Patterns of Mandibular Incisor Agenesis in Japanese Patients With Non-syndromic Oligodontia: A Retrospective Study

Background Most studies on congenitally missing teeth include patients with hypodontia because of limited case numbers, whereas reports focusing exclusively on oligodontia are scarce. Although Japanese individuals are reported to have a relatively high prevalence of missing mandibular incisors, the characteristics of mandibular incisor agenesis in Japanese patients with oligodontia and its relationship with overall severity remain unclear. Methods This retrospective study included patients diagnosed at the Department of Orthodontics, Tokyo Dental College Chiba Hospital, between 1984 and 2022. Patients with non-syndromic oligodontia, defined as congenital absence of six or more permanent teeth excluding third molars, were identified. The prevalence and distribution of missing mandibular incisors, maxillary and mandibular canines, and molars were analyzed. Associations between the total number of congenitally missing permanent teeth and mandibular incisor involvement were evaluated using Spearman's rank correlation and categorical analyses. Results Among 31,913 orthodontic patients, 228 were diagnosed with oligodontia, representing the largest single-institution cohort of Japanese patients reported to date. The prevalence of oligodontia was 0.7% overall (228/31,913), 0.2% during 1984-2011 (50/22,731), and 1.9% during 2012-2022 (178/9,182), the latter period coinciding with the introduction of national health insurance coverage for orthodontic treatment of oligodontia in Japan. Six missing teeth was the most common presentation, and case numbers decreased with increasing severity. Missing mandibular incisors were observed in 41.2% of patients overall (94/228) and in 93.3% of those with severe oligodontia (15-22 missing teeth) (14/15). Regardless of severity, agenesis of three mandibular incisors was the least common pattern (6-14 missing teeth: 5.0% [4/80]; 15-22 missing teeth: 7.1% [1/14]), whereas agenesis of two incisors was the most frequent (6-14 missing teeth: 50.0% [40/80]; 15-22 missing teeth: 50.0% [7/14]). The proportion of four missing mandibular incisors increased with greater overall severity. A significant positive correlation was identified between the total number of congenitally missing permanent teeth and the number of missing mandibular incisors (Spearman's r = 0.3961, p < 0.0001). Furthermore, severe mandibular incisor agenesis (3-4 missing incisors) was significantly more prevalent in patients with 15-22 missing teeth than in those with 6-14 missing teeth (40.0% (6/15) vs. 6.1% (13/213), p = 0.0005). Conclusions In Japanese patients with oligodontia, mandibular incisors are frequently affected, and both the prevalence and severity of mandibular incisor agenesis increase with the overall severity of congenitally missing teeth. These findings indicate site-specific susceptibility of the mandibular incisor region in patients with extensive oligodontia.