Scoliosis is defined as the presence of one or more lateral curves of the vertebral column in the coronal plane, although abnormal curves may affect spinal alignment in all three dimensions. Radiography, computed tomography (CT), and magnetic resonance imaging (MRI) all can play important roles in evaluating scoliosis and determining its underlying cause. Although scoliosis is in 80% of cases idiopathic, various congenital or developmental osseous or neurologic abnormalities may lead to abnormal lateral curvatures of the spine. The selection of the most appropriate imaging modality for a particular examination may be aided by greater familiarity with the imaging manifestations of various causes of scoliosis; furthermore, the image interpretation may be improved by an understanding of the principles underlying the development, progression, and management of scoliosis.
To understand the biomechanics of scoliosis progression, it helps to bear in mind the complex 3D nature of spinal curvature: the lateral scoliotic curvature seen in the coronal plane often is accompanied by perturbations in physiologic spinal alignment in the sagittal and axial planes.
Scoliosis appears to develop in two stages:
- curve initiation, and;
- subsequent progression.
According to the Hueter-Volkmann law, bone growth, in the period of skeletal immaturity, is retarded by mechanical compression on the growth plate and accelerated by growth plate tension. Due to the physiologic curvature in the normal thoracic spine, the compressive force is delivered on the ventrally located part of the vertebral column, whereas the distractive force is delivered on the dorsally located part. The process leading to abnormal spinal curvature is thought to be initiated by the rotation of vertebral bodies in the axial plane, which causes discrepant axial loading between the ventrally and dorsally located portions of the involved vertebrae. Over time, the discrepancy manifests as a change in the directionality of spinal curvature.
Scoliosis is usually classified as primary (ie, idiopathic) or secondary. Idiopathic scoliosis is further classified according to the patient’s age (Infantile, Juvenile and Adolescent), whereas secondary scoliosis is further classified according to the cause.
Leg length discrepancy
|Congenital||Failure of formation or segmentation
Introduced in 1948 by Dr. John Cobb, the Cobb Angle method is used as a standard measurement to determine and track the progression of scoliosis. Cobb suggested that the angle of curvature be measured by drawing lines parallel to the upper border of the upper vertebral body and the lower border of the lowest vertebra of the structural curve, then erecting perpendiculars from these lines to cross each other, the angle between these perpendiculars being the ‘angle of curvature’.
The Cobb Angle helps to determine what type of treatment is necessary:
- A Cobb Angle of 10 degrees is regarded as a minimum angulation to define Scoliosis.
- Between 15 and 20 degrees: Some studies suggest that the patient does not require any specific treatment, but just needs regular check-ups to check if the curve is progressing until bone-maturity.
- Between 20 and 40 degrees: A back brace will generally be prescribed to keep the spine from developing more of a curve.
- 40 – 50 degrees or more: Surgery may be required to correct the curve.
Identification of primary and secondary curves
Major curves, also called primary curves, are the largest abnormal curves in the scoliotic spine and the first to develop. Minor curves, also called secondary curves, are smaller and are considered to develop afterward, to compensate for the perturbation of balance that accompanies the progression of major curves by repositioning the head and trunk over the pelvis to maintain balance.
Assessment of vertebral alignment and balance
The Central Sacral Vertical Line (CSVL) is a roughly vertical line that is drawn perpendicular to an imaginary tangential line drawn across the top of the iliac crests on radiographs, bisecting the sacrum.
The CSVL drawn on radiographs serves as a reference for identifying stable vertebrae, evaluating coronal balance, and determining the curve type, irrespective of the classification system applied (King or Lenke).
The plumb line is a vertical line drawn downward from the center of the C7 vertebral body, parallel to the lateral edges of the radiograph. It is used to evaluate coronal balance on standing frontal radiographs and sagittal balance on standing lateral radiographs. Coronal balance is evaluated by measuring the distance between the CSVL and the plumb line; and sagittal balance is evaluated by measuring the distance between the posterosuperior aspect of the S1 vertebral body and the plumb line. For both coronal and sagittal measurements, balance is considered abnormal if the distance is greater than 2 cm.
To measure coronal balance, a plumb line located to the right of the CSVL is considered to reflect positive coronal balance, whereas a plumb line located to the left of the CSVL is considered to reflect negative coronal balance. For measurements of sagittal balance, a plumb line that is anterior to the posterosuperior aspect of the S1 body is considered to reflect positive sagittal balance, whereas a plumb line that is posterior to the posterosuperior aspect of the S1 body is considered to represent negative sagittal balance.
Use of cross-sectional imaging modalities: When to use CT and MRI
The main purpose of performing CT or MRI in a patient with scoliosis is to identify an underlying cause. In addition, the cross-sectional imaging modalities are useful for guiding surgical treatment and evaluating postoperative complications. Radiography is the method of choice for the initial diagnostic imaging evaluation; it is sufficient to exclude most congenital and developmental osseous anomalies, which account for most cases of scoliosis with an underlying pathologic origin. It is noteworthy that congenital and developmental osseous causes tend to produce curvatures that affect a relatively short segment of the spine on radiographs. Neurofibromatosis with dystrophic curvature, although uncommon, also may produce a short-segment curve. In cases with a complex osseous deformity, radiography alone is inadequate and the use of CT is mandatory, especially when surgery is planned. MRI is used with increasing frequency to evaluate patients with an unusual curve pattern or alarming clinical manifestations.
Multi detector CT with 3D image reconstruction allows the visualization of complex osseous abnormalities of congenital scoliosis. CT can be especially helpful when planning the surgical excision of hemivertebrae because it may depict unexpected osseous anomalies. Pre-operative CT angiography is also useful for determining whether coexistent anomalous vascular conditions are present.
In 90 percent of cases, scoliotic curves are mild and do not require active treatment. Most experts would recommend spinal fusion surgery only when the curve is greater than 40 degrees with signs of progression.
According to the National Scoliosis Foundation, scoliosis affects 2-3% of the population, or an estimated 7 million people in the United States. Approximately 29.000 scoliosis surgeries are performed on adolescents every year in the United States only. (WJO 2015)
There are two primary benefits of successful scoliosis surgery:
- To stop a curve from progressing;
- To reduce rib prominence and diminish spinal deformity.
The success rate of stable fusion and correction of spinal deformity is very high in experienced hands. The average curve correction is approximately 70 percent and the likelihood of complications has been about 2 percent overall.
Risks of spinal fusion include those occurring in any major surgery, such as severe blood loss; urinary infections due to catheterization; pancreatitis; and obstructive bowel dysfunction due to immobilisation during and after surgery. The frequency of specific complications, including death is, however, unknown.
Other risks of scoliosis surgery are listed below:
- Death and neurological damage;
- Loss of normal spinal function;
- Strain on un-fused vertebrae;
- Post surgery pain;
- Infection and inflammatory processes;
- Curvature progression;
- Decompensation and increased sagittal deformity;
- Increased torso deformity.