Abstract
PurposeLong-leg-radiography (LLR) is commonly used for the measurement of lower limb alignment. However, limb rotations during radiography may interfere with the alignment measurement. This study examines the effect of limb rotation on the accuracy of measurements based on the mechanical and anatomical axes of the femur and tibia, with variations in knee flexion and coronal deformity.MethodsForty-five lower limbs of 30 patients were scanned with CT. Virtual LLRs simulating five rotational positions (neutral, ± 10^{circ }, and ± 20^{circ } internal rotation) were generated from the CT images. Changes in the hip–knee–ankle angle (HKA) and the femorotibial angle (FTA) were measured on each image with respect to neutral values. These changes were related to knee flexion and coronal deformity under both weight- and non-weight-bearing conditions.ResultsThe measurement errors of the HKA and FTA derived from limb rotation were up to 4.84 ± 0.66^{circ } and 7.35 ± 0.88^{circ }, respectively, and were correlated with knee flexion (p < 0.001) and severe coronal deformity (p < 0.001). Compared with the non-weight-bearing position, the coronal deformity measured in the weight-bearing condition was 2.62^{circ } greater, the correlation coefficients between the coronal deformity and the deviation ranges of HKA and FTA were also greater.ConclusionsFlexion and severe coronal deformity have a significant influence on the measurement error of lower limb alignment. Errors can be amplified in the weight-bearing condition compared with the non-weight-bearing condition. When using HKA and FTA to represent the mechanical axis and the anatomical axis on LLR, limb rotation impacts the anatomic axis more than the mechanical axis in patients with severe deformities. Considering LLR as the gold standard image modality, attention should be paid to the measurement of knee alignment. Especially for the possible errors derived from weight-bearing long-leg radiographs of patients with severe knee deformities.
Highlights
The lower limb alignment is an important reference for clinical diagnosis and treatment of conditions affecting the knee
Limb rotation caused the deviation ranges of hip–knee–ankle angle (HKA) and femorotibial angle (FTA) measurement to be greater than 3 ◦
Our results demonstrated that the coronal deformity affected the alignment measurement errors greatly
Summary
The lower limb alignment is an important reference for clinical diagnosis and treatment of conditions affecting the knee. Malalignment of the lower extremity is recognized as a critical component of the etiology of knee osteoarthritis [4, 17, 19]. Correct alignment is critical for the outcome of total knee arthroplasty (TKA) [12], especially for personalized surgical design [16, 21]. Lower limb alignment is commonly assessed using measurements of anatomical or mechanical (weight-bearing) axes of the femur and the tibia, as depicted on plain radiographs. Quantification of these axes usually requires the identification of anatomical landmarks from hip to ankle. For evaluation of the knee alignment, long-leg radiographs (LLR) are considered as the gold standard image modality, with high inter- and intra-observer reliability
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