Infraocclusion of primary molars: an epidemiologic and familial study

The prevalence of infraocclusion of primary molars was studied in 1059 Swedish children aged 3-12 years with an even distribution between the age groups. No extractions were performed due to infraocclusion. 94 children (8.9%) showed infraocclusion of primary molars. Infraocclusion was found from 3 years of age. The prevalence varied between age groups, with a maximum of 14.3% in 8- and 9-year-old children and a minimum of 1.9% in 12-year-old children. 49 children had a single tooth in infraocclusion. The primary mandibular molars were affected more than 10 times as often as the maxillary. The prevalence of infraocclusion of the primary mandibular second molar showed a similar pattern but with a 1-2-year delay, up to a maximum in 9-year-old children. After this age the mandibular second molar was the tooth most commonly found ion infraocclusion. In a study of 138 3-12-year-old siblings of 109 children with infraocclusion the prevalence of infraocclusion was found to be 18.1%. When compared with the frequency in the total material (8.9%), the difference proved to be significant, supporting the hypothesis that there is a familial tendency in infraocclusion of primary molars. The mode of inheritance is discussed and it is concluded that the most likely explanation is that the condition is inherited on a multifactorial basis.