Reliability of Root Canal Curvature Measurements and Canal Visibility Ratings over Repeated Calibration Rounds

Objectives: To evaluate the inter- and intra-observer reliability of three methods for measuring root canal curvature and the reliability of canal visibility ratings on periapical radiographs over repeated calibration rounds. Materials and Methods: A total of 360 clinical periapical radiographs were divided into six batches and independently evaluated by three observers. Root canal curvature was measured using the Schneider, Weine, and a novel tangency-based method, while canal visibility was classified on a three-category scale. Inter- and intra-observer reliability were assessed using intraclass correlation coefficients (ICC). Agreement between observers was evaluated using Bland-Altman analysis, and differences between measurement methods were compared using repeated-measures analysis. The association between canal visibility and inter-observer measurement differences was assessed using Spearman rank correlation. Results: Inter-observer reliability for curvature measurements was predominantly good to excellent across all methods, with no consistent improvement over successive calibration rounds. Intra-observer reliability, assessed in one observer, was excellent (ICC 0.90-0.97). Measurement method significantly influenced angle values, with the Weine method producing larger angles than the Schneider and tangency-based methods, whereas the latter two showed comparable results. Agreement on canal visibility remained fair to moderate, and reduced canal visibility showed a weak but significant association with greater inter-observer differences in curvature measurements. Conclusions: All three methods demonstrated comparable inter-observer reliability, although the Weine method yielded systematically larger curvature angles. Agreement did not consistently improve over repeated calibration rounds. Reduced canal visibility may increase measurement variability and should be considered when interpreting root canal curvature on periapical radiographs.