It is most frequently met with in rapidly growing girls of poor physique who are overworked at school or lessons, or on commencing an apprenticeship for which they are physically unfit. In some cases there is nasal obstruction from adenoids, in others the development and free play of the chest are interfered with by tight and ill-fitting garments; in all of them the muscular system is weak and the muscles of the trunk do not take their proper share in maintaining the erect posture. The most important determining factor would appear to be the habitual or repeated assumption of faulty attitudes, partly from carelessness, largely from fatigue, in order to relieve the feeling of tiredness in the back. So far as is known, the condition does not occur in communities living under aboriginal conditions. In some cases there is a hereditary tendency to scoliosis; we have seen it, for example, in a father and his daughters.
The excessive use of one arm in the carrying of weights, the habit of resting on one leg more than the other, or the assumption of a faulty attitude in writing or in playing the piano or violin, doubtless, determine the seat and direction of the curvature, and, when it has once commenced, tend to aggravate and to perpetuate it.
It is probable that the greater frequency of the primary curvature towards the right is associated with the more general use of the right hand and arm, although primary curvatures towards the left are not confined to left-handed persons.
Morbid Anatomy.—The original deviation or “primary curve” is usually in the thoracic region, and has its convexity directed towards the right side. To re-establish the equilibrium of the column, “secondary” or “compensatory” curves, with their convexities to the left, develop in the regions above and below the primary curve. It has been proved experimentally that lateral deviation of the spine is inevitably accompanied by rotation of the vertebræ around a vertical axis, in such a way that their bodies look towards the convexity of the curve, while their spines, laminæ, and articular processes are directed towards the concavity ([Fig. 226]).
As the deformity increases, the individual vertebræ are distorted, the bodies becoming wedge-shaped from side to side, the base of the wedge looking towards the convexity of the curve, while the narrow end looks towards the concavity ([Fig. 228]). As the spine, laminæ, and articular processes also undergo alterations in shape, a line uniting the tips of the spinous processes does not furnish an accurate index of the degree of lateral deviation but minimises it considerably. The muscles and ligaments are altered in length in accordance with the changes in the shape and position of the bones.
In the thoracic region, the ribs necessarily accompany the transverse processes, so that on the side of the convexity they form an undue prominence behind—the “rib-hump” ([Fig. 227]), while on the side of the concavity the chest is flattened and the ribs crowded together so that the intercostal spaces are diminished or even obliterated. The converse—flattening on the side of the concavity—is seen on the front of the chest.
Fig. 227.—Adolescent Scoliosis in a girl æt. 23.
The general shape of the thorax is altered: on the side of the convexity it is longer and narrower than normal and its capacity diminished, while on the side of the concavity it is shorter and broader and its capacity is increased.