A few weeks or months after the onset of infantile paralysis, the spine discloses changes representing a pathological entity,—the paralytic spine. This process can be subdivided into several stages characterized by (1) morphological and (2) functional signs.
1. At the onset we observe enlargement of the intervertebral spaces, followed later on by dullness and cloudiness; the border lines between discs and vertebrae are effaced; and the spaces themselves become uneven and appear markedly narrowed. Ossification of the epiphyseal ring starts in some cases as early as the age of four; the process is very irregular, and the ossification is bulky and confluent. Minute or larger calcium deposits appear in the intervertebral spaces. This process may last for years, accompanied by general bone atrophy; in many cases after a couple of years the process apparently heals spontaneously.
2. Functionally, the paralytic spine is characterized by a high degree of flexibility and compressibility. Because of the increased mobility, univertebral or segmental rotations appear first without any lateral deviation. Due to the flexibility, forward-backward tilting of one or more vertebrae occurs with the disappearance of thoracic kyphosis and lumbar lordosis. As a result of the increased mobility and compressibility—the prime factor in the formation of scoliosis—a translatory shift of the vertebrae appears.
The paralytic spine is the pathological condition preceding paralytic scoliosis. The scoliotic curve requires usually from four to five years. before reaching its final form. Prior to this, the side of the convexity and the direction of the rotation may change several times. The rotation and compression of the spine are the chief factors in preparing the way for paralytic scoliosis.
The rotation is brought about by faulty mechanics of: (a) the pelvis (pelvic rotation); (b) the thorax and shoulder girdle (thoracic rotation); and (c) the respiration.
In pelvic rotation, all spinous processes point to the same side of the body. In thoracic rotation, the thoracic and the lumbar spinous processes point in opposite directions.
The cause of rotation is the pathological imbalance between the two sides of the body in carrying out rotary motions of different degrees during the performance of the daily routine, especially during locomotion. The physiological imbalance, present in every human being, takes advantage of the decreased resistance of the rotary system of the spine—that is, damage of the discs—and causes the predominance of the right thoracic, left lumbar curves.
Paralytic scoliosis is brought about by the imbalance between the two sides of the body exerted on the paralytic spine. Paralytic scoliosis can be differentiated from scoliosis of any other etiology by the uniform density of the spine in the roentgenogram, by the excessive and early rotation of the vertebrae, and by the temporary concave rotation.
Curves resulting from pelvic rotation, except for the sitting curves, have a far better prognosis than the thoracic curves, especially if the latter are associated with respiratory disturbances.