The Effect of Position on Radiographic Angle Measurements of the Lower Extremities

Purpose. Accurately measuring an angle on a lower extremity X-ray is essential for the diagnosis and treatment of knee osteoarthritis (KOA). However, the angle is often affected by position, especially with flexion contracture and rotation. To date, there have been no quantitative analyses examining the relationship between lower extremity angle and patient position and no studies targeting patients with deformities. The aim of this study is to quantify the effect of position on angle measurements in lower extremity X-rays and to compare the effect in patients with different deformities. Methods. Computed tomography (CT) data of 131 patients with knee pain were retrospectively analyzed. The subjects were categorized into the following groups: neutral (hip-knee-ankle angle (HKAA) between 175 and 185 degrees), varus (HKAA less than 175 degrees), valgus (HKAA more than 185 degrees), and flexion (flexion contracture more than 10 degrees). CT images were digitally reconstructed to anterior-posterior X-ray images using an average intensity projection algorithm. The process was then repeated while rotating the reconstruction plane from internal 9 degrees to external 9 degrees. In this manner, X-ray images were reconstructed in different rotational states. The following angles were measured from reconstructed X-ray images: HKAA, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), and femoral valgus angle (FVA). The measurements were then compared according to the degree of rotation. Results. FVA significantly differed according to rotation in all groups (P<0.001), with a difference of 1.3 degrees (+/- 0.4 degrees). HKAA significantly changed only in the flexion contracture group (P<0.001), which showed a difference of 1.0 degrees (+/- 0.7 degrees). However, HKAA in the other groups, LDFA, and MPTA did not significantly differ depending on rotation. Conclusions. Radiographic measurement of FVA is subject to change according to rotation. HKAA significantly changed only in the flexion contracture group, so more care should be taken while obtaining X-rays of patients with flexion contracture.