Total hip replacement (THR) is performed frequently to relieve pain and improve function in patients with advanced hip joint destruction. More than 3,000 centers in the US perform elective THRs in Medicare beneficiaries, and ∼75% of these centers perform <25 THRs/year in the Medicare population. In fact, approximately one-third of all THRs in the US Medicare population are performed in centers with a case load of ≤25 THRs/year (1). Hospital and surgeon THR volume is inversely associated with rates of postoperative mortality and complications (1–3). However, hospital and surgeon volumes do not appear to be associated with patient functional status and satisfaction following surgery (4).
Hip prostheses function well for up to 20 years in >80% of patients, with failure rates of ∼1% per year (5, 6). Studies of prosthesis survival have been performed primarily in single surgical practices or large-volume centers (7, 8). The question of whether failure rates vary by surgeon and hospital volumes has not been studied in detail. One Canadian study that examined the association between the surgeon volume and THR revision rates during the first 3 postoperative years failed to show an association between volume and outcome. However, the cutoff for low surgeon volume in that study was 40 THRs/year (9), which is relatively high by US standards (1).
The goal of this analysis was to examine the association between surgeon volume of THR and revision rates during the first 4 postoperative years in a national sample of US Medicare beneficiaries who underwent primary THR in 1995–1996. We hypothesized that patients operated upon by high-volume surgeons may have lower revision rates compared with patients of low-volume surgeons. Since technical problems are likely to become manifest soon after surgery, we also hypothesized that this association would be more striking during the first 18 months after the index procedure.
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- PATIENTS AND METHODS
The principal findings of this study are that patients operated upon by low-volume surgeons are considerably more likely to undergo subsequent revision of the index THR than patients operated upon by high-volume surgeons. Further, the association between surgeon volume and early failure occurs primarily during the first 18 months after surgery, suggesting technical error as the mechanism of early failure. Finally, among patients operated upon by high-volume surgeons, higher hospital volume is independently associated with lower early failure rates.
These results are consistent with those of a previous study (12). Espehaug and colleagues found that in Norway, patients operated upon in university hospitals (which had median procedure volumes of 11 THRs/year) had higher cumulative revision rates after 4 years than those operated upon in the central (18 THRs/year) and local (27 THRs/year) hospitals (5.5% failure rate in the university versus 3.5% in the central and local centers) (12). The volume effect was seen primarily in patients who received uncemented prostheses. Our study differed from that of Espehaug et al in that in our claims-based study we could not analyze the effect of cemented versus uncemented prostheses, while the Norwegian study did not have accurate data on surgeon volume.
In contrast to our findings, a Canadian study did not show any statistically significant association between surgeon volume and revision rates 1 and 3 years postoperatively (9). The Canadian investigators defined 40 procedures a year as low surgeon volume, as compared with 12 in our study. Since more than two-thirds of elective THRs are performed in the Medicare population (10), even accounting for non-Medicare THRs, our volume threshold was lower than that used in the Canadian study. Hence, the Canadian investigators may simply have missed associations with physician volume because they did not focus on truly low-volume surgeons. In Canada, as in Norway, THRs are concentrated in a smaller number of centers, therefore resulting in a larger overall annual THR caseload per surgeon.
The strengths of our study include the population-based nature of the sample and the flexibility of the analysis to examine the effect of both surgeon and hospital volume during clinically distinct time periods. To the best of our knowledge, ours is the first US-based study examining the association between failure rates and surgeon and hospital volumes of THRs.
Our study has several limitations. Our definition of THR failure was simply the occurrence of a revision THR. Clearly, patients can have a poor outcome of THR yet not receive revision surgery, either because they prefer not to have surgery or because they are poor surgical candidates (13). Other limitations of our study include the fact that Medicare hospital (Part A) claims data, used for this analysis, lack surgical details and reasons for revision. Also, claims data do not specify the side (left or right) of the revision or the index primary THR. Consequently, it is not possible to ascertain whether the revision occurred on the index or contralateral hip. However, the magnitude of misclassification is likely to be slight. Our (unpublished) medical record data on a population-based cohort of patients who had primary or revision THR documents that only 17% of patients undergoing revision THR had a contralateral primary THR performed prior to the index procedure (for details of the cohort composition, see ref. 4). Thus, the opportunity for misclassification is at most 17%. Furthermore, these data indicated that having another primary THR before the index THR procedure did not exhibit association with being revised. Those having prior THR were equally distributed within each hospital and surgeon volume stratum and were as likely to have revisions in earlier (1–18 months) as in later (>18 months) periods. All this information led us to believe that any bias with respect to associations we examined in the study is unlikely.
Restricting the analysis to the Medicare population is unlikely to introduce bias. THRs performed in the Medicare population represent at least 67% of the total number of THRs performed in the US (10). Furthermore, our unpublished data show that the correlation between overall THR hospital and surgeon volume and the volume of THRs performed on Medicare beneficiaries exceeds 0.95.
In conclusion, the majority of THRs performed on Medicare beneficiaries in the US in 1995–1996 were performed by surgeons with annual procedure volumes of <12 THRs/year in the Medicare population. Higher surgeon volume is associated with lower failure rates. This association is most striking during the first 18 postoperative months. Further population-based studies are needed to better understand the role of the type of implant on this association. In addition, studies are needed to determine whether volume influences later failure (e.g., after 10 years of followup). Clinicians should consider surgeon volume among the factors influencing their referrals for elective THR.