Dental Effects of Discordant Thumb-Sucking in Monozygotic Twins: A Case Report

Abstract

This case study examines the dental effects of a persistent thumb-sucking habit by comparing monozygotic twins—one with the habit and one without. With genetic and environmental variables largely controlled, this model isolates the impact of nonnutritive sucking on occlusal development. Clinical and diagnostic assessments revealed that the twin with the habit exhibited an anterior open bite, increased overjet, narrower maxillary arch width, deeper palatal vault, and altered incisor inclinations. A unilateral right-sided functional shift crossbite was also observed. Molar relationships were similar in both twins, whereas canine relationships differed. These findings align with existing literature on the association between digit habits and malocclusion. This genetically controlled comparison strengthens the evidence that persistent thumb-sucking can influence the developing dentition and contributes to a deeper understanding of behavioral influences on occlusal and arch morphology in children in the primary dentition.