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Rotational Projection Errors in Coronal Knee Alignment on Weight-Bearing Whole-Leg Radiographs: A 3D CT Reference

Igor Strahovnik1, Andrej Strahovnik1,2, Samo Karel Fokter1,3,4

  • 1Faculty of Medicine, University of Ljubljana, Kongresni trg 12, 1000 Ljubljana, Slovenia.

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Summary

Whole-leg radiographs (WLRs) may inaccurately assess knee alignment before total knee arthroplasty (TKA), especially in complex morphotypes. A new femoral notch projection ratio (FNPR) better identifies rotational malposition than traditional methods.

Keywords:
3D CTCPAK classificationMDFAcoronal alignmentfemoral notch projection ratio (FNPR) indexprojection-based alignment indexsystematic projection errortotal knee arthroplastyweight-bearing radiographs

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Area of Science:

  • Orthopedic Surgery
  • Radiology
  • Biomedical Engineering

Background:

  • Whole-leg radiographs (WLRs) are standard for assessing coronal alignment before total knee arthroplasty (TKA).
  • WLR accuracy can be compromised in patients with atypical anatomy or limb malrotation.
  • Coronal Plane Alignment of the Knee (CPAK) classification categorizes knee morphotypes, but projection variability exists.

Purpose of the Study:

  • To evaluate discrepancies between WLR and 3D CT scans for knee alignment across CPAK morphotypes.
  • To introduce and validate the femoral notch projection ratio (FNPR) for assessing rotational positioning on WLR.
  • To identify morphotypes most susceptible to projection errors in coronal alignment assessment.

Main Methods:

  • Coronal angles (MDFA, MPTA, FMA, aHKA) were measured using WLR and 3D CT in 187 knees.
  • Rotational positioning on WLR was assessed using the patellar projection ratio (PPR) and the novel FNPR.
  • Knees were classified according to the CPAK system, and projection errors were analyzed.

Main Results:

  • WLR systematically underestimated coronal alignment, with significant bias in CPAK III morphotypes (MDFA° +1.5° ± 2.0°).
  • FNPR was significantly higher in CPAK III and VI, indicating internal limb rotation during imaging.
  • The PPR-FNPR mismatch was greatest in CPAK III (4.1°), suggesting patellar centering can mask rotational malprojection; anterior osteophytes also caused errors.

Conclusions:

  • CPAK III morphotypes (valgus with oblique joint lines) are particularly prone to projection errors on WLR.
  • FNPR is a more reliable indicator of rotational malposition than PPR in specific morphotypes.
  • Consider 3D imaging (CT) or repeat imaging for CPAK III patients to ensure accurate pre-TKA surgical planning.