How Are Scoliosis Curves Classified?

By David K. Wolpert

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An orthopedist may have told you that you have “idiopathic scoliosis, with a right thoracic structural curve and a compensatory lumbar curve.” This section will help you decode such language so that you can better communicate with your surgeon and focus additional research you may conduct on your specific type of scoliosis.

Cases of scoliosis are typically classified in several dimensions: age of onset, cause, the nature of the curve, and the location, type, and convexity of the curve.

Age of onset. This is the age at which your curvature first developed. It is classified either as congenital (scoliosis that developed due to a birth defect), infantile (from birth to age three), juvenile (ages three to ten), adolescent (ages ten to eighteen, or whenever skeletal maturity is reached), or adult (explained below). Most people with scoliosis developed the condition in their teenage years during their growth spurt.

There are two types of adult scoliosis. The more common type is a scoliotic curve that developed prior to age eighteen but continues to progress even though the patient’s bones have finished growing (have reached skeletal maturity). This is called adult idiopathic scoliosis. The less common type is called de novo scoliosis, which is scoliosis that develops later in life, typically after the age of forty. It is believed to be caused by degeneration in the discs and facet joints of the spine and is usually related to arthritis or osteoporosis. It may also be referred to as degenerative scoliosis. De novo scoliosis typically affects only the lumbar region of the spine.

It is important not to confuse the initial onset of scoliosis with the timing of diagnosis. The latter may postdate the former by many years. It is possible to reach adulthood without ever knowing you have scoliosis, even though it probably developed in your adolescent years.

Cause. As mentioned in the previous section, the cause of most scoliosis cases is unknown (idiopathic) but in some cases a cause can be attributed.

Structural versus nonstructural curves. Most people with scoliosis have more than one lateral curve in their spine. Each curve can be classified as either structural (also called primary or major) or nonstructural (usually called compensatory, though also called secondary or minor). Structural curves are the “real” scoliotic curves. They exhibit some degree of permanent curvature because the curve is rigid. They also tend to be the largest curves. To keep the spine as balanced from side-to-side as possible, other sections of the spine may develop a curve in the opposite direction to the structural deformity. These nonstructural, or compensatory, curves generally remain flexible and are thus not “permanent.” Estimating Correction (Chapter 2) provides more information on determining the flexibility of curves.

An individual with a right-bending thoracic curve, for example, may have three curves—a structural curve in the thoracic area, and two compensatory curves, one each in the cervical or upper thoracic area and one in the lumbar region. Most surgeons will only operate on the structural curve because the compensatory curves will generally spontaneously realign once the structural curve is reduced. Surgically correcting only the structural section of a curve is called a selective fusion.

Note that flexible, nonstructural curves can become progressively more rigid as you age. It is thus possible for a flexible compensatory curve to become a rigid structural curve over time, though this curve would still be considered secondary to the primary curve. It is also possible for a primary curve to be nonstructural, if the scoliosis is caused by a neuromuscular disorder, for example.

Location and convexity of the curve. Over 90% of all scoliotic curves exhibit one of four patterns (Figure 3), which are described below.

It is helpful to be familiar with three terms commonly used to describe curves: convex, concave, and apex. Convexity is the direction in which a particular curve bends, or “points” (to the right or to the left). The convex side of the curve is the outer angle; the concave side is the inner angle. The apex of the curve is the most deviated, or off-center, vertebra. If you consider the curve like a mountain, the apex is the peak. There may be more than one apex if you have multiple curves. The common curve types are: