Figure 1: Cobb Method.
All vertebra within the curve are included and the ends marked with lines. The angle of the intersection of these lines is defined as the Cobb Angle. This curve measures 35°
Scoliosis is defined as curvature of the spine in the coronal (front view) plane. Idiopathic scoliosis should be conceptualized as a three dimensional deformity though; twisting of the spine is coupled with curvature producing deformity in both coronal and sagittal (side view) planes. As its name implies, adolescent idiopathic scoliosis occurs between the ages of 10 and 18 and to date, has no known cause. The magnitude of the curve is determined using the Cobb method (figure 1) and conveyed in degrees. Curves measuring more than 10° occur with an approximate worldwide incidence of 0.3% and are distributed equally between males and females. Curves greater than 20° occur with an incidence of 0.3% and have a distribution of five females to every male. Because large degree curves are more likely to require treatment and occur more frequently among females, there is a common misconception that adolescent idiopathic scoliosis, in general, is more common in girls.
The exact etiology of idiopathic scoliosis is yet to be determined, but it is thought to be due to multiple factors. Although the exact genetics is unclear, the observation that idiopathic scoliosis is more common within families suggests the presence of an inherited trait. Research focusing on changes in muscles, the spinal column, rib cage and the chemistry of cartilage in discs suggests that these abnormalities are most likely secondary to the primary scoliosis and not a causative factor. The fact that most curves occur in common patterns, such as right thoracic or left lumbar, raises the possibility that other anatomical asymmetries such as the pulsatile beating of a left sided heart, might have an influence on curve production and progression. We are currently investigating with MRI the relationship between observed turbulent CSF (fluid surrounding the spinal cord) flow at the curve of the apex, differential pressure on the spinal cord, and the influence of these factors on curve progression. For the reader who is interested in more information on the current state of knowledge and research into the etiology of idiopathic scoliosis, a recent Current Concepts Review (Journal of Bone and Surgery, Volume 82A, No. 8, August 2000) will be helpful.
Adolescent idiopathic scoliosis curves are classified by their location in the spine. Curves can occur in the cervical, thoracic, and lumbar spine in various combinations (figure 2).
|Figure 2a: Thoracic||Figure 2b: Double Major||Figure 2c: Thoracolumber|
Structural curves are defined as those curves that incompletely straighten on side-bending. Compensatory curves straighten significantly on side bending and function to produce spinal balance. The location of the structural curve determines the classification of the scoliosis. For example, a structural curve occurring in the thoracic spine with a lumbar compensatory curve is called thoracic adolescent idiopathic scoliosis (figure 3)
Single curves, curves whose apex is at T12 or L1 are defined as a thoraculumbar curve (figure 4), and curves with apices at L2 or L3 are defined as lumbar curves. Structural curves in both the thoracic and lumbar spine are called double major curves. (figure 5) The exact definition of the curve has implications for determining progression and treatment.
|Figure 4a: Apex of curve at T12. This curve is defined as Thoracolumar Adolescent Idiopathic Scoliosis.||Figure 5: Two large structured curves are classified as a Double Major Curve.|
In California , as in many other states, law mandates middle school screening for scoliosis. Scoliosis is detected by observation of a rib prominence during a forward bending test. Girls and boys are most often screened in the 7th and 8th grades, respectively. School screening has effectively reduced both the number of patients requiring surgery and the magnitude of those curves at the time of surgery.
Based on these conditions, this table (figure 7) summarizes current treatment of adolescent idiopathic scoliosis. Small curves measuring less than 20-25° that do not require brace treatment should be observed during periodic examinations of four to six months or 1 year intervals based on their size. Observation remains a form of treatment because any 5° increase in the size of the curve may change the course of treatment.
|Adolescent Idiopathic Scoliosis
Treatment Skeletally Immature Adolescent
|>20° < 25°||4 month X-rays|
|25°-30° with a 5° documented progression||Brace|
Those patients who have curves greater than 40° at presentation and have progressive curves despite bracing should be considered for surgery. Curves between 40 and 50° fall into a relative gray area for surgical indications, but any curve above 50° in a growing child should be surgically stabilized.
The basic principle of surgery is to stop progression of the curve and leave the patient balanced in a frontal and sagittal plane. Cessation of curve progression is achieved with bony fusion between the affected vertebrae while correction is held and supported by spinal instrumentation until healing is complete. All structural curves need to be fused. Depending on the type of instrumentation, a brace may or may not be necessary subsequent to surgery.
Although much attention has been focused on the various approaches to stabilizing curves in adolescent idiopathic scoliosis, certain principles are applicable to all of them. A minimum number of vertebrae should be fused to achieve a balanced spine. Secondly, the extent of the fusion into the lumber spine may negatively impact the future occurrence of low back pain in the patient and, therefore, the fusion should attempt to preserve as many free lumbar segments as possible.
|Figure 8: Pre-op thoracolumber curve treated with anterior spinal instrumentation. Distal lumber motion segments are preserved with this technique.|
In general, either anterior or posterior spinal fusions are employed, based on surgeon preference and curve location. Only in very large curves is it necessary to operate on both sides of the spine. Some approaches clearly make more sense than others. For example, in this author’s opinion a thoracolumbar curve fused through the front generally has a greater chance of preserving distal lumbar fusion levels than a similarly efficacious posterior fusion. (figure 8)
Spinal instrumentation has revolutionized the surgical treatment of progressive curves in adolescent idiopathic scoliosis. Instrumentation serves to correct the curve while holding it stable until bone applied to the spine heals (the fusion). Once the bony fusion occurs, the instrumentation has no function, although it rarely needs to be removed. In the past, Harrington rods provided two points of fixation in the spine and therefore needed to be supplemented with a cast to hold the spine. Contemporary instrumentation techniques utilize segmental fixation which provides attachment to the spine at multiple points.
Unlike the Harrington rod, segmental fixation techniques allow better correction of the curve in both the frontal and sagittal planes (figure 9) (See Sagittal Balance of the Spine and Flat Back Deformity article by Robert S. Pashman, M.D. in Backtalk, June/July 1996.)
|Figure 9: Harrington instrumentation produces distraction forces which may control the curve in the frontal plane (left X-ray), induce forward decompensation and flatback in the (sagittal) side view plane.|
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New instrumentation techniques have, in many instances, proven to be so rigid that postoperative bracing is sometimes not necessary (Figure 10). The type of instrumentation, approach, and the use of post operative braces are based on the surgeon’s experience.
|Figure 10: Pre-op and post-op segmental spinal instrumentation. Frontal and sagittal plane contours have been controlled.|
One exciting, potential advance in the surgical treatment of adolescent idiopathic scoliosis is the use of less invasive techniques, utilizing multiple small incisions for the placement of cameras to view and instruments to correct the scoliosis from the front of the thoracic or thoracolumbar spine. Currently, multi-center studies are under way to establish the safety and efficacy of this type of approach.