Total joint replacement (also known as total joint arthroplasty) is regarded among the most valued developments in the history of orthopaedics. The procedure provides the surgeon the ability to relieve pain and restore function to patients whose joints have been destroyed by trauma or disease.
An ongoing concern in joint replacement, however, focuses on the increased friction and wear of the joint when utilizing man-made implants. “It’s important to recognize that the best bearing surface is human articular cartilage,” notes Douglas E. Padgett, Chief of the Adult Reconstruction and Joint Replacement Service at Hospital for Special Surgery, “but when the damage is too great to work on isolated, smaller cartilage defects, you then look at [man-made] joint replacements.”
Compared to healthy, organic cartilage surfaces, which have a surface friction of nearly zero, the friction between these man-made bearing surfaces are hundreds of times higher. This friction subjects the implant components to wear that can limit the longevity of the joint replacement and induce inflammatory responses in the tissues surrounding the joint itself.
Fundamental to replacing damaged joint surfaces with implants fabricated from man-made materials, then, is the requirement of producing a low friction bearing to minimize surface wear, inflammation in surrounding tissues and possible eventual loosening of the implant, resulting in the need for additional surgery.
In the 1960’s and 1970’s, a number of materials were tried as bearing surfaces in joint arthroplasty, including Teflon® and metallic alloys such as stainless steel and cobalt-chromium alloy. But through the 1980’s and much of the 1990’s, the preferred bearing combination was ultra high molecular weight polyethylene and cobalt-chromium.
Polyethylene results in a low amount of friction when bearing against a highly polished metallic surface. Today, metal-on-metal and ceramic-on-ceramic combinations are being considered as alternative bearing surfaces.
It is the ongoing problem of wear that has led to the reintroduction of a number of alternative bearing combinations.
Clinically speaking, friction and wear result in very small wear particles (typically less than 4/100,000 of an inch, or about 200 times smaller than a grain of sand) that are released to the surrounding joint cavity, initiating an aggressive inflammatory response.
The body mounts a cellular reaction to try to deal with the wear debris, a reaction that unfortunately often leads to unwanted destruction of bone (called osteolysis) around the implant. When osteolysis becomes severe, it can cause pain and loosening of the implant and the need for revision surgery to replace the components. One way to relieve the osteolysis problem and thus increase the longevity of joint replacements is to improve the wear resistance of the bearing materials.
Also facilitating this introduction of materials is the shift toward younger and more active patients seeking and receiving joint replacement. At HSS, the age at which patients undergo knee replacement surgery is rapidly decreasing (see Fig. 1). As a result, a 50-year old patient would benefit greatly from an improved implant with increased longevity. The prioritized impetus for improving implant materials is this change in the demographics of patients seeking knee replacement.
All the materials currently in use in joint replacement are considered safe from a medical standpoint. They are highly biocompatible, meaning that they cause little if any detrimental local or systemic problems due to an immune response or an allergic reaction.
Nonetheless, as total joint replacements remain in service for long periods of time, concern remains that adverse tissue responses could arise as a result of the continual release of worn particles of the prosthetic materials. This concern is often mentioned in conjunction with the reintroduction of metal-on-metal bearing surfaces.
Determining if new forms of bearing surfaces are effective in reducing wear over conventional polyethylene-on-metal bearings is not as easy as it may seem. We are limited in our ability to study wear in the laboratory by our inability to simulate the lubrication conditions, the loads, and the motions that occur over a broad range of daily activities.
Nevertheless, joint simulators are currently the best tools for studying wear. No acceptable animal models exist, and computer analyses of wear have only been proven to be effective for simple constructs such as the ball-in-socket geometry of the hip.
Wear mechanisms differ depending on the joint, so data collected on a hip simulator may have little relevance for wear in a knee, elbow, or disc replacement. Furthermore, the question is not just one of reducing the amount of wear, but also how the size, shape, and surface chemistry of released wear particles differ among bearing surface combinations, since these factors may ultimately influence the biologic reaction and subsequent tendency for osteolysis.
Ultimately, the effectiveness of these surfaces can only be proven on the basis of clinical results from prospective, randomized studies of total joint patients who have received either conventional or new bearing combinations. Some of these data have already started to emerge, so let’s review the current laboratory and clinical data for each of the new bearing surfaces:
Metal-on-metal bearings were among the first to be used in total hip arthroplasty and found clinical success in the 1960s and 1970s. Metal-on-metal hip joints fell out of favor by most orthopaedic surgeons as the clinical results with polyethylene-on-metal joints proved superior.
However, failures during this earlier time period were due in part to poor metallurgy, poor manufacturing techniques, and implant design. The casting process used to make metallic implants suffered from poor quality control, sometimes leading to products that had inferior wear resistance and were prone to fracture in the body. And early metal-on-metal designs often had small head-to-neck ratios. For this reason, it was common to experience impingement between the neck of the artificial femoral component and the rim of the artificial acetabulum. This impingement creates additional wear and can knock the acetabular component loose from the surrounding bone.
Improved metallurgy and manufacturing techniques have led to resurgence in the use of metal-on-metal bearings for hip replacement. Cobalt-chromium alloys with well-controlled grain sizes and finely distributed carbides provide superior hardness and wear resistance compared to earlier versions of the alloy and to stainless steel and titanium alloy.
Clearance and conformity between the bearing surfaces and a smooth surface finish on the metallic bearings are now recognized as important factors that must be controlled as part of the design and manufacturing processes. Laboratory evidence from hip joint simulator studies has confirmed that these bearing surfaces can provide low wear joint implants. The strength of cobalt-chromium alloys in comparison to polyethylene and their increased toughness over ceramics provide additional benefits from the standpoint of hip implant design.
For example, the surface thickness of a one-piece metallic acetabular component can be smaller than modular components made from polyethylene and metal or from ceramic and metal, so larger femoral head sizes can be incorporated, providing an advantage in cases where joint stability is an issue. Similarly, the ability to manufacture large metallic shells allows for surface replacement of the hip joint, a bone-conserving operation geared toward young, active patients with good bone stock in the femoral head and neck.
As with most modern versions of alternative bearings, clinical results for new metal-on-metal hip replacements are available only for the short, intermediate term. Clinical results at four to seven years are similar to those of total hip replacements with a metal-on-polyethylene combination. But only extensive, longer-term experience will determine if reduced wear with metal-on-metal bearings will indeed lower the incidence of osteolysis.
Metal degradation concerns
Concerns continue to exist because the release of ionic debris in metal-on-metal combinations is considerably higher than with other bearing combinations. Metal ions are transported to distant locations in the body, and elevated levels of metals have been found in serum and urine from patients with metal-on-metal hip replacements.
Several factors can influence metal levels in such cases, including the diameter of the joint. For example, resurfacing of the hip is intended to be bone-conserving by employing a large metallic cap secured to the head of the femur and a matching large metallic shell in the acetabulum. But as the size of the head increases, so does the amount of wear; indeed, metal-on-metal resurfacing patients have a significantly greater increase in serum chromium and cobalt levels than patients with a conventional total hip with a much smaller diameter metal-on-metal bearing.
Systemic toxicity and cancer risk are considered possible disadvantages of metal-on-metal bearings, although no direct links to widespread medical problems have been reported with this bearing combination. Continued observation of patients will be necessary to evaluate the link between these elevated metal levels and any chronic adverse effects.
Metal-on-metal bearings are not recommended for patients with poorly functioning kidneys because metal ions excreted through the kidneys can build up in the blood. Also, while being a pregnant woman or a woman of child-bearing age is not necessarily a contraindication for metal-on-metal bearings, studies continue regarding metal ions crossing the placenta.
Clinical evidence is mounting that osteolysis and implant loosening in total hip patients with metal-on-metal bearings may be associated with hypersensitivity to metallic debris, but the direct scientific link between hypersensitivity and loosening remains to be found. Hypersensitivity has not been a significant problem for implants employing metal-on-plastic bearings, so the assumption is that the increased metal burden caused by both bearings being metallic may be responsible for these hypersensitivity reactions but not a direct cause of loosening.
Metal-on-metal bearings have not been applied to many joints other than the hip. Most other joints require different designs to provide adequate function, and therefore suffer from additional wear mechanisms for which metal-on-metal surfaces have few advantages.
“There is no universal agreement by surgeons on the precise indication for metal on metal bearings,” says Steven B. Haas, MD, Chief of the Knee Service at HSS, “but most surgeons agree that younger patients are the best candidates.”
Dr. Padgett notes that his use of metal-on-metal bearings is directed toward young, primarily male, patients with a long life expectancy, who understand the possible long-term concerns of metal degradation.
Surface replacement with metal on metal is a new technology that has gained a great deal of recent interest. Dr. Haas notes that patients receiving surface replacements must have contemporary metal on metal bearings, but that there is some controversy regarding the long-term potential for these surface replacement techniques.
Hip surface replacement preserves more bone in the patient than conventional hip replacement. This has the potential of being a first-line treatment of painful, disabling arthritis in younger, active patients. If revision surgery is needed, the preservation of bone is an advantage in converting to a full hip replacement device. However, long-term results using current technology are not yet available. Prospective outcome research on hip surface replacement is currently being performed at HSS, which will further define the role of surface replacement surgery.
One of the most commonly used bearing surface combinations for joint replacement is metal on polyethylene, a form of plastic that provides marked durability. Dr. Padgett explains that the two groups most traditionally considered for this material are older adults in their seventies or eighties who are relatively sedentary, and younger, active patients who may subject their joints to activities involving repetitive impact.“From a clinical perspective, metal on polyethylene has the longest-term data available for hip replacement,” says Dr. Padgett. “It’s clearly subject to some degree of wear, which is measurable, but the effects of wear degradation related to metal on polyethylene, more often than not, are local phenomena that can readily be followed radiographically. In other words, you know what you’re getting yourself into.”
The goal of finding new means of reducing wear degradation in polyethylene has lead to some interesting developments in this material in the last two decades.
A substantial leap forward was made with the discovery in the late 1980s of the role of oxidation in the wear performance of polyethylene.
Polyethylene components, like most medical devices, are sterilized by exposure to gamma radiation. Unfortunately, the radiation, while penetrating through the component, has sufficient energy to break the chains that form the molecular backbone of the polymer. If the radiation exposure is performed while the component is exposed to air, the broken ends can react with oxygen, causing harmful changes, including a decrease in molecular weight, a dramatic loss of ductility, and a decrease in strength. The combined effect may make the polyethylene markedly more susceptible to wear.
One important form of alternative bearing surface has emerged simply by removing the chance that the polyethylene can oxidize during the sterilization process. Device manufacturers have accomplished this task in two ways:
Though these alterations in sterilization can eliminate degradation, they do not all have the same beneficial impact on wear. Techniques that eliminate irradiation altogether also eliminate the benefit of the additional cross-linking between the molecular chains in the polymer. Indeed, clinical results show increased rather than decreased wear in hip replacements employing polyethylene components sterilized by exposure to ethylene oxide rather than irradiation.
Perhaps the most significant alteration in polyethylene joint replacement components is the inclusion of elevated levels of radiation, beyond those required to simply sterilize the implant. The goal is to induce even higher levels of cross-linking than occur with the conventional sterilization dose. Significantly reduced wear is one of the advantages of elevated cross-linking. Highly cross-linked polyethylenes have been in clinical use since about the turn of the millennium, and early results show a dramatic decrease in wear of between 30 and 96 percent in total hip replacements over those seen with conventional polyethylene.
This increased wear resistance has also renewed interest in larger femoral heads as a means of reducing the risk of dislocation. With a larger head size, sliding distance between the bearing surfaces is increased and the resulting amount of wear is high, so conventional polyethylene-on-metal bearings are typically small in diameter (32 mm or less). Large head sizes are now available with matching large diameter, highly cross-linked polyethylene acetabular components. But these components are thin – less than five millimeters in some cases – making the strength and toughness of highly cross-linked polyethylenes important considerations.
In fact, changes in mechanical properties that accompany increased cross-linking may pose the biggest threat to the clinical effectiveness of these materials. Increased cross-linking, while decreasing wear, makes the material more brittle. The use of this material in knee replacement is limited. “There are little clinical data on the use of highly cross-linked polyethylenes in knee replacement, and some of the laboratory data raise concern,” notes Dr. Haas. Indeed, the types of motion and wear that occur in knee replacement could lead to fracture or other failure of the highly cross-linked polyethylenes in knees.
Reduced toughness shown in laboratory studies suggests greater susceptibility to fracture under extreme conditions. Indeed, broken highly cross-linked total hip components have been reported, though the occurrence is rare. The clinical relevance of these studies is difficult to interpret, because the fracture conditions in implant components depend on geometry and loading conditions that differ noticeably from that used in the laboratory. Thus, close surveillance of the clinical experience will be needed to establish the prevalence of this problem.
Because of this concern, even newer forms of highly cross-linked polyethylenes are being introduced into the orthopaedic community to improve on this technology. These materials use alterations in the cross-linking and thermal treatment processes to impart both increased wear resistance and improved toughness. Clinical use is currently too limited to draw conclusions about the effectiveness of these newer materials in both hips and knees.
Fully dense ceramics, alumina and zirconia, are used in total joint replacements specifically for the purpose of providing more wear resistant bearing surfaces; they have few other mechanical advantages over metallic alloys. Because of their hardness, ceramics can be polished to a very smooth finish and remain relatively scratch resistant while in use as a bearing surface.
The most significant disadvantage of ceramics is their brittle nature, making them susceptible to fracture. As with the case of metal-on-metal bearings, where improvements in metallurgy have sparked renewed interest, improvements in ceramic quality have led to increased interest in ceramic bearings. Increased chemical purity and reduced grain size have lead to increased strength and a dramatic reduction in the number of fractures seen clinically. Nonetheless, strength and toughness remain issues, particularly in acetabular components of ceramic-on-ceramic hip replacements, where impingement of the femoral and acetabular components at extreme ranges of motion can lead to fracture.
With this in mind, most patients whose active lifestyles subject them to repetitive impact are not good candidates for ceramic bearings. Dr. Padgett finds that most patients in their fifties who are looking to remain actively engaged in less-rigorous sports such as golf are typically a good fit for this type of implant.
Three types of ceramic bearing materials are commercially available: alumina, zirconia, and an oxidized zirconium material. Alumina and zirconia components have been used for decades, mostly as femoral heads in total hip replacements. Ceramic-on-polyethylene bearings have been commercially available for some time as alternatives to metal-on-polyethylene. Recently, an oxidized zirconium material has been introduced into both hip and knee replacement components for use against polyethylene; early results are available with this material show excellent wear results.
Long term experience with alumina-on-polyethylene bearings for hip replacement shows reduced wear rates over those typically seen with metal-on-polyethylene bearings, with an associated decrease in osteolysis. Alumina-on-polyethylene bearings in knee replacements have found much more limited use, all of it outside the United States. Only mid term results are available. The absence of direct comparisons with conventional metal-on-polyethylene bearing surfaces of the same design and the lack of long term results make it difficult to assess the clinical benefits of alumina knee replacement components.
The use of zirconia as a bearing surface against polyethylene has not proven as successful clinically as alumina-on-polyethylene bearings. A direct comparison among alumina-, zirconia-, and metal-on-conventional polyethylene bearings in total hip patients revealed the highest wear rate in the zirconia group. The tendency of zirconia - which is as wear resistant as alumina - to change its crystalline structure to a form that is less tough and more prone to wear, is a disadvantage of this material. Examination of the bearing surfaces of failed zirconia implants showed this flaw.
Alumina-on-alumina total hip replacements have been used more extensively in Europe than in the United States.
In general, alumina-on-alumina joints have shown very low wear rates clinically, though the results are design dependent. Recent reports also show excellent wear resistance in young patients, with no measurable wear and no evidence of osteolysis even beyond a decade of follow-up. Furthermore, very few implant fractures have been observed, even in this high demand patient population, lending further credence to the improved mechanical properties of alumina.
One unusual issue affects a small number of patients who have alumina-on-alumina bearings: reports of an audible squeaking sound during normal ranges of motion after surgery. Studies have shown that 1-10% of these patients may experience this problem. While ceramic bearings have been used extensively in hips, the use of ceramic bearings in other joint replacements is much more limited. For joint designs such as knee replacements that require bearings with non-conforming surface shapes to provide the patient with adequate function, the advantages of ceramic-on-ceramic bearings are unclear.
Alumina-on-alumina bearings for knee replacement have been used primarily in Japan. Evidence for the effectiveness of these bearings is very limited, given the short length of follow-up, the lack of comparative data for other bearing surfaces in the same design, and patient selection (for example, one study was limited to only rheumatoid arthritis patients).
Oxidized zirconium, popularly known as Oxinium (which is a brand name applied by orthopedic device manufacturer Smith & Nephew), is a metal-ceramic hybrid technology. Zirconium alloy, a metal, is treated with high pressure and heat in the presence of oxygen; this process converts the metallic surface of the implant to a ceramic. This leads to an implant that has surface properties of ceramic, which is harder, smoother, and has less friction than the metal.
Oxinium is used for femoral heads in hip replacement and the femoral component in knee replacement. Although clinical experience has been promising, Dr. Haas indicates that further testing is necessary. “The two- and five-year clinical data on implant survival and wear in knee replacement have been positive, but longer term studies will be needed to prove that the improved bearing properties of oxidized zirconium lead to longer lasting implants.”
The future of joint replacement may see the implementation of other intriguing surface materials. Surfaces made of diamond have been produced recently, and there is much discussion of the promise of hardened titanium, due to its smooth exterior.
“The final frontier [in man-made joint replacement] is the longevity of the implant, to prevent the problem of osteolysis,” explains Edwin P. Su, MD. “The alternate bearings are an attempt to bring it to the next level and get a further increase in longevity by decreasing the wear degradation problems.”
Is there one best bearing combination that fits all joints in all patients? Probably not. “My own philosophy is that I will use all types of surfaces, recognizing that there are advantages and disadvantages,” explains Dr. Padgett. “At the end of the day, we have to customize what we do for patients on an individual basis.” The age of the patient, the patient’s expected activity levels, and the extent of the patient’s joint problems are among a long list of factors that the surgeon must consider in choosing an implant. And despite the concern over wear and osteolysis, it must be remembered that joint replacement is a highly successful treatment with few complications.
Perhaps one set of bearing materials will eventually emerge as the best for each joint in the body. That will depend on clinical experience as well as continuing research and development to understand the biological consequences of wear and to adopt further improvements in materials as they become available. For now, the decrease in wear afforded by the new bearing surfaces represents an impressive milestone in the further improvement of total joint replacement.
Summary by Dr. Wright with contributions by Mike Elvin