In vivo three-dimensional segmental analysis of adolescent idiopathic scoliosis

Abstract

Introduction: An accurate assessment of three-dimensional (3D) intervertebral deviation is crucial to the better surgical correction of adolescent idiopathic scoliosis (AIS). However, a precise 3D study of intervertebral deviation has not been previously reported.

Objective: The purpose of the present study is to evaluate the intervertebral coronal inclination, axial rotation and sagittal angulation of AIS using 3D bone models and a local coordinate system.

Materials and methods: 3D bone models of the thoracic and lumbar spine of ten AIS patients were constructed using computed tomography. The local coordinate axis was determined semi-automatically for each vertebra. By using these local coordinates, the intervertebral deviation angles were calculated in the coronal, axial and sagittal planes and projected to subjacent local coordinates.

Result: The intervertebral deformity in the coronal plane was larger near the apical region and smaller near the junctional region. Conversely, the intervertebral rotation in the axial plane was smaller near the apical region, and larger near the junctional region. Concerning the sagittal plane deformity, the constant tendency was not recognized.

Conclusion: Using a local coordinate system for the vertebra of AIS, we measured the 3D intervertebral coronal, axial and sagittal deviation of the thoracolumbar spine and found that the change in the intervertebral inclination angle in the coronal plane increased toward the apical region and decreased toward the junctional region, and that the converse tendency was noted for the axial intervertebral rotational angle. This analysis provides an improved 3D guide for the surgical correction of AIS.

Fig. 1

The method used to establish the local coordinate system. a The black sphere is a centroid of the vertebra that is defined as the origin of the coordinate axis. b The cross-sectional surface of the vertebra; the planar approximation of the superior endplate was calculated using the least-squares method, and we estimated a plane parallel to the superior endplate via the origin. c In the cross-sectional plane, the z axis is defined as a line between the centroid and the point that divides the front portion of the vertebral body in half. d A line perpendicular to the z axis and pointing to the left on the plane forms the x axis. Finally, the y axis pointing cranial is defined as a line perpendicular to the z–x plane

Fig. 2

The T8 and T9 bone models and each local coordinate axis. The solid lines present the axis of T8, and the broken lines present the axis of T9. Left the intervertebral coronal inclination. The figure faces the subjacent vertebral x–y plane; the intervertebral coronal inclination angle (arrow) is defined as the angle between two adjacent x axes projected on the subjacent x–y plane. Middle the intervertebral axial rotation. The figure faces the subjacent z–x plane; the intervertebral axial rotational angle (arrow) is defined as the angle between adjacent z axes projected on the subjacent z–x plane. Right the intervertebral sagittal angulation. The figure faces the subjacent y–z plane; the sagittal intervertebral angulation (arrow) is defined as the angle between adjacent y axes projected on the subjacent y–z plane

Fig. 3

The intervertebral deviation of the each adjacent vertebrae of all ten patients. The x axes show the each intervertebral deviation angles of all patients in coronal, axial and sagittal plane. The intervertebral deformity in the coronal plane was larger near the apical region and smaller near the junctional region. Conversely, the intervertebral rotation in the axial plane was smaller near the apical region, and larger near the junctional region. Concerning the sagittal plane deformity, the constant tendency was not recognized

Fig. 4

Two representative cases. The bone models of both the thoracic and lumbar spine are shown in the center, and the left sided graph represents the intervertebral inclination angle in the coronal plane; the right sided graph represents the intervertebral rotational angle in the axial plane. Left case 3, Right case 10