CORR Insights®: What Can We Learn From Surgeons Who Perform THA and TKA and Have the Lowest Revision Rates? A Study from the Australian Orthopaedic Association National Joint Replacement Registry

Where Are We Now?

The Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) is a comprehensive record of nearly 100% of the arthroplasties performed in Australia [2]. By comparison, the American Joint Replacement Registry captures approximately 28% of joint replacement procedures performed in the United States [5]. In 2004, the AOANJRR was one of the first registries to establish a formal system for identifying prostheses with higher than anticipated rates of revision, and in 2007 it was the first to identify a greater rate of revision for the ASR hip resurfacing system (DePuy Synthes) [3]. This led to withdrawal of that implant from the Australian market in 2009, and later confirmation of those findings in other studies resulted in worldwide withdrawal as well. Clearly, studies from the AOANJRR can be impactful, and this example highlights the important role of registry studies that leverage large sample sizes to study rare events and identify outliers.

The findings of registry studies are broadly generalizable because well-run registries are nearly universally inclusive. Still, registry research is observational, and without randomization it’s nearly impossible to mitigate selection bias and the effects of confounders inherent in the heterogeneity of surgeons, patients, prostheses, and healthcare settings. Although high participation rates and decentralization confer generalizability, causality may only be inferred with caution. It is not surprising that reports from registry data that attempt to narrow their focus to clinical questions by using exclusion criteria and limited cohorts have been met with criticism [4, 10, 11].

In the current study, Hoskins et al. [6] present a comparison of surgeon, patient, implant, and practice characteristics between a cohort of surgeons with statistically low revision rates and a cohort including all other surgeons. Surgeons who performed fewer than 50 of either THA or TKA or who had fewer than 5 years of follow-up were excluded. The authors found that THA surgeons with low revision rates were more likely to use hybrid fixation and less likely to use implants that the AOANJRR had identified as poorly performing. TKA surgeons with low revision rates were even more likely to avoid poorly performing implants and instead seemed more likely to use AOANJRR-identified best-performing implant combinations. They were more likely to resurface the patella, more likely to use cemented fixation, and more likely to use highly cross-linked polyethylene. Patients of low-revision-rate THA surgeons were less likely to undergo revision for instability, and patients of low-revision-rate TKA surgeons were less likely to undergo revision for aseptic loosening. Both THA and TKA surgeons in the low revision rate groups often performed more than 100 arthroplasty procedures per year and had 10 or more years of experience. Based on these results, the authors concluded that patient-related differences were not apparent between groups, that modifiable implant factors should be investigated, and that surgeon case volume and experience seemed to be the most obvious difference between low revision rate surgeons and all other surgeons.

Where Do We Need To Go?

Hoskins and colleagues [6] refrained from making statistical comparisons, and that is evidence of their scientific credibility. Although this may initially appear to be a puzzling omission, it is important that an observational study of this type avoid statistical comparisons because these comparisons can be misleading. It may, for example, be tempting to subject the data that authors present in tables and appendices to some ad hoc chi-square tests using any statistical software or online calculator. The type of hospital (public vs. private) is reported in Table 1 in Hoskins and colleagues [6], and the difference between the proportion of THAs performed in public hospitals appears significantly lesser in the low revision rate group (20% vs. 25%; p < 0.001). Similarly, low revision rate surgeons were more likely to have 10 or more years of experience (97% vs. 91%; p < 0.001) and were more likely to use implant combinations that have been identified by the AOANJRR as “best combinations” (6% vs. 2%; p < 0.001). Without adequately controlling for confounders, however, it would be wrong to surmise causality from these findings, and publishing p values and the language of “statistical significance” might wrongly influence naïve readers. For example, are these “best combinations” so well performing because they are often used by low-revision-rate surgeons, or do these surgeons have lower revision rates because they often use these “best combinations”?

Similarly, is a low revision rate due in part to some technical skill acquired over 10 or more years of practice, or could it be more closely related to some other confounder such as a more discerning approach to patient selection? Although the authors state that patient-related differences were not apparent between groups, they only compared age, gender, BMI, and ASA class because those are the only patient characteristics available in the registry. Future studies should control for other patient factors that are known to correlate with risk of revision. Are more experienced surgeons more adept at avoiding higher risk patients, or are they perhaps more likely to direct higher risk patients to seek care from their junior colleagues? Unfortunately, these confounding factors may not be easily disentangled, and future studies of this database may never be able to determine how much of the observed difference in revision rates can be attributed to patient selection.

How Do We Get There?

On November 15, 2020, Lewis Hamilton equalled Michael Schumacher’s record for career Formula 1 drivers’ championships and surpassed the record for career racing victories, thus reigniting an age-old debate. Which factor is more important to a racing driver’s success: the driver or the car? In a Formula 1 racing season, 10 teams each enter two drivers of fundamentally identical cars into a series of approximately 20 races with a number of points awarded to the top 10 finishers in each race, and the driver with the most points at the end of the season is the champion. Successful drivers often stay with a winning team, but it is common for other drivers to change from one team to another throughout their careers. Using these itinerant drivers as controls, researchers have determined that the car and team contribute more to a driver’s success than the drivers themselves; 80% to 90% of their total points are due to team and technology [1].

Despite the authors’ informative registry study, the question remains: Is the arthroplasty surgeon or the implant the more important driver of success? One recent retrospective study from a single practice investigated the effect of implant design on subjective and objective clinical and functional outcomes after TKA [8]. The study found that the choice of implant design did not seem to be as important to outcomes as the surgeons themselves. Some surgeons performed better with a particular design while other surgeons performed better with another. Although a prospective and randomized study would be helpful to validate these findings, there is some evidence that the surgeon is more important than the implant, and most surgeons would readily agree with that [9].

And although most surgeons would agree that they should not use implants that have been identified as poorly performing outliers, should surgeons who use implants with par performance be encouraged to switch to best-performing implant combinations? Is a surgeon’s performance actually enhanced by making the change? Since 2001, the AOANJRR has allowed surgeons to request ad hoc summaries of their individual performance [2], so it is possible for AOANJRR to track the performance of individual surgeons throughout their careers. And a prior study from AOANJRR tracked the performance of implants over time as implant designs changed [7]. Although many surgeons remain in a particular practice setting and use particular implant combinations throughout their careers, some surgeons migrate, change practice settings, switch vendors, or adopt the latest technologies. Similar to these prior studies, a future study from the AOANJRR could track the performance of surgeons after they switch from one implant combination to another. This study could use the stalwart surgeons as controls—similar to the study of Formula 1 drivers and their cars—to determine what proportion of a surgeons’ success is due to the surgeons themselves or the track record of their implants.