Hybrid Hip Joint
New ceramic-metal material combines wear-resistance with strength
Each year, approximately 340,000 total hip replacement surgeries are conducted in the U.S., and 600,000 are performed globally. During a hip replacement procedure, the surgeon removes the top portion of the femur, replacing it with a metal stem with a ball on one end. In 90% of cases, this metal ball is made from cobalt chrome, a tough metal able to withstand the weight-bearing demands of the hip joint. The surgeon also removes the acetabulum, or hip socket, in the pelvis, replacing it with a metal shell and plastic liner. Over time, as the new ball rotates against the plastic liner, the liner wears down and a follow-up surgery, usually 10 to 15 years later, is required to repair the implant.
In the past, surgeons have successfully reduced this life-shortening wear by implanting a hard, low-friction ceramic ball rather than a tough metal ball. The hardened surface of a ceramic ball resists the roughening associated with a metal ball, thus reducing the rate at which it wears down the soft plastic liner. However, although ceramics can extend the life of hip implants by reducing this wear, a major recall of ceramic implants by French manufacturer Saint-Gobain Desmarquest in 2001 reinforced surgeons’ concerns that some ceramic implants may be prone to fracture inside patients. As a result, only 10% of procedures now involve the use of ceramic implants.
“There’s no doubt ceramic implants’ wear-reducing properties can extend the life of an implant since wear is the leading cause of implant failure. But most surgeons believe the risks and potential consequences for patients outweigh the benefits and they are hesitant to use them now. Cobalt chrome, despite its inferior wear characteristics, is the current standard material of choice,” explains Dr. Robert Barrack of Tulane University Medical Center.
Oxinium, made by Smith & Nephew Orthopædics, Memphis, TN, may change that decision. Touted as the newest bearing surface for hip joints in over 20 years, Oxinium can increase the lifespan of hip replacements, thereby decreasing the need for patients to undergo future corrective hip surgeries. Oxinium implants are made from zirconium, an extremely biocompatible metal similar to titanium, the strong, light metal currently used in orthopaedic implants. The metal is heated and infused with oxygen until the outer surface naturally transforms into a ceramic. Rather than converting the entire implant into ceramic, only the outermost portion of the implant — the layer that is in contact with the plastic cup — takes on ceramic’s smooth, hard qualities. The entire component retains the strength and flexibility of the original metal.
As a result, Oxinium hip implants are 4,900 times more abrasion-resistant than cobalt chrome and reduce implant wear by nearly 50%. However, since it is still a metal, the new implants have strength characteristics, such as shatter resistance, similar to cobalt chrome — a unique combination in the orthopaedic industry.
“Oxinium’s best-of-both worlds qualities give back to surgeons the confidence they’ve lacked when choosing between hip implant bearing surfaces,” explains Dave Illingworth, president of Smith & Nephew. “Presently, surgeons have to choose between the strength of metals and the wear resistance of ceramic surfaces. With Oxinium implants, surgeons and patients have the benefits of both.”
Last year Dr. Barrack became the world’s first surgeon to implant an Oxinium hip ball. The company is introducing Oxinium hip implants for use in the United States, as well as in Europe, Australia and Canada, and by year’s end, expects them to account for 25% of all of its hip ball implants.
The introduction of Oxinium hip components builds on the company’s successful application of the technology in knee implants. In five years of clinical use, the company reports no material failures of Oxinium total knee implants. Introduced for sale globally in February 2001, Oxinium knees are now available in all three primary implant variations: cruciate-retaining, macrotextured and posterior-stabilized. Later this year, the company will introduce a revision option, as well as a less-invasive unicompartmental implant, or “uni knee,” made with the Oxinium technology. In 2002, Oxinium knee implants grew in number, with 25% of the company’s knees being made from the new material.
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Smith & Nephew Orthopædics
Artificial organ developed at MIT may lead
to – or prevent – organ replacement
Liver diseases have been difficult to study because liver cells do not function in a petri dish the same way they do inside the body. For example, hepatitis C will not infect human liver cells cultivated in a petri dish. These issues prompted a research team at the Massachusetts Institute of Technology, Cambridge, MA, to develop and patent an artificial mechanical version that would mimic the natural structure and functions of the human liver. This silicon wafer-based “liver chip” will allow researchers to more realistically test the effectiveness of therapeutic drugs and chemical toxicity.
Eastern Plastics, Inc., Bristol, CT, collaborated with Dr. Karel Domansky at MIT to create the miniature 3-D housing with internal channels through which a nutrient-rich liquid is pumped to “feed” the liver cells. The walls of these channels are chemically modified to attract collagen, which in turn provides an anchor point for the liver cells. A filter keeps the liver cells inside the system so that they will anchor.
“A major obstacle in developing the liver chip was designing the channels so that the fluid flows well. Cells need a constant flow, not just for food, but also as a signal that all is well and that they should continue with their business,” says the project’s leader Linda Griffith, professor of chemical and biological Engineering at MIT. Nutrient fluid primes the system before liver cells are injected via syringe; the artificial liver can then be hooked up to a pump that simulates blood flow for the nutrient/oxygen solution. The entire assembly can then be inserted into an incubator that regulates temperature, humidity and concentrations of oxygen and carbon dioxide to best simulate the natural organic environment. The miniature size allows researchers to view the tissue in situ under a microscope.
EPI created the miniature polycarbonate bioreactor housing with integrated channels using very close-tolerance machining techniques and a proprietary multi-layer diffusion bonding process. Diffusion bonding allows EPI to create internal features that would otherwise be impossible to manufacture. The channels are less than 0.035-in. wide and only 0.016-in. deep. EPI stress-relieved the bioreactor so that the polycarbonate better resists cracking and is dimensionally stable.
At just under an inch, the liver chip can be built into other stationary and portable detection and analytical instruments. The device may facilitate development of replacement organs for patients whose own liver has failed. However, the more ambitious goal is to use the device to prevent organ replacement altogether by developing better models of human disease and thus better drug therapies to cure disease.
The MIT investigators have begun to apply the same approach to other tissues. One day, bioreactors of interconnected organs may replace the use of animals in drug development. A version of the liver chip has also been developed to detect battlefield biological weapons. The Department of Defense is a major source of funding for the MIT project, as are E.I. DuPont de Nemours and the National Institutes of Health (NIH). Pharmaceutical companies are eagerly awaiting early results. Researchers hope to pull enough potential users together to facilitate mass production and thereby reduce commercial costs.