Inter- and intraclass correlations for three standard foot radiographic measurements for plantar surface angles. Which measure is most reliable?

Background: The purpose of this study was to evaluate the reliability and reproducibility of three commonly used radiographic measures for plantar surface angles. Methods: The calcaneal angle (CA), calcaneal pitch angle (CPA), and length-height index (LHI) was measured by three independent examiners on two occasions on lateral foot radiographs. Intra- and inter-rater correlations were calculated using a general linear estimate model and post-hoc tests for repeated measures. Bland–Altman's plots with limits of agreement were used for observer differences in scores. Results: The intra-class correlations for the CA ranged from 0.91 to 0.94, for the CPA from 0.93 to 0.98, and for the LHI from 0.96 to 0.97. The inter-class correlations were 0.80 for CA, 0.83 for CPA and 0.93 for LHI. Conclusions: The results of this study strongly suggest that the length-height index was the most consistent and reliable measure for arch height. Level of evidence: Diagnostic Level II, validity.