Third molar agenesis, spacing and crowding o f teeth, and tooth size in caries-resistant naval recruits

1. 1. Third molar agenesis, spacing and crowding of the dental arches, and mesiodistal crown diameters of the lower right first molars were investigated in 195 caries-free white male naval recruits, 17 to 25 years of age, by means of stone study models and roentgenograms. 2. 2. In 25 per cent of the men one or more third molars were congenitally missing. 3. 3. In general, the maxillary teeth were more frequently spaced and less frequently crowded than the mandibular teeth. Only 11.3 per cent of all the men studied had perfectly aligned dental arches (no spacing or crowding in either arch). 4. 4. When third molars were congenitally missing, the teeth in both arches were more frequently spaced and less frequently crowded, and the mean mesiodistal crown diameter of the lower right first molar was smaller than when no third molar agenesis had occurred. 5. 5. The foregoing relationships were most pronounced when all four third molars were congenitally missing. 6. 6. The relationships of third molar agenesis to reduction in tooth number and tooth size were discussed with a view toward their possible association with spacing and crowding of the teeth.