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).

Keywords: 3D simulation; Coronal alignment; Knee flexion; Lower limb rotation; Radiographic measurement.

Fig. 1

Definition of coordinate system—3D model of the right knee joint; left a: implementation of the x-axis (medial–lateral), “best fit” cylinder of the femoral epicondyles with the transepicondylar centre vector as best approximation of the knee’s flexion axis [17, 22]; middle b: implementation of the z-axis (longitudinal): intersecting plane between x-axis and the FHC as best approximation of the MFA; right c: implementation of the y-axis (anterior–posterior): recrossing the x- and z-axes incorporating the FNP → best approximation of the centre of the knee [5, 18]

Fig. 2

Right bone model in different positions, lateral view (blue: reference zero position; white: flexed/rotated model); a zero position; b 15° external rotation; c 10° flexion; d 30° flexion with 15° internal rotation

Fig. 3

Mean differences to the zero-position dependent on rotation and flexion effects measured in the simulation for HKA angle (a), MPTA (b), mLDFA (c) and MAD (d); Negative values caused by internal rotation and positive values by external rotation. Coloured graphs represent different states of flexion; x-axis different states of rotation; MPTA medial proximal tibial angle, HKA hip–knee–ankle angle, MAD mechanical axis deviation, mLDFA mechanical lateral distal femoral angle

Fig. 4

Predicted values (with CI 95%) of the HKA angle (a), MPTA (b), mLDFA (c) and MAD (d); Rotation and flexion effects based on linear mixed model calculation; Negative rotation values  representing internal rotation, positive external rotation; MPTA medial proximal tibial angle, HKA hip–knee–ankle angle, MAD mechanical axis deviation, mLDFA mechanical lateral distal femoral angle