Femorotibial angle scan-rescan reproducibility: A high-precision calculation on a large cohort.

ABSTRACT

PURPOSE: Femorotibial angle (FTA) is a convenient measure of coronal knee alignment that can be extracted from a short knee radiograph, avoiding the additional radiation exposure and specialist equipment required for full-leg radiographs. While intra- and inter-reader reproducibility from the same image has been reported, the full scan-rescan reproducibility across images, as calculated in this study, has not. METHODS: In this study, 4589 FTA measurement pairs from 2586 subjects acquired a year apart were used to estimate FTA scan-rescan reproducibility using data from the Osteoarthritis Initiative. Subjects with radiographic progression of osteoarthritis or other conditions that may cause a change in coronal knee alignment were excluded. Measurement pairs were analysed using paired-samples  t $t$ tests to detect differences and compared to symptomatic changes in Western Ontario and McMaster Universities Arthritis Index scores for joint pain, stiffness and physical function to detect correlations. RESULTS: The 95% limit of agreement and the paired-samples correlation were calculated with high precision to be [-1.76°, +1.78°] and 0.938, considerably worse than the corresponding figures for intra- and inter-reader reproducibility, without relation to symptomatic or radiographic changes in knee condition. This error will weakly attenuate R 2 ${R}^{2}$ and r $r$ values from their true values in correlative studies involving FTA. The realistic maximum value for R 2 ${R}^{2}$ is 87% and for Pearson's r $r$ is 93%. CONCLUSION: The scan-rescan reproducibility in FTA is almost double the intra- and inter-reader reliability from a single scan. At almost ±2° accuracy, FTA is inappropriate for surgical use, but it is sufficiently reproducible to produce good correlations in studies predicting disease incidence and progression. LEVEL OF EVIDENCE Level II, retrospective study.