Tooth morphology across patterns of third molar agenesis: a geometric morphometric study in a French orthodontic...

Associations between tooth number and size have been reported in hypodontia, in line with developmental and evolutionary models. This study aimed to evaluate the relationship between patterns of third molar agenesis and mandibular postcanine crown morphology, and to explore the potential influence of sex.

This retrospective cross-sectional study included 160 participants aged 13-17 years, recruited from a French orthodontic practice. Participants were allocated to four groups (n = 40 each) based on the pattern of third molar agenesis: unilateral mandibular, bilateral mandibular, complete third molar agenesis, and sex-balanced controls. Two-dimensional geometric morphometric analyses were performed on the right mandibular permanent first premolar (LP1), second premolar (LP2), first molar (LM1), and second molar (LM2) using anatomical landmarks and sliding semi-landmarks. Multiple linear regression, Procrustes MANOVA, and RRPP pairwise comparisons were used to assess differences in size and shape, adjusting for sex.

No significant association was found between third molar agenesis patterns and size across the postcanine dentition. Allometric effects were negligible. A significant sex effect was observed for both size and shape of LP1. Significant shape differences according to agenesis pattern were identified for LP1, LM1, and LM2. No clear severity-dependent gradient was observed across agenesis patterns.

These findings indicate that third molar agenesis is associated with tooth-specific shape modifications rather than a generalized size reduction across the postcanine dentition.

While crown size appears unaffected, subtle lingual and distal crown outline variations in the mandibular postcanine teeth of individuals with third molar agenesis may influence restorative and prosthetic planning.