THA has become one of the most commonly performed operations in Medicare patients, with over 160,000 primary and revision THA procedures reported in Medicare patients in 2007 [1
]. Successful THA allows elderly patients who suffer from disabling hip disease to regain their function and maintain a more active, healthy lifestyle. Nevertheless, increased utilization and rising costs have led to concerns regarding the appropriateness of using ‘premium’, more expensive implant technologies in Medicare patients. Hard-on-hard bearings are associated with lower wear rates in the laboratory setting [15
]. However, wear-related failures, such as bearing surface wear, osteolysis, and mechanical loosening, may be less of a concern among Medicare patients when compared with younger, more active patients who undergo primary THA [8
]. Therefore, the higher cost associated with hard-on-hard bearings may not be justifiable in terms of improved patient outcomes and lower reoperation rates in Medicare patients, unless they are associated with fewer short-term complications, such as dislocation. One example of a potential short-term benefit of M-M bearings that is often cited is the ability to use large diameter femoral heads, which theoretically could result in a lower risk of dislocation when compared with M-PE bearings. We therefore compared the short-term risks of complication and revision THA among Medicare patients having a primary THA with M-PE, M-M, and C-C THA bearings.
Our findings are limited by the use of an administrative database, where bearing surface type is an optional modifier code that can be reported in conjunction with the primary procedure code (primary or revision total hip arthroplasty), which introduces a potential source of bias into our study. However, this limitation is somewhat mitigated by the matched cohort study design, where patients with known bearing types were matched by age, gender, and U.S. census region. Second, other variables of interest which may influence patient outcomes, including surgical approach, surgeon experience, and implant design are not accessible from administrative claims, and we were only able to evaluate complications which are captured in administrative claims data, such as DVT, dislocation, infection, mechanical loosening, and revision surgery, rather than pain and patient reported functional outcomes. Third, since the optional bearing surface modifier administrative codes were not introduced until October, 2005, our study was limited to outcomes and complications that occurred within two years of the index procedure. However, the rationale for our study was that presumably the reason for using alternative bearings in Medicare patients is to reduce short-term complication rates (eg, dislocation), rather than long-term complications (eg, wear/osteolysis), which are more of a concern in younger patients than in Medicare patients. Furthermore, the Australian hip registry [2
] and other reports in the literature [6
] have raised concerns regarding higher than expected short-term complication and revision rates in patients with hard-on-hard bearings. Nevertheless, further study is necessary to evaluate differences in long-term clinical outcomes and revision rates as additional data becomes available.
We found the adjusted risk of short-term complication and revision THA among Medicare THA patients was similar, regardless of bearing surface, with the exception of patients who had M-M bearings, who had a slightly higher risk of periprosthetic joint infection than patients who had C-C bearings. The reasons underlying the increased risk of infection for M-M bearings compared with C-C bearings, even after adjusting for medical comorbidities and hospital factors, remain unclear. It is possible that some of these patients may have been misdiagnosed as having a periprosthetic joint infection when in fact they had a local soft tissue inflammatory reaction related to the M-M articulation [6
]. The findings in some patients with such reactions mimic periprosthetic joint infection, as reported previously by Mikhael et al. [22
]. Although the higher risk of infection in the M-M cohort compared with the C-C cohort was significant (Hazard Ratio 3.03, CI = 1.02 − 9.09), the clinical importance of this difference is unclear, especially given the relatively low incidence of infection (0.59% versus 0.32%, respectively).
The relatively high incidence of M-M bearings reported in the Medicare population is somewhat surprising, given M-M bearings are thought by some to be primarily indicated for younger, more active patients who are at higher risk for bearing surface wear and osteolysis with M-PE bearings [14
]. However, one theoretical advantage of M-M bearings in older patients is improved stability (eg, lower risk of dislocation) due to the ability to use larger-diameter femoral heads. Our data do not demonstrate an advantage of M-M bearings in terms of lower short-term risk of dislocation when compared with either M-PE or C-C bearings. This could be related to the increasing trend of using 32- and 36-mm heads in M-PE bearings, which may be sufficient to substantially reduce the risk for dislocation. Although the database used in this study did not include information about femoral head size, according to industry sources [21
], the proportion of femoral heads implanted in the US sized 32 mm or greater has increased from approximately 12% to 79% between 1998 and 2008.
In summary, we found Medicare THA patients with hard-on-hard (M-M, C-C) bearings had a similar risk of complications and revision THA compared to patients who had M-PE bearings during the first 2 years after primary THA. These findings provide a basis for additional analyses of the comparative effectiveness of THA bearing surfaces in the Medicare population.