Which Bearing Surface Should Be Used: Highly Cross-Linked Polyethylene versus Metal or Metal versus Ceramic on Ceramic?

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Chapter 84 Which Bearing Surface Should Be Used: Highly Cross-Linked Polyethylene versus Metal or Metal versus Ceramic on Ceramic?

RICHARD A. HOCKING, BSc (MED), MBBS (Hons), FRACS (orth), STEVEN J. MACDONALD, MD, FRCSC

Total hip replacement changes the lives of more than one million patients worldwide each year. In wellselected patients, the success of the implant has historically meant that for nearly 90% of patients, their primary hip replacement is their only hip replacement.¹ This level of success has been reached with multiple implants including cemented monoblock femoral stems with conventional cemented, all-polyethylene, acetabular components of several different designs. If this is the case, why are we looking for alternative bearings?

First, the levels of success in historical data were achieved only in select groups of patients. Many historical studies had an inbuilt selection bias. Surgeons were aware that there were certain diagnoses that fared poorly with the implants and techniques available to them, and in particular, younger patients did not respond as well in terms of survivorship of the implant. As such, patients who were perceived to be at high risk for unsuccessful results were not offered a total hip replacement. National registries are probably the best tool for observing the effect 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, they have observed an overall diminishing revision burden, which they speculate may be caused by better surgical training, an improvement in the performance of modern implants compared with their historical counterparts, and surgeons modifying their practice to use combinations of implants that have performed well in the registry.² Despite these favorable conditions for hip arthroplasty, there has been a relative increase in the revision rate of total hip replacements performed for inflammatory arthropathies, childhood hip disorders, and post-traumatic arthritis²—all diagnoses found predominantly in young people having total hip arthroplasties.

Second, patient demands have significantly changed since the introduction of total hip arthroplasty. Today’s patients and surgeons are unlikely to proceed with operations such as arthrodesis and resection arthroplasty. Patients perceive that total hip replacement is almost universally successful and offers a return to near-normal function. This perception has contributed to the fact that greater numbers of younger patients are seeking total hip arthroplasty than ever before; in addition, the activity level and expectations of all patients undergoing total hip replacement has significantly increased.

Therefore, patients not only expect to be more active than their historical counterparts, data demonstrate that these patients will live longer. All of this leads to greater tribological loads on the bearing surface; hence, there have been concerted efforts by the orthopedic scientific community to develop new bearing surfaces that will meet the demands of the patients who are currently seeking total hip replacement. To this end, advances have been made with polyethylene-bearing surfaces with the development of highly cross-linked polyethylene, and there has been renewed interest and advances in both engineering and the understanding of metal on metal in both total hip and resurfacings and ceramic-on-ceramic bearings.

WHAT ARE THE TRIBOLOGICAL DEMANDS ON TOTAL HIP ARTHROPLASTY ARTICULATIONS?

Historically, wear particles from conventional polyethylene gamma irradiated in air, coupled with small-diameter metal or ceramic heads, has led to osteolysis and failure of the components after 5 to 15 years of use.^3,⁴ In a study of Charnley hips retrieved at time of revision for aseptic loosening,⁵ the mean time to revision was 12 years, and the mean volumetric wear at failure was 785 mm.⁴ The same polyethylene had been tested in another study using a hip simulator, and this degree of volumetric wear was obtained after 12 years of wear when it was assumed that 1.5 million steps were taken in a year.^6,⁷ Assuming 1.5 million steps per year as average use of the joint replacement, conventional articulations should have a total lifetime of between 10 and 20 million cycles before failure.

Young people have higher levels of activity than older patients by up to an additional million steps per year.⁸ They should also expect to live 20 to 40 years longer than the conventional recipients of total hip replacements. Younger patients are also planning to return to more vigorous activities with their hip replacements, and as such, we would expect that they would be at a greater risk for polyethylene failure and osteolysis, as well as an increased risk for dislocation than their historical counterparts. To improve both range of motion and stability, larger and larger diameter femoral heads have been developed.⁹ A progression has occurred from the 22.25-mm to the 26-, 28-, 32-, and now 36- and 38-mm metal heads for use in metal/poly articulations. These larger heads have increased wear associated with the increased surface area of the articulation.^4,¹⁰ In hip simulator data, this is seen with conventional polyethylene and significantly reduced with highly cross-linked polyethylene. Even with the biggest heads, the wear of the highly cross-linked polyethylene is still significantly less than the historical wear rates of smaller heads against conventional polyethylene. Metal-on-metal articulations have even greater available bearing diameters and are now up to 44 mm with conventional stems and shells, and anatomic head sizes with surface replacement components.

Thus, the lifetime tribological demand that young people 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 the younger or more active patient, there would need to be at least a 10-fold improvement in the wear characteristics of the alternative bearing couple compared with conventional polyethylene for the bearing couple to survive without need for revision. This is the driving force behind the development of alternative bearing surfaces.¹¹

Evidence

Hip simulator studies have been used extensively since the late 1990s to support the research and development of new bearing systems for hip prostheses.¹²^–¹⁵ Total hip replacements often fail because of polyethylene wear debris-induced osteolysis.¹⁶ Most studies comparing cross-linked polyethylene and conventional polyethylene have concentrated only on volumetric wear. Osteolysis is, however, dependent on the size, shape, and chemical activity of the wear debris generated.¹⁷ To compare alternative bearing surfaces, one must determine the relative biological activity of the debris generated and then relate it to the volumetric wear of the articulation. This then gives a means of directly comparing the tribological and biological performances of different alternative bearings.^5,¹⁷^–²⁵

CROSS-LINKED POLYETHYLENE

Ultra-high-molecular-weight polyethylene comprises 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 amount of cross linking is determined by the type of irradiation the polyethylene is exposed to, the dosage of radiation applied, the atmosphere in which the polyethylene is exposed to the radiation, and the postradiation treatment of the polyethylene.²⁶ Cross linking improves the resistance of the polyethylene to adhesive and abrasive wear. This should result in decreased linear and volumetric wear in cross-linked polyethylene articulations compared with conventional polyethylene.

In hip simulator studies, there is an eightfold reduction in volumetric wear of cross-linked polyethylene compared with conventional polyethylene.^19,²⁴ Improvements in wear rate by an additional 40% are seen when ceramic heads are used against the cross-linked polyethylene.^19,²⁴ In clinical studies, a reduction in linear and volumetric wear has been seen using cross-linked polyethylene ranging from 28% to 95%.²⁶^–³⁰ 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.^19,²⁴ Thus, even though there is an eightfold reduction in volumetric wear, there is only a fourfold improvement in the functional biological activity of the bearing couple. Thus, in vitro, cross-linked polyethylene articulations do not reach the 10-fold improvement in wear characteristics that are estimated to be required by younger patients having 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.

In a randomized, prospective study with minimum 4-year radiographic follow-up, the incidence of osteolysis was significantly less in the cross-linked arm of the study.²⁷ This observation was more pronounced on the femoral side where the difference between the cross-linked and noncross-linked group was highly significant (P = 0.001). However, when the volume of the osteolytic area was examined, one finds that although the total volume of osteolysis was less in the cross-linked group, the difference was not significant (P = 0.4). In another 5-year retrospective study comparing cross-linked and conventional polyethylenes, there were no osteolytic lesions observed around the femoral components in the cross-linked group²⁸ (Level of Evidence IV).

Thus far we have concentrated on the improved material property of wear resistance that cross-linked polyethylene has compared with conventional polyethylene. There are, however, downsides to crosslinking polyethylene, namely, the physical properties of yield strength, overall tensile strength, and resistance to elongation or crack propagation are all reduced.³¹ These physical properties are important to the overall survival of the implant because they each represent a mode through which the implant may fail. One needs to remember that wear-related osteolysis and aseptic loosening are not the only reasons that an implant may fail. Impingement of the neck of the femoral component on the polyethylene insert may result in catastrophic failure caused by fracture of the polyethylene. Fatigue failure of the locking mechanism may lead to loosening of the insert within the modular shell again, causing catastrophic failure.³¹ These failure mechanisms are rarely seen with conventional polyethylene; however, the diminished mechanical properties of cross-linked polyethylene raise concerns that failures because of these mechanisms may become more frequent.³² A balance therefore needs to be reached between the improved wear properties and the reduced physical properties of cross-linked polyethylene. To date, this balance has not been determined.