Scoliosis is a condition defined by a side-to-side curvature of the spine that measures at least 10 degrees on an X-ray, sometimes involving a rotational twist of the vertebrae. Since the visual appearance of the back is insufficient to gauge the true severity of the curve, medical professionals rely on specific charts and measurement tools. These standardized methods allow doctors to accurately track the condition’s progression over time and determine the most appropriate course of management.
Understanding Scoliosis Curve Types
Before measuring severity, the curve is first classified by its physical structure and location within the spine. Scoliosis is broadly categorized into structural and non-structural types. Structural scoliosis is the more common form, involving a permanent, three-dimensional change with sideways bending and vertebral rotation. Non-structural, or functional, scoliosis is a temporary, two-dimensional curve without rotation, often caused by factors like muscle spasms or a leg length discrepancy, and it may resolve when the underlying cause is addressed.
Structural curves are described by their location, such as thoracic (mid-back), lumbar (lower back), or thoracolumbar (a curve spanning both regions). The pattern of the curve is also noted: a single curve is often described as a C-shape, while a double curve—like one in the thoracic and a compensating one in the lumbar spine—is known as an S-shape. The most common curve in the thoracic spine typically bends to the right, a pattern called dextroscoliosis.
Measuring Severity: The Cobb Angle
The Cobb angle is the primary metric used to quantify the magnitude of the spinal curvature, and it is measured directly from a standing X-ray image. To determine this angle, a doctor first identifies the two most tilted vertebrae at the ends of the curve, known as the end vertebrae. A line is drawn parallel to the top of the superior end vertebra, and a second line is drawn parallel to the bottom of the inferior end vertebra. The Cobb angle is the angle formed by the intersection of lines perpendicular to these two endplate lines, expressed in degrees.
A measurement of less than 10 degrees is considered a minor spinal asymmetry and not a formal diagnosis of scoliosis. Mild scoliosis is classified as a curve between 10 and 25 degrees, requiring only observation. Moderate curves fall between 25 and 45 degrees, which is often the threshold where intervention like bracing may be considered, especially in a growing patient. A curve measuring over 45 to 50 degrees is classified as severe, representing the range where surgical correction may become an option.
Assessing Progression Risk Through Skeletal Maturity
The Cobb angle alone is insufficient to determine the risk of a curve worsening, as the patient’s remaining skeletal growth potential is a major factor. The most common tool to assess this is the Risser sign, a grading system that uses the spinal X-ray to examine the ossification of the iliac crest, the curved top edge of the hip bone. The iliac crest ossifies, or turns to bone, in a predictable pattern that mirrors the overall growth of the spine.
The Risser sign is assessed on a scale from 0 to 5. Risser 0 indicates no ossification has started, meaning the patient has substantial growth remaining and a higher risk of curve progression. As the iliac crest ossifies, the score increases, with Risser 5 indicating complete fusion and the end of skeletal growth. A low Risser score combined with a moderate Cobb angle suggests a high probability that the curve will progress and requires careful monitoring.
Translating Measurements to Treatment Decisions
The combination of the Cobb angle and the Risser sign creates a decision matrix that guides clinical care. For a skeletally immature patient (Risser 0–2) with a mild curve (less than 20 degrees), the standard approach is observation and regular monitoring to detect any change. If that patient has a moderate curve between 25 and 40 degrees, bracing is often recommended to prevent progression during the remaining growth period.
The threshold for considering surgical intervention is a curve that has progressed beyond 40 to 50 degrees, especially if the patient is still growing. Even in skeletally mature patients (Risser 4–5), a curve measuring over 50 degrees may still be watched closely, as larger curves can continue to progress slowly throughout adulthood. The treatment plan is a dynamic decision, balancing the measured severity of the curve with the patient’s biological potential for future growth.