Mandibular Second Molar Fusion With A Mandibular Third Molar: A Clinical and Radiographic Diagnostic Challenge

Abstract

Dental developmental anomalies comprise a heterogeneous group of conditions manifesting as alterations in the number, size, shape, and structure of teeth. Among these, dental fusion the developmental union of two or more separately developing tooth germs presents a significant clinical challenge because of its rarity in the permanent posterior dentition. This report presents a rare instance of unilateral fusion between a mandibular second molar and a mandibular third molar in a 37-year-old Chinese male. The patient presented for a routine checkup, where clinical examination identified a "double crown" morphology with seven distinct cusps. Radiographic evaluation, initially via panoramic and periapical views and subsequently through cone-beam computed tomography, confirmed the diagnosis of fusion. The imaging revealed a complex internal architecture characterized by three parallel roots and a shared pulp chamber communicating with three separate root canals. A confluence of dentin and a vertical radiopaque line within the crown were pathognomonic features distinguishing this anomaly from gemination and concrescence. This case is particularly notable given the history of extractions of third molars in all other quadrants, reinforcing the diagnosis of true fusion of the second and third molar anlagen rather than union with a supernumerary tooth. The report discusses the multifactorial aetiology of fusion, the application of Mader’s "two-teeth" rule, and the importance of advanced three-dimensional imaging in guiding management. Long-term prognosis for such teeth depends on rigorous hygiene to mitigate risks of caries and periodontal disease associated with irregular morphology.