The Spacing Index (TSI): A diagnostic classification for spaced dentitions

To test the validity and reliability of The Spacing Index (TSI).

A single-centre diagnostic study.

Orthodontic Department, Tameside and Glossop Integrated Care NHS Foundation Trust, Manchester, UKMethods:TSI was outlined based on the principle of mild spacing (0-4 mm), moderate spacing (5-8 mm) and severe spacing (>8 mm). A total of 53 anonymised study models were selected as a convenience sample of patients attending hypodontia multidisciplinary clinics. A range of cases with different amounts of spacing in each arch was chosen. Each model was measured by ML-C in millimetres using calibrated Vernier callipers and then subdivided to give the gold standard spacing TSI categorisation of mild, moderate or severe. Two test raters (CB and NM) then visually assessed the same study models and rated the spacing as mild, moderate or severe. Comparison of the gold standard rating and the test rating was made to assess the validity of TSI. The reliability of the index was assessed by both the gold standard and test raters 2 weeks later.

The weighted kappa score was 0.79 (substantial agreement) between the gold standard rater and the test raters for the mild, moderate and severe TSI categories. The weighted kappa scores were greater than 0.80, demonstrating almost perfect intra-observer reliability over time.

The proposed TSI has been shown to be valid and reliable and could be used to categorise spacing during routine orthodontic assessment.