Abstract: Fused teeth usually occur in the mandibular anterior tooth region, which share the same root canal system or have individual pulp canal systems. Fused teeth in mandibular first premolars are rare. The root canal system of mandibular first premolar has great variation, usually single canal, and four canals are exceedingly rare. This paper reports a rare case of a fused mandibular first premolar with four root canals. Cone-beam computed tomography was used to examine the affected tooth, and microscopic examination was employed to accurately locate the root canal orifices. Nickel-titanium instruments were used for mechanical preparation and ultrasonic irrigation, followed by tight filling of the root canals using thermoplastic gutta-percha. 3-12 months' follow-up after surgery showed excellent treatment outcomes for the affected tooth.

Key words: first mandibular premolar, fused teeth, root canal retreatment, cone-beam computed tomography