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Patterned Asymmetry in Tooth Emergence Timing

Although attention has been directed to asymmetries in mesiodistal and buccolingual crown dimensions, beginning with Ballard (Angle Ortho 68:67, 1944), there are also left-right differences in tooth emergence timing that are of investigative interest and possible clinical concern. Since isomeres may emerge several years apart in some cases, there is a need to ascertain the extent to which emergence asymmetry is patterned, the degree of correspondence in asymmetry between the jaws, and the possible relationship between permanent tooth emergence order and the degree of emergence timing asymmetry. To accomplish this we have made use of serial longitudinal tooth emergence data on Michigan boys and girls (Garn and Smith, J Dent Res, in press, 1980), calculating emergence asymmetry both as left-right correlations (r) using sex-specific Z-scores for emergence age and as R.M.S. asymmetry (ad) using absolute age difference in the emergence of antimeres. The first of these two measures corrects the differences in emergence timing variability, and the second does not. For means of comparison we ranked the coefficient of alienation (1-r2) and R.M.S. asymmetry (ad) from lowest to highest, for seven teeth within each jaw. Finally, we ranked permanent tooth emergence order using the male order of means as shown in the table. Where values of r, 1-r2 and (ad) asymmetry are given to two decimal places, the rankings of left-right asymmetry are somewhat different for the two measures within a jaw. They are, however, very similar between the jaws, as indicated by a rank order correlation (rho) for upper and lower