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

Abstract

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. 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  <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow><mrow><mi>t</mi></mrow> </mrow> <annotation><math altimg="urn:x-wiley:09422056:media:ksa12352:ksa12352-math-0001" wiley:location="equation/ksa12352-math-0001.png" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mi>t</mi></mrow></mrow></math></annotation></semantics> </math> 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. 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow> <mrow><msup><mi>R</mi> <mn>2</mn></msup> </mrow> </mrow> <annotation><math altimg="urn:x-wiley:09422056:media:ksa12352:ksa12352-math-0002" wiley:location="equation/ksa12352-math-0002.png" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msup><mi>R</mi><mn>2</mn></msup></mrow></mrow></math></annotation></semantics> </math> and <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow><mrow><mi>r</mi></mrow> </mrow> <annotation><math altimg="urn:x-wiley:09422056:media:ksa12352:ksa12352-math-0003" wiley:location="equation/ksa12352-math-0003.png" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mi>r</mi></mrow></mrow></math></annotation></semantics> </math> values from their true values in correlative studies involving FTA. The realistic maximum value for <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow> <mrow><msup><mi>R</mi> <mn>2</mn></msup> </mrow> </mrow> <annotation><math altimg="urn:x-wiley:09422056:media:ksa12352:ksa12352-math-0004" wiley:location="equation/ksa12352-math-0004.png" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><msup><mi>R</mi><mn>2</mn></msup></mrow></mrow></math></annotation></semantics> </math> is 87% and for Pearson's <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics> <mrow><mrow><mi>r</mi></mrow> </mrow> <annotation><math altimg="urn:x-wiley:09422056:media:ksa12352:ksa12352-math-0005" wiley:location="equation/ksa12352-math-0005.png" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mrow><mi>r</mi></mrow></mrow></math></annotation></semantics> </math> is 93%. 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 II, retrospective study.