The loss of tooth structure volume is an objective parameter that can be used to predict the probability of success of root canal retreatments. At 1-year follow-up, the percentage of unfavourable outcomes of root canal retreated teeth was significantly higher when less than 30% of the original tooth tissue structure was present at baseline.

The proportions of favourable outcomes of primary root canal treatments and retreatments assessed with CBCT were lower when compared to periapical radiographs, and also lower than those historically reported by periapical radiograph-based outcome studies. Considering the very high favourable outcome of anterior teeth and premolars compared to molar teeth, future studies assessing the effect of new materials and techniques on the outcome of root canal treatments should be based on preoperative and postoperative CBCT images, and focus on molar teeth.

AimFirst, to examine the impact of the residual volume of coronal tooth structure in posterior teeth measured with an intra‐oral scanner on the 4‐year clinical survival of root canal retreated teeth. Secondly, to assess retrospectively the effectiveness of the Dental Practicality Index (DPI) in predicting the survival of root canal retreated teeth.MethodologyA total of 156 posterior root canal treated teeth (140 patients) had baseline periapical radiographs (PA) and cone‐beam computed tomography (CBCT) scans taken prior to root canal retreatment. These teeth were followed up with a clinical examination at 1, 2, 3 and 4 years (T12, T24, T36 and T48) with periapical radiographs and CBCT images taken at T12, and PA taken at T24, T36 and T48 where appropriate. Root canal retreated teeth were dichotomized into ‘survived’ versus ‘extracted’. Fisher’s exact test was used to determine the association between the volume of remaining coronal tooth structure and the 4‐year tooth survival. The Dental Practicality Index for each tooth was established using the preoperative clinical and radiographical data. Fisher’s exact test was used to establish a relationship between categorical variables, the total score of DPI vs tooth outcome.ResultsThe percentage of extractions associated with teeth with <29.5% remaining coronal tooth structure was 3 times higher (12.5%) compared to that of teeth with a residual tooth structure > 29.5% (3.5%), but with no significant difference (P = 0.073). There was a significant correlation between the outcome of root canal retreatments at 1 year, assessed by both PA and CBCT, and the 4‐year survival (Fisher’s exact test, P = 0.007 and P = 0.001, respectively). Teeth with DPI scores ≥ 6 were more likely to be extracted than teeth with DPI score < 6 (18.8% vs. 3.9%) (Fisher’s exact test, P = 0.045).ConclusionTeeth with <30% of remaining tooth structure were associated with a survival rate above 80% and teeth with more than 30% of residual tooth structure survived in more than 94% of the cases. The radiographic outcome of root canal treatment can also help to predict tooth survival with teeth having an unfavourable outcome at 1 year more likely to be extracted within 4 years of completion of treatment. The DPI score can potentially be used to identify teeth with failed root canal treatment, which are likely to be extracted following retreatment and cuspal coverage.