Detecting and treating scoliosis is as much an art as a science. The spine is an elegant structure, with complex curves that interact to form a delicate balance.
Now that computer-assisted techniques have shed new light on the intricate workings of the spine, doctors are looking for more advanced ways to measure and evaluate scoliotic curves.
Since 1948, the Cobb method has remained the most widely used radiographic measurement system for determining the severity of scoliosis. On an x-ray of the spine, doctors measure the angle between two lines—one drawn parallel to the superior endplate of one vertebra above the fracture, and another drawn parallel to the inferior endplate of the vertebra one level below the fracture.
Known as the Cobb angle, this technique was introduced in the 1940s by orthopedic surgeon John Robert Cobb, who used it to measure coronal plane deformity on antero-posterior plane x-rays. It quickly became the standard by which scoliosis cases are classified.
Today, Cobb angle measurement plays a key role in diagnosis and treatment. As the most frequently referenced barometer for spinal curve correction, it drives clinical decision making for patients with scoliosis and has served as the primary basis for all natural history studies as well as subsequent research on orthotic treatment and surgery indicators.
However, the dawning realization of the Cobb angle’s shortcomings have led some orthotic experts to question whether scoliosis treatment should revolve so heavily around the Cobb angle. Some argue it’s time to shift the focus to achieving overall balance in the spine.
Limitations of the Cobb Angle
The Cobb angle measures spinal curves on just two vertical planes: the sagittal and coronal. (The sagittal plane divides the body in half from left to right, while the coronal divides it from front to back.)
But orthotists now have a better understanding of scoliosis as a three-dimensional condition that also involves the horizontal axial plane, which divides the upper part of the body from the lower. As this three-dimensional understanding of scoliosis unfolds, vertebral rotation is increasingly becoming the linchpin of scoliosis study.
“Above all, measurement of vertebral rotation is of key significance in the prognosis and treatment of scoliotic curves,” says one group of researchers from Edmonton, Canada. “It may act as an indicator of curve progression, thus being clinically applicable for both preoperative and postoperative assessment.”
While Cobb angle measurement is useful for quantifying the severity of a scoliosis curve, its inability to encompass the axial plane renders it “limited in scope to a two-dimensional angular relationship between two angulated vertebrae” and unable to “relay the complex nature of a three-dimensional individual curve,” asserts the American Academy of Orthotists and Prosthetists.
For example, researchers noted more than two decades ago that patients with the same Cobb angle measurement can have wholly different curvature patterns. Treatment based solely on reducing the Cobb angle, without taking into account the full three-dimensional scope of the condition, could result in less-than-desirable outcomes.
“If we straighten out the curves, but they become less balanced in the process, we haven’t necessarily achieved a good result,” said Don Katz, CO, director of Orthotics at Texas Scottish Rite Hospital for Children.
Alternative Scoliosis Measurement Techniques
Although the Cobb angle remains the most ubiquitous measurement system for scoliosis, it’s not the only one. Other techniques include:
The scoliometer is the most commonly used clinical tool for measuring spinal rotation. While the patient bends at the waist, with straight knees and dangling arms, the scoliometer measures the degree of rotation at each spinal level. One advantage of this technique is that it doesn’t require an x-ray, so patients aren’t exposed to unnecessary radiation. Although it’s not reliable enough to guide patient diagnosis and management on its own, it’s useful as an initial screening tool for scoliosis.
Like the Cobb angle, the Nash-Moe technique involves analyzing an x-ray.
However, it measures vertebral rotation based on the position and orientation of vertebral pedicle shadows. (Pedicles are the tube-like parts of the vertebrae that extend out the sides and curve toward the back.) One study concluded that the shortcomings of the Cobb angle are partly eliminated by this technique. Its main weakness is that the results can vary considerably depending on the observer.
The Perdriolle torsion meter also measures vertebral rotation on an x-ray. But where the Cobb angle fails to address the axial plane, this method picks up the slack. The torsion meter measures the vertebral pedicle shadow offset against the edges of the drum-like vertebral bodies. On the down side, some controversy surrounds this method regarding its reliability and accuracy.
Without the Cobb angle, scoliosis research would not have advanced as far as it has today. However, to effectively treat scoliosis, doctors need a more complete understanding of vertebral rotation. While each of these measurement systems can help contribute to the overall picture, researchers are still looking for reliable ways to measure scoliosis in its full, three-dimensional expression.