Mandibular Incisor Crowding in Twins and Siblings, a Retrospective Assessment

Objectives: To assess the heritability of lower incisor crowding by evaluating differences between monozygotic twins (MZ), dizygotic twins (DZ), and siblings (SIB).
Methods: Thirty pairs of twins or siblings, DZ (n=10) and SIB (n=8), ranging from 11 to 18 years old, were matched to MZ pairs (n=12) for age and sex. Mandibular casts were randomly sampled from the Forsyth/Moorrees Twin Study and digitally converted to three-dimensional models. Contact points for each lower incisor were marked and Little’s Irregularity Index was measured. One operator measured all contact points and irregularity indices on all subjects. Intra-examiner reliability was tested by re-measuring 10 casts after 14 days. Descriptive statistics and paired t-tests evaluated Little’s Irregularity Index between the different zygosities, controlling for family. An ANOVA evaluated the average difference between different zygosity samples.

Results: The mean age for the sample was 14.7 years (+/-2.03 months). On average, there was no statistically significant difference between DZ (0.08) or MZ (-0.37) twins in Little’s Irregularity Index (p=0.62, p=0.93, respectively). However, when examining the Irregularity Index between siblings in the SIB group (2.91), there was a statistically significant difference (p=0.04). Additionally, ANOVA and post-hoc tests showed the average of the difference in Little’s Irregularity Index between the MZ and SIB groups was statistically significant (p=0.04).
Conclusions: MZ and DZ twins have similar lower incisor irregularity while that of siblings is significantly different. On average, DZ twins share half their genetic material, which is the same amount shared between any two siblings. The differing lower incisor irregularity of MZ and SIB groups and not the MZ and DZ groups supports further investigation into the heritability of tooth position.
(Supported by the American Association of Orthodontists Foundation)