Are arthroplasties (orthopedic implants) best anchored to the contiguous bone using acrylic bone cement

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Are arthroplasties (orthopedic implants) best anchored to the contiguous bone using acrylic bone cement?

Viewpoint: Yes, acrylic bone cement has been used successfully in arthroplasties for several decades.

Viewpoint: No, newer materials such as bioceramics will eventually offer a safer and more durable anchor for arthroplasties.

The total hip replacement (THR) operation, which can restore mobility to people with damaged or arthritic joints, is one of the glowing success stories in the history of modern medicine. Rather than repairing or reconstructing the ball-and-cup joint of the hip, either of which is a very difficult operation, the entire unit is replaced with two artificial prostheses. This procedure has also been successfully applied to other joints such as fingers and knees, and the operation is without complications in more than 95% of cases.

However, there have been doubts raised as to the long-term durability of the most commonly used components of THR operations, in particular the cement that glues the artificial prosthesis to the bone. Wear and tear, and a general degradation of the cement over time, often result in hip replacement failure. More worrying are the additional complications that can arise from cement debris generated as the bond between the bone and the prosthesis breaks down. Particles of cement debris can damage surrounding tissue or enter the bloodstream, leading to serious problems.

The hip consists of two main parts: a ball (femoral head) at the top of the thighbone (femur), which fits into a rounded cup (acetabulum) in the pelvis. Ligaments connect the ball to the cup, and a covering of cartilage cushions the bones, allowing for easy motion. There is also a membrane which creates a small amount of fluid to reduce friction in the joint, and allow painless motion. The hip is one of the body's largest weight-bearing joints, and because of the heavy load it bears, the hip can become badly worn. The cartilage is particularly vulnerable to wear, and without this layer of cushioning, the bones of the joint rub together and become irregular, causing stiffness and pain.

In a total hip replacement operation, surgeons replace the head of the thigh bone with an artificial ball, and the worn socket with an artificial cup. Such operations began to be regularly performed in the 1950s, when medical technology and engineering allowed the use of safe and durable components. Although THR is considered a miracle of modern medicine, the wealth of data collected on the procedure has shown that there is a lack of durability in the components after about 10 years. At first this was not seen as a great problem, as the vast majority of patients were elderly arthritis sufferers, and 10 years of service from a new hip was certainly better than nothing. However, with the increasing longevity of Western populations and a corresponding rise in the physical expectations for life and leisure, the 10-year life span of hip prostheses is now seen as too short for vast numbers of patients.

The main culprit in the decay of the artificial hip appears to be the cement used to bond the replacement parts to the bone. Over time, tiny cracks in the cement weaken the bond between bone and prosthesis, and the parts become loose. As a result the patient can suffer discomfort, pain, and a range of dangerous complications arising from the movement of particles of cement within the body, often causing heart problems and other life-threatening conditions.

There is obviously a need for new solutions, but opinion is divided as to the best way to improve the life span of artificial hips. Some doctors advocate using artificial components that can be fixed in place without cement. By encouraging the surrounding bone to grow into the replacement parts, a more durable bond can be created. However, the drawback of this solution is that the bone must be given time to grow into its bond with the prosthesis, and thus recovery times for such operations are measured in months, rather than days. Although it does seem that the bonds created in this process are more durable, there is a lack of long-term data, since the earliest operations using this method were carried out in the 1980s.

Another proposed solution is to cement only one part of the replacement hip, the femoral head (ball), while using the cementless bone-growth attachment for the acetabular (cup). As the breakdown usually occurs first in the acetabular prosthesis, this method strengthens the weakest part of the artificial joint. However, some critics argue that such hybrid total hip replacements would have drawbacks associated with both methods—a long recovery time, and a small projected increase in the life-span of the prosthesis, as yet unknown.

The third solution is to attempt to improve the materials in cemented artificial hips, and thereby increase their durability. New cements, adjustments to the way the cement is mixed, and additives are all being tested. As the process by which the cement breaks down is not fully understood, it is hoped that research will be able to shed light on the process, and allow for minor changes in the current cement to solve the problems. Since cemented prostheses allow for quick recovery times and a stronger initial bond, if the lifetime of the cement could be improved, such a method would have many advantages. However, clinical trials of new ingredients or methods will have to wait many years before their full impact of this solution can be known.

The total hip replacement is the best way to restore mobility to damaged joints, but the question remains as to which method will serve the patient best. The long-and short-term needs of the individual must be considered, and the pros and cons of each option weighed. The increasing life-spans of patients, as well as a rising culture of continuing physical leisure activities into later life, makes such decisions difficult.

—DAVID TULLOCH

Viewpoint: Yes, acrylic bone cement has been used successfully in arthroplasties for several decades.

Arthroplasties are orthopedic implants used to relieve pain and restore range of motion by realigning or reconstructing a joint. Arthroplasty surgery can restore function in a hip or a knee when disabling joint stiffness and pain are produced by severe arthritis. Most total joint replacement surgery is performed using acrylic bone cement to mechanically hold the parts in place. Patients receiving new hips can stand and walk without support almost immediately after arthroplasty surgery when this type of bone cement is used.

The year 2001 was the 40th anniversary of the clinical use of acrylic bone cement in total joint replacement. The particular acrylic used, polymethylmethacrylate (PMMA), was first introduced for use in low-friction hip replacement in the 1960s by the British orthopedic surgeon, Sir John Charnley (1911-1982). By 2001, total hip replacement was described by the American Academy of Orthopaedic Surgeons (AAOS) as an orthopedic success story, and the use of acrylic bone cement has done much to make it so. More than 500,000 hip replacements are performed worldwide every year, most of them using PMMA bone cement.

History of Joint Replacement

Arthroplasty surgery was developed to relieve the pain and immobility from arthritic hips, although it has since been adapted to other joints, particularly knees. The hip is described as a ball-and-socket joint because the head of the femur (thighbone) is spherical and it moves inside the cuplike acetabulum (socket) of the pelvis. Both the ball and the socket become impaired with arthritis, but in most early arthroplasty operations only the ball was treated or replaced.

Hip replacement was first attempted in 1891. There is not much information on the outcome, but through the years the disabling nature of arthritis has led to a great deal of innovative surgery to relieve the intense pain and immobility it causes. In 1925, Marius Nygaard Smith-Petersen (1886-1953), a surgeon in Boston, tried covering the ball at the end of the femur with a molded piece of glass. The glass did provide the necessary smooth surface on the ball to give some relief, but was not durable. The process, called "molded arthroplasty," was also tried with stainless steel. In 1938, two brothers, Drs. Jean and Robert Judet of Paris, tried a new commercial acrylic plastic. By the 1940s, molded arthroplasty was the best hope for sufferers.

In 1936, a cobalt-chromium alloy was used in orthopedics and provided dramatic improvement. It is still used today in various prostheses, for it is both very strong and resistant to corrosion. Although mold arthroplasty and the new alloy were improvements, the resurfacing of the ball was not enough to give predictable relief. In addition, many patients had limited movement, so the search for more improvements continued.

In the 1950s, Frederick R. Thompson of New York, and Austin T. Moore of South Carolina, independently developed replacements for the ball at the end of the femur. The end of the femur was removed and a metal stem with an attached metal ball was placed into the marrow cavity of the femur. The acetabulum was not replaced. The procedure was called hemiarthroplasty. The results were not entirely successful, since the socket problems remained and the implant was not secured to the bone. In 1951, Dr. Edward J. Haboush of New York City tried anchoring a prosthesis with an acrylic dental cement.

In England, Dr. John Charnley was working on improving the treatment, and in 1958 he attempted to replace both the socket and the ball, using Teflon for the acetabulum. When that was not satisfactory, he tried another plastic, polyethylene, which worked very well. Charnley used the acrylic bone cement polymethylmethacrylate (PMMA) to secure both the socket and the ball prostheses. Dr. Charnley had succeeded in performing a total hip replacement (THR), a development which benefited so many sufferers that he was knighted by Queen Elizabeth II, becoming Sir John Charnley.

Implant Construction

The total hip replacement used by Charnley in 1962 involved a 0.86-in (22-mm) stainless steel ball on a stem, inserted into the femur to replace the ball side of the joint, and a high density polyethylene socket to replace the acetabular. He secured both components in place with PMMA. Since Charnley's first replacements, balls of different sizes and materials and different stem lengths have been fashioned to accommodate individual patients.

Total hip replacements today use ball portions made of highly polished cobalt/chromium alloys or ceramic materials made of aluminum oxide or zirconium oxide. The stem portions are made of titanium-or cobalt/chromium-based alloys. The acetabular socket is made of metal or ultrahigh molecular weight polyethylene, or polyethylene backed by metal. The prosthetic parts weigh between 14 and 18 oz (400 and 500 g), depending on the size. It is important to use biocompatible materials that can function without creating a rejection response. The materials must also be resistant to corrosion, degradation, and wear. One concern with the use of acrylic bone cement is particulate debris, which in a some cases has been generated over time as the implant parts move against each other.

The implant parts must also have the same mechanical properties as healthy versions of the structures they replace. The Charnley total hip replacements have consistently produced pain relief and rapid recovery because the implants are fixed in place. The patient has nearly immediate mobility. The success of the Charnley method has led to total joint replacement surgery being performed on knees, ankles, fingers, wrists, shoulders, and even elbows.

Acrylic Polymers

PMMA, polymethylmethacrylate, is one of a large family of acrylic polymers that have in common some association with a compound called acrylic acid, a rather small molecule with the empirical formula C 3 H 4 O 2. The name "acrylic" relates to the acrid odor of the acid, which is not present in the very large molecules that are called acrylic polymers. Poly means "many," mer means "parts." Therefore in PMMA, P (poly-) means "many," and the rest of the name, MMA (methylmethacrylate), relates to the particular parts that are chemically bonded over and over again to make this polymer. These individual parts that combine to make a polymer are called monomers.

Proteins are complex natural polymers. Acrylic polymers are synthetic molecules and not as complex as proteins. The term polymer is more generally used with synthetic materials. They are also commonly called plastics.

There are two basic ways to make a polymer, depending on whether the monomers just join one after another with no byproduct, or whether the monomers join and produce a small molecular byproduct. The process depends on the properties of the particular monomers. In addition polymerization, a reaction proceeds, usually under the influence of a catalyst, to link identical monomers to each other in long chains. Very reactive groups such as peroxides are common catalysts, and polyethylene is one example of an addition polymer.

Condensation polymerization produces very large molecules, as monomers join by producing a small molecule byproduct, part of the byproduct molecule coming from each of the monomers as they join. Nylon 6-10 is an example of a condensation polymer. The small molecule byproduct is HCl, hydrogen chloride (hydrochloric acid in solution). Condensation polymers are thermosetting, that is they use heat to cure and set and become infusible when heated. Sometimes catalysts are used for condensation polymers. The first commercially prepared plastic was Bakelite, in the early 1900s.

PMMA

Commercial names for PMMA include Lucite and Plexiglas. PMMA is an amorphous, transparent, colorless plastic that is hard and stiff, but brittle. PMMA was first synthesized by Dr. Otto Rohm and Otto Haas early in the twentieth century and first commercialized in the 1930s. It was used for a variety of applications to replace hard rubber, including dentures. During World War II, PMMA was used for aircraft windshields and canopies. The original synthesis of PMMA required heat, but in the 1940s, a "cold-curing" process was developed that made it possible to use it in orthopedic applications.

Bone cements are made of a powder containing methacrylate and a liquid monomer of methylmethacrylate. The powder contains a peroxide initiator, and the liquid contains an activator. The polymerization process begins at room temperature with mixing of the powder and liquid. Opaque agents are added to the powder for radiographic contrast so the surgeon can check the cement in place. An antibiotic is also added to the powder. Chlorophyll is added to the components for optical marking of the bone cement during the operation. The long-term success of the acrylic bone cement depends at least in part on the exactness of the mixing and application procedures.

The polymer is mixed in the operating room and inserted into the femur during the polymerization process. The implant is then inserted into the hardening cement. A similar procedure is used if cement anchors the acetabular in place. As the polymerization process proceeds, the PMMA fills all the spaces to mechanically anchor the prosthesis. The bone cement does not chemically bond to the bone or the prosthesis.

Long-Term Prospects

A total hip replacement is irreversible, but it may not last forever. The cement in cemented total hip replacements, or in other joints, may break down over time. The time varies for a number of reasons, not all well understood. One of the factors being researched is the influence of mixing techniques on the physical properties of PMMA. There is considerable evidence that mixing procedures play a significant role in the quality of the bone cement.

Breakdown of the cement occurs when microcracks in the cement appear, and the prosthetic becomes loose or unstable. The problem is most often with the acetabular component. The rubbing of the ball against the cup produces microscopic debris particles, which are absorbed by the cells around the joint, and an inflammatory response results. This response can lead to bone loss, and the implant becomes loose as a result. Only about 10% of the total hip replacements fail within 10 years, but the large number of implants being performed makes that a significant number of patients in distress.

Cementless implants were introduced in the 1980s. These implants are larger and have a surface structure that is supposed to induce new bone growth. Recovery is slow and there has not been enough time to evaluate the success of these procedures. The age of patients having total hip replacement surgery has dropped considerably since Charnley introduced the procedure, and the longer and more active life of recipients has contributed to the number of replacements that have begun to fail.

The American Academy of Orthopaedic Surgeons (AAOS) addressed the problem of in vivo (within the body) degradation of acrylic bone cement in total hip arthroplasty at their annual meeting in 1999. Factors identified as contributing to failure include enzymatic activity and local pH, mechanical loading on the joint, porosity, and the initial molecular weight or other characteristics of the cement that may influence the susceptibility of PMMA to in vivo degradation.

One study, on the micrometer-sized filler particles put into cement to make it opaque to x rays (so the orthopedic surgeon can monitor the cement over time), was done through Harvard Medical School in Boston. The filler particles commonly used are either barium sulfate or ceramic particles such as zirconium oxide. The microsized particles tend to clump together and cause voids, which eventually lead to breakdown in the bone cement. When smaller particles of nano-sized aluminum oxide were studied, they stayed dispersed, suggesting this may be one solution to the problem. Other studies indicate the type of sterilization used on the cement components has a direct effect on the longevity of the prosthesis. Gamma irradiation was found to reduce the cement's toughness and resistance to fracture, but ethylene oxide sterilization did not reduce the quality of the cement. In other studies, the benefits of adding reinforcing fibers was investigated. Such fibers are added routinely to plastics to increase their toughness for many other applications.

There is another possible option to consider when deciding whether or not to use acrylic cement, hybrid total hip replacements that use a cemented femoral component but a cementless acetabular one. This procedures eliminates what many consider the weak point. Hybrid hips are now widely used and producing good results according to reports in the Great Britain, where this whole success story started.

—M. C. NAGEL

Viewpoint: No, newer materials such as bioceramics will eventually offer a safer and more durable anchor for arthroplasties.

A surgeon can either repair, reconstruct, or replace a patient's damaged joint. All three operations are difficult, and the history of medicine is replete with failures in this area. Early in the twentieth century, surgeons learned that the best remedy for a diseased or injured joint is to replace it with a prosthesis, but that was not possible until the late 1950s.

History of Joint Replacement Surgery

The first successful operation to rebuild a joint was performed in 1826 by John Rhea Barton (1794-1871) in Philadelphia. He repaired a disabled ball-and-socket joint in a sailor's hip by reshaping it into a hinge joint without using a prosthesis. Barton's achievement was dependent upon the features of that particular case and was essentially unrepeatable.

Royal Whitman (1857-1946) reported another successful but isolated case of successful hip arthroplasty without prosthesis in 1924, but by then surgeons were beginning to understand that their goal of strong, dependable, durable artificial joints would only be met by developing low-friction prostheses rigidly attached somehow to natural bone.

Sustained research in replacing hip joints with prostheses began in the 1920s. Surgeons left the head of the femur alone, but inserted an artificial cup into the acetabulum, the socket of the pelvis. Selecting proper materials for prostheses is always a problem. Materials must resist wear, abrasion, breakage, and rejection; they must adhere well to bone; and they must not damage, irritate, or infect living body tissue. Between 1923 and 1939, experimental acetabular prostheses were made of glass, celluloid, Viscaloid, a celluloid derivative (1925), Pyrex (1933), and Bakelite (1939).

In 1937, a Boston doctor, Marius Nygaard Smith-Petersen (1886-1953) began trying metals for acetabular cups. He reported in 1939 his design of a successful cup made of Vitallium, a relatively inert, generally biocompatible, alloy of cobalt, chromium, and molybdenum. Several researchers tried stainless steel or steel-reinforced acrylics in the 1940s and 1950s. Artificial femoral heads became possible with these new substances, but friction and deterioration remained significant problems.

In the 1950s, the British surgeon John Charnley (1911-1982) experimented with acetabular cups made of polytetrafluorethylene (PTFE), a low-friction, nearly inert substance usually called Teflon. Combining these cups with femoral heads made of stainless steel and other metals, Charnley succeeded in producing slippery, well-lubricated, and less troublesome artificial hip joints. For the first time, both the acetabulum and the head of the femur could be replaced with artificial materials. Charnley's landmark 1961 article, "Arthroplasty of the Hip: A New Operation," in the British medical journal The Lancet, announced the first genuine total hip replacement (THR), that is, the manufacture and safe implanting of strong, durable, biochemically inert, artificial joints.

The total replacement of knees, fingers, and other joints soon became possible after Charnley's breakthrough in low-friction prosthetic arthroplasty. But early in the 1960s, Charnley realized that Teflon was inadequate for the socket because it would erode and discharge irritating debris into the joint and surrounding tissues. His subsequent experiments with other synthetic low-friction materials led him in 1963 to high density polyethylene, which is still preferred by most THR surgeons.

Clinical Problems

Most arthroplastic complications occur in cases of hip or knee arthroplasty, simply because the hip and knee are larger and must bear more weight than other joints, and thus come under more stress. Complications occur more often with artificial hips than with artificial knees because the natural movements of the ball-and-socket joint in the hip are more versatile and complex than those of the hinge joint in the knee, again producing more stress, and more kinds of stress. Nevertheless, THR gives most patients distinct relief from their degenerative or painful hip conditions, and the long-term prognosis for THR remains among the most optimistic of all surgical procedures, about 96% free of complications in all cases.

The natural ball-and-socket hip joint consists of the head of the femur, rotating in the acetabulum, lubricated by synovial fluid, supported by ligaments and muscles, and cushioned by cartilage. The aim of hip arthroplastic science is to mimic and preserve this combination to the greatest extent possible, for the sake of the comfort, safety, and mobility of the patient.

Typically the acetabular prosthesis is just a hollow hemispheric cup made of ultrahigh-molecular-weight polyethylene. The femoral prosthesis is much more elaborate, mimicking the head and neck of the femur and one or both trochanters, and terminates in a thin cylindrical stem 4 or 5 in (10 or 12 cm) long, made of stainless steel, titanium, titanium alloy, cobalt-chromium alloy, cobalt-chromium-molybdenum alloy, or some other strong, noncorrosive metal. During THR, the acetabulum is reamed, the cup prosthesis is inserted, the part of the femur to be replaced is cut away either above both trochanters, through the greater trochanter, between the trochanters, through the lesser trochanter, or just below the lesser trochanter. Then the shaft is reamed, and the stem inserted into this new hollow. The stem is sometimes fluted to prevent rotation, further elongated to restrict jiggling, or made porous or roughened to improve adhesion.

The convex side of the acetabular prosthesis and the stem of the femoral prosthesis must be reliably attached to the pelvis and femur. There are two common ways to accomplish these attachments—by cementing the prosthesis to the reamed inner surface of the bone, or allowing the bone to grow into the prosthesis without cement. There is also a procedure called hybrid fixation, in which one component, usually the femoral, is cemented, while the other, usually the acetabular, is not.

Each method has its advantages and disadvantages. The question is whether cementing these components to the residual bone or adhering them in some cementless way is better for the patient. Bone will usually grow into the porous or roughened surface of a prosthesis and create a very strong, safe, and durable bond without cement, but this takes some time, at least several months. Bone cement provides a quicker bond that is stronger in the short term, but may be more likely to weaken over time or to cause infection or injury to surrounding tissues.

Among the most common and most significant complications of THR are: the loosening of a prosthesis; the erosion of the prosthesis or its cement so that debris is generated; sepsis (infection); nerve damage; thrombosis (blood clot) or other cardiovascular difficulties; and cancer. All these complications, except for cancer, are most often associated with cemented prostheses. One of the main problems with THR, the gradual dis-integration of the materials of the artificial ball-and-socket joint, is more likely to occur with cemented prostheses because of their tendency to loosen and allow unwanted movement of components within joints.

The standard bone cement is polymethylmethacrylate (PMMA), which Charnley introduced in the early 1960s. It cures quickly and adheres well, but there are two common problems associated with its use. First, as it cures it gets hot enough to damage surrounding soft tissues. Second, it has an effective longevity of only about 10 years, after which it is likely to loosen and generate debris. Incidents of cardiac arrest, thrombosis, and other serious cardiovascular complications have been associated with using PMMA.

Loosening of the prosthesis can be caused by failure of the prosthesis itself or its cement, by the natural aging and withering of the remaining bone, or by erosion of bone as the prosthesis aggravates it. The failure of the cement not only causes pain as the prosthesis loosens, but can also be very dangerous to the health or life of the patient, as tiny particles of bone, dried cement, metal, plastic, fat, or other substances are released into surrounding tissues and/or the cardiovascular system. This debris creates either physical damage, obstruction, or infection. Cobalt or chromium debris has specifically been associated with the onset of cancer. Even though rates of complication such as infection or induced thrombosis dropped significantly after the 1980s because of more effective prophylactic and anticoagulant drugs, earlier and more thorough physical therapy, and shorter hospital stays, physical damage to peripheral nerves still occurs sometimes from inadequately controlled extrusions of cement.

The most serious side effect of cemented prostheses is bone cement implantation syndrome (BCIS), a condition first recognized and named in the early 1970s. BCIS consists of debris extruding from the hardening or hardened cement in the bone marrow and getting into the blood vessels. Symptoms may include low intraoperative blood pressure, pulmonary embolism (obstruction of a blood vessel in the lung), cardiovascular hypoxia (low oxygen level), pulmonary artery distress, or cardiac arrest. BCIS is the leading cause of death as a complication of arthroplasty.

Additional arthroplasties are sometimes necessary to replace defective, deteriorated, or improperly installed implants. Revision arthroplasties are also often indicated for cemented bonds, because of the short effective duration of bone cement.

A Wide Range of Solutions

Research into debris control, friction control, the bone/prosthesis bond, and other joint replacement problems generally centers around the materials used. Cement adheres a prosthesis to bone more quickly, but natural bone growth around and into the implant ultimately provides a stronger and more durable bond. Ideally, then, the best implant would be bioactive in such a way as to stimulate rapid bone growth into its porous surface without recourse to cement. Current research is proceeding in that direction, and the most promising research concerns bioceramics.

Bioceramics are materials designed to promote or facilitate the growth of living tissue into themselves to form strong bonds. They are used mostly in arthroplasty, but also in plastic surgery, heart surgery, and dentistry. There are four kinds of bioceramics: bioinert, such as alumina or zirconia; resorbable, such as tricalcium phosphate; bioactive, such as hydroxyapatite; and porous, such as some coated metals. The methods employed for coating implants with bioceramics include sol-gel and plasma spraying processes.

Alumina and zirconia are commonly used for the ball of the femoral component and the inside of the cup of the acetabular component, where bioactivity is not wanted. Several bioactive kinds of bioceramics are used as ingredients in new bioactive bone cements, which are less toxic and less thrombogenic (likely to cause blood clots) than PMMA. They are also used in cementless arthroplastic applications.

With many minor improvements, ultrahigh molecular weight polyethylene for the socket has been used since Charnley's time, but methods of attaching it to the acetabulum vary. Recently acetabular components backed with porous ceramic-coated metal have shown the best results. In 2001, a team of surgeons in Osaka, Japan, led by Takashi Nishii, reported that firm adhesion of the acetabular cup without cement could be achieved even in cases of bone degeneration.

The femoral stem is sometimes coated with hydroxyapatite, because it is chemically similar to bone, increases the porosity of the stem, and therefore adheres well without cement. The stems of implants can also be coated with a fine layer of biodegradable polymer which encourages bone growth and gradually disappears as the bone grows into its space. A prosthesis coated with hydroxyapatite has a surface that is both porous and bioactive, thus better promoting vigorous growth of bone into the prosthesis to create a natural biochemical bond. This bioactivity and porosity is the main advantage of hydroxyapatite over cement in these applications. In 2001, another surgical team in Osaka, led by Hironobu Oonishi, reported significantly reduced rates of prosthesis loosening when bone cement was used in conjunction with tiny granules of hydroxyapatite.

F. Y. Chiu's Taiwanese team reported in 2001 that mixing bone cement with the antibiotic cefuroxime greatly reduced the incidence of infection caused by the cement in elderly patients. Also in 2001, Alejandro Gonzales Della Valle's team in Buenos Aires reported that bactericidal agents such as vancomycin and tobramycin mixed with PMMA yielded excellent results against gram-positive infections.

The bottom line for surgeons is that choosing the right method of prosthesis implantation and fixation for each particular patient dramatically reduces the risk of complications for that patient. Some patients are right for cementing; some not, and this medical determination must be made on a case-by-case basis.

The quality and viability of the residual bone is a major factor in deciding whether or not to cement. The lifestyle of the patient also matters. Since a cemented prosthesis in an active patient would typically require revision arthroplasty after 10 years, it would not be best for, say, a 50-year-old cyclist. But older patients who are not likely to survive beyond the time when revision arthroplasty would be indicated, or whose bones are thinner, weaker, and less robust, often do better with cemented prostheses, especially if their lifestyles are less active. In 2001, Ewald Ornstein's team in Hassleholm, Sweden, reported disappointing results in 18 cemented revision arthroplasty cases. Even so, bone cements are probably best for most older patients, and uncemented prostheses for younger, but even for patients in their 80s and 90s, each clinical case must be evaluated according to its own set of indications.

—ERIC V.D. LUFT

KEY TERMS

ACETABULUM:

Hemispheric cuplike socket on each side of the pelvis, into which the head of the femur fits.

ARTHROPLASTY:

From the Greek words arthron (joint) and plassein (to form, to shape, or to create); any surgical procedure to rebuild a joint, especially the hip or knee, usually by replacing the natural joint with a prosthesis.

BIOACTIVITY:

Property of a chemical substance to induce a usually beneficial effect on living tissue. An artificial joint component is said to be bioactive if it is osteogenic, i.e., if it stimulates the growth of bone to form a biochemical bond with the implant.

CAPUT FEMORIS:

"Head of the femur"; the hemispheric protuberance that fits into the acetabulum to create the ball-and-socket joint of the hip.

COLLUM FEMORIS:

"Neck of the femur"; the short, stout rod of bone between the head and shaft of the femur, projecting from the shaft at an angle of about 125°.

MONOMER:

Single chemical unit that combines over and over in large numbers to form a polymer, or plastic.

NANO:

Unit of size referring to one billionth, as in 1 nm (nanometer) is one-billionth of a meter.

POLYMER:

Compound made of a great number of repeating smaller units called monomers; also commonly called plastic.

POLYMERIZATION:

Process by which monomers join to make polymers. Polymerization occurs in two ways: addition polymerization where monomers combine directly (usually the reaction needs a catalyst to occur); and condensation where a small byproduct molecule is created as two monomers bond.

REVISION ARTHROPLASTY.

Second joint replacement operation required because of the failure of the first.

SHAFT:

Elongated cylindrical part of a bone, such as the femur, tibia, fibula, humerus, radius, or ulna.

SOL-GEL PROCESS:

Low-temperature chemical method that synthesizes a liquid colloidal suspension ("sol") into a solid ("gel"), yielding a high-purity glass or ceramic substance composed of fine, uniform, and often spherical particles. The sol-gel process facilitates the manufacture of bioactive materials such as hydroxyapatite.

THA OR THR:

"Total hip arthroplasty" or "total hip replacement," one of the most common and most successful of all modern surgical procedures.

TROCHANTER:

Either of two rough protuberances, called the greater, major, or outer trochanter and the lesser, minor, or inner trochanter, near the upper end of the femur between its shaft and neck. Their main purpose is to serve as attachments for hip muscles.

JOHN CHARNLEY DEVELOPS TOTAL HIP REPLACEMENT SURGERY

In the late 1950s and early 1960s, the British orthopedic surgeon John Charnley (1911-1982) developed successful techniques and materials for total hip replacement (THR) surgery, to substitute artificial ball-and-socket joints for diseased or injured femur-pelvis joints. Many surgeons consider THR the greatest surgical advance of the second half of the twentieth century, because it provides long-term pain relief, restores mobility and functionality, dramatically improves the quality of life for millions of patients, and is relatively free of complications. In 1950, a 40-year-old patient with degenerative hip disease could expect to be in a wheelchair by age 60. In 2002, a similar patient, after THR, can walk, play golf, and ride bicycles in his or her 70s and 80s.

With bachelor of medicine (M.B.) and bachelor of surgery (Ch.B.) degrees in 1935 from the Victoria University of Manchester School of Medicine, Charnley became a Fellow of the Royal College of Surgeons in 1936. He volunteered for the Royal Army Medical Corps in 1940, and from 1941 to 1944 served as an orthopedic surgeon to the British forces in North Africa.

At Wrightington Hospital of the Manchester Royal Infirmary in 1958 Charnley used polytetrafluorethylene (PTFE), better known as Teflon, and stainless steel to achieve "low friction arthroplasty," that is, the manufacture and safe implanting of strong, durable, biochemically inert artificial joints. By the mid-1960s THR was a routine surgical procedure.

—Eric v.d. Luft

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