Several theories have been proposed implicating each structural component of the spine (muscle, bone, ligaments, and/or disc) as a cause of Scoliosis.
These theories are based on observations that conditions such as Marfans Syndrome (fibrillin disorder), Duchenne muscular dystrophy (muscle disorder), and fibrous dysplasia of bone are each associated with the development of Scoliosis.
The well know collagen fiber defects of Marfan Syndrome and Osteogenesis imperfecta have led researchers to investigate the connective tissues in AIS as well. A histomophometric study of intevertebral discs has shown changes associated with the distribution of collagen fibres in Scoliotic patients, but these changes were probably secondary to the Scoliosis rather than a primary cause. Subtle changes in the composition of the discs, however, certainly might allow the disc wedging seen in Scoliosis to occur.
Several investigators have proposed that scoliosis could be secondary to a functional deficit of muscular components. Changes in the distribution of muscle fiber type have been reported with decreased levels of type I and type II fibers in Scoliotic patients. Several other studies have demonstrated structural changes in muscle fibers with asymmetric changes found on the concavity and convexity of Scoliotic curves. However, these convexity versus concavity modifications seem more likely to be secondary adaptations of the muscle in reaction to the structural changes of Scoliosis.
Spinal Growth and Biomechanical Theories
Abnormalities in spinal growth mechanisms also provide an attractive etiologic theory because Scoliosis development and progression are temporarily related to the time of rapid adolescent growth. Differential growth rates between the right and left side of the spine could generate assymmetry that would be accentuated with assymmetric biomechanical loading and the Huetter-Volkman principle, which states that growth is retarded by increased compression and accelerated when compressive leads are reduced.
This mechanical modulation of growth may explain progressive deformity observed in some patients during growth.
Other investigators have postulated that the etiology of Scoliosis relates to the development of relative thoracic Lordosis. The theory suggests that anterior spinal growth outpaces posterior growth, producing hypokyphosis with subsequent buckling of the vertebral column, which leads to the rotational deformity of Scoliosis. However, the cause for this theorized mismatch of anterior and posterior spinal column growth has not been presented. It may relate to differences in the rates of endochondral and intramembranous growth of the vertebral elements. Interestingly, it has been documented that thoracic kyphosis tends to decrease in normal children during the adolescent growth may be important in the development of Scoliosis. In addition, several studies suggest that adolescents with Scoliosis are taller than their peers!!!