What is the Cobb angle?
The Cobb angle was first described in 1948 by Dr. John R Cobb, an American orthopedic surgeon, to measure the angle of spinal curves. It was originally used to measure coronal plane (frontal plane, vertical plane that divides the body into ventral and dorsal (front & back) sections) abnormality on x-rays when classifying scoliosis. It has since been modified to analyze sagittal plane (vertical plane which passes from ventral to dorsal dividing the body into right and left halves) abnormality. It is a measurement used for evaluation of scoliosis curves on an AP radiographic projection of the spine.
How is the Cobb angle measured?
A curve is measured beginning with the apical vertebra. This is the most likely displaced and rotated vertebra with the least tilted end plate. The end/transitional vertebrae are then identified above and below. The end vertebra are the most superior and inferior vertebra which are least displaced and rotated and have the maximally tilted end plate. A line is drawn along the superior endplate of the superior end vertebra and a second line drawn along the inferior end plate of the inferior end vertebra. If the end plates are indistinct the line may be drawn through the pedicles. The angle between these two lines or the lines drawn perpendicular to them is measured as the Cobb angle.
For S-shaped scoliosis, where there are two contiguous curves, the lower end vertebra of the upper curve will represent the upper end vertebra of the lower curve.
What is the significance of Cobb angle?
The Cobb angle is a measure of the curvature of the spine in degrees which helps the doctor to determine what type of treatment is necessary. Typically, a Cobb angle of 10 is regarded as a minimum angulation to define scoliosis. For scoliosis curves between 10-15 degrees, orthopedic doctors usually do not prescribed treatment, aside from regular check-ups, until the patient has gone through puberty. If the curve is between 20-40 degrees, orthopedic doctors will generally prescribe one of several types of braces that are either worn for 18-23 hours a day. However, at the Hudson Valley Scoliosis Correction Center we treat large and small curves with a treatment plan focused on scoliosis exercises and neuromuscular rehabilitation. Our goal is to stabilize the spine at a curve less than 10 degrees and to reduce and stabilize the curve before it reaches 30 degrees. Each curve is examined and an individualized treatment plan is created.
Why do Cobb angles appear to vary?
Cobb Angle is used worldwide to measure and quantify the magnitude of spinal abnormalities, particularly scoliosis. The Cobb angle measurement has become the “gold standard” of scoliosis evaluation endorsed by Scoliosis Research Society and as such is used as the standard measurement to quantify and track the progression of scoliosis. The forward bending test is typically used to screen for scoliosis prior to puberty. If the test reveals symptoms of scoliosis an X-ray is taken and, if present, the Cobb angle is measured. However, because the Cobb angle reflects curvature only in a single plane, it fails to account for vertebral rotation and so it will not accurately demonstrate the severity of three dimensional spinal curvature. Patients often ask, “Why does the Cobb angle calculation vary from doctor to doctor?” One important reason is that the patient placement is critical to a correct measurement.That’s why it is important to have the scoliosis evaluated beyond simply knowing a “Cobb Angle”. Scoliosis is a 3 dimensional distortion and the care plan must reflect that reality of which Cobb angle is only a small part.