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Background: One of the difficulties in assessing the causes of root resorption is to separate the contribution made by genetic factors from those due to environmental factors, such as treatment. Objective: The aim of this study was to investigate the genetic contribution to orthodontic root resorption. Methods: The subjects of this investigation were 16 monozygotic (MZ) and 10 dizygotic (DZ) twins. All twins had zygosity diagnosed using microsatellite analysis. Each twin-pair had same type of malocclusion, were treated with the same type of appliance, and by the same clinician. The lengths of the maxillary incisors, mandibular incisors, and mandibular molars were assessed both qualitatively and quantitatively, on pre-and post-treatment panoramic films. The genetic contribution to external apical root resorption was assessed using concordance and heritability estimates. Results: Concordance estimates from the qualitatively and quantitatively measured external apical root resorption were 44.9 per cent and 49.2 per cent respectively for the MZ twins, and 24.7 per cent and 28.3 per cent respectively for DZ twins. An overall heritability estimate of 0.34 was obtained.

Conclusions: External apical root resorption was not dependent on the pretreatment root length. Qualitatively and quantitatively determined estimates of concordance indicate a genetic component to root resorption. A larger sample is needed before models of heritability can be used to determine the components contributing to the variance.(Aust Orthod J 2004; 20: 1–9)