Significant changes in lower limb alignment due to flexion and rotation—a systematic 3D simulation of radiographic measurements

Abstract

Background Many radiographic lower limb alignment measurements are dependent on patients’ position, which makes a standardised image acquisition of long-leg radiographs (LLRs) essential for valid measurements. The purpose of this study was to investigate the influence of rotation and flexion of the lower limb on common radiological alignment parameters using three-dimensional (3D) simulation. Methods Joint angles and alignment parameters of 3D lower limb bone models (n = 60), generated from computed tomography (CT) scans, were assessed and projected into the coronal plane to mimic radiographic imaging. Bone models were subsequently rotated around the longitudinal mechanical axis up to 15° inward/outward and additionally flexed along the femoral intercondylar axis up to 30°. This resulted in 28 combinations of rotation and flexion for each leg. The results were statistically analysed on a descriptive level and using a linear mixed effects model. Results A total of 1680 simulations were performed. Mechanical axis deviation (MAD) revealed a medial deviation with increasing internal rotation and a lateral deviation with increasing external rotation. This effect increased significantly (p < 0.05) with combined flexion up to 30° flexion (− 25.4 mm to 25.2 mm). With the knee extended, the mean deviation of hip–knee–ankle angle (HKA) was small over all rotational steps but increased toward more varus/valgus when combined with flexion (8.4° to − 8.5°). Rotation alone changed the medial proximal tibial angle (MPTA) and the mechanical lateral distal femoral angle (mLDFA) in opposite directions, and the effects increased significantly (p < 0.05) when flexion was present. Conclusions Axial rotation and flexion of the 3D lower limb has a huge impact on the projected two-dimensional alignment measurements in the coronal plane. The observed effects were small for isolated rotation or flexion, but became pronounced and clinically relevant when there was a combination of both. This must be considered when evaluating X-ray images. Extension deficits of the knee make LLR prone to error and this calls into question direct postoperative alignment controls. Level of evidence III (retrospective cohort study).

Linear correlation between patellar positioning and rotation of the lower limb in radiographic imaging: a 3D simulation study

“…In zero position, mean value for HKA angle was 180.1° (SD: ± 3.1°) and for the MPTA angle 87.7° (SD: ± 2.6°). The mean mechanical axis deviation (MAD) was measured to be 6.2 mm (SD: ± 8.4 mm), which is also within the range reported by Paley et al as the physiological norm (10 mm ± 7 mm) [2, 17, 18]. Exclusion criteria were advanced osteoarthritis of the hip or knee joint, radiographic evidence of previous realignment surgery, fractures, and any lower extremity joint replacement.…”

“…However, the determination of angles in LLRs is highly sensible to rotational inluence and prone to error, as several previous studies have demonstrated a signiicant decrease in medial proximal tibial angle (MPTA) and hip-knee joint angle (HKA) due to rotation [1,2,7]. In particular, deformities, osteoarthritis, and restricted mobility in general result in malrotation between 20° of external and internal rotation [9][10][11].…”

Significant difference in femoral torsion between coronal plane alignment of the knee type 1 and 4

“…Other investigators mostly performed standard digital long-leg radiographs [13,23,25,26]. Here, torsional malalignments, extension deficits of the knee joint and inaccurate adjustment of the projection plane led to measurement errors, which could be prevented by using EOS imaging [2,17].…”

The coronal alignment differs between two‐dimensional weight‐bearing and three‐dimensional nonweight bearing planning in total knee arthroplasty

“…The preoperative planning was performed using a specific software by one manufacturer and based on manually determination of bony landmarks in preoperative NWB CT. By using standardised definitions for each landmark the reliability to obtain reproducible alignment parameters is high, but cannot simply be transferred to all 3D models [ 20 ]. Preoperative passive extension deficit was considered as clinical parameter, since extension deficits of the knee can significantly influence the coronal lower limb alignment especially in combination with (mal‐)rotation [ 33 , 35 , 36 ]. The differences between planned and achieved (postoperative) coronal correction as well as the influence of the surgeon dependent adjustment was not part of this study.…”