Both hard-on-hard bearing options were developed and used before the short- to medium-term results of highly crosslinked polyethylene were reported. The wear rate of highly crosslinked polyethylene is 23%–95% lower than conventional polyethylene [3, 4, 13], even for thinner liners, allowing for use of larger femoral heads, neutralizing the purported advantages of the hard bearings to a degree. However, these data emerged over time, as hard bearings became available in the marketplace and grew rapidly, peaking to about 40% use during THA by 2006 [1].

Since then, concerns with use of hard-on-hard bearing surfaces in THA have steadily increased [2] due to unique complications reported with varying frequency. For ceramics, the major unique complications are fracture and noise generation (squeaking). For metals, the major concerns are adverse tissue reactions of various descriptions and, to a lesser degree, failure of ingrowth or early loosening of components. Various authors reported the frequency of fracture of contemporary devices between 0.004% and 0.19% [27, 30, 31] and squeaking between 1% and 21% [3, 16, 28]. The incidence of major tissue reactions is unknown currently, but some believe it to be around 1% or less [2] and that of early loosening to be between 1% and 6% [11]. However, the incidence of these complications leading to revision arthroplasty is not well established by prior literature.

For the COC cohort, we used one of six different acetabular shell types between 2002 and 2009, with the two most common types being Trident^® PSL^® (806 of 1757 patients, 45%) and Trident^® Hemispherical^® (692 of 1757, 39%) (Stryker Orthopaedics, Mahwah, NJ, USA). Between 2002 and 2009, we used 10 types of femoral stems, with the most common being Accolade^® (1338 of 1757 patients, 76%) (Stryker Orthopaedics). Femoral head sizes ranged from 28 to 36 mm. Minimum followup was 6 months (mean, 50.4 months; range, 6–96 months) in the COC cohort, with 1697 records (97%) available. Six different surgeons performed the procedures, each utilizing a modified Hardinge approach [10] while the patient was in a supine position. Patients in the COC cohort returned for followup visits at 6 weeks, 6 months, and 1 year. Afterwards, we followed up with patients at either 1- or 2-year intervals to assess for component failure. At each postoperative visit, the surgeons evaluated patients and obtained radiographs. Sixty patients from the COC cohort were lost to followup at an average of 65 days (range, 0–179 days).

Between 1996 and 2006, the surgeons at one center performed 1589 MOM primary THAs, of which a minimum 2-year followup (mean, 60.2 months; range, 24–178 months) was available for 1210 (76%). The surgeons utilized three systems of acetabular construct: a modular titanium shell with a CoCr insert and a 28- or 32-mm inner diameter (351 patients, 22%), a CoCr monoblock shell of increasing thickness mated with a 38-mm CoCr head (750, 47%), and a solid “resurfacing style” CoCr monoblock thin (3-mm) shell with anatomic heads of increasing diameter (40–60 mm) (488, 31%). Four surgeons performed the procedures, all utilizing a modified direct lateral approach (modified Hardinge) with the patient in the lateral decubitus position. Patients were followed at 6 weeks and then seen yearly thereafter. At each postoperative visit, the surgeons evaluated patients and obtained radiographs. Three hundred seventy-nine patients from the MOM cohort were lost to followup at an average of 234 days (range, 3–690 days). From the medical records, we determined whether the patient had a revision and those lost to followup.

This study had several limitations. First, our study population was from two joint arthroplasty centers and did not represent the general population, as hard-on-hard bearings are more likely to be used in younger patients who need THA. The surgeons most likely selected patients with high levels of activity to receive hard-on-hard bearings, the current indication for the use of these bearing surfaces. Thus, we could not match these patients for age and activity level with a cohort receiving other bearing surface to conduct a comparative analysis. Second, a variety of devices were utilized, including different femoral stems and acetabular components, as well as different varieties of COC and MOM bearing surfaces. Hence, the failure of THA in this cohort was due to a multitude of reasons, most of which related to prosthetic devices, and not all failures occurred because of the bearing surface. Third, many surgeons contributed patients, with the potential that each surgeon performed the procedure differently. We had no standard protocol for performing the THA and the use of hard-on-hard bearing surfaces. Fourth, despite having a digital database that minimizes errors in place at both institutions, the retrospective character of our study in which recollection of data may be inaccurate. Some of the patients in one or both institutions may have sought evaluation and treatment for possible complications elsewhere. Fifth, while the overall cohort was relatively large, the subcohorts were too small to perform a multivariable analysis to determine differences in the groups while controlling for potentially confounding variables.

A review of recent COC outcomes demonstrated survival rates of 84%–98% [9, 25] (Table 2). Our data support these findings with a survival rate of 98%. The etiologies for revision varied depending on the study and length of followup. In our analysis, the most common reason for revision THA was not bearing related, which was consistent with the previous studies reporting outcomes of COC THA. A review of recent literature reporting outcome of MOM THA demonstrated an overall survival rate between 93% and 99% [6, 23] (Table 3). The failure rate of MOM THA in our cohort (5%) was similar to these previous reports. The majority of the studies reported aseptic loosening as the most common cause of revision for the MOM THA, again consistent with the outcomes of our study [17, 23].

We found only 13% of failures of COC THA were potentially attributable to bearing surface, and the majority of failures in this cohort were acetabular or femoral component related. We found squeaking and fracture were the most common bearing surface-related complications. The etiology of impingement was not clear in all cases but believed to have occurred as a result of excessive anteversion of the acetabular component in patients with elevated ceramic acetabular liner. Impingement may be an important contributor to wear of any bearing surface if components are malpositioned and several recent articles have specifically discussed accelerated wear with the use of COC bearing surfaces [8, 29]. Hence, failure to recognize malpositioning of acetabular components could have accelerated failure for all bearings, despite improved wear rates of hard-on-hard compared to metal on polyethylene. The results from the MOM cohort demonstrated the use of the hard bearing surfaces attributed to 26% of revisions. The use of MOP could have helped to avoid adverse tissue reactions requiring revision surgery. Studies have reported failures with specific MOM designs, and surgeons should consider these before implantation and widespread use [22]. Despite the revisions attributed to hard-on-hard bearings, the data highlight the initial workup of a failed hard-on-hard bearing should mirror that of MOP. Infection, aseptic loosening, component malposition, and recurrent dislocation all are potential reasons for failure of THA and should be investigated before attributing the failure to the bearing surface alone.

Despite the complications due to the use of hard-on-hard bearing surfaces, the overall short-term revision rates in our study were 2.2% for COC and 5.4% for MOM. These rates were consistent with other studies documenting similar success with the use of alternative bearings [18, 31]. Our study retrospectively reviewed a combined 2869 patients who underwent hard-on-hard THA. We had 439 patients between both cohorts lost to followup.