CHAPTER SYNOPSIS
Three alternate bearing choices to conventional polyethylene currently are used in North America. The first is highly cross-linked polyethylene, which is now entering its second decade of clinical use. The second is metal on metal, which has clinical applications that date back to the 1960s, but as a bearing has seen significant changes and advances over many years. The third is ceramic on ceramic, which again has decades of clinical use and similarly has undergone significant changes and improvements over this period. This chapter reviews the applications of each bearing and its pros and cons pertaining to primary total hip replacement.
IMPORTANT POINTS
- 1
Considering the wear, patient demand, and functional stability of each bearing is paramount in matching one to a patient.
- 2
The cross-linking of highly cross-linked polyethylene has improved wear characteristics, which improves the polyethylene’s resistance to adhesive and abrasive wear, with an expected reduction in both linear and volumetric wear compared with conventional polyethylene.
- 3
Ceramics have seen a significant advance in manufacturing with the process of hot isostatic pressing, resulting in a much improved ceramic product from those used a decade ago.
- 4
Metal-on-metal bearings have wear properties that depend on the exact composition of the alloy, the clearance between the components, the diameter of the articulation, the time after implantation, and orientation of the components.
CLINICAL/SURGICAL PITFALLS
- 1
Simply relying on a large-diameter head for stability should be avoided if a small amount (less than 6 mm) of crossed-linked polyethylene cannot be used or if using a large metal-on-metal head that is not aligned appropriately for stability, which can increase wear particle production.
INTRODUCTION
Total hip replacement clearly is one of the most successful surgical procedures available. Globally more than 1 million patients annually receive a total hip arthroplasty. The long-term success of the majority of these implants has been excellent, and in approximately 90% of patients their primary hip replacement is their only hip replacement and will last a lifetime. These results have been reproduced with many implant designs from the earliest prostheses, including cemented monoblock femoral stems with conventional cemented all-polyethylene acetabular components, to the many cementless femoral and acetabular components used in North America during the past 3 decades. With this being the case, questioning the drive to look at alternate bearings in total hip arthroplasty would be reasonable.
Of note, the levels of success in historic data were only achieved in a select groups of patients. Many historic studies had an inbuilt selection bias, with surgeons being aware that certain diagnoses fared poorly with the implants and techniques available to them. This was particularly the case with younger patients, who demonstrated decreased implant survivorship. Patients who were perceived to be at higher risk of failure were not offered a total hip replacement. National registries are an excellent resource when evaluating the effects that age and diagnosis have on the revision rate. The Swedish registry has been collecting data relating age and diagnosis to the success of hip arthroplasty since 1979. Over this period an overall diminishing revision burden has been observed, which has been speculated to be attributable to several interrelated factors: better surgical training, an improvement in the performance of modern implants compared with their historic counterparts, and modifications to surgical practice to use combinations of implants that have performed well in the registry. Despite these favorable conditions for hip arthroplasty, the revision rate has relatively increased for total hip replacements performed for inflammatory arthroplasties, childhood hip disorders, and posttraumatic arthritis —all diagnoses that affect young people undergoing total hip arthroplasties. The effect of age and implant longevity takes years to become evident in patients receiving modern total hip replacements. The 2007 Australian National Joint Replacement Registry, reporting on 104,234 primary total hips, demonstrates that at 6 years’ follow-up no statistical difference in the revision rate is seen with regard to age. However, long-term series have previously demonstrated that this effect takes many years to become evident.
Patient demands have significantly changed over the 40-year history of total hip arthroplasty. In the past, procedures such as arthrodesis and resection arthroplasty were accepted by both patients and surgeons. In 2008 both of these procedures are basically of historic interest and only used for the rarest of indications. Patients perceive that total hip replacement is almost universally successful and offers a return to near-normal function. Although this is true for the majority of patients, it has lead to greater numbers of younger patients seeking total hip arthroplasty. In addition, surgeons have more confidence in current implants and are willing to use them in these younger patients. A further challenge, however, is that many of these patients have higher activity levels and expectations than their historic counterparts.
A dual challenge exists, however, because data demonstrate these younger and more active patients will live longer. The combination of higher activity levels and increasing life span leads to a greater tribologic load on the bearing surface. This has been a significant driver to the orthopedic scientific community in developing new bearing surfaces that will meet the demands of patients who are currently seeking total hip replacement.
Three alternate bearing choices to conventional polyethylene currently are used in North America. The first is highly cross-linked polyethylene, which is now entering its second decade of clinical use. The second is metal on metal, which has clinical applications that date back to the 1960s, but as a bearing has seen significant changes and advances over many years. The third is ceramic on ceramic, which again has decades of clinical use and similarly has undergone significant changes and improvements over this period.
ISSUES DRIVING ALTERNATE BEARING DEVELOPMENT
Wear
Historically, wear particles from conventional polyethylene gamma irradiated in air, coupled with small-diameter metal or ceramic heads, have lead to osteolysis and the potential for component failure. In studies of retrieved Charnley implants that had failed because of aseptic loosening, the mean time to revision was 12 years and the mean volumetric wear at failure was 785 mm. That same polyethylene, tested in other studies with a hip simulator, demonstrated that same degree of volumetric wear after the equivalent of 12 years of wear, assuming that 1.5 million steps are taken in a year. Assuming 1.5 million steps per year as an average use of the joint replacement, conventional polyethylene would have an expected longevity of approximately 20 million cycles, or less than 15 years, before the potential for component failure. Conventional polyethylene alone can therefore not predictably provide patients with a long-lasting bearing; this has been a significant impetus in alternate bearing development.
Patient Demands
Younger patients have higher levels of activity than older patients by up to an additional 1 million steps per year. In addition, the younger the patient, the longer is the predicted life expectancy. Younger patients also may have different expectations and a desire to return to more vigorous activities with their hip replacements. This combination of factors clearly leads to the potential for a greater risk of polyethylene failure and osteolysis. In addition, patients of all ages have increased expectations and demands of their prostheses that were not present decades ago. This changing patient profile has been another driver in the emergence of alternate bearing couples in total hip arthroplasty.
Instability
Hip instability and dislocation are significant postoperative concerns to both patients and surgeons alike. Although many authors quote a dislocation rate of 1%, a Medicare database review demonstrated that the 6-month dislocation rate was actually 3.9%. To improve both range of motion and stability, larger and larger diameter femoral heads have been developed. The original 22.25-mm metal head has increased to the 26-mm, 28-mm, 32-mm, and now 36-mm and 38-mm metal heads for use in metal/polyethylene articulations. These larger heads have increased wear associated with them because of the increased surface area of the articulation. This correlation of increasing wear of conventional polyethylene with increasing femoral head size has been demonstrated in both hip simulator and clinical data. Once again patient and surgeon desires and needs, in this case for improved hip stability, highlight the deficiencies of conventional polyethylene and lead to the evolution of bearings that do not demonstrate this linear relation of wear to increasing head diameter. Metal-on-metal and ceramic-on-ceramic bearings, and to a lesser degree cross-linked polyethylene, meet these demands.
Current estimates are that the lifetime tribologic demands that younger patients place on their hip replacements may have increased to between 100 and 200 million cycles. If a single implanted articulation is to cope with the expected increase in demand of these younger and more active patients, at least a 10-fold improvement in the wear characteristics of the alternative bearing couple would be needed compared with conventional polyethylene for it to survive without need for revision.
The development of alternate hip bearings has been intimately coupled to excellent basic science research. In particular, hip simulator studies have been extensively used over the last decade to support the research and development of new bearing systems for hip prostheses. The most common cause for failure of total hip replacements is widely accepted to be polyethylene wear debris-induced osteolysis. The majority of the research efforts have therefore been focused on the accurate measurement and reduction of linear and volumetric polyethylene wear. However, the development of osteolysis is also well understood to depend on the size, shape, and chemical activity of the wear debris generated. Therefore, to compare alternative bearing surfaces, the relative biologic activity of the debris generated must be determined and then related to the volumetric wear of the articulation. This then provides a means of directly comparing the tribologic and biologic performance of different alternative bearings.
A critical point to bear in mind when reading the summaries of the three alternate bearings is the tremendous variability in manufacturing techniques among implant manufacturers. This applies to all the bearings, but particularly to highly cross-linked polyethylene and metal on metal. These should be viewed as classes of bearings and not a single bearing because the results from one design type may be quite different from another. Generalizations would be a significant error when reading the results of any one study. It would be akin to seeing the results of carbon-reinforced polyethylene and concluding that all forms of polyethylene are flawed. Only with a thorough understanding and working knowledge of the exact manufacturing of these bearings and the variations present can we begin to compare the results among devices and move the science of bearing technologies forward.
Highly Cross-Linked Polyethylene
As a material, ultra-high-molecular-weight polyethylene (UHMWPE) is composed of long chains of polyethylene molecules, some in parallel and others in random orientation. When the polyethylene molecules are exposed to gamma irradiation, free radicals are released. With the release of free radicals from adjacent polyethylene molecules, two chains may link at the site from which the free radicals were released. The degree to which cross-linking occurs is influenced by many factors: the type of irradiation to which the polyethylene is exposed, the amount of radiation applied, the atmosphere in which the polyethylene is exposed to the radiation, and the postradiation treatment of the polyethylene. Cross-linking improves polyethylene’s resistance to adhesive and abrasive wear, with an expected reduction in both linear and volumetric wear compared with conventional polyethylene.
Historically, cross-linking of polyethylene has been present for years as a byproduct of the sterilization method when polyethylene was exposed to low doses of radiation (2.5 mrads). Therefore conventional polyethylene that was sterilized in this fashion, although being known as simply “conventional polyethylene,” truly is low-dose, cross-linked polyethylene. By increasing the radiation exposure (range, 5 to 10 mrads), the polyethylene cross-linking is significantly increased; this polyethylene is referred to as highly cross-linked polyethylene .
The intentional cross-linking of polyethylene is not a new concept. Grobbelaar et al demonstrated minimal wear in 56 of 64 cases at long-term follow-up with acetabular components gamma irradiated with 10 mrads. An extremely high dose (100 mrads) of gamma irradiation in air was used by Oonishi and Kadoya, who reported a significant reduction in wear compared with a control group that had polyethylene sockets that were not irradiated. Oonishi et al more recently reported a very low wear rate with polyethylene irradiated at lower levels of radiation (6.0 mrads). Lastly, Wroblewski et al reported on polyethylene that was chemically cross-linked and articulated against ceramic heads, demonstrating a significant reduction in wear rates.
Beginning in the late 1990s, most orthopedic equipment manufacturers began producing highly cross-linked polyethylene with techniques very different from those discussed in the earlier series. Tremendous variability in the manufacturing processes of the polymer itself were present, as well as the methods to reduce or eliminate the potential for oxidative degradation through an annealing or remelting process. Despite these differences, the claims of wear reduction were remarkably consistent ( Table 13-1 ). Therefore, when referring to polyethylene as highly cross-linked, an understanding that this is a class of devices that may perform very differently in vivo over time is crucial. An additional challenge is that before 10-year results were published on any given highly cross-linked polyethylene, most manufacturers have released, or soon will, their next generation of this material, the so-called second-generation highly cross-linked polyethylene . These polyethylenes were developed to improve the mechanical properties, improve the wear resistance, and decrease the potential for polyethylene oxidation seen in some first-generation highly cross-linked polyethylenes. Although advances in understanding of the material properties in vitro have occurred, the long-term clinical results will be critical to the evaluation of this bearing.
| Implant | Marathon (DePuy) | XLPE (Smith & Nephew) | Longevity (Zimmer) | Durasul (Sulzer) | Crossfire (Stryker-Osteonics-Howmedica) |
|---|---|---|---|---|---|
| Starting Product | Extruded bars | Extruded bars | Compression molded sheets | Sheets to Pucks | Extruded bars |
| Radiation | Gamma (5Mrads) | Gamma (5Mrads) | Electron beam (10Mrads) | Electron beam (9.5Mrads) | Gamma (7.5Mrads) |
| Thermal Stabilization | Remelted (155° x 24hrs) | Remelted (150° x 2hrs) | Remelted (150° x 2hrs) | Remelted (150° x 2hrs) | Annealed 120° |
| Final Sterilization | Gas plasma | Ethylene oxide | Gas plasma | Ethylene oxide | Gamma 2.5 – 4Mrads in Nitrogen |
| Claimed Wear Reduction | 85% | 90% | 89% | >95% | 90% |
In hip simulator studies there is up to an eightfold reduction in volumetric wear of highly cross linked polyethylene compared to conventional polyethylene. Improvements in hip simulator wear rates by an additional 40% may be seen when ceramic heads are used against the highly cross linked polyethylene. In clinical studies, a reduction in linear and volumetric wear has been variable, ranging from 28 to 95%. These observations would suggest that if volumetric wear alone determined osteolysis potential, cross linked polyethylene should be a much better articulation than conventional polyethylene.
Unfortunately cross linked polyethylene produces wear debris that theoretically may have a specific biological activity double that of conventional polyethylene. This is an estimate by some authors and refuted by others. One could therefore use this argument to state that although there is an eightfold reduction in volumetric wear, there may only be a fourfold improvement in the functional biological activity of the bearing couple. Again, at this point, the clinical data is simply not there. However, in vitro, cross linked polyethylene articulations may not reach the ten fold improvement in wear characteristics that would be ideal and might be required by younger patients undergoing total hip replacement. In vitro experiments thus predict a reduction in osteolysis when highly cross linked polyethylene is used, however the reduction may not be proportionate to the observed reduction in the volumetric wear.
The mid-term results of several studies have demonstrated reduced wear with cross linked polyethylene, but similar clinical results between conventional and cross linked polyethylene groups when a control group is present. Similarly differences in osteolysis development have been difficult to see between groups. Once again, at only mid-term followup, this is an expected result as both groups are usually well below the 0.1mm/year of polyethylene wear normally required to produce osteolysis (the “osteolysis threshold” ). However employing a CT technique, Leung et al, have recently reported a decreased volume of pelvic osteolysis in a cross linked polyethylene cohort, compared with a conventional polyethylene group.
As with all alternate bearings there are pros and cons, and highly cross linked polyethylene is no exception. Secondary to the cross linking techniques, the physical properties of yield strength, overall tensile strength, and resistance to elongation or crack propagation are all reduced. These physical properties are important to the overall survival of the implant as they each represent a mode through which the implant may fail. While a focus has been maintained on wear related osteolysis and aseptic loosening, it is critical to bear in mind that an implant may also fail secondary to mechanical causes. The highly cross linked polyethylenes have an increased risk of fracture. This is exacerbated in cases of cup malposition, in combination with very thin liners, when the force on the rim of the polyethylene is significant. Additionally, any femoral component to polyethylene impingement increases the probability of a fractured polyethylene insert. Another possible failure mode is a fatigue failure of the locking mechanism that could lead to secondary loosening of the polyethylene insert within the modular shell resulting in a component failure. These failure mechanisms are rarely seen with conventional polyethylene; however the diminished mechanical properties of cross linked polyethylene raise concerns that failures due to these mechanisms may become more frequent. Therefore it is this balance, between the improved wear properties and the reduced mechanical properties of cross linked polyethylene that manufacturers and researchers continue to explore and understand.
Metal on Metal
The concept of a metal on metal bearing being used in a hip arthroplasty application dates back more than 60 years. In the 1960s a number of papers discussed the early results of its clinical use. These bearings have been followed for decades, and therefore of all the alternative bearings, metal on metal has data with the longest follow-up, with clinical data available on the McKee Farrar prosthesis with nearly 30 years follow-up. Currently there are two main iterations of the metal on metal articulation in North America: a modular metal insert which fits into a traditional press fit shell and mates to a metal femoral head and a femoral component or a non modular monoblock acetabular component of the resurfacing variety which couples to a large diameter resurfacing type femoral head which is either the femoral component of a resurfacing system or is a modular femoral head that links to a femoral component.
While early metal on metal implants were initially quite popular in terms of clinical use, they gradually fell out of favour as the early results of the Charnley prosthesis were published and were excellent. The early metal on metal implants also had less than ideal designs. There were issues with inadequate clearances, component impingement, poor manufacturing tolerances and poor material selection. Beginning in the late 1980s there was a resurgence in interest in metal on metal bearings with the introduction of so-called second generation metal on metal implants that attempted to address the original design concerns.
Metal on metal articulations have the most complex tribology of the alternative bearings. The wear properties are dependant on the exact composition of the alloy, the clearance between the components, the diameter of the articulation, the time after implantation, and orientation of the components. In simulator studies, high carbon alloys have been compared with low carbon alloys and are clearly superior with low carbon alloys having a 6 times greater volumetric. From this observation the investigators recommended that low carbon alloys should not be used in metal on metal articulations. This has been supported by other authors. Many of the original second generation metal on metal designs did indeed use a high carbon liner, but a conventional low carbon femoral head was also used. As discussed, this bearing couple is not ideal and may compromise long term results of these earlier designs. Metal on metal articulations feature a significant running in phenomenon. In the first million cycles, the articulation has a significantly increased wear rate compared with the steady state wear rate observed after this. Paradoxically, in contradistinction to metal on polyethylene bearings, articulations of larger diameter have less volumetric wear than smaller articulations. This applies both to the wearing in and the steady state wear rates. Lubrication analysis revealed that as the head size increased, the potential to achieve fluid film lubrication also increased, and resulted in reduced wear.
Radial clearance has also been examined as a contributor to the volumetric wear rate. As the radial clearance increases from 20 to 150 microns and beyond, the volumetric wear in both the running in and the steady state phase increases. Lubrication analysis revealed that as radial clearance increased, fluid film decreased which explains the worsening wear rates for increased radial clearance. The ideal radial clearance is a much debated topic between implant designs, but most accept the premise that as the clearance increases, so does the potential for increased wear, if all other factors are kept equal.
The clinical results of current metal on metal bearings have been equal to those seen with metal on polyethylene designs. It would be reasonable to assume that it will not be until the second decade of clinical use that any differences between bearing results would begin to emerge. The challenge will be that the articulation designs continue to undergo evolutions and improvements and current design results may not equate to those earlier versions.
Metal on metal articulations have the advantage that they produce low volumetric wear rates, that increasing the diameter of the articulation improves the volumetric wear rates even more, and the articulations can be manufactured in a large variety of diameters as either conventional total hip replacements or surface replacements. Is this then the ideal bearing surface for the young adult who intends to be active with their total hip replacement? What are the disadvantages of metal on metal articulations?
A concern with the use of metal-on-metal bearings continues to be the elevated levels of metal ions measurable in patients’ blood and urine following implantation, particularly cobalt and chromium. Many studies have evaluated the circulating heavy metal ions ( Table 13-2 ). There are, however, multiple complex issues associated with the analysis of metal ions, including collection techniques, analysis, statistical methodologies, and reporting of results. To date, the literature on this topic has been characterized by significant variability in all of these factors. Attempts have been made to standardize these methods but significantly variability continues in current publications. Between implants, while measured ion levels do vary, there is quite a consistency to the results ( Table 13-2 ). It should be emphasized, that despite the concerns, there has not been a single report of an adverse event secondary to these elevated metal ions in the blood and urine of patients having received a metal on metal bearing. While the concern regarding metal ion carcinogenicity has been expressed, over 700,000 metal on metal total hip arthroplasties have been performed globally with no causal links to cancer.
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