During the 1970s and 1980s, the great majority of hip prostheses in clinical use incorporated a polyethylene acetabular liner bearing against a femoral ball of metal or ceramic. By the early 1990s, it had become apparent that the most common cause of failure of this type of hip prosthesis was an osteolytic reaction to the hundreds of billions of sub-micrometer polyethylene particles generated by wear1, and several types of alternative bearings were introduced with, hopefully, substantially greater resistance to wear. These alternatives included highly crosslinked, thermally stabilized polyethylenes (XL polyethylenes) against metal or ceramic, metal-on-metal, and ceramic-on-ceramic. In their study, Milošev and colleagues compared the clinical performance of a single type of hip prosthesis (an SL-PLUS grit-blasted titanium alloy femoral stem and a BICON-PLUS grit-blasted commercially pure titanium acetabular shell [Plus Orthopedics, Rotkreuz, Switzerland, now Smith & Nephew Orthopaedics]) using three types of bearings: either a conventional polyethylene (i.e., gamma-radiation sterilized in a nitrogen atmosphere) bearing against a stainless steel ball (MoP), a cobalt-chromium-molybdenum alloy liner and ball (MoM), or a Biolox forte alumina ceramic (CeramTec, Plochingen, Germany) liner and ball (CoC), all with 28-mm diameters.
In a maximum clinical follow-up of ten years, with revision for any reason as the end point, the survivorships were 0.984, 0.956 and 0.879 for the hips with the MoP, CoC, and MoM designs, respectively. With revision for aseptic loosening as the end point, the survivorships were 0.995, 0.990 and 0.894, respectively. Despite the use of conventional polyethylene, only one of the MoP hips was revised due to aseptic loosening. The single greatest cause of revision of the CoC bearings was component fracture (four of nine revisions; 1.8% of the population), with only two of the nine revisions due to aseptic loosening. With MoM hips, five of six revisions were due to aseptic loosening.
The strengths of this study are that the three bearing surfaces were compared with use of identical designs of acetabular and femoral components, and the surgeries were done in a single center, which would tend to reduce confounding variables. However, there are a number of factors that limit our ability to generalize these results. Although the authors reported on the incidence and extent of osteolysis in the hips that were revised, this was not assessed in the overall population, so one wonders how many of each type of hip might have had lysis that was substantial but not yet sufficient to cause loosening or require revision. Furthermore, the authors did not sufficiently distinguish between osteolysis and aseptic loosening. That is, while severe lysis can lead to aseptic loosening, each of these can occur independently of the other.
Since the authors did not compile data on the positioning of the acetabular components, and they did not inspect the retrieved components for signs of impingement, the incidence and potential influence of neck-socket impingement on the loosening failures could not be assessed. This raises the question of whether the loosening of the MoM hips and/or the fracture of the CoC hips might have been secondary to neck-socket impingement, which can have less of an effect on MoP hips due to the lower stiffness of the polyethylene. It also should be noted that the Biolox forte alumina used in the present study has been supplanted by Biolox delta, a composite of alumina and zirconia with increased resistance to fracture2.
In addition to these details, there are two major factors that must be considered when interpreting the results of this study. First, the clinical problem of adverse local tissue reactions with MoM hips, which has been increasingly reported during the past few years3, has primarily involved large-diameter total hip replacements and, to a lesser extent, MoM surface replacements. Adverse local tissue reactions have seldom been reported in conjunction with small-diameter MoM bearings, such as the BICON-PLUS (used in the present study) or the Metasul (Zimmer, Warsaw, Indiana)4.
Second, as in the present study, many surgeons prefer to use hard-on-hard bearings rather than polyethylene in young and/or highly active patients. However, given the excellent clinical performance of the XL polyethylenes to date5, one has to question the continued rationale for this preference. Since the introduction of the XL polyethylenes in the late 1990s, several million hips with XL polyethylene liners have been implanted and, presumably, a substantial percentage of these are in young and/or active patients. Nevertheless, we are not aware of any reports of revisions due to osteolysis caused by wear of an XL polyethylene liner. With the maximum follow-up period for these patients now being greater than fourteen years, we can expect that surgeons will become increasingly more confident in using XL polyethylene bearings in young and/or active patients.