Infraocclusion of primary molars. An epidemiological, familial, longitudinal clinical and histological study.

The aims of this thesis were to estimate the prevalence and familial tendencies of infraocclusion of primary molars and analyse the effect of extraction on occlusal development, and to characterise histologically the ankylosis in infraoccluded primary molars and recommend guidelines for treatment. The thesis is based on five investigations. In the prevalence and familial studies, 1059 children and 138 first-degree sibs were investigated. The three longitudinal clinical studies comprised 215 primary molars in infraocclusion and the children were subjected to clinical and radiographic examination and analysis of model casts every six months. The histological study comprised 62 teeth in infraocclusion and 40 in normal positions. In addition, bone biopsies were analysed histologically and by enzyme histochemical methods. The prevalence of infraocclusion of primary molars was found to be age-related and was significantly higher in sibs than in the studied population, which indicates a familial tendency. Infraoccluded primary molars with a successor present generally exfoliated normally. The permanent successors usually erupted six months later than on the normal contralateral side. Extraction of primary molars in infraocclusion meant definite space loss in some cases. With aplasia of the successor, the infraoccluded tooth did not exfoliate within the normal time range and the root resorption was found to be very slow, especially after 12-13 years of age. Very few of the previously reported negative effects of non-treatment of primary molars in infraocclusion on exfoliation, eruption and occlusal development could be confirmed in this study. Most infraoccluded primary molars were ankylosed histologically. No differences were found in the bone biopsies from the infraoccluded and control group. The results indicate that ankylosis in infraoccluded primary molars is not a static condition but part of a remodelling process following normal resorption of the roots. The general treatment policy should be to wait for normal exfoliation of infraoccluded primary molars to take place if the permanent successor is present and in cases with aplasia consider the aplasia per se, the degree of primary root resorption and the risk of progression of the infraocclusion.