title: Do long leg supine CT scanograms correlate with weight-bearing full-length radiographs to measure lower limb coronal alignment?
abstract: BACKGROUND
  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.
  
  
  METHODS
  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° (165°-200°) for the CTS and 181° (164°-202°) for the FLX. The CTS MA angle data between the assessors were highly correlated (r=0.971, p <0.001) as were FLX 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 FLX were in valgus alignment. Malalignment >5° was revealed by 18.8% of the CTS and 35.4% of the FLX.
  
  
  CONCLUSIONS
  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 FLX still remains a vital imaging modality. CTS should be used with caution in view of the under-detection of malalignment.
authors.affiliation: Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK. Electronic address: gbej1@yahoo.com.
authors.forename: H O
authors.initials: HO
authors.lastname: Gbejuade
authors.affiliation: University of The West Of England, Bristol, UK.
authors.forename: P
authors.initials: P
authors.lastname: White
authors.affiliation: Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK.
authors.forename: M
authors.initials: M
authors.lastname: Hassaballa
authors.affiliation: Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK.
authors.forename: A J
authors.initials: AJ
authors.lastname: Porteous
authors.affiliation: Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK.
authors.forename: J R
authors.initials: JR
authors.lastname: Robinson
authors.affiliation: Avon Orthopaedic Centre, Southmead Hospital, Bristol, UK.
authors.forename: J R
authors.initials: JR
authors.lastname: Murray
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D017760:Bone Malalignment / Q000000981:diagnostic imaging*; D006801:Humans; D007867:Leg Bones / Q000000981:diagnostic imaging*; D008875:Middle Aged; D015588:Observer Variation; D015203:Reproducibility of Results; D016683:Supine Position / Q000502:physiology; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing / Q000502:physiology*
keywords: Alignment; CT scanogram; Knee; X-ray