Aetiology

Although many theories have been proposed and extensively tested, the aetiology of adolescent idiopathic scoliosis (AIS) remains unknown.

There is agreement among investigators that there is a genetic component, as the prevalence of scoliosis in the daughters of women with AIS is almost 30%.[14] Additionally, twin concordance studies have shown a prevalence of 73% to 92% in monozygotic twins and 36% to 63% in dizygotic twins.[14][15][16] Despite these observations, specific genetic abnormalities have not been consistently shown.[17][18][19][20] The true inheritance pattern of AIS is likely to be multifactorial.

Hormonal imbalances, most notably those of decreased melatonin or increased growth hormone levels, have also been proposed as a contributing factor. However, results from studies are inconsistent.[16][17][18][19][20][21]

Theories involving structural tissue abnormalities within the spine have also been tested. These theories are based upon the knowledge that conditions involving decreased structural integrity of muscle, bone, or ligaments (e.g., spinal muscular atrophy, polyostotic fibrous dysplasia of the axial skeleton, and Marfan's syndrome, respectively) have an increased propensity towards the development of scoliosis.[22][23]

Pathophysiology

The pathophysiological process behind the development of AIS is unknown. The observation that curve development and progression correlate with the period of rapid adolescent growth appears to support a biomechanical contribution. However, multiple theories exist that attempt to explain the process by which the development takes place, and while each makes sense from a biomechanical standpoint, it has been difficult to directly correlate these theories to the in vivo adolescent scoliotic spine.

The general concept suggested by the collective literature is that the process begins with a multifactorial propensity towards the development of scoliosis that is accelerated through the complex biomechanical environment of the rapidly growing adolescent spine.

Classification

Early versus late[2]

Idiopathic scoliosis can be classified as early onset or late-onset.

Early onset idiopathic scoliosis

  • Develops before 10 years of age.

Late-onset idiopathic scoliosis

  • Develops after 10 years of age.

Infantile versus juvenile versus adolescent[2]

Idiopathic scoliosis can also be classified as infantile, juvenile, or adolescent.

Infantile idiopathic scoliosis

  • Develops before 3 years of age.

  • Often occurs with other congenital abnormalities and, in approximately 90% of cases, the curve resolves without treatment.

  • Approximately 90% of cases present with a left thoracic curve.

Juvenile idiopathic scoliosis

  • Develops between 3 and 10 years of age.

  • Curves are often progressive with the potential for severe truncal decompensation (severe, clinically apparent deformity in which the trunk is no longer centred over the pelvis, resulting from coronal imbalance in severe curves) and subsequent cardiac and pulmonary complications.

  • Nearly 90% of these patients will require surgical arthrodesis secondary to the magnitude of the curvature and the degree of curve progression.

Adolescent idiopathic scoliosis

  • Develops between 10 and 18 years of age.

Descriptive system of curve classification

Spinal deformities in scoliosis show a variety of curve patterns. This classification system provides a description of the curvatures based on the location of the apical vertebra and the direction in which the curve deviates from the midline. For example, the most common curve is a right-sided thoracic curvature (where the apical vertebra is located between T2 and T11 and the convex side of the curve is to the right).

  • Cervical: curve apex located up to C6 and C7

  • Cervicothoracic: curve apex located between C7 and T1

  • Thoracic: curve apex located between T1 and T2 and T11 and T12

  • Thoracolumbar: curve apex located between T12 and L1

  • Lumbar: curve apex located up to L1 and L2.

King-Moe system

Previously the most widely used classification for scoliosis, this system was developed by surgeons to determine which portions of the spine would require instrumentation in order to correct the overall deformity. It was not designed to be a complete classification system for scoliosis.[3]

There are 4 curve types described within this classification system.

  • Type I: lumbar dominant and S-shaped (approximately 10% of curves).

  • Type II: thoracic dominant and S-shaped (approximately 33% of curves).

  • Type III: thoracic dominant, lumbar, and thoracic curves do not cross the midline (approximately 33% of curves).

  • Type IV: described as long or double thoracic (approximately 10% of curves).

Lenke and associates system

A complex classification system providing guidance on surgical decision making for all scoliosis curve patterns.[4][5][6][7] It takes into account both coronal and sagittal plane deformity and is composed of a number (1-6) describing the type of curve, a lumbar curve modifier (A, B, or C), and a thoracic sagittal modifier (based on degree of kyphosis present).

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