and with polyethylene this may show up after 10 to 15 years, resulting in joint inflammation and bone loss. As a result, new wear resistant polyethylene liners have been introduced, called “highly cross linked polyethylene.” During the manufacturing process the plastic is treated with a short burst of radiation to help the cup resist wear.
Wear rates: Metal-on-Polyethylene implants wear at a rate of about 0.1 millimeters each year.
Ceramic Ball and Polyethylene Liner:
Ceramic heads are harder than metal and are the most scratch resistant implant material. The hard, scratch resistant, ultra-smooth surface can greatly reduce the wear rate on the polyethylene bearing. The wear rate for this type of implant is less than Metal-on-Polyethylene. The implant shown to the right utilizes vitamin E stabilized highly cross linked 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 crosses linked polyethylene liners.
Wear rates: Ceramic-on-Polyethylene implants wear at a rate of about 0.05 millimeters each year, i.e. 50% less than Metal-on-Polyethylene. The newer, highly cross linked polyethylene liners have shown wear rates as little as 0.01 millimeters each year
Biomet, which introduced its Arcom XL highly cross linked polyethylene in 2005, has introduced another new polyethylene offering: the UHMW E-Poly material, which is stabilized with vitamin E, a natural antioxidant. The material is expected to reduce oxidation.
E-Poly was developed in conjunction with Massachusetts General Hospital (MGH) and Cambridge Polymer Group in Boston. The technology is also co-licensed to Zimmer. In laboratory tests, Biomet E-Poly hip liners demonstrated 95-99% less wear. No oxidation has been found in the lab tests due to vitamin E antioxidant. It has much better mechanical properties and better fatigue strength than the first generation of cross linked PE.
The X3 sterilization process involves sequential cross linking, where it is irradiated and then annealed three times for a 9 Mrad total dose of irradiation. The molecular chains stay closer to each other with sequential cross linking & therefore are more easily re-cross linked after heat treatment. X3 technology is the highest cross linked product on the market today for knees.
The US and European hip and knee implant markets are the two largest markets for hip and knee implants globally, with the US accounting for nearly 50% and Europe contributing to around 30% of total procedures worldwide. Hip and knee replacements are mature markets in the US and Europe with knee procedures and revenues growing at a higher rate than hip replacement procedures and revenues. The 2005 revenues for hip implants in the US were US$2 bln and US$1.4 bln in Europe. Knee implant revenues for 2005 were US$2.4 bln in the US and US$774 mln in Europe. The average prices for hip implants grew by 2-3% from 2004 to 2005 while that of knees grew by 3-4%. Future market growth is expected to be driven by increasing demand for hip and knee replacements as the baby boom generation nears retirement and an increasing number of younger patients undergo hip and knee replacement procedures.