The EOS stereoradiography system has shown to provide reliable varus/valgus (VV) measurements of the lower limb in 2D (VV2D) and 3D (VV3D) after total knee arthroplasty (TKA). Validity of these measurements has not been investigated yet, therefore the purpose of this study was to determine validity of EOS VV2D and VV3D.
EOS images were made of a lower limb phantom containing a knee prosthesis, while varying VV angle from 15 degrees varus to 15 degrees valgus and flexion angle from 0 degrees to 20 degrees, and changing rotation from 20 degrees internal to 20 degrees external rotation. Differences between the actual VV position of the lower limb phantom and its position as measured on EOS 2D and 3D images were investigated.
Rotation, flexion or VV angle alone had no major impact on VV2D or VV3D. Combination of VV angle and rotation with full extension did not show major differences in VV2D measurements either. Combination of flexion and rotation with a neutral VV angle showed variation of up to 7.4 degrees for VV2D; maximum variation for VV3D was only 1.5 degrees. A combination of the three variables showed an even greater distortion of VV2D, while VV3D stayed relatively constant. Maximum measurement difference between preset VV angle and VV2D was 9.8 degrees, while the difference with VV3D was only 1.9 degrees. The largest differences between the preset VV angle and VV2D were found when installing the leg in extreme angles, for example 15 degrees valgus, 20 degrees flexion and 20 degrees internal rotation.
After TKA, EOS VV3D were more valid than VV2D, indicating that 3D measurements compensate for malpositioning during acquisition. Caution is warranted when measuring VV angle on a conventional radiograph of a knee with a flexion contracture, varus or valgus angle and/or rotation of the knee joint during acquisition.