A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements.

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.

Wear of gamma-crosslinked polyethylene acetabular cups against roughened femoral balls.

Crosslinking of ultrahigh molecular weight polyethylene has been shown to markedly improve its wear resistance in clinical studies and laboratory tests using hip joint simulators. However, because most of the laboratory studies have been done under clean conditions using prosthesis-quality, highly polished counterfaces, there is concern regarding how well an intentionally crosslinked polyethylene acetabular cup will resist abrasion by a femoral ball that has been damaged by third-body abrasion in vivo. To investigate this, conventional and radiation crosslinked-remelted acetabular cups of ultra-high molecular weight polyethylene were tested in a hip joint simulator bearing against smooth femoral balls and against balls with moderate and severe roughening. Cups were tested with and without aging to accelerate any oxidative degradation. The crosslinked cups were produced by exposing extruded GUR 4150 bar stock of ultrahigh molecular weight polyethylene to 5 Mrad gamma radiation under a partial vacuum and then the bars were remelted to extinguish residual free radicals. Artificial aging at 70 degrees C under 5 atm oxygen for 14 days induced negligible oxidation in the crosslinked and remelted material. Against smooth balls, the wear of the crosslinked cups, with or without aging, averaged approximately 15% of that of the conventional cups. Against the moderately rough balls, the wear rate of the conventional cups was unchanged, whereas the wear rate increased slightly for the nonaged and aged crosslinked cups, but was still only 26% and 20% of that of the conventional cups, respectively. Against extremely rough balls, the mean wear rates increased markedly for each material such that during the final 1 million cycle interval, the average wear rates of the nonaged and the aged crosslinked cups were 72% and 47% of that of the conventional cups, respectively. That is, the crosslinked polyethylene showed substantially better wear resistance than conventional polyethylene across the range of ball roughnesses, with or without accelerated aging.

Quantification of third-body damage and its effect on UHMWPE wear with different types of femoral head.

We examined stainless-steel, cobalt-chrome, titanium and alumina and zirconia ceramic femoral heads retrieved at revision surgery. All the heads had articulated against ultra-high-molecular-weight-polyethylene (UHMWPE) acetabular cups. We studied the simulation of third-body damage and the wear of UHMWPE against the various materials used for the heads. The surfaces of the retrieved heads were analysed using a two-dimensional contacting profilometer. Third-body damage was characterised by the mean height of the scratches above the mean line (Rpm). The alumina ceramic and zirconia ceramic retrieved heads were found to have significantly less damage. In laboratory studies the ceramics were also more resistant to simulated third-body damage than the metal alloys. We studied the wear of UHMWPE against the damaged counterfaces in simple configuration tests. The damaged ceramics produced less polyethylene wear than the damaged metal counterfaces. The wear factor of UHMWPE against the damaged materials was dependent on the amount of damage to the counterface (Rp). Our study has shown the benefit of using the harder and more damage-resistant ceramic materials for femoral heads.

The wear of ultra-high molecular weight polyethylene sliding on metallic and ceramic counterfaces representative of current femoral surfaces in joint replacement.

A number of studies have investigated the influence of surface roughness on the wear of ultra-high molecular weight polyethylene (UHMWPE) in total joint replacement. The results of these studies have shown that the wear factor is proportional to the counterface roughness raised to a power greater than one. In this laboratory study, the effect of surface finish of several biomaterials on the wear of UHMWPE was studied. The study was conducted using reciprocating pin-on-plate wear tests with bovine serum as a lubricant. The biomaterials investigated as the counterface material included stainless steel, cast cobalt chrome (CoCr), CoCr (ASTM F799), alumina ceramic and zirconia ceramic. The counterface topographies of the wear plates were produced using techniques representative of current manufacturing methods. The surface roughness of the wear plates was varied in the range Ra = 0.005-0.04 micron; this was representative of femoral heads and femoral knee components currently used clinically. Metals and ceramics with a similar surface roughness produced a similar wear rate of UHMWPE. For the limited range of smooth counterfaces used in this study only a moderate correlation was found between the surface roughness and the wear factors. For a change in counterface roughness Ra of 0.005 to 0.04 micron, the wear factor increased from 7.4 +/- 1.6 to 16.5 +/- 2.4 x 10(-9) mm3/N m (mean +/- standard error). This variation in counterface roughness had much less effect in wear than previously reported for rougher counterfaces. For an extended range of counterface roughness, a stronger correlation was found using an exponential function for the regression fit. The exponential function shows the benefits of decreased wear with decreased surface roughness. Although the wear rate decreased less rapidly with decreased counterface roughness for Ra values below 0.05 micron, there were significant advantages to be gained from improved femoral head roughness to below 0.01 micron Ra.