Introduction The volume-outcome relationships in failed root canal treatments (RCTs) are rarely studied. Thus, we aimed to examine the association of failed RCTs with dentist and institutional volumes. Methods Ambulatory dental claims (2005–2010) of 1 million beneficiaries randomly selected from the National Health Insurance Research Database of Taiwan were used. This study is a population-based cohort study using a marginal Cox proportional hazards regression model in which the outcome variables cluster around individual patients. A total of 458,557 teeth that received first-ever RCT from 2005 to 2010 and were followed up until the end of 2010 were identified as the tooth cohort. These teeth were aggregated to 244,368 patients, 10,901 dentists, and 7122 institutions for analysis. Results Dentists in the second, third, and highest quartiles all showed a significantly lower hazard ratio of failed RCTs than those with the lowest case volume, and their adjusted hazard ratios (aHRs) were 0.93, 0.92, and 0.90, respectively. Medical institutions in the second, third, and highest quartiles also showed a significantly lower hazard ratio of failed RCTs than those with the lowest case volume, and their aHRs were 0.93, 0.89, and 0.81, respectively. Teeth with a history of periodontitis also exhibited a significantly higher aHR of failure events. The use of rubber dams significantly reduced the rates of failed RCTs. Conclusions Dentists and institutions with high care volumes tended to show good RCT outcomes. Such an effect was prominent and robust among dental institutions.
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