Bilateral asymmetry of tooth formation is elevated in children with simple hypodontia

Tooth formation normally progresses symmetrically between sides; the goal in this study was to test the clinical impression that left-right asymmetry in tooth formation is elevated in children with simple hypodontia.

Data from panoramic X-rays of American white children (5-14 years of age) with simple hypodontia (n=158) were compared to a comparable group from the same practises with all teeth present (n=206). Children with hypodontia were otherwise phenotypically normal, with no cleft or recognized syndrome. Crown-root formation of each tooth (ignoring third molars) was scored using an 11-grade scheme. Analysis relied on chi-square goodness-of-fit tests and odds ratios.

Hypodontia typically occurs unilaterally; it is more common in girls than boys; and it most frequently affects second premolars (omitting third molars). No evidence of a side preference was found as regards absence of the tooth or tooth formation. Tooth formation was decidedly more frequently asymmetric in those with hypodontia, though again the distribution by side was random in the sample. Summed over all tooth types, asymmetric formation occurred in 18.6% of cases with hypodontia compared to 11.9% in controls, and this is significant by chi-square (P=0.03), with an odds ratio of 1.43 (CL: 1.02, 2.04). All tooth types exhibit elevated developmental asymmetry in the hypodontic sample despite only one or a few teeth being agenic.

Increased asymmetry suggests a breakdown in the rigour of developmental timing in cases with simple hypodontia. In concert with increased frequencies of other growth issues in such cases, such as side differences in size and morphology, hypodontia is best viewed as a symptom of an anatomically broad relaxation of developmental canalization between homologous structures, not an isolated dental feature.