Measuring Scoliosis Using two Different Methods with the Cobb Angle

» Measuring Scoliosis Using two Different Methods with the Cobb Angle
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For many years, X-rays have been used to diagnose and measure scoliosis (curvature of the spine). No matter how young or old the patient is or where the curve is located, this technique has continued to be simple and reliable. In this study, researchers from the Scoliosis Research Institute in Korea take another look at the Cobb angle measurement. They compare two different starting and ending points used in measuring the angle of the curve.

The Cobb angle is defined as the angle formed between a line drawn parallel to the superior endplate of one vertebra above the curve and a line drawn parallel to the inferior endplate of the vertebra one level below the curve. Superior means above and inferior refers to below. The endplate is a flat piece of cartilage that comes in direct contact with the disc as it sits in between two vertebrae.

An alternate way to measure the Cobb angle is by using the pedicles as the bony landmark instead of the endplates. The pedicles are used when the endplates don't show up distinctly on X-rays (usually because of the young age of the developing child). The pedicle connects the body of the vertebra to the vertebral arch or ring behind the vertebral body. The vertebral arch goes around the spinal cord to protect it, leaving an opening called the spinal canal for the spinal cord to travel from the brain down to the bottom of the spine.

This is the first study to compare the results using both the endplate and the pedicle method of measuring the Cobb angle. The authors thought perhaps age and location of the curve might make a difference in the Cobb measurements. But as it turns out, after measuring over 300 X-rays using both methods, there was no statistical difference between the two methods. And to make sure the results were accurate, there were three different examiners using a digitized computer system to make the measurements and compare them. Each measurement was calculated three separate times by each of the three examiners. The examiners included two spine surgeons and a spine fellow with years of experience reading X-rays.

Only children with a thoracic (middle spine) scoliosis were included and only one curve (the major or biggest one) was measured. The researchers paid attention to the ages of the children and the location of the curves for each age group. The groups were divided into under seven years old, seven to 10 years old, and older than 10. The curves were divided into mild, moderate, and severe categories affecting the upper, middle, or lower thoracic spine.

They even analyzed how accurate each examiner was from one measurement to the other (since they had to repeat each angle three times). This is called intraobserver reliability. In order to be accurate enough to consider the measurements reliable, the measurements must be the same (or very close) each time. And then they compared interobserver reliability -- that's to see how close the three examiners came to each other in their measurements.

The results were still the same: no statistical difference using pedicle versus endplate as the top and bottom for the angles. Minor differences were noted that didn't amount to much. For example, there was less intraobserver variability when the pedicle method was used. And the reliability seemed to improve when using the pedicle method for the youngest group (less than seven years old). Interobserver results were pretty much the same no matter which method was used and for all age groups and curve locations. Larger curves (more than 40 degrees) seemed to be easier to measure reliably and accurately using the pedicle method.

The authors make note of the fact that they tried to eliminate as many places for error as possible in the study. Instead of letting the examiners choose where to begin and end the measurements, everyone was told which vertebrae to use as the top and bottom. Using digital radiographs also helped by eliminating the need for special tools (e.g., protractors for measuring angles), pencils, and markers. The consistency in measurements obtained may really directly link these factors.

Cobb angles are important when evaluating scoliosis because they help guide the surgeon in making treatment recommendations. Regardless of the method used to measure the Cobb angle, there are still challenges to face. Indistinct landmarks, vertebral bodies that are tilted, and unusual shapes of the endplates can affect measurements that rely on the endplate method. Having an alternate method using the pedicles may be helpful in some cases. Without digital computerized radiograms, the authors cannot guarantee the methods will yield the same results. That would require another study making these same comparisons without the availability of a digitized system by using standard (noncomputerized) X-rays.

Reference: Satyen S. Mehta, MS, MRCS, et al. Interobserver and Intrabserver Reliability of Cobb Angle Measurement: Endplate Versus Pedicle as Bony Landmarks for Measurement: A Statistical Analysis. In Journal of Pediatric Orthopaedics. October/November 2009. Vol. 29. No. 7. Pp. 749-754.

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