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Scoliosis is considered to be present when there is a coronal plane curvature of the spine measuring at least 10°. However, treatment is not generally instituted unless the curvature is > 20-25°. The curvature may be balanced (returning to midline) or unbalanced. The vertebrae at the ends of the curve are designated the terminal (or end) vertebrae, whereas the apical vertebra is at the curve apex.
Curvatures are described by the side to which they deviate. A dextroscoliosis is convex to the right, with its apex to the right of midline. A levoscoliosis is convex to the left, with its apex to the left of midline.
Curvatures can be categorized as flexible (normalizing with lateral bending toward the side of the curve) or structural (failing to correct).
Most scoliotic curvatures are associated with abnormal curvature in the sagittal plane. These are described as kyphosis (apex dorsal) or lordosis (apex ventral).
Scoliosis due to fracture, congenital anomaly, or infection typically has an angular configuration. Other causes of scoliosis tend to have a smooth curvature. Scoliosis most commonly involves the thoracic spine, followed by the thoracolumbar spine. In the past, curves were categorized as primary and secondary (compensatory), but it is often difficult to make the distinction. Therefore, these designations are no longer commonly used.
The Cobb method is most commonly used to measure scoliosis. The vertebrae at each end of the curve (the terminal vertebrae) are chosen. These are the endplates with the greatest deviation from the horizontal. The curvature is the angle between a line drawn along the superior endplate of superior terminal vertebra and a line along the inferior endplate of the inferior terminal vertebra. In severe curvatures, the endplates are often difficult to see. In that case, the inferior cortex of the pedicle can be used as the landmark for making the measurement. If measurements are made on hard copy radiographs, it is usually necessary to draw lines perpendicular to the endplates and measure the angle between the perpendicular lines. On most PACS, the measurements can be made directly from the endplates.
The Ferguson method is another way to measure scoliosis. In this method, lines are drawn from the center of the apical vertebra to the center of each terminal vertebrae. The angle of the scoliosis is the angle between these 2 lines.
Scoliosis is almost always associated with abnormal curvature in the sagittal plane. The most common finding is loss of normal thoracic kyphosis. The Cobb method can be used to determine sagittal plane deformity. Rotational deformity is often present but can only be grossly assessed on radiographs. It can be measured on CT scan by superimposing the apical and terminal vertebra.
Normally, the T1 vertebra is centered over the L5 vertebra in both the coronal and sagittal planes. Coronal or sagittal plane imbalance can be measured as the horizontal distance between the center of the L5 vertebral body and a plumb line drawn through the center of the T1 vertebral body.
Rotational deformity is present in most types of scoliosis. This is difficult to measure. Measurement is most easily made by superimposing axial images of the terminal and apical vertebrae or by using 3D CT.
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