2 Adolescent Idiopathic Scoliosis: Classification and Natural History
Children and adolescents between the ages of 10 and 18 can suffer from scoliosis caused by syndromic, congenital, neuromuscular, or neurologic conditions. However, if an etiology for the scoliosis is not found, it is classified as adolescent idiopathic scoliosis (AIS). 1 AIS is the most common type of spinal deformity in this population. Scoliosis is defined as a curvature of more than 10 degrees in the coronal plane. The normal alignment of the spine is straight in the coronal plane, whereas in the sagittal plane the thoracic kyphosis averages ~ 30 degrees and the lumbar lordosis averages ~ 55 degrees. AIS is much more common in females than in males and has a prevalence of about 1 to 3% for curves measuring 10 degrees or more. 1 AIS has a prevalence of ~ 0.1% for curves measuring at least 40 degrees. 2 , 3
In general, management of AIS consists of observation, bracing, or surgery. For skeletally immature patients, observation is considered for curves less than 25 degrees, bracing is indicated for curves between 25 degrees and 40 to 45 degrees, and surgery is considered for curves greater than 40 to 45 degrees. Patients who are skeletally mature are typically not treated in a brace. In these patients, observation is indicated for curves up to 40 degrees and surgery is considered for curves greater than 45 to 50 degrees.
In 1983, King et al 4 presented a classification system for selecting fusion levels in patients with idiopathic scoliosis. This classification system focuses on treating patients with posterior surgical fusion of the thoracic spine using Harrington rod instrumentation. Based on review of 405 patients, the authors identified five types of curves and recommended fusion levels.
An S-shaped curve in which both the thoracic and lumbar curves cross the midline. The lumbar curve has a higher Cobb angle than the thoracic curve by at least 3 degrees on standing X-rays, or the thoracic curve is more flexible on side-bending films. Both the thoracic and lumbar curves are fused in these patients. The lowest instrumented vertebra (LIV) is L4 or higher in all cases.
An S-shaped curve in which both the thoracic and lumbar curves cross the midline. However, the thoracic curve has a higher Cobb angle on standing X-rays, or the thoracic curve is less flexible on side-bending films.
King et al suggested either fusing both curves or performing a selective thoracic fusion. A type II curve of less than 80 degrees could safely undergo selective thoracic fusion if the LIV is stable.
Primarily a thoracic curve in which the lumbar curve does not cross the midline. Only the thoracic curve is fused in these patients.
A long thoracic curve in which L5 is centered over the sacrum but L4 tilts into the long thoracic curve. The thoracic curve is fused in these patients.
A double thoracic curve with T1 tilted into the convexity of the upper curve. The upper curve is structural on side-bending films. King et al recommended fusing both thoracic curves in this type of curve.
Thoracic curves that do not fit the above five categories.
For type III, IV, and V curves, when performing a fusion, the stable vertebra is selected as the LIV. For patients undergoing thoracic fusion only, fusion distally beyond both the neutral and stable vertebra is not recommended.
The King classification system became widely used for the classification and treatment of idiopathic scoliosis even though it had poor to fair interobserver reliability. 5 However, the King classification was based solely on coronal radiographs and placed emphasis on thoracic curves. Lenke et al 6 recognized the need for a more reliable and comprehensive classification system that would consider the three-dimensional deformity encountered in idiopathic scoliosis, would have high inter- and intraobserver reliability, and would help in surgical decision making. They proposed a new, modular classification system in 2001 with three main components: curve type (1 through 6), lumbar spine modifier (A, B, C), and sagittal modifier (–, N, +). Thus, this new classification system aimed to define all types of curves while considering the deformity in multiple planes. The Lenke classification system is currently the most commonly used classification for AIS. Four radiographs are essential to classify curves using the Lenke classification: standing coronal and sagittal X-rays of the entire spine, as well as right and left side-bending views.
The coronal Cobb angles are measured on the upright coronal radiograph. Proximal thoracic (PT) curves are defined as those with the apex located between T3 and T5. Main thoracic (MT) curves have an apex between T6 and the T11-T12 intervertebral disk. The thoracolumbar (TL/L) curves have an apex between T12 and L1, whereas lumbar curves have an apex between L1-L2 and L4.
Side-bending radiographs are used to define structural curves. Curves ≥ 25 degrees on side-bending films are defined as structural curves. Additionally, the sagittal radiographs are used to assess structural curves. A PT curve is considered structural if the kyphosis from T2 to T5 measures ≥ 20 degrees. MT and TL/L curves are considered structural if the kyphosis from T10 to L2 measures ≥ 20 degrees. The major curve is the MT or TL/L with the larger Cobb angle on the standing PA radiograph. Thus, six main types of curves are defined:
Type 1: Main Thoracic
The MT curve is the only structural curve and is the major curve. This is also found to be the most common type of curve (40%).
Type 2: Double Thoracic
The MT curve is the major curve. However, the proximal thoracic curve is structural.
Type 3: Double Major
The MT curve is major, although both the MT and TL/L curves are structural.
Type 4: Triple major
The PT, MT, and TL/L curves are all structural. The major curve could be either the MT curve or the TL/L curve.
Type 5: Thoracolumbar/Lumbar Curve
The TL/L curve is the only structural curve and is the major curve.
Type 6: Thoracolumbar/Lumbar–Main Thoracic
The TL/L is major, although both the MT and TL/L curves are structural.
The lumbar modifier is assigned based on the position of the apical TL/L vertebra relative to the center sacral vertical line (CSVL):