research _ working length I _Nekoofar MH, Ghandi MM, Hayes SJ, Dummer PM: The Fundamental Operating Principles of Electronic Root Canal Length Measurement Devices. International Endodontic Journal 2006; 39(8):595–609. _Mounce R: Determination of True Working Length. Endodontic Practice 2007; 10(1):18–22. _Decision-making factors Each time we need to determine WL we are faced with various challenges and factors influencing our decision of where, when, why and how to locate the apical terminus. For one, there are factors dictated by nature that lie beyond our influence: the anatomy of the root canal system, the morphology of the apical region and its variations, and the pathological state of the pulp and periodontal tissues. Additionally, there are factors that we can and should control, namely our knowledge, skills and equipment. Our daily practice brings us experience and moulds our preferences, however, after years of practising, certain prejudices can develop that in some cases can lead to errors. Looking at root canal anatomy, the first fact is that root canals always deviate from the long axis of their roots and the apical foramen almost never coincides with the principal axis of the root (Fig. 1). Anatomical details and variations of the apical region are central to determining WL. The anatomical foramen is seldom (in less than 50 % of cases) located at the anatomical apex. In other words, the anatomical foramen is not always located at the anatomical apex (Fig. 2), which has been proven in numerous studies that have presented figures of 50, 80, 92 and up to 98 % of cases with the anatomical foramen 0.2 to 3.8 mm short of the anatomical apex. Therefore, it is a fact that the anatomical foramen is neither at the anatomical nor at the radiographic apex. Consequently, the instrument placed into the root canal exits through the apical foramen at various angulations from 10° up to 90° (Figs. 3a & b). In other words, root canals deviate and exit mesially and Fig. 4a Fig. 4b distally, something that can easily be revealed on a clinical radiograph. Unfortunately, canals also deviate bucally and lingually. According to the literature, this is the case in 20 to 55 % of teeth, depending on their morphological type (Figs. 4a & b). Additionally, a majority of root apices have multiple foramina, causing apical delta and difficulty in locating the endodontic terminus. The histology of the apical cementum and cemento-dentinal junction (CDJ) and their variations is morphologically intriguing. In only 5 % of teeth, cementum extends at the same level of two opposite walls of the same canal. The extent of those layers of cementum on different walls could vary from 0.5 to 3.0 mm into the root canal, and variations of the CDJ in each individual tooth range from 200 to 800 µm (Figs. 5a & b). The CDJ is seldom well defined and sometimes it is very difficult to differentiate dentine from cementum. Therefore, most of the eminent authors consider the CDJ an inconsistent feature, even histologically. Throughout the entire life and function of a tooth, the apex is constantly remodelled by cementum deposition and resorption. This remodelling process leads to illusory dislocation of the apical foramen but actually increases the length of a root. Thus, even the CDJ is Figs. 4a & b_Root canals deviate bucally and lingually in 20–55 % of all cases. Figs. 5a & b_The depth of the layers of cementum on different walls of the root canal varies from 0.5 to 3 mm. Figs. 6a & b_The apical constriction is always located coronally to the CDJ. Fig. 5a Fig. 5b Fig. 6a Fig. 6b roots 4 _ 2009 I 31