Symmetry of the Final Working Length in Multicanal Teeth: A Review of Anatomical Principles and Clinical Implications
Accurate determination of the final working length (WL) is central to successful endodontic therapy in teeth with multiple root canals. Because the apical constriction cannot be visualised directly, clinicians rely on anatomical landmarks, radiographs, electronic apex locators (EALs) and, increasingly, on the observation that canals within the same tooth, and homologous teeth on opposite sides of the arch, tend to terminate at comparable depths. This review examines the anatomical evidence underpinning this principle of ‘working length symmetry’ in multicanal teeth and its clinical relevance. A narrative literature search of PubMed/MEDLINE, Scopus and Google Scholar was performed using combinations of the terms working length, apical constriction, symmetry, multicanal, contralateral and bilateral. Three levels of symmetry emerged from the literature: (1) orifice-level symmetry, described by Krasner and Rankow’s laws of pulp-chamber-floor anatomy; (2) intraradicular symmetry, in which the apical constrictions of two canals within the same root (for example, the first and second mesiobuccal canals) occupy comparable positions relative to the anatomical apex; and (3) bilateral, or contralateral, symmetry, in which homologous teeth on the two sides of the same arch show a high, though incomplete, correspondence in root length, canal configuration and apical anatomy. Micro-computed tomography and cone-beam computed tomography studies broadly support these observations, although exceptions are frequent enough that symmetry should be used only as a corroborative, not a definitive, guide. Combining anatomical symmetry with radiographic and electronic confirmation offers the most reliable approach to establishing the final working length in multicanal teeth, reducing the risk of under- or over-instrumentation and of missed canals.
Keywords: Working length; apical constriction; root canal symmetry; multicanal teeth; electronic apex locator; endodontics
