By David K. Wolpert
The text below is protected under copyright. Do not copy or forward without the permission of the publisher.
An orthopedist has probably told you that you have a curve of some specific degree measurement, such as a "50 degree curve." This section provides an explanation of curve measurements. The standard measurement of scoliotic curves is called the Cobb angle, named after the orthopedic surgeon who created this approach to measurement (the Cobb method).
The first step in determining the magnitude of your curvature is to take a full-length standing P/A x-ray of your back. "P/A" (or posteroanterior) stands for posterior/anterior, which in this context means "from the back to the front of the body." This type of x-ray picture is taken with the patient standing up, with his or her back facing the x-ray machine. The picture spans the entire back, from the top of the neck all the way down to the pelvis. It may also be necessary to take A/P x-rays (anterior/posterior, taken with the patient facing the x-ray machine), although, for females in particular, this presents a higher risk of radiation exposure to breast tissue.
Your orthopedist will study the x-ray to determine the two end-vertebrae in your curve—the vertebrae at the upper and lower extremes of your structural curve. Next, two straight lines are hand-drawn on your x-ray film, one at the top of the highest vertebra in the structural curve and one at the bottom of the lowest vertebra in the structural curve. The orthopedist will then draw lines perpendicular to those lines. The perpendicular lines intersect at an angle. The degree measurement of that angle is the magnitude of your curvature. As mentioned previously, many people with scoliosis have more than one curve, each of which may be either structural or compensatory. Your orthopedist will measure all the curves, but structural curves are the most important.
When documenting a Cobb angle, it is important that the orthopedist or radiologist record not only the upper and lower end-vertebrae used for the calculation, but also which method (e.g. the Cobb method) was utilized. The same vertebrae should be used on subsequent occasions to measure curvature, thereby ensuring that future analyses of your curve's progression are valid "apples-to-apples" comparisons. This information is usually found in the radiographic report, which is a short summary of the findings written shortly after the x-rays are taken. These reports become part of your chart (a file of your medical records from your interactions with a specific doctor) and therefore last far longer than the x-ray films, which are often discarded from medical records after a few years.
Be aware that the Cobb method is not an exact science. It is prone to human error. One possible error is selecting the wrong vertebrae as the end-points for the top and bottom of the structural curve. An orthopedist may also make small errors in drawing straight lines or in using a protractor or other tool to measure the angle, thus resulting in an inaccurate angle estimation. In addition, the structural measurement of your curvature may be slightly off due to factors beyond the control of the orthopedist, such as precisely how straight you were standing when the x-ray was taken, or even muscle fatigue that can slightly increase your curvature temporarily.
With these points in mind, you should consider the measurement of your curve to be a good approximation, realizing that the actual curve may vary by a few degrees. According to a study done by the Scoliosis Research Society, differences in measurement by a particular orthopedist of an identical x-ray may vary over time by as much as 5 degrees, while differences in measurement between orthopedists may vary by as much as 10 degrees. The implication of this variance is that you should not be alarmed if your curve measurement increases by a degree or two. Likewise, a slight reduction in curve measurement does not necessarily mean your curvature has actually improved.
In addition to the lateral curvature, the rotation of the spine can also be measured. This is done by observing how far the pedicles of the vertebra at the apex of the curve deviate from the midline. The pedicles are seen on x-ray films as small oval indentations on either side of the vertebra. The midline is an imaginary vertical line drawn down the middle of a hypothetically straight spine. The two pedicles in a non-rotated vertebra should be equidistant from the midline. An arbitrary scale of 0-4 is used to describe the relative proximity of the pedicles to the midline. The amount of rotation would be reported as something like a "+3 rotation."