Objectives: The aim of this study was to quantify variation in the emergence of primary lateral and central incisors in a cohort of Australian twins, and to examine sources of asymmetry in the emergence of antimeric pairs.
Methods: Data for 95 twin pairs were obtained from parental records of primary tooth emergence. One randomly-selected twin from each pair was used to calculate descriptive statistics for incisor emergence times. Asymmetrical emergence was assessed by calculating the percentage of antimeric pairs that showed differences of 1, 3, 7, 10, 14, 21 and 30 days respectively.
Results: Considerable variation in emergence sequence was noted. The most common sequence was: mandibular central incisors, maxillary central incisors, maxillary lateral incisors, mandibular lateral incisors. Males were developmentally advanced and less variable than females for all teeth. There was no significant effect of zygosity on mean emergence times, although monozygotic twins were more variable than dizygotic twins. Premature birth did not have a significant effect on tooth emergence.
Except for mandibular lateral incisors, at least 50% of all antimeric pairs emerged symmetrically when allowing a 10 day interval between sides, with mandibular lateral incisors more likely to emerge asymmetrically for all time intervals. There was a slight advancement of left relative to right for those antimeric pairs displaying asymmetry, except for the mandibular central incisors. . The maxillary lateral incisors displayed the greatest degree of directional asymmetry.
Conclusions: Considerable variation in timing and sequence of emergence of primary incisors was observed, including evidence of directional asymmetry, reflecting an interplay of both genetic and environmental influences.
The support of the NHMRC of Australia, the ADRF, and the Dental Board of SA are gratefully acknowledged.