Cusp size, sexual dimorphism, and heritability of cusp size in twins

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Abstract
Overall measures of mandibular molars reflect the combined size contributions of the component cusps and ridges. Until now, the size hierarchy of primary and permanent mandibular molar cusps remained unclear. This paper utilizes the relative plane surface areas (basal area dimensions) of the individual molar cusps, as assays of cusp size to demonstrate cusp size variations within populations, antimere cuspal variations, sexual dimorphism, and the heritability of cusp size. Duplicate dental casts from 199 pairs of like-sexed twins provide the raw data. Defined anatomic landmarks on the occlusal surfaces were reduced to X-Y rectangular coordinates prior to the computation of the basal areas dimensions. The results establish a cusp size hierarchy specific for molar type, i.e., five-cusped molars with a distal fovea and distal marginal ridge (5fd), five-cusped molars without a distal fovea and without a distal marginal ridge (5o), and four-cusped molars (4c). Sexual dimorphism in cusp size in apparent in 5fd molar cusps but not in 5o molar cusps. However, males have a significantly higher frequency of 5fd molars. Females have a higher frequency of smaller 5o and 4c molars which have fewer crown components. Moreover, female 5o molars have cusps as large as or larger than 5o male molar cusps. Right-side-left-side differences exist between antimere cusps based on relatively low correlations. The mirroring of molar types occurs infrequently. When observed, most intrapair differences for cusp size, using F-ratios, indicate a low component of hereditary variability.