Little's irregularity index: photographic assessment vs study model assessment

Abstract

Introduction: The aim of this study was to investigate whether photographic images are accurate and reliable to assess Little's irregularity index (LII) compared with study model measurements.

Methods: Ten plastic models with different levels of incisor irregularity were photographed occlusally at 9 angulations ranging from 70° to 110° on 3 occasions. Clinical occlusal photographs and study models of 72 dental arches were selected from 16 patients participating in a study of incisor alignment. LII was measured from the models by using digital calipers and from the digital images with ImageJ software (Rasband WS, National Institutes of Health, Bethesda, Md; http://rsb.info.nih.gov/ij/, 1997-2006).

Results: Photographs of the plastic models and the direct measurements of the models showed high levels of reliability and repeatability (intraclass correlation coefficients [ICC], 0.999 and 0.983). Validity was shown by the limits of agreement between the plastic models and the photographs at 90°, 70°, and 110°, which were -0.2621 to 0.9547, -0.1961 to 1.3205, and -0.7399 to 2.0405, respectively, and the ICC were 0.993, 0.983, and 0.969, respectively. The intraexaminer and interexaminer ICC for the clinical photographs were 0.997 and 0.987, and intraexaminer agreement for the study models was 0.995. The limits of agreement and the ICC between the photographic method and study model method were -1.9656 to 1.2861 and 0.986.

Conclusions: LII can be measured from photographs with good reliability and repeatability. The limits of agreement indicate that, for the mean value of a group of subjects, the photographic method is valid, but care is needed in interpreting an individual measurement.