Background: Maxillary midline diastema (MMD) is a gap between the maxillary central incisors, posing cosmetic and psychosocial concerns for most patients. MMD results from multifactorial causes, including labial frenum, microdontia, genetics, and various oral habits. The purpose of this article was to review MMD, analyze its prevalence, describe a new shape-based classification, and identify potential causes with a focus on evidence-based practice.

Materials and methods: This cross-sectional descriptive study utilized orthodontic clinical records of Saudi adults aged 18-30 years who had not sought orthodontic treatment. MMD presence was confirmed by examining dental models. Diastema size was measured at gingival, middle, and incisal levels using a Mitutoyo calliper with 0.1 mm accuracy. Pairs of Angle Class I models were used to ensure equivalent tooth size for specimens. Patient records were reviewed to determine etiologies, and Bolton's tooth size ratio was calculated for analysis. Statistical analyses included Z-test, Shapiro-Wilk test, paired/independent t-tests, and post-hoc Bonferroni tests, conducted using SPSS version 26.

Results: The overall prevalence of MMD was 25% (n = 96), with 67% occurring in males. Among malocclusions, MMD prevalence was 42% in Class I, 32% in Class II, and 26% in Class III. MMD size distribution was 0.5-1 mm (18%), 1-2 mm (38%), and >2 mm (44%). A new shape-based classification identified trapezoidal (44%), triangular (40%), rectangular (12%), inverted triangular (2%), and biconcave (2%) forms. Major contributors included labial frenum elongation (25%), microdontia (19%), and midline shifting (72%). No significant differences in tooth sizes were found between MMD cases and Angle Class I controls (P > 0.05). Differences at the three measurement levels were negligible (P > 0.05) for both gender and malocclusion.

Conclusion: This study highlights the polygenic nature of MMD and introduces a new shape-based classification. Key predisposing factors, such as midline shift and microdontia, were identified. These findings will guide clinicians in treating patients diagnosed with MMD.