Interrelation between fusions in the primary dentition and agenesis in thesuccedaneous permanent dentition seen from an embryological point of view

The purpose of the present stud?; was to elucidate the relationship between fusions in the primary dentition and the occurrence of agenesis in the suc cedaneous permanent dentition in a Danish child population and to elucidate this relationship from the recently described normal embryological develop ment of the anterior parts of the human maxilla and mandible. The material included radiographs, either as intraoral film or as orthopant omograms from a total of 19 primary dentitions with a total of 21 fusions. Radiographs of the permanent dentition in the fusion regions were available for all 19 dentitions. Of 21 fusions, a total of 20 were in the mandible and one in the maxilla. I n 15 cases, the fusions were between primary incisors and in six cases betw een lateral incisors and canines. Agenesis of a permanent lateral incisor always occurred when there had been fusion of a primary lateral incisor and a canine in the primary dentition. When fusion had been between primary incisors. there was only agenesis of an incisor in the permanent dentition in a few cases. The degree of fusion between the involved teeth was not related to the occurrence of agenesis. It is suggested that the intra-jaw differences are related to the recently reported prenatal developmental patterns of the alveoli of the incisors and canines. Moreover, it is suggested that neural crest developmental field d ifferences between the developing maxilla and mandible may explain the inte r-jaw differences in phenotypic abnormalities.