Four Year Follow-up of a Mandibular Second Premolar Tooth with Three Canals and Large Periapical Lesion After Retreatment: A Case Report

90% of mandibular second premolars typically have a single root canal from the orifice to the apical foramen. However, there may be rare cases where different canal numbers are present. In this instance, a four-year follow-up of a three-canal mandibular second premolar tooth with a rare anatomical variation and a large periapical lesion that had previously undergone root canal treatment was presented. The retreatment was performed using ProTaper Universal D1, D2, and D3 retreatment files and irrigated using 2.5% sodium hypochlorite, 17% ethylenediaminetetraacetic acid, and saline. During the patient’s 7th-month follow-up, significant bone formation was observed in the periapical region, and at the 48th-month follow-up, the lesion size had substantially reduced. Radiographic examination revealed changes in densities within the lesion, reconstruction of the trabecular structure, formation of lamina dura in the apical region, asymptomatic formation of the teeth during clinical examination, and healthy soft tissues, all indicating a successful treatment. It is essential to note the significance of the Cone Beam Tomography imaging system in identifying canal variations, which should be used appropriately to increase treatment success. As demonstrated in this case, the first approach for a root canal-treated tooth with a large periapical lesion should be non-surgical endodontic treatment.