Background: During total hip arthroplasty (THA) in lateral decubitus, perioperative radiography allows the surgeon a simple evaluation of the leg length difference (LLD) by measuring the proximal femoral length. However, the effect of femoral malpositioning on proximal femoral length measurements during the evaluation of perioperative radiographs is not adequately understood. We aimed to (1) investigate the effects of malposition on proximal femoral length using three-dimensional computer simulations and (2) verify whether a simple correction formula can improve the accuracy of LLD evaluation on perioperative radiographs.
Methods: We analyzed 86 patients who underwent THA. Digitally reconstructed radiography (DRR) images were reconstructed in various limb positions (femoral abduction, adduction, and flexion), and proximal femoral length measurements in those malpositions were simulated. Additional morphological measurements of the femoral neck torsion angle in the sagittal plane were performed to elucidate the simulation findings. The malposition angle of abduction-adduction was evaluated with actual perioperative radiographs, and trigonometric correction was attempted.
Results: The leg length measurement decreased as the femoral DRR image shifted from neutral to abduction and adduction, demonstrating approximately 1 mm per 10° of abduction or adduction. The leg length measurement increased as the femoral image shifted from neutral to 10° and 20° of flexion, demonstrating approximately 3 mm per 10° of flexion. With a peak at 20° of flexion, the proximal femoral length measurement decreased in the DRR images at 30°, 40°, 50° and 60° of flexion. The femoral neck torsion angle was 21.1 ± 5.6° on the operative side. The effect of coronal malposition on leg length discrepancy was so small that the difference following trigonometric correction was not statistically significant (p=0.108).
Conclusion: In the present simulation, coronal malposition had a small effect on LLD evaluation. As the femoral neck has a torsion of approximately 20°, the proximal femoral length is projected the longest when the femur is flexed 20°. With careful positioning of the limb in the coronal plane, the use of a correction formula for LLD evaluation would not be necessary. Surgeons should ensure that both lower limbs are in the same position in the sagittal plane during THA in lateral decubitus.
Keywords: digitally reconstructed radiography; hip; leg length; malpositioning; total hip arthroplasty.
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