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Case 13

History: 16 year old female, scoliosis and back pain, please assess Cobb angle, sagittal balance, and coronal balance

Exam requested: Scoliosis study

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All radiologists are familiar with Cobb angles, but everyone may not be familiar with the concept of coronal and sagittal balance. Let’s review all of these concepts before taking on this case.

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Murtagh et al. Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery: How I Do It. n Radiology: Volume 278: Number 3—March 2016

The Cobb angle is calculated as the angle created by a line drawn along the superior endplate of the most superior vertebral body of the curve (superior terminal vertebral body) and a line drawn along the inferior endplate of the most caudal involved segment (inferior terminal vertebral body).

Cobb angle

The Cobb angle is a well-established technique for measuring scoliotic curvature.

The curve is calculated by identifying the vertebral bodies at the superior and inferior margins of the curve (also known as the terminal vertebral bodies).

The terminal vertebral bodies are the cranial and caudal vertebral bodies with the greatest degree of tilt. Once identified, a line is drawn along the superior endplate of the most cranial terminal vertebral body, and another line is drawn along the inferior endplate of the caudal terminal vertebral body. The resultant angle is the Cobb angle.

In the adolescent population, progressive scoliosis with a Cobb angle between 25° and 45° will be managed conservatively, while Cobb angle greater than 50° is typically treated surgically.

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Murtagh et al. Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery: How I Do It. n Radiology: Volume 278: Number 3—March 2016

Normal coronal alignment. The C7 plumb line (black line) intersects the midpoint of the superior endplate of S1 (central sacral vertical line) demonstrating neutral coronal balance

Abnormal coronal alignment. There is 6.5 cm of coronal plane decompensation (CPD) to the patient’s right, demonstrated as the distance between the C7 plumb line (black line) and the central sacral vertical line (red line).

Coronal plane decompensation

In neutral position, the midpoint of the inferior endplate of C7 is directly superior to the midpoint of the superior endplate of S1.

The coronal plane decompensation is calculated first by drawing a plumb line (which is a line drawn perpendicular to the floor) from the inferior midpoint of C7. The central sacral vertical line is then identified.

The central sacral vertical line is a plumb line that passes through the midpoint of the superior endplate of the sacrum.

Coronal plane decompensation is the horizontal difference between these two lines. Coronal plane decompensation is described as being “right” or “left” depending if the shift is to the patient’s right or left.

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Murtagh et al. Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery: How I Do It. n Radiology: Volume 278: Number 3—March 2016

Normal sagittal alignment. Neutral sagittal balance in which the plumb line (yellow line) from the midpoint of the inferior endplate of C7 (red dot) passes through the posterior superior corner of S1 (black dot)..

Abnormal sagittal alignment. 4.5 cm of positive sagittal balance calculated as the distance between the plumb line from the midpoint of the inferior endplate of C7 (yellow line) and the plumb line through the posterosuperior corner of S1 (green line).

Sagittal plane decompensation

Spinal vertical alignment is measured as the distance between a plumb line through the midpoint of the inferior endplate of C7 and a plumb line through the posterosuperior corner of S1.

In neutral position, the plumb line of C7 should normally intersect with the posterosuperior corner of S1.

The mean spinal vertical alignment in asymptomatic adults is 0.5 cm ± 2.5 (standard deviation) and increases with normal aging.

By SRS-Schwab Classification System, a spinal vertical alignment of less than 4 cm is graded as “0” or “non-pathological” sagittal alignment, that 4 cm to 9.5 cm is graded as “+” or “moderate” deformity, and that greater than 9.5 cm is graded as “++” or “marked” deformity.

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Back to our case…

16 year old female, scoliosis and back pain, please assess Cobb angle, sagittal balance, and coronal balance

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What are the important findings on the frontal radiograph?

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What are the important findings on the frontal radiograph?

Mild dextroscoliosis of the thoracic spine with a Cobb angle of 5 degrees from the superior endplate of T5 to the inferior endplate of T12.

There is also minimal 0.8 cm of coronal plane decompensation (CPD) to the patient’s right.

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What are the important findings on the lateral radiograph?

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What are the important findings on the lateral radiograph?

There is exaggerated lumbar lordosis with 4.7 cm of negative (or posterior) sagittal balance,

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Case 13: Comprehensive assessment of spine deformity

Historically, surgeons have focused on coronal alignment and planned surgery accordingly to correct scoliotic deformity with the goal of surgery to reduce the degree of coronal deformity by using rods held in place by hooks, screws, or a combination of both.

It is now apparent that sagittal alignment correlates highly with quality of life scores and that failure to address sagittal alignment, in addition to the coronal deformity, can lead to persistent pain and deformity.

Imaging of scoliosis is no longer just measuring Cobb angles on roller boards full of “stitched” frontal radiographs. Radiologists need to understand the concepts of coronal and sagittal deformity and should appreciate the role that the pelvis plays in global balance.
When standing upright, there is minimal energy and maximal comfort when C7 is centered over S1. Normally the spine is straight in the coronal plane, and this position is maintained with minimal effort. In the sagittal plane, C7 is normally positioned comfortably over S1 as a result of a series of lordotic and kyphotic curves.

Malalignment in either plane, defines sagittal and coronal imbalance

Murtagh et al. Pelvic Evaluation in Thoracolumbar Corrective Spine Surgery: How I Do It. n Radiology: Volume 278: Number 3—March 2016