ORIGINAL ARTICLE New model for dental age estimation: Willems method applied on fewer than seven mandibular teeth Ivan Bedek 1 & Jelena Dumančić 2,3 & Tomislav Lauc 4,5,6 & Miljenko Marušić 7 & Ivana Čuković-Bagić 8 Received: 25 October 2018 /Accepted: 9 April 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Willems method measures the developmental stages of the seven left permanent mandibular teeth and is frequently used for dental age estimation. The aim of this study was to test its accuracy on a large sample of the Croatian population and to develop new models for estimation based on one to seven mandibular teeth. The developmental stages were evaluated on the digital, standardized orthopantomograms of 1868 Croatian children aged 5 to 16. Univariate regression was used for age estimation based on one tooth and regression with forward and backward elimination for selection of the best combination on two to six teeth. Tested in parallel with Willems method, the accuracy of the new models was assessed within intervals of ± 0.5, ± 1, ± 1.5 and ± 2 years and shows the percentage of correct estimations. Using Willems method, the average overestimation was 0.41 years for boys and 0.22 years for girls. Newly developed models that use two to seven teeth proved to be significantly more accurate (p < 0.001). The accuracy of age estimation increases significantly with the number of teeth in a model. Predictably, within the interval ± 0.5 years, a model with three teeth has 3% fewer accurate age estimations than a model including seven teeth. In both theory and practice, Willems method has been considered suitable for age estimation among Croatian children. However, these newly developed models significantly surpass its accuracy. Models using two to seven teeth represent a simple, reliable, and accurate method for age estimation, even in cases with missing mandibular teeth. Keywords Dental age estimation . Willems method . Croatia . Forensic odontology . Newly developed models Introduction Dental age estimation plays an important role in forensic den- tistry, physical anthropology, bioarcheology, and clinical den- tistry. There are two basic approaches to dental age assessment in children: (1) assessing teeth eruption in the oral cavity and (2) assessing the development and mineralization of crowns and roots on dental radiographs. Most methods use a different number of the developmental stages of permanent teeth [1–4]. The number of teeth required for a specific method varies ac- cording to the parameters determined by the authors for the assessment. The most frequently used method for dental age estimation in children is the Demirjian method [3], which is based on eight developmental stages (A to H) of the seven left permanent mandibular teeth. Willems et al. [4] modified this method after a survey which revealed that Demirjian method overestimated age in the Belgian Caucasian population. The adapted method later became known as Willems method. Willems method represents a relatively simple system for assessing dental age. Although based on the earlier premise of * Jelena Dumančić dumancic@sfzg.hr 1 Bedek Dental Clinic, Zagreb, Croatia 2 Department of Dental Anthropology, School of Dental Medicine, University of Zagreb, Zagreb, Croatia 3 Department of Dental Medicine, University Hospital Centre Zagreb, Zagreb, Croatia 4 Study of Anthropology, Faculty of Social Sciences and Humanities, University of Zagreb, Zagreb, Croatia 5 Department of Dental Medicine, Faculty of Dental Medicine and Health, Osijek, Croatia 6 Apolonija Dental Clinic, Zagreb, Croatia 7 Department of Mathematics, Faculty of Science and Mathematics, University of Zagreb, Zagreb, Croatia 8 Department of Paediatric and Preventive Dentistry, School of Dental Medicine, University of Zagreb, Zagreb, Croatia International Journal of Legal Medicine https://doi.org/10.1007/s00414-019-02066-5