Is the research emphasis on the wear of ultra-high molecular weight polyethylene bearing components in joint replacements warranted, given that the longevity of such replacements depends on a multitude of factors?

Viewpoint: Yes, the research emphasis on the wear of ultra-high molecular weight polyethylene bearing components in joint replacements is warranted: not only is physical wear the most important factor in the aging and malfunctioning of joint replacements, but it is closely tied with chemical and biological factors.

Viewpoint: No, the current research emphasis on the wear of polyethylene bearing components in joint replacements is unwarranted, given that alternative materials are available that can provide not only greater longevity but biological tolerance as well.

Prosthesis is the development of an artificial replacement for a missing part of the body, including arms and legs, and the area of medicine devoted to the study and development of prostheses is called prosthetics. Since ancient times, people have attempted to create substitutes for missing limbs, but the origins of prosthetics as a scientific discipline dates back only to the sixteenth century and the work of French surgeon Ambroise Paré (1510-1590).

Despite his efforts, Paré was ultimately faced with the limits of sixteenth-century knowledge, both in terms of antisepsis (keeping a body part free of germs) and engineering. Nevertheless, physicians of the early modern era were able to develop replacements for missing parts in the upper extremities—for example, metal hands fashioned in one piece, and later, hands with movable fingers. Still, the technology that would make it possible for a patient to actually move his or her own fingers the way a person with a whole limb would—in other words, through neuromuscular activity originating in the brain—still lay far in the future.

The two world wars of the twentieth century brought with them great improvements in prosthetics, including the development of more serviceable mechanical joints and the implementation of materials that were lighter in weight and therefore more usable. Today, prostheses are typically made of polyethylene, a dense form of plastic also used in manufacturing everything from electrical insulation to bottles. Other materials include metal alloys, such as cobalt-chromium, as well as ceramics, and often one of these is combined with polyethylene in creating a prosthetic device.

One of the biggest challenges in developing successful prostheses involves the interaction between the artificial material and the truncated body part to which it is attached. Below-knee prostheses, for instance, are held in place either by means of a strap that passes above the kneecap or by rigid metal hinges that attach to a leather corset wrapped round the thigh. Aboveknee prostheses may be attached to a belt around the pelvis or slung from the shoulder, or they may be held in place by means of suction. In cases of amputation from the hip joint, it may be necessary to use a plastic socket. Arm prostheses may include a shoulder harness with a steel cable, which the patient can operate by shrugging the shoulder, thus tightening the cable and opening or closing the fingers of the artificial hand.

More sophisticated prostheses, developed in the latter half of the twentieth century, actually established an interface between the patient's muscles and the artificial limb. This is achieved through a surgical operation called cineplasty. It is also possible, in some cases, to use electric current to operate an artificial hand. In such devices, known as myoelectrical control systems, electrodes are built into the prosthesis itself and are activated by contractions of the patient's muscle.

In creating and using a device that attaches to a living body, there are a whole host of challenges, most of which are either physical or biochemical in nature. On the physical level, there is the problem of simply ensuring that the device will do what it is supposed to do, and at the same time withstand the ordinary wear that will be exerted on it in the course of daily use. Much more complicated, however, are the biochemical concerns, which can be extremely serious.

First of all, there is the matter of ensuring sterility, such that no foreign biological material is introduced to the patient's body. There is also a problem that crosses the lines between the physical and the biochemical, resulting from the friction between the body part and the prosthesis. This friction can and often does cause the displacement of microscopic particles from the prosthetic device—particles that may invade the patient's body or may serve as a medium for invasion by microbes.

Such are the problems that confront makers of prosthetic devices, who are engaged in an extremely complex and vital endeavor that brings together disciplines rooted both in the medical sciences and in engineering. Therein lies the basis for the controversy at hand, which revolves around the use of polyethylene components in artificial joints. In the essays that follow, several issues are examined. In particular, there are the questions of whether polyethylene is the best material for such components, and whether the present emphasis on physical wear in the testing of polyethylene prosthetic devices is warranted.

With regard to the second of these questions, regarding the matter of wear testing, it is worth considering whether physical concerns alone should weigh as heavily as they do in such studies, when the devices in question have an obvious and critical biological function. Unlike the primitive devices of Paré's time, which were as separate from the patient's body as an item of clothing, many modern prostheses are actually connected to living tissue. Therefore it is wise to question whether physical testing alone can protect against the possible biochemical effects that may result from the scattering of particles.

As to the first question, regarding the viability of polyethylene as a material, there is a body of opinion that maintains that metal alloys and/or ceramics might function more safely and efficiently in this capacity. Polyethylene has been in wide use for prosthetics since the early 1970s, but that period has also seen vast strides in the development of prosthetic technology. Improvements include the use of cobalt-chromium alloys in conjunction with polyethylene, but even this technology is subject to significant wear in a number of ways.

As the technology of prosthetic devices has changed, so has the market for them. Whereas in the past, patients equipped with prosthetic devices tended to be elderly, by the end of the twentieth century, prosthetic surgeons were outfitting artificial body parts for patients in middle age. Such patients, who in many cases lost their limbs in the course of vigorous physical activity, continue to be more active than the elderly prosthetic patient, even after being outfitted with the artificial limb. This change in the population makeup of prosthetic patients likewise poses new challenges to medicine and engineering in the quest to develop better and safer prosthetic devices.

—JUDSON KNIGHT

Viewpoint: Yes, the research emphasis on the wear of ultra-high molecular weight polyethylene bearing components in joint replacements is warranted: not only is physical wear the most important factor in the aging and malfunctioning of joint replacements, but it is closely tied with chemical and biological factors.

Just a few minutes into the classic 1967 film The Graduate occurs one of the most famous quotes in motion-picture history. A nervous guest of honor at a party thrown by his parents, Ben Braddock (Dustin Hoffman) is accosted by one of his parents' friends, a Mr. McGuire (Walter Brooke), who tells him, "I just want to say one word to you… just one word." That one word, which contains a large piece of the future for business, the economy, and even society, turns out to be—"Plastics." Though there is an unmistakably satirical edge to the interchange, the truth is that plastics have changed the quality of human life, and in few areas is this more apparent than that of prosthesis, or the development of artificial substitutes for missing limbs or other body parts.

The idea of developing prosthetic limbs goes back to Roman times, but the origins of medical prosthetics as a science dates only to the sixteenth century. These early prostheses were primitive affairs: metal hands that did not move, or even a hook in place of an appendage, an image that became attached to pirates and other exotic characters. Only in the wake of the twentieth-century world wars would scientists make the technological breakthroughs necessary to develop the type of workable prosthetics in use today. For that to become possible, it was necessary to make use of an entirely new type of material: plastics.

At the time of their initial appearance in the 1930s, plastics were among the first everyday materials that seemed to be completely artificial, in contrast to the wood, stone, cotton, wool, leather, and other natural materials that made up most people's worlds. Yet plastics are formed of hydrocarbons, or chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms, and since the presence of carbon and hydrogen identifies a substance chemically as organic, they are in fact organic materials.

Their unique physical characteristics result from the fact that plastics are made up of polymers, or long, stringy molecules that are in turn made up of smaller molecules called monomers. Plastics are by definition high-molecular weight substances, but by the latter part of the twentieth century, it became possible to create plastics of extraordinarily high molecular weight, which made them much more durable. Long chains of 10,000 or more monomers can be packed closely to form a hard, tough plastic known as high-density polyethylene, or HDPE, used, for instance, in making drink bottles—and, eventually, in constructing prostheses.

By the end of World War II, plastics had become the standard for prostheses, though usually the plastic had to be reinforced either by a metal frame or even by glass. By the time HDPE made its appearance, surgical prosthetics had begun to go far beyond the point of simply attaching fake limbs to real ones: in certain cases, physicians were able to achieve an interface between living and non-living material, for instance by attaching the biceps muscle to a prosthetic arm, thus giving the user greater control over movement. This interface, naturally, introduced an entirely new level of complexity to the nature of prosthetics, and raised concerns regarding the long-term usefulness of prosthetic body parts in light not only of possible physical wear, but of deterioration resulting from chemical and biological factors as well.

"Fighting Wear Has Become More Important Than Ever"

It is now possible, with this background, to properly join the present debate over the research emphasis on wear—that is, on physical factors, as opposed to chemical or biological ones—in ultra-high molecular weight polyethylene joint replacements. Discussing the state of prosthetics at the end of the twentieth century, as well as this emphasis on wear testing, John DeGaspari wrote in Mechanical Engineering: "Artificial hip joints must perform reliably for many years of use and millions of cycles," a cycle being a completed movement of the body part—for instance, taking a step. "A typical patient with a hip replacement joint may take as many as a million steps a year."

As DeGaspari went on to note, most modern hip replacement prostheses—and this is the case with numerous other prosthetic parts as well—are made from a combination of metal with ultra-high molecular weight polyethylene. However, not just any metal will do. According to William L. Healy and others in the American Journal of Sports Medicine, "First-generation, stainless steel hip and knee implants were associated with fracture rates that were not acceptable. Since the introduction of cobalt-chrome alloys and titanium alloys, implant breakage is not common."

Nor are plastics and metal the only materials necessary to developing a wear-resistant prosthetic. Reported Joseph Ogando in Design News, "Biomedical engineers once had feet of clay when it came to ceramic knees. Ceramics, with their low coefficients of friction and exceptional hardness, could certainly reduce the wear that shortens the life of artificial knees, but these brittle materials just could not handle the contact stresses found in even the best implant designs." Ogando went on to report that Smith & Nephew of Memphis, Tennessee, had created a new type of knee implant using a zirconium substrate with a zirconia surface—in other words, a metal interior with a ceramic exterior. "This hybrid material," he noted, "called 'oxidized zirconium,' pairs the mechanical properties of a metal with the wear-fighting capabilities of a ceramic."

Ogando also noted that "fighting wear has become more important than ever." This is a point made in many sources, each of which appears to take as a given the idea that wear should be the focus of prosthetic research. Thus, Timothy W. Wright began an article in the American Academy of Orthopaedic Surgeons Bulletin by declaring that "Wear-related failures of total joint arthroplasties [surgery to realign or reconstruct a joint] remain a serious problem." Likewise, Healy and his co-writers, noting that an international gathering of scientists from 35 nations and 44 states had declared the first 10 years of the twenty-first century the "Bone and Joint Decade," observed that "As a new millennium begins, new polyethylenes, ceramics, metals, and composite materials are being evaluated for improved wear characteristics at the bearing surface of joint replacements."

The Connection Between Physical, Chemical, and Biological Properties

The writers in the American Journal of Sports Medicine also commented that "The wear factor and generation of particles may be the most important variable for patients and orthopaedic surgeons to consider when discussing athletic activity after joint replacement operation." This matter of "generation of particles" has causes that are tied to chemical factors, but the result appears (at least in part) in the form of physical wear.

In establishing the interface between mechanical prostheses and biological material, sterilization is essential. This sterilization has in the past been achieved through the use of gamma radiation, the bombardment of the polyethylene cup (which attaches to living tissue) with extremely high-energy particles that remove all possible contaminants. But this process of irradiation has the unintended consequence of removing hydrogen atoms from the hydrocarbon chains, resulting in the generation of free radicals. The latter term refers to electrons that have been released, bringing about a net electric charge and creating an unstable chemical situation. If oxygen, a highly reactive element that can bring about such corrosive effects as rust, comes into contact with the free radicals, the result is a chemical reaction that can be detrimental to the prosthesis. The polymer begins to lose molecular weight, and the surface of the polyethylene becomes less resistant to wear, such that infinitesimal particles of the plastic begin to break loose, creating inflammation in the body.

The above is an excellent example of how physical, biological, and chemical factors interact in prostheses, rendering these various factors ultimately inseparable. The final result, however, is physical, in the form of accelerated wear. This relationship between chemical and/or biological and physical factors is reinforced by findings published on the website of the Institute of Orthopaedics in London, on which it is stated (and corroborated with experimental data) that "The wear of polyethylene increases with the level of oxidation."

Wright, in the American Academy of Orthopaedic Surgeons Bulletin, likewise noted the relationship between physical and biological factors in causing wear. "Biologically," he observed, "the local reaction to wear debris"—the separation of particles from the prosthesis, as we have already noted—"results from the activation of phagocytosis by macrophages." In other words, the physical phenomenon of wear brings about biological damage through the action of microorganisms.

Protecting Against Biological and Chemical Complications

From these findings and others, it is clear that the physical phenomenon of wear is inseparable, in a real-world sense, from chemical phenomena such as oxidization, or biological phenomena such as infection. Therefore, the direction of research regarding high-molecular weight polyethylene bearing components in joint replacement should remain focused on wear, since this is likely to be the source of biological or chemical complications.

What, then, is to be done? Or, to put it another way, how can prosthetic manufacturers develop materials that, by resisting wear, protect against oxidization, infection, and other chemical or biological factors? One promising area is in the crosslinking of polymers, a technique pioneered by polymer chemist Orhun Muratoglu at Massachusetts General Hospital in Boston. By heating polyethylene to 257°F (125°C)—slightly below its melting point—then irradiating it, the Massachusetts team has been able to recombine the free radicals, resulting in a product that shows no measurable signs of wear after 20 million cycles.

At the same time, research conducted by Thierry Blanchet, associate professor of mechanical engineering at Rensselaer Polytechnic Institute in Troy, New York, indicates that immersion of the prosthesis in an acetylene-ethylene-hydrogen atmosphere during heating produces a much more wear-resistant product. Other techniques are under investigation, and though many of them involve attempts to alter the chemical properties of the prosthetic, the emphasis is on wear. This is no doubt as it should be, since the prosthesis is ultimately a physical component. It must be biologically suited to the user, and must not induce chemical reactions that may render it less than fully effective, but its ultimate design is as a physical part of the body—an arm or hand that grasps, for instance, or a hip or leg that makes it possible for the user to walk like a person with two healthy legs.

—JUDSON KNIGHT

Viewpoint: No, the current research emphasis on the wear of polyethylene bearing components in joint replacements is unwarranted, given that alternative materials are available that can provide not only greater longevity but biological tolerance as well.

The Six Million Dollar Man once entertained television viewers with superhuman feats made possible by man-made replacement parts surgically implanted throughout his body. If the makers of these parts relied on the same materials primarily used in joint replacements today, the six-million-dollar man would be sitting in a rocking chair, unable to walk to the refrigerator without a cane. At best, he has been to see the doctors for numerous tune-ups and likely a few major repairs.

In the world of joint replacement prosthetics, especially those involving the hips and knees, polyethylene has been used for 30 years as a relatively successful bearing surface in combination with a metallic or ceramic counterface. The combination allows for total range of movement by forming a highly functioning articulation (the joint or juncture between bones or cartilages). Over the years, joint replacement manufacturers switched from using high-density polyethylene to ultra high molecular weight polyethylene (UHMWP) for improved mechanical properties and performance. UHMWP remains the most popular materials used in conjunction with a metal alloy like cobalt-chromium (Co-Cr) to form the articulation. UHMWP's higher molecular weight and fewer polymer chain branches result in a polymer with a higher abrasive resistance and good chemical resistance for durable wear properties. Nevertheless, UHMWP does have drawbacks, and emphasizing research to focus on the improvement of polyethylene is unwarranted, especially when other materials may be more durable and safer.

The Problem with the Current Emphasis

The old saying, "don't put all your eggs in one basket," is most often used in terms of money and investments. But it applies to most aspects of life, including science. The use of polyethylene in joint replacements has served orthopedic patients well, but their long-term use has revealed several problems. For example, because joint prostheses are essentially bearings, friction and wear are major problems. High friction forces can produce shearing stresses and wear. The use of polyethylene and metal involves three types of wear. Abrasive wear results from the direct contact between metal and plastic components and can result in the metal abrading the plastic. Adhesive wear is similar to a welding and tearing of the contacting surfaces between the metal and polyethylene. Pitting wear is the most common type of wear and results from incongruent contact between the metal implant and plastic bearing. A major clinical concern in joint replacements has been the production of wear debris from contact between the articulating surfaces.

Although many efforts have been made to modify polyethylene to reduce wear, most have produced similar results as the conventional polyethylene. Furthermore, polyethylene components undergo oxidative degradation during manufacturing, which can lead to harmful changes in the material's physical and mechanical properties. Oxidative degradation of polyethylene can continue to cause components to be more and more brittle and less resistant to fracture and fracture-related wear damage.

Another approach used to improve polyethylene's wear resistance is to induce crosslinks into the polyethylene morphology. Crosslinking is a method involving either ionizing radiation or chemical methods to improve UHWMP's abrasion resistance. Essentially, it bonds molecules together to make a stronger material. However, many of the new crosslinked polyethylenes have reduced ductility and fracture resistance compared to the conventional polyethylenes. Despite all these attempts, no improved polyethylene has yet been proven in the clinical setting.

Durability has also become a major issue concerning the use of metal-polyethylene joint prosthetics. For many years, these prosthetics were used primarily in the elderly, that is, people in their 70s and older. However, joint replacement patients now include many who are in their 40s and 50s. These more active patients have placed extra stress on the replacement, and the metal-polyethylene combination has not stood up to the strain.

Studies have shown that polyethylene wear debris induces osteolysis as a long-term complication, especially in younger, more active patients. The metal and polyethylene combination can produce up to 40 billion particles per year. As this debris accumulates, it usually results in an aggregation of macrophages that attempt to phagocytize it. The resulting inflammatory response include the release of lytic enzymes, pro-inflammatory cytokines, and bone-resorbing mediators that result in osteolysis, or bone resorption. The osteolysis, in turn, causes aseptic (not involving infections or pathogens) loosening and the implant's clinical failure. One approach to this problem, for example, is to reduce the ball size of the polyethylene femoral in hip replacements. Although reducing the size of the polyethylene head components minimizes this wear, it has induced stability problems in some patients.

The Next Generation

Orthopedic surgeons and scientists are looking at ways to increase prosthetic durability and reduce the volume of particulate debris released into the tissues surrounding the joints. This reduction, in turn, lowers the occurrence of osteolysis and aseptic loosening. Many researchers are focusing on bearing materials that wear significantly less than the standard metal-polyethylene combination. Their primary goal is to find a material or combinations of materials that have good wear properties and low coefficients of friction. The ultimate goal is to develop joint prosthetics that reliably relieve pain, allow unlimited function, and last throughout the patient's lifetime.

Although the use of metal-on-metal (M/M) bearings in joint prosthetics dates back to the late 1960s and ceramic-on-ceramic (C/C) bearings have been around for nearly as long, early primitive designs (primarily involving fixation of the stem or socket) led to inferior results compared to metal-on-polyethylene. However, recent advances in the use of metals and ceramics have led many researchers to consider replacing polyethylene with these materials. For example, scientific analyses of components made with these newly engineered parts show that the occurrence of wear can be at least 100 times less than polyethylene components.

Although poor design and manufacturing methods led to problems, M/M articulations have been undergoing a renaissance worldwide. First-generation metal-on-metal devices that have survived for more than two decades have shown low wear rates and very little change in surface finish and dimension. Studies of twenty-to thirty-year-old M/M hip prosthesis have shown a notable absence of osteolysis around the implant.

With today's modern technology, metal-bearing surfaces can be designed to be almost perfectly smooth and round, which further minimizes wear. In the case of hip replacements, M/M components can include larger head/cup sizes without large increases in wear volumes for improved range of motion and the use of the large implant sizes needed for surface replacement of the hip. Furthermore early findings have shown that M/M wear particles are in the 20-60 nm size range, which is an order of magnitude smaller than the polyethylene wear particles produced in the metal-polyethylene articulations. In fact the worst case estimate of combined femoral and acetabular (cup-shaped socket) linear wear in hip replacements was 4.2 microns per year, which is about 25 times less than the wear that typically occurs with polyethylene.

Clearly, close attention to design detail and manufacturing quality has led to revised interest in M/M joint prosthetics. These prosthetics have enormous potential for greatly reduced wear and fewer problems with osteolysis. Studies of second-generation M/M bearings have revealed very good results, including little evidence of metal wear and few wear particles found in histologic sections.

Ceramic-on-ceramic (C/C) bearings have been used in Europe since the early 1970s. Very early on, ceramic materials were associated with a large grain structure, which could lead to fractures. Overall, poor implant design, acetabular component loosening, and low-quality ceramic, resulting in fracture and debris generation, dampened enthusiasm for their use. However, like M/M bearings, early problems and failures were related less with the alumina material used than to inefficient design and, in some cases, the surgical procedure itself. Improved manufacturing processes have created a much stronger ceramic material with a small grain size. Overall, ceramic bearings made of alumina have shown to have the lowest wear rates of any of the bearing combinations, both in simulator cases and in clinical studies.

In a study conducted over 20 years beginning in 1977, C/C bearings showed excellent results in the younger population, with an 86% prosthetic survival rate at 15 years in patients less than 50 years of age. Osteolysis was found in less than 1% of patients. The linear wear rates were very low. Histological studies also confirmed that the little alumina ceramic debris that did occur was tolerated well biologically.

As a result of these and other findings, C/C is perhaps the most promising bearing material for use in young and active people. Some findings show that an exceptionally long-term survival rate without any activity limitations can be expected for C/C bearing prosthetics, with the fracture risk estimated to be at 1/2000 over a 10-year period. Furthermore, the design of the newer generation C/C bearings makes surgical implantation for correct positioning much easier.

The Biocompatibility Issue

To be considered biocompatible, a material should be well tolerated in the body, causing little response from the person's (or host's) body, including metabolic, bacteriologic, immunologic, and neoplastic responses. Although all of the different materials used in joint replacements have been tested and approved in terms of biocompatibility, problems still occur. Extensive research into this area has shown that tissues and cells exposed to particulate polyethylene produce numerous inflammatory mediators associated with bone resorption (or loss). It is generally accepted that most cases of aseptic loosening and osteolysis are the result of macrophagic response to polyethylene wear particles.

In contrast, patients have shown a very low incidence of hypersensitivity to M/M implants made with a cobalt-chrome alloy. Even though studies have shown higher levels of cobalt and chromium in patient's hair, blood, and urine than in metal-polyethylene cases, researchers found that the cobalt ions are rapidly transported from the implant site and mostly eliminated in the urine. Nevertheless, chromium tends to be stored in the tissues for longer periods. Another concern with M/M implants was the question of carcinogenicity. Clinical studies to date have shown that CoCr implants do not induce cytotoxic effects. These findings are supported by epidemiological studies as well. Furthermore, ongoing studies of early M/M implants found that the clinical incidence of sarcomas in M/M patients is extremely low.

C/C bearings have the advantage of bulk alumina being chemically inert and, as a result, having excellent biocompatibility. Although a foreign body response is still induced by particulate alumina in tissues, the inflammatory mediators released by the immune system in response to particulates is generally less than those stimulated by polyethylene particles. As shown by cell-culture studies, particulate alumina is not toxic.

The Future of Joint Replacement Research

The adverse biological effects of wear debris from polyethylene bearing components coupled with high wear rates and shorter longevity paint a clear picture that research into joint replacement bearing materials should expand beyond polyethylene. Several alternatives to metal-polyethylene implants have been advocated, including the sole use of ceramics or metals. As a bearing surface, ceramic has great potential because of its hardness, which can tolerate rigorous polishing, resulting in greater scratch resistance. Furthermore, the bearing is wettable, which allows for less friction. Metal bearings have also shown superior durability compared to the use of polyethylene. The volume of wear debris in both metal and ceramic bearings is also expected to be minimal, resulting in less injury to the bone around the prosthesis.

Experience with metal-polyethylene bearings has shown a typical survival rate of about 15 years. Often, patients with these joint replacements return to a life of pain and restricted mobility that can be even worse than their pre-surgical conditions. Metal and ceramic bearings promise to prolong the durability of joint replacements, especially in younger patients. Of course, rigorous mechanical testing and hip simulator research are required, as are further controlled clinical trials for specific products. But by expanding research to focus more on metal-on-metal and ceramic-on-ceramic bearing components, joint prosthetics will one day last as long and be tolerated as well as the normal healthy joint.

—DAVID PETECHUK

Further Reading

DeGaspari, John. "Standing Up to the Test." Mechanical Engineering 121, no. 8: 69-70.

"Durability of Biomaterials." Institute of Orthopaedics, London. May 30, 2002 [cited July 22, 2002]. <http://www.fractures.com/institute/bme/durabil.htm>.

Healy, William L., Richard Iorio, and Mark J. Lemos. "Athletic Activity After Joint Replacement." American Journal of Sports Medicine 29, no. 3: 377-88.

Jazrawi, Laith M., and William L. Jaffe."Ceramic Bearings in Total Hip Arthroplast: Early Clinical Results and Review of Long Term Experiences." Arthroplasty Arthroscopic Surgery 10, no. 1 (1999): 1-7.

Lerouge, S., et al. "Ceramic-Ceramic vs. Metal Polyethylene: A Comparison of Periprosthetic Tissue from Loosened Total Hip Arthroplasties." Journal of Bone and Joint Surgery 79 (1997): 135-39.

McNeill, Bridgette Rose. "Getting Hip to Joint Replacement." Southwestern Medicine (The University of Texas Southwestern Medical Center at Dallas). May 30, 2002 [cited July 22, 2002]. <http://www.swmed.edu/home_pages/publish/magazine/hip/joint.html>.

Morawski, David R., et al. "Polyethylene Debris in Lymph Nodes after a Total Hip Arthroplasty." Journal of Bone and Joint Surgery 77-A, no. 5 (May 1995).

Ogando, Joseph. "A Knee for the Long Haul." Design News 56, no. 12: 76-77.

Rimnac, Clare M. "Research Focuses on Polyethylene Wear." The American Academy of Orthopaedic Surgeons Bulletin 49, no. 1 (February 2001).

Waddell, James P. "Improving the Durability of Total Joint Replacement." Canadian Medical Association Journal 161 (1999): 1141.

Wagner, M., and H. Wagner. "Medium-Term Results of a Modern Metal-on-Metal System in Total Hip Replacement." Clinical Orthopaedics 379 (October 2000): 123-33.

Wright, Timothy. "Investigators Focus on Wear in Total Joints." American Academy of Orthopaedic Surgeons (AAOS). May 30, 2002 [cited July 22, 2002]. <http://www.aaos.org/wordhtml/bulletin/dec00/fline1.htm>.

KEY TERMS

ACETABULAR:

The cup-shaped socket in the hip bone.

CLINICAL STUDIES:

Studies that involve or depend on direct observation of patients.

CYCLE:

In the realm of prosthetics, a cycle is a completed movement of the body part—for instance, taking a step.

CYTOKINES:

Immunoregulatory substances secreted by the immune system.

FREE RADICAL:

An atom or group of atoms that has one or more unpaired electrons, such that it is highly reactive chemically.

HDPE:

High-density polyethylene, a high-molecular weight variety of plastic, made up of long chains of 10,000 or more monomers packed closely to form a hard, tough substance.

HISTOLOGIC:

Tissue structure and organization.

HYDROCARBONS:

Chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Plastics are composed of hydrocarbons.

IRRADIATION:

Bombardment with high-energy particles. In the case of prosthesis, this is for the purpose of sterilizing the prosthetic.

NEOPLASTIC:

Having to do with the growth of non-functioning tissues such as tumors.

OSTEOLYSIS:

The dissolution of bone; most often associated with resorption.

PHAGOCYTIZE:

The ingestion of foreign material or bacteria in the body by phagocytes, a special kind of cell.

POLYMERS:

Long, stringy molecules that are made up of smaller molecules called monomers.

PROSTHETICS:

The branch of medical science devoted to the development and implementation of artificial substitutes for missing limbs or other body parts.