A Critical Evaluation of the Concept of Bilaterality of Caries and Its Statistical Analysis

¹ Department of Biostatistics, University of Oregon Dental School, Portland, Oregon

The concept of caries bilaterality could be logically defined by two aspects, namely, symmetry and association. By "symmetry" it is meant that the caries rates of the two members of a given pair were similar. By "association" it is meant the said caries rates were not independent of each other. Symmetry was said to exist if the caries rates were not significantly different from each other. No symmetry was claimed if the difference were not significant. The existence of association was tested by a X²-test, and the extent of association was measured by the coefficient of association, x²/n.

Analysis of data of Bertram et al.³ indicated the presence of symmetry and association in every pair. The differences between the results of Bertram's² and Nanda's⁷ analyses are probably due to differences in the types of data used, that is, the former author used tooth pairs as units, while the latter used tooth-surface pairs. The hypothesis that, in the long run, symmetry and association occur with simultaneity, the lack of either one being probably a developmental phenomenon, might be tested with a longitudinal experiment.

Analysis of Nanda's⁷ published data indicated the following findings: Out of 14 maxillary surface pairs, symmetry with association was observed between 9 9, symmetry without association was observed between 3 3, and association without symmetry was observed between 2 2.

Out of 7 mandibular pairs, symmetry with association was observed between 5 5, and one each was observed in the categories of symmetry without association and association without symmetry. In no case were symmetry and association both absent. The patterns of symmetry and association not only differ between surface pairs of different teeth but also occur between surface pairs of the same tooth. The teeth whose mesial and distal surface pairs differed in pattern were the upper first and second deciduous molars, first permanent molar, lateral incisor, and the lower first permanent molar.

Although symmetry and association were observed to occur frequently together in esse, the fact that they were separable was demonstrated by the cases in which only one of them was present, that is, the mesial surface of the deciduous first and second upper molars, the distal surface of the upper first permanent molar, and the distal surface of the deciduous lower first molar had only symmetry, and the distal surface of the second upper deciduous molar, the mesial surface of the upper permanent lateral incisor, and the mesial surface of the second lower permanent molar had only association.