Tooth eruption symmetry in functional lateralities

Abstract

Dental casts and oral photographs from a cross-sectional sample of 2092 young North Americans with detailed information on functional lateralities (eyedness, handedness and footedness) were examined to compare the proportions of symmetrical and asymmetrical eruption of the antimeric (left–right, contralateral pair) permanent teeth using a four-grade eruption scale. The proportion of symmetrically erupting antimeric teeth was higher for some teeth in those with non-right-sidedness of the feet and eyes, but not significantly so in the case of handedness. Left-footedness was significantly (95% confidence interval) associated with an increased proportion of symmetrical pairs of the maxillary first molar and mandibular lateral incisor, and non-right-eyedness with an increased proportion of symmetrical eruption and left/right non-balanced proportions of asymmetrical eruption in maxillary central incisors. True right-sidedness (hand, foot and eye) was significantly (P⩽0.05) associated with advanced eruption of the left mandibular first molar. It is suggested that while the timing of antimeric tooth emergence and clinical eruption is primarily programmed before crown mineralization, starting approximately at the 30th gestational week in the case of first permanent molars, symmetrical/asymmetrical tooth emergence and eruption may provide information a posteriori on prenatal and early postnatal growth and development.

Introduction

Clinically is very common to see contralateral differences in antimeric tooth emergence. One- to 6-month differences between the left- and right-hand sides are normal and usually there is no great harm to the structural or functional development of the child's dentition. Eruption symmetry has not been examined extensively in population studies and only a few of the comparisons of contralateral tooth eruption have emphasized on directional/fluctuating contralateral differences. According to Steggerda and Hill (1942), Tomes (1859) believed that teeth on the left-hand side erupt earlier, while Bean (1914) was of the opinion that some teeth erupt earlier on the left- and others on the right-hand side; Cattel (1928) attempted to verify these facts, but found such asymmetry only in the maxillary first premolar (references in Steggerda and Hill). Although the left–right dental asymmetry was later regarded as random (fluctuating), it has been suggested that both dimensional (crown size) asymmetry and temporal (eruption) asymmetry might be structured (Garn et al., 1981). Symmetry and asymmetries in antimeric tooth eruption were shown to be sex- and race-specific in a study by Heikkinen et al. (1999), where girls were more symmetrical than boys in the eruption of incisors, canines and maxillary first premolars, Afro-Americans were more symmetrical than caucasoids in mandibular incisors and canines, and directional asymmetry appeared as a right-side advantage in the eruption of the upper first premolar in Afro-American boys.

The processes that cause a tooth to emerge from its crypt and to erupt through the gum are only partly understood in spite of the wide research in this area (Davidovitch, 1994, Davidovitch et al., 1998). Despite the fact that genetic factors appear in the differential timing of mean tooth emergence between sex and race groups, tooth eruption is multifactorial, including local controlling factors (crowding, premature or delayed loss of deciduous teeth), secular trends and social background. As early as 1969, Friedlender and Bailit suggested that their findings could be interpreted as pointing to an early susceptible stage in tooth development after which most influences cease to have any effect. This critical stage probably occurs prenatally in those teeth in which mineralization commences before or soon after birth, i.e., the first molars and permanent incisors. In permanent canines and first premolars this critical stage probably occurs during the first postnatal years, and in second molars and premolars 2 or 3 years later. Other important later stages and mechanisms regulating tooth eruption, including those involved in the continuous blood supply, the growth of the periodontal ligament, alveolar bone and the nerves of the tooth, cannot be overruled. Recently, the dental follicle has been a major focus in tooth eruption studies, e.g in early alveolar eruption of the tooth (Cahill and Marks, 1980), and cascades of signalling molecules and other substances have been discovered to accelerate tooth eruption during intra-alveolar bone resorption /apposition (Wise, 1998, Wise et al., 1994).

Human laterality has been shrouded in myth since the beginning of history, and handedness, for example, has been held to be multifactorial in origin. Sex difference has been observed in many traits expressing laterality, with females exhibiting more symmetry, but any such differences are too small to be classed as sexual dimorphism (Morgan, 1980). The main themes of research in human structural and functional laterality, including handedness, footedness and eyedness, have had genetic, environmental, social, psychological, neurophysiological and anatomical perspectives. In general, a strong current towards a genetic background in human laterality exists (Annett, 1981) and the conclusions of reviews, such as those of Corballis (1983) and Geschwind and Galaburda (1985), focus on the largely unknown role of gestational development in determining human laterality, and cite as an example the many entodermal structures that are more asymmetrical than tissues of ectodermal or mesodermal origin. Left-cerebral dominance exists for the human-specific skills, such as language, and the right hemisphere is more active during emotion; facial movements, for example, are better controlled on the left-side of the face, which means a simultaneous increase in the activity of the right hemisphere (Chaurasia and Goswami, 1975). It is not well documented whether these associations between nervous and functional systems could be genetic or environmental, forming a determined basis for the start of the function, or later, developing during the ‘training’ period.

The teeth provide a useful instrument for the study of development due to their unique structure and long period of sequential development. We have suggested in earlier papers that the association of some gestational and postnatal variables, such as maternal smoking during pregnancy and affected motor development in infancy, are involved in the reduction of crown dimensions in some deciduous and permanent teeth and an accelerated eruption of mandibular incisors (Heikkinen, 1996, Heikkinen, 1998, Heikkinen et al., 1995, Heikkinen et al., 1997). With the exception of second premolars and third molars the effects of maternal smoking on the permanent dentition of the child could be based on the sensitive period of crown development reaching from the middle gestation until birth, when most of the permanent teeth in sequence have began their development. Nicotine, for example, is a classic cholinergic agonist and may have differential effects on the left- and right-hand sides of developing neural system (Domino, 1995). Prenatal exposure to tobacco smoke has been shown to increase fluctuating permanent tooth crown asymmetry (Kieser and Groeneveld, 1998). The sensitivity of each tooth in terms of size reduction, crown morphogenesis, asymmetry of antimeres, as well as that of the fetus during gestation, is associated with the sex and the race of the child, and with buffering mechanisms, such as the size of the fetus during critically sensitive moments of development (Heikkinen 1996).

Our aims now were first to explore the symmetry and the asymmetries of tooth eruption in functional lateralities, and second to elucidate the role of tooth eruption sequences as an indicator in the timing of overall laterality.