EOS measurements

The EOS sterEOS® workstation 2020 with EOS software was used to measure the PTS on EOS radiographs. During EOS image acquisition, the patient stood with the right foot slightly in front of the left foot. This facilitated side identification and avoided or reduced overlay. The 3D mode of the software was used for the measurements. The physician was able to look at both 2D planes. However, setting the cursor on one plane automatically set a mark on the other plane, which allowed for 3D identification of structures. This feature made it possible to differentiate between the lateral and medial tibial compartments as well as to determine the exact location of a chosen point. All measurements were performed on the sagittal image plane, and the coronal plane was used for 3D identification. The TA was determined according to the method of Lipps et al. [16]. A circle was drawn in the proximal tibia, within the proximal, anterior, and posterior cortical borders. A second circle was drawn in the distal tibia in the same manner, touching the anterior, posterior, and distal cortices. The line between the midpoints of these circles is the TA (Fig. ​(Fig.1A).1A). Two additional axes were defined. The ATC is a line at the anterior outer tibial cortex that connects a point 8 cm distal to the knee joint with a point 5 cm proximal to the ankle joint (Fig. 1A). The intramedullary axis (IMA) is a line that connects the centre of the proximal tibia (proximal circle) with the midpoint of the medullary canal 15 cm distal to the knee joint (Fig. ​(Fig.1B).1B). The angles between the TA, ATC, and IMA were measured. To measure the PTS, a line connecting the highest anterior and posterior points of the lateral and medial tibial plateau was drawn (Fig. ​(Fig.1D,1D, F). The points were double-checked for correctness on the coronal plane. Then, the PTS was measured using the TA as a reference.

Depiction of the reference axes and slope measurements on EOS radiographs. (A–F) The yellow line is the TA on the sagittal image plane. (A, B, D, F) The green line is the ATC. (A) The blue line is the IMA. (B) The medial (C, D) and lateral (E, F) PTSs were measured on the sagittal image (D, F). The coronal image (C, E) was used mainly to identify the highest anterior point of the tibial plateau. The yellow dots are linked 3 dimensionally in the coronal and sagittal images