Ferguson's concept of a primary curve was the first important step in the solution of the problem of scoliosis. However, the misuse of the terms primary curve and secondary curve has caused confusion.
The normal spinal curves in the anteroposterior plane of the erect spine may be used as a basis for defining three types of lateral curves:
1. A structural curve is one with structural changes in bone, nerve, muscle, or other tissues and is a curve which the patient cannot correct and maintain corrected when he is erect;
2. A functional curve is one without structural changes that results from the normal available angulation between vertebrae. It is a curve which the patient can correct and maintain corrected when he is erect;
3. A combined structural-functional curve is one which has some structural and some functional characteristics which the patient can partly correct when erect.
Structural and functional curves have definite characteristics easily noted on a single erect anteroposterior roentgenogram. The cardinal roentgenographic
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signs of a structural curve are abnormal wedging, angulation, rotation, or position of the vertebrae in the curve.
If we remember the characteristics of a normal functional curve and observe the relative amount of abnormal wedging, and angulation, and rotation, and the changes in position of the vertebrae in any curve, it is easy, with a single erect anteroposterior roentgenogram, to determine which curves are major structural curves (primary), which are functional (secondary) curves, and which are combined structural functional or minor structural curves.
Although roentgenograms of the spine bent to either side or sitting with the pelvis tilted are helpful before arthrodesis in deciding how much the opposite curves will reduce and how much the major curve may be reduced without loss of balance, they are not necessary to determine which curves are major structural (primary) curves.
There are multiple curves in most patients with scoliosis and in view of the confusion caused by the misuse of the terms primary curve and secondary curve it might be helpful to use the terms structural curve and functional curve. Since there are very few pure functional curves and most spinal curves have some structural changes we may describe those with major structural changes as major structural curves and those with relatively minor structural changes as minor
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structural curves. Thus we could describe a common triple-curve pattern as a single major structural curve with minor structural curves above and below, indicating that the middle curve has the greatest structural changes but the curves above and below also have some structural elements. We could describe another problem as double major structural curves with minor structural or functional curves above and below.
In scoliosis it is important to estimate the relative structural changes in the curves and the resultant displacement of the head from its central spinal axis.
Copyright 1960 by The Journal of Bone and Joint Surgery, Incorporated