The effect of lower limb rotation on radiographic femoral and tibial joint line obliquity measurements, and the association between the measurements and their changes: a digitally reconstructed radiograph evaluation

Background Lower limb rotation at the time of imaging may affect the measurement of joint line angles using plain radiographs, potentially compromising measurement accuracy. Accurate joint line angles are important for orthopaedic surgery planning and limb alignment. This study aimed to investigate the changes in distal femoral and proximal tibial joint line angles in response to limb rotation, and evaluate the correlation between these changes using digitally reconstructed radiographs (DRRs) generated from computed tomography (CT) images. Methods Preoperative CT data from 50 knees scheduled for TKA or unicompartmental knee arthroplasty (UKA) at our institution were analysed using a TKA planning software. The femur and tibia were aligned perpendicularly to their mechanical axes in the coronal and sagittal planes. The surgical epicondylar axis (SEA) and Akagi’s line were used as references for femoral and tibial rotation, respectively, with 0° defined as neutral rotation. Each bone was rotated from 20° external to 20° internal rotation in 5° increments using the software, and DRR images were generated at each position. Lateral Distal Femoral Angle (LDFA), Medial Proximal Tibial Angle (MPTA), and Posterior Tibial Slope (PTS) were measured at each rotational angle. The absolute values and their variations with rotation and correlations between each angle and their respective changes were analysed. Results At 0°, the mean ± SD values were 87.5 ± 2.4°, 83.9 ± 2.6°, and 8.4 ± 3.2° for LDFA, MPTA, and PTS, respectively. The mean changes across 20° external to 20° internal rotation were 0.3° (1.8°–0.6°), 1.1° (1.0°–1.5°), and 6.2° (1.9°–2.4°) for LDFA, MPTA, and PTS, respectively. MPTA was negatively correlated with ΔPTS (r = –0.7). Based on the MPTA values, the patients were categorized into: <82.5°, 82.5–84.5°, and >84.5° groups. ΔPTS was statistically significant different between the <82.5° (8.6 ± 2.3°) and 82.5–84.5° groups (6.9 ± 1.6°) and between the 82.5–84.5° and >84.5° groups (4.0 ± 1.9°) (p<0.05 and p<0.001, respectively). Conclusions Limb rotation significantly affects PTS, particularly in cases with greater medial inclination of the tibial plateau. Therefore, caution must be exercised when using plain radiographs for preoperative planning or postoperative evaluation.