Comparison of Permanent Mandibular Molar Crown Dimensions between Mongolians and Caucasians

The aims of this study were to compare crown dimensions of mandibular first molars (M1) and second molars (M2) between Mongolians (belonging to the Khalkha-Mogol grouping) and Caucasians (Northern European ancestry) and to attempt to explain any observed differences in phylogenetic and ontogenetic terms. Materials in this study comprised dental casts of 48 Mongolian female subjects with a mean age of 20.5 years and 50 Caucasian female subjects with a mean age of 21.5 years. For M1, the buccolingual diameters of both mesial and distal crown components in Mongolians were significantly larger than in Caucasians. For M2, the mesiodistal and buccolingual diameters of the (1990) observed this dental pattern in populations of Northern China, Mongolia, and Southern Siberia. Even though frequencies of occurrence and degrees of expression of nonmetric morphological crown features have been described in many Asian populations, including Mongolians (Scott and Turner, 1998; Turner, 1990; Manabe et al., 2003), there have been only a few studies describing mesiodistal and buccolingual crown diameters in Mongolians (Matsumura, 1995; Matsumura and Hudson, 2005; Hanihara, 2005). Recently, more emphasis has been placed on describing how the various components of the dental crown contribute to overall crown size, with studies focussing on intracoronal components rather than traditional mesiodistal and buccolingual crown diameters. However, as far as we are aware, no such study has been carried out in Mongolians. Therefore, the aim of this study was to compare not only overall crown size but also the sizes of various crown components (i.e., talonid and trigonid) of mandibular distal crown components in the Mongolian sample were significantly larger and the mesiodistal and buccolingual diameters of mesial components were significantly smaller compared with those of Caucasians. Common environmental effects, possibly related to the prenatal environment, as well as genetic influences, may be contributing to the differences in buccolingual dimensions of M1 between Mongolians and Caucasians. Given that the M2 develops later and over a longer period of time than the M1, it is reasonable to assume that this tooth may be subject to greater environmental pressures than applied to the M1. Dental Anthropology 2007;20:1-6. 2 Y. HASEGAWA ET AL. first and second molars between a sample of modern young female adult Mongolians and a sample of female Caucasians of similar ages, and to attempt to explain any observed differences in phylogenetic and ontogenetic terms. The study forms part of a larger investigation of the Mongolian dentition being undertaken by researchers from the Health Science University of Mongolia, Mongolia, and the Nippon Dental University School of Life Dentistry, Japan. MATERIALS AND METHODS Materials in this study comprised dental casts of 48 Mongolian female subjects and 50 Caucasian female subjects. The Mongolian dental casts were produced from impressions collected by a survey team from the Nippon Dental University School of Life Dentistry, Japan. This material is stored at the School of Dentistry, Health Science University of Mongolia, Ulaanbaatar, Mongolia. The ages of the Mongolian subjects ranged between 18.4 and 25.0 years, with a mean age of 20.5 years. Mongolian dental casts were collected from students attending colleges and universities in Ulaanbaatar, who were born in Ulaanbaatar or its suburbs and who belonged to the Khalkha-Mogol grouping. The Caucasian dental casts are stored in the School of Dentistry, The University of Adelaide, and were obtained from dental students between 20.8 and 24.5 years, with a mean age of 21.5 years. For the Caucasian group, only those students with Northern European ancestry were chosen. Dental casts were used only if mandibular first and second molars (M1 and M2) had no caries, no dental treatment, no anomaly of crown morphology, and only if the cusp tips, central pits and occlusal grooves were not noticeably affected by tooth wear. According to ethical standards, it was necessary for Mongolian students to be told the purpose of the study and agreements were obtained from them before impressions for dental casts were taken. Casts of the dentitions of the Adelaide students were obtained as part of their dental course requirements and were selected from a larger collection. M1 and M2 were measured using a pair of sliding digital calipers to an accuracy of 0.05 mm. The selected dimensions of the tooth crowns that were measured are shown in Figure 1. The methods adopted to measure mesiodistal and buccolingual crown diameters were as described by Fujita (1949). Mesiodistal crown diameters of the trigonid and talonid were recorded as described by Yamada (1992), and buccolingual crown diameters of the trigonid and talonid followed the definitions given by Kondo et al. (1998). A suggestion made by Yamada (1992) was adopted to make it possible to define the border between the trigonid and talonid by defining the midpoint between the mesial central fossa and the intersection of the buccal groove. Comparisons of mean values for mandibular molar crown dimensions between the Mongolian and Caucasian samples were made using Student’s t-test. F-tests were used to compare variances. Statistical significance was set at alpha = 0.05. Descriptive statistics including distribution parameters were calculated with StatView (SAS institute, version 5.0 for Macintosh). Measurement errors were analyzed by a procedure of double determination measurements using paired t-tests (statistical significance set at alpha = 0.05) for systematic errors and the method described by Dahlberg (1940) for random errors.