Evaluation of a new low-dose biplanar system to assess lower-limb alignment in 3D: a phantom study

Knee coronal alignment is routinely assessed on a full-length radiograph of the lower limbs. However, poor positioning of the knee during the procedure affects the accuracy of this kind of measurement, particularly in cases combining knee rotation and flexion. The purpose of this study was to assess the value of a three-dimensional assessment of the hip-knee-ankle (HKA) angle based on a biplanar radiographic system. A biplanar slot scanning system was used to take radiographs of three lower-limb synthetic models with similar frontal deviation (5A degrees valgus) but different flexion angulations (0A degrees, 9A degrees, and 18A degrees). Biplane acquisitions were done with lower-limb axial rotations ranging from 20A degrees of internal rotation to 20A degrees of external rotation on each of the lower limb models. Three independent observers performed standard 2D measurements of the HKA angle from each anteroposterior (AP) image and also modeled the lower limb in 3D for each biplane acquisition with dedicated software. The HKA angle was automatically calculated from the 3D models. The results of the 2D and 3D techniques were compared. Axial rotation provoked 2D HKA measurement errors up to, respectively, 1.4A degrees, 4.7A degrees, and 6.8A degrees for the lower extremities with 0A degrees, 9A degrees, and 18A degrees flexion, while it never affected the 3D HKA measurement for more than 1.5A degrees. Interobserver errors were 0.7A degrees (SD = 0.5A degrees) for the 2D measurements and 0.6A degrees (SD = 0.4A degrees) for the 3D measurements. The 3D modeling allows for a more accurate evaluation of coronal alignment compared to 2D, eliminating bias due to wrong knee positioning.