| Summary: | <h4>Background</h4>Although prior research into the relationship between volume and outcome indicates that this relationship is not linear and that an optimal volume should be specified, consensus is lacking regarding the ideal value of this optimal volume. The purposes of this study were to use a visual method to identify surgeon- and hospital-volume thresholds and to examine the relationships of surgeon and hospital volume thresholds to 30-day readmission.<h4>Methods</h4>A retrospective nationwide population-based study design was adopted. Patients who received total knee replacement surgery between 2007 and 2008 in any hospital in Taiwan were included. After adjusting for patient, physician, and hospital characteristics, a restricted cubic spline regression model was used to identify optimal surgeon- and hospital-volume thresholds. Further, a patient-level mixed effect model was conducted to test the respective relationships between these thresholds and 30-day readmission.<h4>Results</h4>A total of 30,828 patients who had received their surgeries from 1,468 surgeons in 437 hospitals were included in this study. Thresholds of 50 cases a year for surgeons and 75 cases a year for hospitals were identified using a restricted cubic spline regression model. However, only the surgeon volume threshold was associated with 30-day readmission using a patient-level mixed effect model after adjusting for patient-, surgeon- and hospital-level covariates.<h4>Conclusions</h4>According to the results of the restricted cubic spline models, the optimal volume thresholds for surgeons and hospitals are 50 cases and 75 cases a year, respectively. However, only the surgeon volume threshold is associated with 30-day readmission.
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