Crown and root lengths, and root-crown ratios of lower incisor teeth of mongoloid, non-monoloid retarded and normal individuals.

Crown and root lengths were measured and root-crown ratios calculated for lower incisor teeth of mongoloid (M), non-mongoloid retarded (NM) and normal (N) subjects. The teeth from all subjects had been extracted for periodonlal disease except those teeth from normal children which had been extracted for orthodontic reasons. Both crowns and roots of the M teeth were shorter than NM and N teeth. Crowns of the teeth from older mongoloids and non-mongoloids tended to be shorter than those of younger patients, probably due to attrition. However the roots of the teeth of older mongoloids were longer than those of younger mongoloids; in all cases root formation was complete. Regression analysis showed that the root-crown ratios of the mongoloid central and lateral incisors and non-mongoloid lateral incisors were positively related to age (P < .05). This association was related to attrition in the NM and N teeth, whereas for M teeth, the association was influenced by root length. It was considered that short roots and unfavourable root-crown ratios of incisor teeth in the mongoloid children may contribute to early mobility and subsequent tooth loss.