Introduction: In order to properly diagnose and treatment plan, an orthodontist needs to be knowledgeable about the changes that occur to the maxillary and mandibular dental arches throughout growth. The purpose of this study is to provide an analysis of morphological shape differences seen in the maxillary and mandibular dental arches individually, as well as together, from birth to 3 years of age.
Methods: Dental casts from the Iowa Infant Growth study ranging from 2 months to age 4 were photographed in the occlusal plane. The images were landmarked with 3 standard landmarks and 10 sliding semi-landmarks along the curvature of the maxillary and mandibular arches. TpsRelW was used to slide the semi-landmarks and superimpose the date to facilitate shape analysis. MorphoJ was used to determine the degree to which size influences dental arch shape. Once the effects of allometry had been removed, a principal component analysis was run on the residuals to display major features of shape variation in the dataset. Finally, a two block partial least squares analysis was run to determine the degree to which the maxillary and mandibular arches were integrated throughout early growth.
Results: Allometry accounts for 9.63% of symmetric shape variation in the maxilla, while it accounts for 56% of symmetric shape variation in the mandible. Asymmetric shape variation is independent of allometry as it only affects 0.34% of the maxillary and 1.46% of the mandibular shape variation. Principal component one accounts for over 60% of all shape variation seen in maxillary and mandibular residuals. Principal component one of symmetric residuals results in a longer, wider dental arch or a shorter, narrower one. Principal component one of asymmetric residuals results in a dental arch with one posterior side being longer and wider while the contralateral side is shorter and narrower. The first three time points (2 months- 1 year) do not display significant integration between the maxillary and mandibular arches. Integration increases with age, displaying significant integration at the last three time points, with the most integration being displayed at 2.5 years.
Conclusions: Allometry affects some of the symmetric shape variation in the maxilla, but over half of the symmetric shape variation in the mandible. The asymmetric components are independent of allometry. Integration of the maxillary and mandibular arches increase with age from 2 months to 3 years, peaking at a time point of 2.5 years.