Shape variation and covariation of upper and lower dental arches of an orthodontic population

Abstract

Objectives: This study aimed to quantify the patterns of shape variability and the extent and patterns of shape covariation between the upper and lower dental arch in an orthodontic population.

Methods: Dental casts of 133 white subjects (61 males, 72 females; ages 10.6-26.6) were scanned and digitized in three dimensions. Landmarks were placed on the incisal margins and on the cusps of canines, premolars, and molars. Geometric morphometric methods were applied (Procrustes superimposition and principal component analysis). Sexual dimorphism and allometry were evaluated with permutation tests and age-size and age-shape correlations were computed. Two-block partial least squares analysis was used to assess covariation of shape.

Results: The first four principal components represented shape patterns that are often encountered and recognized in clinical practice, accounting for 6-31 per cent of total variance. No shape sexual dimorphism was found, nevertheless, there was statistically significant size difference between males and females. Allometry was statistically significant, but low (upper: R(2) = 0.0528, P < 0.000, lower: R (2) = 0.0587, P < 0.000). Age and shape were weakly correlated (upper: R(2) = 0.0370, P = 0.0001, lower: R (2) = 0.0587, P = 0.0046). Upper and lower arches covaried significantly (RV coefficient: 33 per cent). The main pattern of covariation between the dental arches was arch width (80 per cent of total covariance); the second component related the maxillary canine vertical position to the mandibular canine labiolingual position (11 per cent of total covariance).

Limitations: Results may not be applicable to the general population. Age range was wide and age-related findings are limited by the cross-sectional design. Aetiology of malocclusion was also not considered.

Conclusions: Covariation patterns showed that the dental arches were integrated in width and depth. Integration in the vertical dimension was weak, mainly restricted to maxillary canine position.

Figure 1.

Upper and lower dental arch form and landmarks digitized in this study. Canine landmarks are shown as tetrahedrons for easy identification in all subsequent figures.

Figure 2.

Plots depicting the sample in shape space, showing absence of shape sexual dimorphism, for upper (a) and lower (b) dental arch form. Blue: males, red: females.

Figure 3.

Thin plate spline (TPS) grids depicting the effect of varying each of the four most significant PCs for upper arch form. Each figure presents the deformation at 3 SD in the negative (left) and positive (right) direction. Blue: shape consensus, red: −3SD/+3SD configurations.

Figure 4.

TPS grids depicting the effect of varying each of the four most significant PCs for lower arch form. Each figure presents the deformation at 3 SD in the negative (left) and positive (right) direction. Blue: shape consensus, red: −3SD/+3SD configurations.

Figure 5.

Plots depicting the sample in form space, showing size difference between males and females, for the upper (a) and lower (b) dental arches. The line represents the vector of pure size change. Blue: males, red: females.

Figure 6.

TPS grids showing shape changes under the influence of allometry for the upper and lower arch form. The upper arch is shown from the occlusal and frontal aspects to show vertical position of canines. Blue: shape related to small size, red: shape related to large size.

Figure 7.

TPS grids showing shape changes under the influence of age for the upper and lower arch form. Blue: shape related to small age, red: shape related to older age.

Figure 8.

The main pattern of covariation of the sample. Blue: shape consensus, red: −3SD/+3SD configurations.