Idiopathic Scoliosis: Etiology and Anatomy

Stefan Parent, M.D., PhD, FRCSC
Montreal, QC 

Idiopathic scoliosis is a three-dimensional curvature of the spine that occurs with no clearly identifiable cause and remains a subject of active investigation. Progress has been made, especially in the fields of genetics and molecular biology, which could provide new insights into the etiology and pathophysiology of scoliosis development. The goal of this chapter is to review the current etiological theories and anatomical changes associated with idiopathic scoliosis.

 

Etiology
The cause of idiopathic scoliosis remains unknown. Different etiological theories have been proposed and studied extensively. Genetic factors, hormonal factors, growth abnormalities, biomechanical and neuromuscular theories, as well as different tissue disorders of bone, muscle and fibrous tissue, have all been proposed as possible causes of idiopathic scoliosis.

Genetic Factors
Several studies demonstrate a genetic component in the development of idiopathic scoliosis. Population studies have shown an increased incidence in families of patients with idiopathic scoliosis compared to the general population3,4,15,20,23,30,31. In one study, daughters of women with idiopathic scoliosis had a 27% prevalence of scoliotic curves greater than 15° 5. Studies of twins presenting with scoliotic curves show a prevalence of scoliosis in 73%-92% of monozygotic twins compared to 36%-63% in dizygotic twins6,8

Despite the accumulating evidence of a genetic etiology, the exact inheritance pattern, genes and gene products causing scoliosis remain unknown. The most likely inheritance pattern is multifactorial20. Genomic screening and statistical linkage analysis could provide new research avenues to help identify different genes responsible for scoliosis7.

Hormonal Factors
Melatonin deficiency has been proposed as a cause of scoliosis, as removal of the pineal gland (primary site of melatonin secretion) resulted in scoliosis in chickens12,13. In addition, patients with progressive scoliosis had a decrease of 35% in night-time melatonin secretion compared to patients without scoliosis progression14. Others have not been able to confirm these clinical findings1. More recently, a melatonin signalling dysfunction has been established in a series of patients with scoliosis17. In any case, the mechanism by which altered melatonin activity loads to scoliosis is unknown.

Spinal Growth/Biomechanical Theories
Abnormalities of spinal growth mechanisms also provide an attractive etiologic theory, because scoliosis development and progression occur during rapid adolescent growth10,11. Differential growth rates between the right and left side of the spine generate asymmetry that would be accentuated with asymmetric biomechanical loading in conformity with the Huetter-Volkman principle16,21,22,28 which states that growth is retarded by increased compression and accelerated when compressive loads 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 lordosis2,18,22,27. Willner and Johnson believe that anterior spinal growth outpaces posterior growth, producing hypokyphosis with subsequent buckling of the vertebral column, leading to the rotational deformity of scoliosis29. However, the cause for this theorized "mismatch" of anterior and posterior spinal column growth has not been presented. 

Tissue Abnormalities
Several theories have been proposed implicating each structural component of the spine (muscle, bone, ligaments and/or disc) as the causative agent of scoliosis. These theories are based on observations that conditions such as Marfan syndrome (fibrillin disorder), Duchenne muscular dystrophy (muscle disorder) and fibrous dysplasia of bone are each associated with the development of scoliosis. 

Central Nervous System Theories
Neuromuscular theories are based on the observations that patients with neuromuscular disease often develop scoliosis, and thus sub-clinical dysfunction of the central nervous system could also result in scoliosis development. Syringomyelia is associated with an increase in scoliosis incidence, possibly secondary to direct pressure on the sensory or motor tracts of the spinal cord26,33. Irritation of the brain stem may result from Chiari malformation or enlargement of the fourth ventricle and result in scoliosis. Postural equilibrium and vestibulo-ocular system dysfunction have been observed in patients with scoliosis24,25,32.

Anatomy
Idiopathic scoliosis is a three-dimensional deformity affecting the orientation and position of the spinal elements in space. The regional and global changes are characterized by a deviation in the frontal plane, a modification of the sagittal profile, as well as alterations in the shape of the rib cage. The most characteristic feature of scoliosis is the coronal plane curvature of the spine, the most common being a convex right deviation of the thoracic spine. Although, originally thought to be associated with kyphosis, in most cases the apical region of thoracic scoliosis is in fact hypokyphotic. The "apparent kyphosis" results from the convex side rib prominence that results from axial rotation of the vertebra in the transverse plane. Maximal at the apex, vertebral rotation alters the shape and orientation of the ribs, creating the rib prominence that makes the trunk appear kyphotic.

With increasing use of pedicle screws for fixation, the need for knowledge regarding pedicular anatomy has increased. The size and orientation of the thoracic and lumbar pedicles vary depending on the level. Lumbar pedicles are smallest in the upper lumbar spine while thoracic pedicles are narrowest through the mid-thoracic region. The transverse plane angulation is greatest in the upper thoracic spine, while a relatively constant sagittal inclination of 20° exists for all the thoracic pedicles. Knowledge of the location and orientation of each pedicle relative to the local surface anatomy of the posterior elements is critical in safely placing pedicle screws.

A morphometric analysis of anatomical specimens with scoliosis has demonstrated deformation of the vertebrae. This altered shape consists of frontal plane vertebral body wedging that is maximal at the apex of a typical scoliotic curve. In addition, the pedicle width is smaller on the concave side of the scoliotic curve (Figure 1)9,19. Because of the scoliosis, the spinal cord tends to lie closer to this smaller concave apical pedicle.

Parent_Figure_1
Figure 1. Characteristic vertebral deformity with smaller pedicle on the concave side. Note the approximate position of the spinal cord. On the right, a normal thoracic vertebra.

The etiology of idiopathic scoliosis remains a subject of active research. A strong genetic component exists in scoliosis and both parents and patients with scoliosis should be advised about the risk of their offspring developing scoliosis. A good understanding of the anatomy of the deformity is essential to help the surgeon provide the best surgical treatment possible.

References 

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