In the Spanish population, the Willems method was the most accurate for estimating age. In order of precision, the most appropriate methods for application in boys were the Willems, Nolla and Demirjian methods, and in girls were the Willems, Demirjian and Nolla methods.
In Spain, the Demirjian method has been used based on the development of the third molar [15, 27]. However, in the medical-legal environment, the Demirjian method is used based on the stages of the teeth between the left central incisor and the second left molar of the mandible . Its application has been very frequent, and, as in our study, the tendency of the calculation of dental age is towards overestimation [14, 16, 17, 22, 28, 29]. In the study of Melo et al. , the precision is very similar to ours, 0.86 and 0.70, respectively. In the case of the study of Feijoo et al. [14, 28], our results obtained greater precision in boys (0.68 versus 0.87) and lower precision in girls (0.70 versus 0.55).
There are two studies in which the Demirjian and Nolla methods have been used together [16, 17]. For both, our results coincide regarding the tendency of the Demirjian method to overestimate, although we obtained greater precision in boys , and our results also agreed regarding underestimation with the Nolla method [16, 17].
There is only one study carried out in a population of Spanish origin of 266 children, where the Willems method is applied together with the Demirjian method and the Cameriere method . The results obtained are similar to ours, finding that the Willems and Demirjian methods overestimate and that the Willems method has greater precision. However, our study has a larger sample size, confirming the first results published in the Spanish population.
The methods used in this study have been studied worldwide. Most of the findings reported on the calculation of dental age coincide with the trend shown by our results, favoring the external validity of the methods used.
In the case of the Nolla method, we also find underestimation when applied in Brazilians and Croats , in Malaysians , in Turks , in Bangladeshi and British  or in Indians . However, there are conflicting results in some of the aforementioned populations, with overestimation being found in the study of Lopes LJ et al.  with Brazilians and in the study of Mohammed RB et al.  in Indians.
In the case of the Demirjian method, we also find overestimation in Brazilians and Croats , in Malaysians , in Turks , in Bangladeshi and British  or in Indians .
In the case of the Willems method, we also find overestimation in Bangladeshi and British  and in Indians . However, the study carried out by Mohammed RB et al.  also in the Indian population reports underestimation with the Willems method.
The application of the Demirjian, Willems and Nolla methods in the same design has been carried out only in three studies [2, 33, 34]. Maber et al.  analyzed 946 radiographs of children aged 3 to 16.9 years and Hegde S et al.  analyzed1200 radiographs in children between 5 and 15 years. As in our case, the Willems method was the most accurate, and together with the Demirjian method, they overestimated the chronological age. The Nolla method was underestimated in both cases.
In the study by Mohammed RB et al. , 760 radiographs were analyzed in children aged 6 to 16 years, and the results showed overestimation by the Demirjian method. However, unlike our findings, overestimation was found with the Nolla method and underestimation with the Willems method.
Studies such as that of Melo et al.  analyzed samples of 2641 patients aged between 7 and 21 years. However, the Demirjian method allows the estimation of age only up to 16 years; therefore, the valid sample of this study was 956 children (up to 18 years). In our study, we studied children from 4 to 14 years old, with an equal proportion of boys and girls and a valid sample for the application of the methods used to estimate the dental age.
Dental age could be calculated with the regression models constructed in this study, as on other occasions they have been used in the studies of Diz et al. . Given the findings presented here, it would be desirable to use the Willems method in the Spanish population to estimate the dental age.