3D quantitative tooth shape analysis in hypodontia

Objective: Assessment of tooth morphology is essential in the diagnosis and management of hypodontia patients. Several techniques have been used to quantify tooth shape in hypodontia patients and these have shown smaller tooth dimensions and anomalous tooth shapes in these patients when compared with controls. However, previous studies have mainly used 2D images and provided limited information. In the present study, 3D surface-imaging and statistical shape analysis were used to evaluate tooth form differences between hypodontia and control patients. Methods: Eighteen anatomical landmarks were recorded on the clinical crown of the lower left first permanent molar of 3D scanned study models of hypodontia and control subjects. The study sample comprised 120 hypodontia patients (40 mild, 40 moderate and 40 severe hypodontia patients) and 40 age- and sex-matched controls. Procrustes coordinates were utilized to scale and superimpose the landmark coordinate data and then were subjected to principal component analysis (PCA). Subsequently, shape differences were tested statistically using MANOVA. Results: Significant interaction was found between the two factor variables “group” and “gender” (p< 0.002). Overall expected accuracies were 66% and 56% for females and males respectively in the cross-validated discriminant-analysis using the first 20 PCs. Hypodontia groups showed significant shape differences compared with the control subjects (p<0.0001). Significant differences in tooth crown shape were also found between sexes (p<0.0001). There appears to be progression in shape variation from mild, to moderate to severe hypodontia, while the controls are isolated from all three groups. Severe hypodontia has less prominent cusp tips, a flatter gingival margin and more tapered proximal margin when compared to the control subjects. Conclusions: The degree of variation in tooth shape was associated with the degree of the severity of the hypodontia. Quantitative measurement of the tooth shape in hypodontia patients may enhance the multidisciplinary management of those patients.