Assessment of the plane of maximum curvature in adolescent idiopathic scoliosis using 3D ultrasound : an exploratory study.

AIMS: The size of the curves in patients with adolescent idiopathic scoliosis (AIS) is traditionally assessed using 2D Cobb angles, which may underestimate the extent of this complex 3D deformity. The plane of maximum curvature, measured using 3D reconstructions of the spine, including 3D ultrasound, offers more accurate 3D representation. However, the optimal 3D ultrasound-based method of measuring the plane of maximum curvature remains unknown. We evaluated the reliability and agreement of three methods of measuring the plane of maximum curvature (the maximum-curvature, endplate-apex-endplate, and maximum-axial-rotation methods) to provide radiation-free and severity-specific clinical recommendations for 3D ultrasound measurements of the plane of maximum curvature using the SRS-Lenke-Aubin 3D classification.

METHODS: A total of 88 patients with right major thoracic AIS, stratified by the severity of the curve (mild 29; moderate 34; severe 25), underwent 3D ultrasound. Conventional 2D parameters were quantified from the 3D ultrasound reconstructions. For each method of measurement of the plane of maximum curvature, we recorded the ultrasound curve angle (UCA) on the plane, its orientation, and the difference between it and the coronal UCA (ΔUCA). The reliability of all variables, the agreement between the three methods of measurement, comparisons between the severity groups, and correlations between the parameters of the deformity were analyzed.

RESULTS: All parameters showed excellent intraobserver reliability. The maximum-curvature and endplate-apex-endplate methods showed high agreement in the UCA measurements of the plane of maximum curvature and were consistent across the different severities of curve. For both methods, the orientation of the plane of maximum curvature was positively correlated with the coronal curvature and negatively correlated with the thoracic kyphosis. The mean ΔUCA exceeded the clinically relevant threshold of 5° in mild curves. In contrast, measurements using the maximum-axial-rotation method showed greater bias in the UCA measurement of the plane of maximum curvature and inverse associations; the orientation of the plane of maximum curvature showed negative correlations with the coronal and sagittal curvatures as well as with the apical and maximum axial vertebral rotations. The ΔUCA obtained by the three methods did not differ significantly across severity groups.

CONCLUSION: 3D ultrasound-based measurements of the plane of maximum curvature are reliable for the evaluation of the deformity in patients with AIS. The endplate-apex-endplate method is recommended for routine assessment of the plane of maximum curvature in moderate and severe curves, offering an automatic, reliable, and radiation-free alternative for the 3D monitoring of the scoliotic spine.