Etiology

Congenital scoliosis

Congenital scoliosis is the most frequent congenital spinal deformity. It is present at birth as the result of embryologic or intrauterine maldevelopment of vertebral elements. The term “congenital” is slightly misleading because it implies that the curvature is apparent at birth, but this is not necessarily so. The vertebral anomalies are present at birth, and the clinical deformity develops with spinal growth and may not become apparent until later childhood . These may be caused by failure of formation or failure of segmentation. They are commonly associated with cardiac or urologic abnormalities that develop during the same period (before 48 days of gestation) ( Fig. 1 ) . Vertebral maldevelopment can be classified as defects of segmentation or defects of formation. Curve progression is strongly related to the type of vertebral abnormality with the poorest prognosis for unilateral unsegmented bars with contralateral hemivertebrae (up to 10°/yr progression), a less severe progression in cases of hemivertebrae or double hemivertebrae (1–2.5° and 2–5°/yr, respectively), and a least severe progression in patients who have block and wedge vertebrae (<1°/yr progression) ( Fig. 2 ). The most common anomaly is hemivertebrae, which is seen in about 40% of cases.

A 5-year-old boy who has Klippel-Feil syndrome type II. Klippel-Feil syndrome occurs in a heterogeneous group of patients unified only by the presence of a congenital defect in the formation or segmentation of the cervical spine . Numerous associated abnormalities of other organ systems may be present. As a consequence of these fusion and segmentation anomalies, this patient has a congenital cervical and thoracic scoliosis. Klippel-Feil type I shows massive fusion of cervical and upper thoracic vertebrae. Type II shows fusion of a limited number of vertebrae and hemivertebrae. Occipitoatlantal fusion and other lower thoracic anomalies are present. Type III shows cervical fusions and lower thoracic or lumbar fusions. C2-C3 fusion, also present in this case, is the most common form of congenital fused cervical vertebrae and is probably dominant with variable expression. This patient also shows hemivertebrae and unilateral fused vertebrae.

Postero-anterior plain film ( A ) and multiplanar reconstruction 3-D reformation of a multi-row detector CT in a patient who has congenital thoracic scoliosis with wedge vertebrae and hemivertebrae ( B ). Curve progression is strongly related to the type of vertebral abnormality, with the poorest prognosis for unilateral unsegmented bars with contralateral hemivertebrae (up to 10°/yr progression), a less severe progression in cases of hemivertebrae or double hemivertebrae (1–2.5°/yr and 25°/yr, respectively), as in this case, and a least severe progression in patients who have block and wedge vertebrae (<1°/yr progression). Associated rib anomalies are seen frequently.

Scoliosis in generalized diseases and syndromes

Neural axis abnormalities have a prevalence between 19.5% and 26% among infantile and juvenile scoliosis cases . Several factors have been identified that correlate with a higher incidence in scoliosis, such as equilibrium and vestibular dysfunction, melatonin deficiency, syringomyelia ( Fig. 3 ), Chiari malformation, and spinal tumors. Neuromuscular disorders (eg, cerebral palsy and muscular dystrophy) and some generalized diseases and syndromes (eg, Marfan, neurofibromatosis, rheumatoid disease, or bone dysplasia) are associated with scoliosis. Scoliosis is seen in 8.7% of patients who have Down syndrome .

Clinically suspected infantile scoliosis is confirmed by plain film ( A ). All cases of infantile and juvenile idiopathic scoliosis require further work-up with MR imaging because they are commonly associated with a variety of neural axis abnormalities and neuromuscular disorders, including syringomyelia (as in this case) ( B ).

Curve location

Curve location is defined by its center, known as the apex, which is the most lateral disc or vertebra of the curve. Usually the apical vertebra is also the most horizontal. Scoliosis can be classified as cervical (apex between C2 and C6), cervicothoracic (C7–T1), thoracic (T2–T11), thoracolumbar (T12–L1), lumbar (L2–L4), or lumbosacral (L5 and below).

Age at onset

Age at onset or diagnosis is used to classify scoliosis as the following types: type 1, infantile (0–3 years); type 2, juvenile (4–10 years, mean age of diagnosis of juvenile scoliosis is around 8 years); type 3, adolescent (11–17 years); and type 4, adult (≥18 years).

Curve types

Primary curves are the first to develop. Secondary curves develop as a means to balance the head and trunk over the pelvis, not only in the frontal but also in the sagittal plane. At the time of diagnosis, it is not always possible to differentiate primary curves from secondary curves ( Fig. 4 ). Structural curves (as opposed to nonstructural curves) cannot be corrected with side-bending or traction. Nonstructural curves can be secondary curves or functional curves (postural, secondary to short leg, muscle spasm).

Follow-up of scoliosis in an adolescent girl. ( A ) 13 years. ( B ) 14 years. ( C ) 14.5 years with brace. ( D ) 15.5 years. Imaging during bracing confirms that the thoracic curve is structural and that the thoracolumbar curve is secondary. The thoracic curve progresses 7° between ages 13 and 14, after which bracing is started. During bracing, the secondary thoracolumbar curve can be partially corrected. The thoracic curve only progresses 2° over the following 1.5years during bracing.

Several classifications according to different curve patterns have been proposed as a preoperative assessment. The use of these classification schemes allows scoliosis practitioners to compare various treatments of similar curve patterns and to recommend selective fusions of the spine when appropriate. The most widely used classification, developed by Moe and reported by King and colleagues (the King-Moe Classification), was designed primarily to help the clinician to decide when to instrument the thoracic curve alone and when to instrument the thoracic and lumbar curves. This classification is not comprehensive, and a more complete and reliable classification was proposed by Lenke and colleagues . The Lenke Classification considers three components: curve type (types 1–6), a lumbar spine modifier (A, B, or C), and a sagittal thoracic modifier (−, N, or +). The six curve types have specific characteristics on frontal and sagittal radiographs that differentiate structural and nonstructural curves in the proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar regions (TL/L). The major curve is the one with the largest Cobb measurement and is included in fusion surgery for idiopathic adolescent scoliosis. The minor curves are all other nonmajor curves. One of the main debates in scoliosis surgery is whether to include those minor curves in the fusion. Thus, six curve types are distinguished in this system based on whether the PT, MT, and TL/L regions are major, minor structural, or nonstructural, including type 1, MT; type 2, double thoracic; type 3, double major; type 4, triple major; Type 5, TL/L; Type 6, TL/L-MT . Many other classification types exist, such as the Scoliosis Research Society Classification for Adult Spinal Deformity, but these usually differ mainly on surgical interpretation points.

Congenital scoliosis

Congenital scoliosis shows progression in 75% of cases. The poorest prognosis is for thoracic curves and for multiple hemivertebrae and a convex unilateral bar (failure of segmentation) opposite the hemivertebrae. Block and wedge vertebrae show progression <1°/yr, hemivertebrae show a mean progression of 1° to 2.5°/yr, double hemivertebrae increase at a double rate, and unilateral unsegmented bars with contralateral vertebrae may progress up to 10°/yr. The management of congenital scoliosis requires frequent clinical and radiographic follow-up to detect progression. Curve progression or severe vertebral anomalies known to cause curve progression require prompt treatment to prevent deformity and morbidity, such as thoracic insufficiency syndrome .

Infantile idiopathic scoliosis

The vast majority of these curves are self-limiting. The few that progress (usually double structural curves) can be difficult to manage. In cases where the rib vertebral angle difference is larger than 20°, progression is likely. The rib vertebral angle difference is defined as the difference in angulation of the left and right ribs on the apical vertebra as measured on an anteroposterior (AP) radiograph.

Juvenile idiopathic scoliosis

Juvenile idiopathic scoliosis is often progressive (70%). The potential for trunk deformity with cardiac and pulmonary compromise exists especially in scoliosis with onset before 5 years of age. Curves of greater than 30° are almost always progressive, at a rate of 1° to 3°/yr before 10 years of age and at a rate of 4.5° to 11°/yr after 10 years of age. If the scoliosis is in the thoracic region, surgery is required in more than 95% of these cases.

Adolescent idiopathic scoliosis

Roughly 2% of adolescents have a scoliosis (>10°), but only 5% of these have a progression of the curve to greater than 30° ( Fig. 4 ). The progression of scoliosis is dependent on the growth velocity and the magnitude of the curve at the first visit. Progression is most notable with a growth velocity of greater than 2 cm/yr, between 9 and 13 years of age, at bone ages between 9 and 14 years, at Risser signs 0 to 1, and between 0.5 and 2 years before menarche .

The key risk factors for curve progression are the remaining spinal growth (skeletal immaturity) combined with the curve magnitude at a given time and, to a lesser degree, female gender . Skeletal immaturity or remaining skeletal growth is determined by age, menarchal status, and Risser sign (radiologic) or Tanner staging (clinical). The main progression occurs at the time of most rapid skeletal growth. This occurs at 11 to 13 years of age for girls and at 13 to 15 years of age for boys. Determining the peak height velocity (PHV) timing by accurate serial heights measurements is difficult. Usually, patients present without accurate prior height measurements. In these cases, other markers have to be used. The Risser stage is still 0 at the time of PHV, and this phase of accelerated skeletal growth is split into halves by the closure of the triradiate cartilage . Other markers can be used, including digital uncapped phalangeal epiphyses, which are indicative of pre-PHV, and fused epiphyses, which are indicative of post-PHV. Capped but nonfused epiphyses are indeterminant. Tanner stage 1 for breast strongly indicates pre-PHV. Stage 3 for breast and pubic hair occurs at or after the PHV, and stage 4 occurrs after PHV . In the decelerating growth phase, a Risser less than 1 is associated with progression in 60% to 70% of patients, whereas Risser 3 has a risk for progression in less than 10%. Curve pattern has also been identified as an important variable for predicting the probability of progression. Primary thoracic curve scoliosis, especially King types II, III, and V, progress more than primary lumbar curve scoliosis.

Adult idiopathic scoliosis

Curves of less than 30° usually show no progression. Curves measuring 30° to 50° at skeletal maturity progress at an average of 10° to 15° during a normal lifetime. Curves between 50° and 75° show a continuing rate of nearly 1°/yr. In untreated patients, an increased mortality rate due to cardiopulmonary disorders is seen most frequently in scoliosis of greater than 90°.