Do long leg supine CT scanograms correlate with weight-bearing full-length radiographs to measure lower limb coronal alignment?

Introduction: The gold standard for measuring knee alignment is the lower limb mechanical axis (MA) using weight-bearing lower limb full-length x-ray (FLX). However, CT scanograms (CTS) are becoming increasingly popular in view of lower radiation exposure, speed of data acquisition and supine positioning. We compared the correlation and degree of agreement of knee joint coronal alignment using these two imaging modalities. Method: From our series of complex primary and revision knee arthroplasty patients, we selected those with both FLX and CTS recorded onto digital PACS. The coronal alignments were assessed in 24 knees and the valgus/varus angles relative to the MA were measured. Results were analysed statistically using the paired samples t-test, Pearson's correlation coefficient, intra-class correlation coefficient, Cohen's kappa and Passing and Bablok regression to assess potential equality of methods. Results: The mean MA was 180.5 degrees (165 degrees -200 degrees) for the CTS and 181 degrees (164 degrees -202 degrees) for the FLX. The CTS MA angle data between the assessors were highly correlated (r = 0.971, p <0.001) as were FIX MA angle measurements (r = 0.988, p <0.001). 41.7% of the CTS and 37.5% of the FLX were in varus alignment, while 50% of the CTS and 43.8% of the FIX were in valgus alignment. Malalignment >5 degrees was revealed by 18.8% of the CTS and 35.4% of the FIX. Conclusion: Overall, good agreement was observed in MA angle data between the two imaging modalities, but reproducibility may be problematic. In the malaligned limb, weight-bearing FIX still remains a vital imaging modality. CTS should be used with caution in view of the under-detection of malalignment. (C) 2013 Elsevier B.V. All rights reserved.