Tooth crown dimensions and cusp number in hypodontia: assessed by a new three-dimensional technique

Abstract

Background: Development of the dentition is a valuable model for studying craniofacial and general development. It is a Complex Adaptive System (CAS) in which the outcome of interactions between genetic, epigenetic and environmental factors, at the molecular, cellular and tissue levels leads to a phenotype with variation in tooth number, size, shape and mineralization. These variations are important as they underpin evolutionary change. This study is part of a major international collaborative project investigating hypodontia: a variation of tooth number. The project aims to investigate the development of hypodontia from genotype to phenotype in the same group of patients. The phenotype of hypodontia is more extensive than agenesis. The present study investigates part of the phenotype of hypodontia, the relationship of congenitally absent teeth and the crown size and shape of the formed teeth. Aim: Compare the crown dimensions and cusp numbers in patients with mild or moderate hypodontia to matched controls with normal numbers of teeth. Materials and Methods: The sample consisted of 69 patients, 36 females and 33 males, with between 1 and 5 congenitally missing teeth and a set of matched controls. From imaging the dental study casts 3D digital models were produced. Linear measurements were made of the mesio-distal (MD), bucco-lingual (BL) and crown height (CH) dimensions. In addition, the cusp numbers of premolar and molar teeth were counted. Statistical methods used included linear mixed effect models and generalized estimating equations. The new method was validated against traditional 2D calipers, the measuring tool software was tested for repeatability, and for the intra and inter-operator reliability. Results: Intra-class correlation coefficient (ICC) and technical error of measurements (TEM) were used to determine reliability. ICC values were above 0.75 in almost all analyses, and the TEM was negligible, which is indicative of high agreement. The crown dimensions of the hypodontia group were statistically significantly (p<0.05) smaller than the control group in the majority of all three dimensions (MD, BL and CH). There were fewer cusps present on the occlusal surfaces of the first premolar and first molar teeth in the hypodontia patients than in the control group. Interestingly, patients with hypodontia of one upper lateral incisor who retained the antimeric incisor, had significantly reduced crown dimensions when compared to the remaining hypodontia group. Conclusion: The findings of this study confirm that the phenotype of hypodontia includes reduction in all three tooth crown dimensions and in cusp numbers of existing teeth as well as agenesis. The results support the concept that dental development is a Complex Adaptive System whose outcomes are a range of variations of number, size and shape of teeth. These variations are compatible with evolutionary changes and the suggestion of recent reductions in the human dentition.