Heritability of 39 orthodontic cephalometric parameters on MZ, DZ twins and MN-paired singletons

The twin method is one of the most effective methods available for investigating genetically determined variables in orthodontics, as well as in other medical fields, depending on the variance in the shape and the size of skull and teeth, both on genetic and environmental influences. The former have been extensively evaluated and, in particular, most of the facial and dental cephalometric parameters have shown high heritability, the vertical parameters have a higher genetic control compared with the horizontal ones. Nevertheless, most of the results provided by twin research have been considered arbitrary if directly transferred to a singleton population and in further studies, including extensive analysis of the parents, familial and nutritional habits have been recommended. In this study, heritability of 39 lateral orthodontic cephalometric parameters has been estimated by both statistical method of path analysis and Dahlberg' quotient in three orthodontic samples of young monozygotic and dizygotic twins and same-sex pairs of singletons living together, matched for sex and age, to evaluate genetic versus environmental factors affecting heritability of craniofacial features in the aim to obtain results not only statistically significant but also transferrable to a singleton population. Different inheritance trends, showing the highest concordance of values between monozygotic twin pairs when compared with dizygotic twin pairs or the same-sex singletons paired group, come from the 39 selected lateral cephalometric parameters, confirming the hypothesis that strong genetic control is exerted especially on the vertical ones. Heritability seems to have more influence on anterior vertical parameters than posterior. Mandibular structure seems to be more genetically determined than mandibular size. All five Ricketts' cephalometric typological parameters confirm high heritability coefficients, the same applies to the linear horizontal McNamara's measurement and lower incisor to A-Pg line. The two tested statistical methods showed relevant concordance of results.