DIFFERENCES IN THE MORPHOLOGY AND SIZE OF THE TEETH OF A CARIES-SUSCEPTIBLE AND A CARIES-RESISTANT STRAIN OF RATS

¹ Ontario Dept. of Health, University of Toronto, and Harvard School of Dental Medicine, Boston, Mass.

Measurements of the size and certain morphologic, characteristics of the upper first molar teeth have been compared between a caries-susceptible and a caries-resistant group of rats (Harvard strains). Measurements included the greatest mesiodistal diameter of the crown, the mesiodistal diameter of the crown at the cervix, the greatest buccolingual diameter of the crown, and the depth and the angle of the sides of the mesio-occlusal fissure measured both from a lingual view of the whole crown and from a hemisection of the tooth.

No significant differences were found for any of the tooth characteristics measured, between male and female animals.

Within the caries-resistant group significant differences were found between litters for all measurements except the angle of the sectioned fissure. Within the caries-susceptible group no differences of significance were found between litters for the fissure angle, whether measured from the lingual view of the crown or from the section, or for the fissure depth. The other measurements did differ significantly between litters in this group. Except for the angle of the fissure from the lingual view, the variation between litters for each characteristic was less in the susceptible group than in the resistant group.

When the differences in measurements were tested between susceptibility groups, all were found to be significant except the buccolingual diameter of the crown and the angle of the fissure sides as measured from the lingual view. The difference in mesiodistal diameter at the cervix was of more significance than was the difference in greatest mesiodistal crown diameter.

The angle and the depth of the mesio-occlusal fissure were concluded to be separate factors apparently related to caries susceptibility. While fissure depth seemed to be related to the size of the crown at the cervix, it was also established that of these two factors, fissure depth seemed more strongly related to caries susceptibility.

Suggestions have been made as to the nature of possible relationships between the depth and angle of the sides of the fissure, and caries susceptibility.