The most common form of scoliosis is idiopathic scoliosis, with which no congenital spinal deformities are present, or any diseases, that may explain the scoliosis (such as muscle diseases). The prevalence of idiopathic scoliosis for curvatures of 10° is 2-3% and for curvatures of 20° it is 0.3-0.5%. The prevalence is lower for larger curvatures.
The number of degrees is measured on x-rays according to Cobb’s method [Figure 67]. A tangent line is drawn along the uppermost end plate of the most tilted vertebra, at the upper side of the scoliotic curve. Another tangent line is drawn along the lower end plate of the most tilted vertebra, on the lower side of the scoliotic curve. The plumb lines placed on the tangent lines together form an angle that is called the scoliosis angle. Cobb’s angle measure of 10° or more is classified as scoliosis.
The larger scoliotic curvatures occur more frequently in girls compared to boys.
Figure 67
With structural idiopathic scoliosis, a number of characteristics are visible, which depend on the location.
The Thoracic Type
This is the most common type of scoliosis. Thoracic scoliosis nearly always has convexity (bulging) to the right. It is not known why this is the case. Only with development of idiopathic scoliosis at a very young age (the infantile type), or with scoliosis that is caused by intraspinal abnormalities (syringomyelia or spinal cord tumours), can the scoliosis be convex to the left side. There should always be suspicious of a special cause of scoliosis when it concerns an atypical form (scoliosis in the wrong direction or very rigid scoliosis).
In upright standing position, the following is observed with a usual right-sided convex form. The right scapula is positioned in a more lateral and backwards direction compared to the left scapula. If there is also a high thoracic left convex contralateral curvature, the left shoulder may be positioned slightly higher. Otherwise, the right shoulder is positioned higher or there is no shoulder difference. With the more severe forms, the whole thoracic dome is pointed to the right compared to the rest of the torso and the plumb line, hanging from the spinous process of C7, will end up to the right side of the natal cleft [Figure 68]. This is known as disequilibration, which is expressed to the nearest 0.5 centimetre. Not much deformity is often visible from the front, only a discrete asymmetry of the chest. For women, there may also be a difference in chest size (the left breast is nearly always larger).
When the patient bends forward (stoop test), with the arms hanging down and palms of hands loosely against each other, the rib deformity is visible clearly. During this, the examiner is seated in front or behind the patient and glances over the back to discover an asymmetry. The right rib arch protrudes to the back (the ribs on the right are seen to be slightly elevated) in relation to the left rib arch, which has actually dropped. On the one hand, this happens due to rotation of the vertebrae in the middle of the scoliotic curve, and the rib attachment to the rotated vertebrae. On the other hand, it is because of deformity of the ribs themselves, as part of the scoliosis. This gibbus deformity can be measured using the gibbus height and gibbus rotation. The gibbus height is the difference in height between the highest ribs on the right and the lowest ribs on the left, at equal distance from the spinous process of the apex vertebra (this is the most rotated vertebra in the middle of the scoliotic curvature) measured with a scoliometer [Figure 68].
The gibbus rotation is the angle that the apex vertebra and most rotated ribs make in relation to the plane of the floor. This angle can be measured with a special rotation meter, a type of level, which expresses a rotational angle in degrees. These meters are only used by school doctors and orthopaedists specialising in scoliosis. It should be noted that with these measurements, a positive gibbus result can be obtained on a person with a completely straight back when only the vertebrae rotate.
Often, a single curvature with structural characteristics (rotation, stiffness) has a compensatory opposite curve, that lacks structural characteristics and hence, is called compensatory. On anterior-posterior x-ray images (taken from the dorsal side) with the patient in a position of maximum lateroflexion to the left and then right, the compensatory curve will straighten completely. This opposite curve is needed for the patient to stand in a perpendicular position. If the opposite curve does have structural characteristics, the diagnosis should be S-form scoliosis instead of C-form scoliosis.
Figure 68
The Thoracolumbar Type
This type of scoliosis is less common than the thoracic form. Here, the curvature is located lower with the top of the curve (the apex) often at the level of L1 or L2. This type of scoliosis can be point in a left or right direction. The scoliosis is visible with the patient standing upright, because of the waist triangle difference, and asymmetrical silhoutte. On the convex side, the waist triangle is completely or partially flattened, whilst the waist triangle on the concave side is deeper. In severe scoliosis, this may even lead to skin folding. The low scoliosis is often confused with pelvic tilting, which is usually absent but appears to be present. During the stoop test, the gibbus can be detected, although it is often less profound than with the thoracic form due to the lack of rib involvement. The gibbus is formed by the rotated vertebral elements and the muscles covering the back. The gibbus is expressed as follows: gibbus height (cm) and gibbus rotation (degrees).
The Combination of Thoracic and Thoracolumbar S-Scoliosis
This type is even rarer than the previous two types. Here, two opposite scoliotic curvatures are present. The thoracic curvature is virtually always convex on the right, with the thoracolumbar convex curvature in the left direction. If a convex thoracic scoliosis to the left is present, one must be suspicious of an underlying cause for the scoliosis, as already discussed for the thoracic type. With this scoliosis, both the scapula asymmetry as well as the waist triangle difference are visible and during anteflexion, both the thoracic and lumbar gibbus can be detected. S-shaped scoliosis is often well equilibrated (the plumb line will hang inside the natal cleft) and the deformity of the back will have a better cosmetic appearance than with a single curvature. In the case of thoracic scoliosis with compensatory thoracolumbar opposite curve, no lumbar gibbus will be visible and with a single thoracolumbar scoliosis with compensatory thoracic opposite curve the thoracic gibbus will be absent.