The reliability of Little's Irregularity Index for the upper dental arch using three dimensional (3D) digital models

To investigate the inter-examiner variability of contact point displacement measurements (used to calculate the overall Little's Irregularity Index (LII) score) from digital models of the maxillary arch by four independent examiners.

Maxillary orthodontic pre-treatment study models of ten patients were scanned using the Lava(tm) Chairside Oral Scanner (LCOS) and 3D digital models were created using Creo(®) computer aided design (CAD) software. Four independent examiners measured the contact point displacements of the anterior maxillary teeth using the software. Measurements were recorded randomly on three separate occasions by the examiners and the measurements (n=600) obtained were analysed using correlation analyses and analyses of variance (ANOVA).

LII contact point displacement measurements for the maxillary arch were reproducible for inter-examiner assessment when using the digital method and were highly correlated between examiner pairs for contact point displacement measurements >2mm. The digital measurement technique showed poor correlation for smaller contact point displacement measurements (<2mm) for repeated measurements. The coefficient of variation (CoV) of the digital contact point displacement measurements highlighted 348 of the 600 measurements differed by more than 20% of the mean compared with 516 of 600 for the same measurements performed using the conventional LII measurement technique.

Although the inter-examiner variability of LII contact point displacement measurements on the maxillary arch was reduced using the digital compared with the conventional LII measurement methodology, neither method was considered appropriate for orthodontic research purposes particularly when measuring small contact point displacements.