Maxillary second molar root fusion: incidence and relationship with alveolar bone loss in ICR mice

Fusion of molar roots is a common variation of root morphology known to adversely affect periodontal disease prognosis. Among methods for inducing experimental periodontal disease in animal models, the ligature-induced periodontitis (LIP) mouse model is most frequently employed. However, few studies have examined root abnormalities in experimental mice. This study aimed to investigate the incidence of root fusion in the maxillary second molars of mice and determine its relationship with alveolar bone loss, offering new insights into animal LIP models.

From 2019 to February 2024, 222 Institute of Cancer Research mice (444 maxillary second molars), which served as control animals in periodontitis experiments conducted by the Department of Periodontology and the Department of Anatomy at the College of Dentistry, Gangneung-Wonju National University, were initially examined. After excluding fractured or missing teeth, 402 teeth were included in the final analysis. Root fusion was evaluated using micro-computed tomography and categorized by the degree of fusion. To examine the relationship between fused roots and alveolar bone loss, the distance from the cementoenamel junction to the alveolar bone crest was measured on the mesial and distal sides of the maxillary second molars.

Root fusion was found in 10.4% of animals and 6.5% of teeth. Two types of fusion were observed: mesio-palatal (MP) and disto-palatal. Partial root fusion was more frequent than complete fusion. Within the ligation group, teeth with MP fusion exhibited greater alveolar bone loss than those without root fusion (P<0.05).

Within the limitations of this study, maxillary second molars with MP root fusion accelerated alveolar bone loss in ligated teeth in a LIP mouse model. Future research should consider deviations resulting from MP root fusion.