8 Adolescent Idiopathic Scoliosis Abstract This chapter on adolescent idiopathic scoliosis (AIS) provides an in-depth review of the diagnosis, nonsurgical management, and surgical management of the disease. The most up-to-date basic science that is foundational to treating AIS is summarized and condensed for the reader. This chapter includes information on measuring curve progression, modern treatment paradigms, and criteria for observation, bracing, and surgical correction. It also covers classification systems, surgical decision making, and surgical techniques. Keywords: adolescent idiopathic scoliosis, Lenke’s classification, nonsurgical management, spinal deformity, surgical treatment, thoracolumbar spine Clinical Pearls • Curve magnitude and skeletal maturity are the major factors determining treatment of patients with adolescent idiopathic scoliosis (AIS). • Surgical treatment is indicated for curves that are unacceptably deforming, symptomatic, or likely to progress beyond skeletal maturity. • For patients who require surgical correction of their AIS deformity, two critical questions must be answered when developing a treatment strategy: What is the proposed upper level of instrumentation, and what is the proposed lower level of instrumentation? Scoliosis is the most common spinal deformity and is diagnosed when a coronal curve of 10 degrees or greater is present on a standing posteroanterior 36-inch radiograph. Identifiable etiologies of scoliosis include congenital, neuromuscular, degenerative, and iatrogenic, but when no discernible etiology exists, idiopathic scoliosis is diagnosed. Idiopathic scoliosis is the most common type of scoliosis and can be further subclassified as infantile (presenting from birth to 3 years), juvenile (age 4–10 years), adolescent (age 11–18 years), or adult (age > 18 years). Adolescent idiopathic scoliosis (AIS) is the most common type, with a prevalence of 2 to 3% for all curves greater than 10 degrees and 0.1% for curves greater than 40 degrees. The overall female-to-male ratio is 3.6:1 among those with AIS, but this ratio varies from 1:1 among those with very small curves to 10:1 among those with curves greater than 30 degrees. Although the term “idiopathic” implies that no discernible cause is known, numerous studies demonstrating familial clustering of AIS strongly suggest a genetic etiology.1 Curve severity and skeletal maturity are the major factors determining treatment of patients with AIS. The progression of curve severity is primarily driven by skeletal growth, with further curve progression rare among those who have achieved skeletal maturity. Pubertal growth includes phases of both acceleration and deceleration, with the greatest rates of curve progression occurring during the growth acceleration phase (the crucial period). As such, it is important in the diagnosis and prognosis of patients with AIS to be able to accurately measure bone age. Numerous methods have been described for identifying these pubertal phases of growth in patients with AIS. These include sequential measurements of sitting height (much more precise than standing height as it separates trunk growth from leg growth) and radiographic assessments of bone maturity. Numerous radiographic tests of bone maturity have been described, each of which grades skeletal maturity at various phases of pubertal growth. The crucial period of skeletal growth (pubertal growth acceleration) is best identified using the olecranon method. This method relies on the correlation of various phases of elbow closure, specifically of the olecranon, with stages of the pubertal acceleration phase. As the olecranon moves through phases of double ossification, semilunar, quadrangular, partial fusion, and complete fusion morphologies, the patient similarly progresses through the pubertal acceleration phase ( Another widely known method for grading skeletal maturity is the Risser grade. Risser’s grading is based on the progressive fusion of the iliac apophysis from lateral to medial, with grade 0 representing no ossification, grades 1 to 4 representing quartiles of ossification, and grade 5 representing complete ossification. Risser’s grading has also been shown to correlate strongly with various phases of hand maturation, with Risser 1 corresponding to ossification of the distal phalanges, Risser 2 to the metacarpal phalanges, and so forth. It is important to note that Risser grade 0 comprises the first two-thirds of all pubertal growth, including the entirety of the crucial acceleration phase. Risser grade 1 heralds the beginning of the pubertal growth deceleration phase ( Another important variable to measure is annual curve progression velocity. When curve velocity is considered against the stage of skeletal maturity, which prognosticates potential for future curve progression, one can accurately predict the probability of a patient developing a progressive curve that will require treatment. For example, 33% of patients who are in the accelerating phase of puberty with a curve velocity of less than 6 degrees per year will go on to require surgery, whereas 71% with curve velocities of 6 to 10 degrees per year and 100% with velocities greater than 10 degrees per year will require surgery.2 AIS typically causes mild or no symptoms during adolescence. However, the deformity itself can have highly negative impacts on adolescent mental health, leading to measurable detriments in quality-of-life measures that extend into adulthood. Severe curves (curves > 50 degrees) furthermore carry the risk of curve progression into adulthood, and eventual development of pain and neurologic deficits. Very severe curves (curves > 90 degrees) also carry the risk of restricted pulmonary function. As such, treatment is indicated for curves that are unacceptably deforming, symptomatic, or statistically likely to progress beyond skeletal maturity. The goals of treatment are to correct the deformity, prevent further progression of the curve, restore trunk symmetry and balance, and minimize morbidity and pain. The treatment goals of infantile and juvenile idiopathic scoliosis are to control the curve until the patient has at least reached the age of pulmonary tree maturity (typically 10 years of age) so that fusion of the spine does not result in stunting of pulmonary maturity, which could lead to inadequate pulmonary function once adulthood is reached. Fig. 8.1 Illustration showing pubertal growth acceleration according to the olecranon method and deceleration according to the Risser grade. Pubertal growth acceleration may be characterized by progression of ossification of the olecranon from the double to semilunar to quadrangular to partial and complete fusion morphologies in the elbow. The Risser grade is based on the progressive fusion of the iliac apophysis from lateral to medial, with grade 0 representing no ossification, grades 1 to 4 representing quartiles of ossification, and grade 5 representing complete ossification. (Used with permission from Barrow Neurological Institute, Phoenix, Arizona.) Table 8.1 Current treatment paradigm for AIS
8.1 Introduction
8.1.1 Curve Progression
Fig. 8.1).
Fig. 8.1).
8.2 Treatment Paradigms
Curve magnitude (°) | Skeletally immature | Skeletally mature |
10–25 | Observe | No follow-up needed |
25–45 | Brace | Observe to adulthood |
> 45 | Surgical treatment | Surgical treatment |
The treatment strategies available for patients with AIS can be broadly categorized as observation, bracing, or surgical correction. The choice of treatment strategy to pursue is based on both curve severity and skeletal maturity and is summarized in 
Table 8.1. Other indications for surgical intervention include curves that progress despite bracing and curves that are unacceptably disfiguring for the patient (especially lumbar curves).
Bracing is a treatment strategy for patients with curves of an intermediate severity who are progressing toward the surgical treatment threshold. The goal of bracing is to stabilize progressive curves, and therefore avoid the need for surgical correction. In 2013, Weinstein et al3 published the results of the BrAIST (Bracing in Adolescent Idiopathic Scoliosis Trial), a randomized clinical trial, showing that among female AIS patients treated with bracing, 72% (105/146) stabilized and therefore avoided the need for surgical correction, whereas only 48% (46/96) of those treated without bracing avoided surgical treatment. Furthermore, brace adherence and outcome were directly correlated, with more than 90% of those who wore the brace more than 13 hours per day successfully reaching skeletal maturity without the need for surgery. These data are the highest quality of evidence currently available for bracing, and firmly establish bracing as an effective treatment strategy for patients with intermediate-severity AIS.
8.3 AIS Classification
The King classification system was originally designed in 1983 to categorize AIS and assist with surgical decision making.4 However, the King system is limited in its utility because it fails to account for sagittal alignment and has poor intraobserver and interobserver reliability. The Lenke classification system was developed in 2001 to overcome these important limitations and help define a standardized treatment based on objective criteria.5
The Lenke classification system requires analysis of both the coronal and sagittal plane to characterize the curve based on three main components: (1) curve type, (2) lumbar spine modifier, and (3) sagittal thoracic modifier.
The Lenke curve type is categorized into six types (1–6) based on the characterization of the major and minor curves (
Table 8.2). The major curve is defined as the curve with the largest Cobb angle and is always structural in nature. However, the minor curves may be either structural or nonstructural based on flexibility on lateral bending radiographs. Structural curves are rigid and are defined as those curves with residual Cobb angles of greater than or equal to 25 degrees on lateral-bending radiographs. Nonstructural curves are flexible curves with residual Cobb angles of less than 25 degrees on side-bending radiographs. In types 1 to 4, the major curve is in the main thoracic (MT) region (apex between T6 and T11–T12 disc). An MT curve (type 1) has proximal (apex between T3 and T5) and thoracolumbar (apex between T12 and L1) curves that are nonstructural. A double thoracic curve (type 2) has a structural proximal curve and nonstructural thoracolumbar curve. A double major curve (type 3) has a nonstructural proximal curve and a structural thoracolumbar curve. A triple major curve (type 4) has both structural proximal and thoracolumbar curves. In types 5 and 6 curves, the major curve is in the thoracolumbar region. A thoracolumbar/lumbar curve (type 5) has nonstructural proximal and MT curves. A thoracolumbar/lumbar MT curve (type 6) has structural MT and nonstructural proximal thoracic (PT) curves.
The degree of lumbar coronal deformity is assessed with the lumbar spine modifier and is an important determinant of spinal balance and success of surgical treatment. Modifiers A to C describe the relationship of the apex of the lumbar curve to the central sacral vertical line (CSVL). The CSVL bisects the cranial aspect of the sacrum and is perpendicular to the true horizontal. Modifier A can only be used in patients with major curves in the MT region (types 1–4) and describes a lumbar curve in which the CSVL runs between the pedicles of the lumbar spine (L1–L4). Modifier B also can only describe types 1 to 4 curves and represents a lumbar curve in which the CSVL passes between the medial border of the concave pedicle and the lateral border of the apical vertebral body. Modifier C can be used to describe types 1 to 6 curves and describes a lumbar curve in which the CSVL falls medial to the apical vertebra.
The sagittal thoracic modifier takes sagittal alignment into account and is another component of the Lenke classification that is imperative in surgical planning. Thoracic kyphosis is measured on a standing lateral radiograph from the superior endplate of the fifth thoracic vertebra to the inferior endplate of the twelfth thoracic vertebra. Normal kyphosis is defined as + 10 to + 40 degrees of thoracic kyphosis. Hypokyphosis (−) is defined as less than + 10 degrees of kyphosis. Hyperkyphosis (+) is defined as greater than + 40 degrees of kyphosis.
Several strengths of the Lenke classification system include excellent intraobserver and interobserver reliability, inclusiveness of all patterns of AIS, and facilitation of communication between surgeons. However, one of the biggest limitations of this classification system is the failure to address the rotational component and the three-dimensional nature of the deformity. With new technology that allows for three-dimensional analysis of the spine, the Scoliosis Research Society recognizes the need for a three-dimensional classification system to further understand and guide treatment of this complex pathology.
8.4 Surgical Decision Making
For patients who require surgical correction of their AIS deformity, two critical questions must be answered when developing a treatment strategy: What is the upper limit of instrumentation (upper instrumented vertebra [UIV])? What is the lower limit of instrumentation (lower instrumented vertebra [LIV])? A number of important considerations must be taken into account when choosing the UIV and LIV, including the types of curves present, the direction of those curves, whether those curves are structural or compensatory, and the presence of any sagittal deformity. As such, the preoperative radiographic work-up in AIS should include 36-inch standing posteroanterior and lateral plain radiographs, as well as lateral-bending radiographs. These radiographs can be used to identify structural and compensatory curves (curves that correct to < 25-degree Cobb angle on lateral-bending radiographs should be considered compensatory), as well as important vertebral landmarks including the end-of-Cobb vertebra (EV), neutral vertebra (NV), and stable vertebra (
Fig. 8.2).
