Clinical Biomechanics: Scoliosis
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This is Chapter 13 from RC’s best-selling book:
“Clinical Biomechanics:
Musculoskeletal Actions and Reactions”Second Edition ~ Wiliams & Wilkins
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Chapter 13: Scoliosis
In traditional medicine, scoliosis is commonly ignored until gross cosmetic effects or signs of structural destruction are witnessed. In chiropractic, however, even minor degrees of distortion should be considered at the time of spinal analysis because of their subtle biomechanical and neurologic consequences, and to halt potential progression at an early stage. To give a better appreciation of these points, this chapter describes the general structural, examination, and biomechanical concerns that should be considered, along with the highlights of conservative therapy.
GENERAL CONSIDERATIONS
The Spinal Curves [1-9]
A curved column has increased resistance to compression forces. This is just as true in the spine, as for a rib or long bone. Most authorities consider the spine to have four major curves: anteriorly convex curves at the cervical and lumbar areas and, anteriorly concave curves at the thoracic and sacral levels. Cailliet considers the coccyx a curve, but this curve is usually considered an extension of the sacral curve. A few authorities consider the atlanto-occipital junction as a separate anteriorly convex curve. Regardless, the spinal curves offer the vertebral column increased inflexibility and shock-absorbing capability while still maintaining an adequate degree of stiffness and stability between vertebral segments (Fig. 13.1).
Structural vs Functional Curves
The adult thoracic and sacral anteriorly concave curves are firm structural arcs as the result of their vertebral bodies being shorter anteriorly than posteriorly. The normal kyphosis of the adult thoracic and sacral curves is quite similar to that of the fetal spine. This is not true for the anteriorly convex cervical and lumbar regions where the curves are essentially the result of their soft tissue wedge-shaped IVDs. It is for this reason that the cervical and lumbar curves readily flatten in the supine position, while the thoracic kyphosis reduces only a slight amount.
There is a clinical correlation of disc wedging to disc disease. Most disc lesions are found in the cervical and lumbar regions where the greatest degree of physiologic wedging occurs. This appears to be true in both hyperlordosis and an exceptionally flat cervical or lumbar curve.
Effect of Bipedism
An adult discless spine would resemble that of the newborn. Since animals that walk on four legs and infants prior to assuming the erect position do not have the physiologic curves of the erect adult, it can be assumed that these curves are the result of bipedism. In the erect position, the lower lumbar area is especially subjected to considerable shearing stress. [10, 11]
Overall Balance
Although the spine is often considered as the central pillar of the body, this is only true when the spine is viewed from the anterior or posterior aspect. When viewed laterally, the spine lies distinctly posterior to the thoracic body mass essentially because of the space-occupying heart (Fig. 13.2), It lies much more centrally in the cervical and lumbar regions. An abundance of body mass also lies anterior to the midline in the head, which must be held by erector and check ligament strength if a thoracic “hump” or a flattened cervical curve are to be avoided.
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From a balance standpoint, the 7 cervical vertebrae and the 5 lumbar vertebrae arc anteriorly from the central gravity line to compensate for the 12 thoracic vertebrae curved posteriorly from the gravity line. The female spine, below the age of 40 years, arcs less than that of the male, and the thoracic arc increases with age regardless of gender. The extent of deviation from the midline of the cervical and lumbar curves is controlled to a great extent by the strength of the antigravity extensors, the weakness of the flexors, and the ability of the flexors to stretch. Keep in mind that the fetus is curved like a crescent. When the infant gains the erect position, the anterior lumbar curve and pelvic tilt are governed essentially by the strength of the lumbar-pelvic erectors and how far the iliopsoas and iliacus will elongate.
The Base Effect
The greater the SI or T1 angles, the sharper the lumbar or cervical curves must be to bring the spine back toward the center of gravity. Thus, the sacral angle and wedge shape of the LS disc determine the angles of the lower lumbars and the compensatory upper lumbar angles, and the Tl plane and shape of its disc generally control the cervical curve. Ideally, this base effect would progress up the spine from the sacrum so that the odontoid process would be in line with the gravity line. However, because the thoracic area allows minimum mobility to the anterior or posterior, considerable stress is placed at the L1-T12 and T1-C7 junctions.
Spinal Stability [12-17]
The stability of the spine depends upon a number of factors, but essentially it is maintained by the relationship of the vertical gravity line to the segments. When weight is in balance to the gravity line, muscular activity is minimal. When in a chronic unbalanced state, fatigue and structural deformity are induced (Fig. 13.3). This is readily brought out when vertebral asymmetry horizontally or a short leg produces a lateral tilt that must be compensated by scoliosis. In short time, the muscles on the concave side of the curve become fatigued, the vertebral bodies rotate toward the side of convexity, and the spinous processes rotate toward the side of concavity.
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