Dowling, A.H., Burns, A., Macauley, D., Garvey, T.M., Fleming, G.J.

Objectives: To compare contact point displacement measurements used to determine the Little’s Irregularity Index (LII) score on study casts and digital models of study casts by an independent examiner.

Methods: The contact point displacement measurements of the six maxillary anterior labial teeth were measured on ten study casts, using digital callipers and their associated digital models, and using Creo Parametric software on five occasions following scanning using a LAVA chairside oral scanner (LCOS) three-dimensional (3D) intra-oral scanner. Means, standard deviations and co-efficients of variation (CoV) were determined, and data analyses (Pearson’s correlation coefficients [PCCs] and Intraclass correlation coefficients [ICCs]) and statistical analyses (three and two-way analyses of variance [ANOVAs] and Independent Sample Student’s t-tests) were carried out (p<0.05).

Results: Significant positive correlations for the contact point displacement measurements were evident between all measurement time points for the study casts (r>0.978; p<0.0001, and ICC>0.910; p<0.0001) and the digital models (r>0.963; p<0.0001, and ICC>0.986; p<0.0001). The CoV results showed that the contact point displacement measurement data from the digital models was more reproducible than the study casts. Of the 50 Independent Sample Student’s t-tests, 21 significant increases (p<0.042) were reported in contact point displacement measurements <2.9mm for the digital models compared with the study casts.

Conclusion: The use of 3D digital models can improve the reliability of LII measurements by reducing the subjectivity associated with choosing the anatomic tooth contact points, and the awkwardness of measuring the contact point displacements on study casts using a cumbersome calliper technique.

Clinical significance: Intra-examiner variability in the measurement of LII is still evident, with digital models suggesting that either improved software specifically aimed at the orthodontic community be identified or a new method for measuring anterior incisor crowding be sought.

Journal of Dentistry 2013, September 3. [Epub ahead of print]