Multilevel analysis of left-right symmetry in caries development over time

Objectives: Caries has been reported to develop in the same groups of teeth, symmetrically across the midline, in the same jaw. This paper investigates the utility of caries on a tooth surface, as a predictor of the development of caries on the same surface type of the opposite tooth in the same jaw. The statistical method used is a multilevel modelling technique, allowing for the clustering of surfaces and teeth within individuals. Methods: The data were from a 3 year randomised controlled trial of a chlorhexidine varnish, on adolescents in the UK. A logistic multilevel model was fitted using MLwiN software. Three levels were specified, individual participant, tooth, and surface. Only surfaces which were sound at baseline were included. The outcome variable was the development of caries into enamel or dentine, after three years. Covariates in the model were intervention group, tooth position, and the caries status of the corresponding opposite surface at baseline. Results: 53146 surfaces were included, of which 2050 (4%) had a decayed corresponding opposite surface. The probabilities of different teeth developing caries varied greatly. A surface on the lower incisors had a predicted probability of less than 1% of developing caries if the corresponding opposite surface was sound, and 4% if decayed, whereas a surface on a lower first molar had a predicted probability of 14% of developing caries if the corresponding opposite surface was sound, and 49% if decayed. Conclusions: This multilevel modelling technique provides a clinically useful estimate of the probability of a surface developing caries during a longitudinal study if the same surface on the corresponding opposite tooth is decayed at baseline, while controlling for the natural clustering in tooth surface data. This work has benefited from a research donation from Unilever Oral Care.