Effect of impaction on the performance of the CBCT-based third molar maturity index (I3M) for the 18-year age threshold

The third molar maturity index (I3M), the ratio of the summed open-apex widths to tooth length of the mandibular third molar, assesses whether an individual has reached 18 years. We examined how impaction affects the classification performance of the I3M at the 18 year threshold. The I3M measurements were performed on 744 mandibular third molars on cone-beam computed tomography (CBCT) scans. Sensitivity, specificity, and AUC were compared between impacted and non-impacted molars, and the effect of impaction on correct classification was tested using age-adjusted generalized estimating equations. Discrimination did not differ significantly between groups (AUC, p = 0.12); instead, impaction shifted the error pattern. At the 0.08 cut-off, impacted molars showed lower sensitivity (0.68 vs 0.91) and reduced odds of correct classification (adjusted OR 0.34, 95% CI 0.19–0.59; p < 0.001), misclassifying adults as minors, whereas non-impacted molars more often misclassified minors as adults. Specificity reached the ≥ 90% level required in criminal contexts for impacted (0.95) but not non-impacted molars (0.54), although the latter was based on only 24 molars from minors. Impaction status should be considered when applying the I3M; a lower cut-off may be needed for non-impacted molars, but larger, age-balanced studies are required to establish it.