Using Little's Irregularity Index in orthodontics: outdated and inaccurate?

Little's Irregularity Index (LII) was devised to objectively score mandibular incisor alignment for epidemiological studies but has been extended to assess the relative performance of orthodontic brackets, retainer or treatment modalities. Our aim was to examine the repeatability and precision of LII measurements of four independent examiners on the maxillary arch of orthodontic patients. The hypothesis was that the reproducibility of individual contact point displacement measurements, used to calculate the LII score, are inappropriate.

The displacement of the anterior contact points, of the six upper labial segment teeth of the maxillary arch on each of ten casts, were randomly assessed by four examiners at three time-points using LII.

Significant correlations were evident between the six examiner-pairs (r > 0.413; p < 0.001) for contact point displacements of >0.5 mm. The coefficients of variation showed that 516 of the 600 individual contact point displacement measurements differed by >20% of the mean. Analyses of variance revealed significant differences (p < 0.047) between examiners for 46% of the contact point displacement measurements.

The reproducibility of individual contact point displacement measurements, used to calculate the LII score, is poor such that using LII to assess the performance of orthodontic brackets, retainers or treatment modalities must emphatically be discouraged.

The use of LII by the orthodontic community to predictably determine the outcome of orthodontic treatment modalities in clinical practice cannot be advocated due to the limited accuracy and precision of the technique.