Torus mandibularis has frustrated several attempts to make family patterns of variation fit modifications of Mendelian models. It is suggested that the quasi-continuous model of inheritance provides a rational explanation for the diverging opinions. The model implies an underlying continuous and normally distributed variable, 'liability', with a threshold value beyond which individuals will be affected. Both genetic and environmental factors determine liability, making the system multifactorial. The incidence of variable degrees of torus was examined in two groups of patients with different stresses on the jaws: one group with bruxism and one comparison group. The transformation of incidences to group means and variances of liability was demonstrated. Muscular forces during bruxism were shown to influence liability. The relative importance of environmental and genetic components of variance could, however, not be estimated directly from the entire groups, since both were mixed with regard to the genetic predisposition. To achieve materials with uniform genotypes, all individuals without torus were omitted. The estimate of the genotypic variance (VG) was obtained by subtracting the variance of the bruxism sub-group--the environmental component associated with bruxism (VEB)--from the total phenotypic variance of the comparison sub-group (VP). The estimate of the genetic determination of torus (VG/VP) turned out to be about 30%, whereas approximately 70% of the causes seemed to be attributable to environmental influence in terms of occlusal stress. Gene effects on the morphologic level are usually pleiotropic, and it is suggested that the correlation of torus mandibularis with other clinical variables might make an interesting subject for further investigation.

The variation in lorus palatinus (TP) prevalence was investigated among a total of 2010 denial patients over 10 yr of age native to two different regions in Norway, the Lofoten Islands in northern Norway and the Gudbrandsdalen valley, an inland area in the southeastern part of the country. The two groups were presumed to belong to the same Caucasian stock, but were extremes with regard to the fish‐to‐meat ratio of the diet, which was 3:1 in Lofoten and 1:3 in Gudbrandsdalen, respectively representing a soft type of diet, and a tough type requiring greater muscular forces during mastication. The study revealed: 1) the prevalence of TP was higher among women than men; 2) TP seemed likely to arise from a multifactorial liability, with part of the genetic factors residing on the X chromosome; 3) the prevalence of TP appeared to be higher among natives of Lofoten, who consume the softer food, than among natives of Gudbrandsdalen, the higher prevalence being hypothesized to have some connection with nutrient substances present in saltwater fish, possibly Ω3 polyunsaturaled fatty acids and vitamin D; and 4) TP seemed to be a dynamic phenomenon capable of growth and subject to resorption remodeling.

The prevalence of torus mandibularis was assessed in two groups of dental patients, altogether 2010 individuals over 10 yr of age: 1181 individuals native to the Lofoten Islands in North Norway, situated at 68 degrees latitude; and 829 patients indigenous to Gudbrandsdal, an inland district in the southeastern part of the country at 61 degrees latitude. Both groups were supposed to be of the same Caucasian stock and, therefore, to have similar genetic predisposition to torus on the average. The following observations were found: 1) the prevalence of torus mandibularis was much greater in Gudbrandsdal than in Lofoten (P much less than 0.001); 2) the prevalence decreased among persons above 50 yr of age as compared with those of the age classes 10-49 yr (P less than 0.01); and 3) it was smaller among women than men (P less than 0.05), mainly due to such a decrease in Lofoten. In a recent investigation of people living in Gudbrandsdal the fraction of the variation of torus that was attributable to genetic differences was estimated as about 30%, whereas approximately 70% of the causes seemed to be ascribable to environmental influences in terms of occlusal stress. It is suggested that dietary habits and number of existing teeth seemed to be environmental variables with an influence on the observed variation of torus prevalence between geographical regions, age classes, and sexes. The question about a possible sexual difference as to the genetic component of liability to torus mandibularis was outside the scope of the present study.

— Comparison of incidence of torus mandibularis and average number of present teeth among 2010 consecutive dental patients over 10 yr of age revealed that these variables seemed to be positively related: patients with torus mandibularis had on the average more teeth present than those without torus (P<0.0000001). The difference was statistically significant in all age classes except 50 years and over. In the adolescent age class, 10–19‐yr, patients with torus mandibularis had on the average fewer unerupted canines than those without torus indicating that the hyperostosis seemed to be associated with more adequately developed jaws. The decreasing incidence of torus mandibularis with age approximately paralleled the decreasing number of present teeth, indicating that number of functioning teeth seems to be a factor of importance for the maintenance of the trait.