The prevalence of infraocclusion of primary molars was measured using serial plaster casts of 925 children (449 males and 476 females) who had been under regular dental care at the Pediatric Dental Clinic, Tohoku University School of Dentistry.
The teeth were diagnosed as having infraocclusion when the marginal ridge was 1 mm or more below the adjascent teeth.
Serial plaster casts with severe infraocclusions of primary molars were then measured with use of the three dimensional system to determine positional changes of the affected teeth and the adjacent teeth.
The following results were obtained.
1) The prevalence of infraocclusion in primary molars was 14.5% and it was age-related with high prevalence in 6 to 8 year old children.
2) Single instances of infraocclusion in any affected dentition occured more than multiple instances. The highest number of infraocclusion in an affected dentition was six.
3) Infraocclusions of primary molars occured more frequently in mandible than in maxilla and the prevalence was not different between first primary molars and second primary molars.
4) The prevalence of infraocclusion affected bilaterally was 88.7% of the subjects who had 2 or more infraocclusions and 66.5 % of them were affected symmetrically.
5) 43.5% of the infraoccluded teeth had been restored with preformed crowns when they were recognized as infraocclusions and 45.5% of the teeth covered with crowns had been given the endodontic treatments at the same time.
6) The vertical positions of affected teeth and their adjaecnt teeth changed longitudinally in various patterns.
7) The space loss of the mesio-distal distance of the affected area was between 1 and 2 mm in the highest ratio of infraocclusions.