Validation of the third molar maturity index cut-off value of <0.08 for indicating legal age of 18 years in Eastern Chinese region

Having a reliable, inexpensive and simple method to estimate 18 years of age would be of help in the forensic field. This study aimed to test the accuracy of the third molar maturity index (I3M) in indicating the legal adult age of 18 years. This retrospective cross-sectional study analysed 450 digital panoramic images of Eastern Chinese children and young adults (226 females and 224 males) aged between 14 and 22 years. A cut-off value of I3M < 0.08 was tested in discriminating adults from minors for both sex. For females, the sensitivity of the test (Se) was 75.0%, with a 95% confidence interval (95%CI) of 67.5% to 82.5%. The specificity of the test (Sp) was 100%. The proportion accurately classified (Ac) individuals was 85.8% (95%CI, 81.3% to 90.4%). The Bayes post-test probability was 100% (93.6% to 100%). For males, Se, Sp and Ac were 91.9% (95%CI, 87.1% to 96.7%), 92.0%(95%CI, 86.7% to 97.3%) and 92% (95%CI, 88.4% to 95.5%), respectively. The Bayes post-test probabilities was 92% (95%CI, 88.4% to 95.5%). Males were ahead in the development of third molars comparing to females according to I3M. A stepwise logistic regression analysis showed that both I3M and sex contribute to the regression model to discriminate adults (≥18 years) from minors (<18 years), while a receiver operating curve (ROC) analysis indicated some better accuracy of I3M < 0.12 in females, without statistically significant difference when compared to I3M < 0.08. The results of this study show that the cut-off value of I3M < 0.08 may help to discriminate Eastern Chinese adults from minors. However, further study should evaluate the usefulness and possible variability of I3M cut-off value in a specific population before used for legal and forensic procedures.