Objectives: The most common configuration of the mandibular second molar is the presence of two roots, although the presence of one and three roots has also been reported. The objective of this work was to present a rare anatomic configuration with four roots diagnosed during an endodontic therapy. Materials and methods: Endodontic treatment was performed using a dental operating microscope. The anatomic configuration was confirmed with a cone beam computed tomography image analysis. An electronic database search was conducted to identify all the published reported cases of mandibular second molars with four roots. Results: The cone beam computed tomography analysis showed the presence of three mature normal length roots plus a smaller and conical radix entomolaris. The authors were unable to identify a single case in population's characterization researches with large samples by search in the available literature. Only one in vivo case report was available in the literature. Conclusions: A configuration with four roots is rare for the mandibular molars. This configuration has an incidence of 0.04% in the first lower molar with only three in vivo case reports available. For the second lower molar, no incidence ratio was found.
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