Conventional Radiographs

Standing frontal (anteroposterior or posteroanterior) and sagittal (lateral) whole spine (i.e., 14 × 36-inch-long cassette or digitally stitched) radiographs are important for the proper evaluation of patients with spinal deformity. Radiographs should show the occiput and shoulders superiorly and the pelvis, including femoral heads, inferiorly. Standing views should be taken in a standardized position with the patient’s hips and knees in extension to remove potential compensation of sagittal imbalance. Frontal radiographs are oriented with the patient’s right side on the right side of the screen or view box. This orientation allows the observer to view the film as if he or she is standing behind the patient examining the spine, or as he or she would see it while performing surgery using a dorsal approach. Sagittal radiographs are oriented with the patient facing toward the right. Proper orientation of full-length scoliosis radiographs is demonstrated in Figure 150-2 .

Proper orientation for standing posteroanterior ( A ) and lateral ( B ) radiographs of an adult patient with scoliosis. Both the occiput and femoral heads are clearly visible.

A comparison of the degree of deformity between weight-bearing and non-weight-bearing films (i.e., supine) gives some information regarding the rigidity of the deformity. Additional views to help determine the stiffness of the deformity can also be useful. These views include supine lateral bending films, bending films over a bolster, fulcrum bending films, and push and traction views. In the evaluation of spondylolisthesis, dynamic lateral views ( Fig. 150-3 ) of the lumbar spine are used to help determine the degree of instability. Additional plain radiographic views that are useful in deformity include oblique views to visualize the pars interarticularis, the Ferguson view to better visualize the sacral region, and the Stagnara or Leeds view (an oblique view through the apical region of a scoliotic curve that accounts for rotation) to better visualize the pedicles.

Examples of dynamic extension ( A ) and flexion lateral ( B ) radiographic views demonstrating spondylolisthesis of L3 over L4.

Advanced Imaging

Advanced imaging has become nearly standard in the evaluation of patients with symptomatic deformity. Common modalities include CT, CT myelography, CT angiography, and MRI. MRI gives excellent soft tissue detail and is useful in demonstrating disc disease, spondylotic changes, and intraspinal anomalies. MRI should be obtained in all cases associated with neurologic compromise unless contraindicated. CT gives excellent bony detail and is extremely useful in preoperative planning. CT myelography has the added benefit of providing intraspinal information in addition to high-resolution bony detail. CT angiography and MR angiography are useful in the evaluation of vascular anatomy, which may have surgical approach and technique implications.

Advances in imaging software such as multiplanar rendering (MPR) allow the surgeon to view multislice CT and MRI data in multiple and adjustable planes. This allows for more accurate preoperative measurement of pedicle diameter and a greater appreciation of the disease process ( Fig. 150-4 ). Three-dimensional reconstructions can easily be created to aid in operative planning. It is also possible to have models of an individual patient’s spine fabricated and surgery simulated prior to the patient reaching the operating room. In our opinion, thin-cut CT myelography plus MPR provides extremely useful information in operative planning for deformity.

CT lumbar spine in a patient with scoliosis and a pars defect. Multiplanar rendering is used to better visualize the pathology.

Diagnostic Imaging Conventions and Measurements

The location of a sagittal or coronal deformity is defined by the location of the apex of the curve. The apex is the disc or vertebra that is maximally displaced and minimally angulated. A deformity is considered thoracic if it has an apex between the T2 and the T11-12 disc, thoracolumbar if the apex lies between the T12 and L1 vertebrae, and lumbar if the apex is at or distal to the L1-2 disc. This characterization varies in different classification systems of adult spinal deformity. In scoliosis, the curve is described based on the side of its convexity. Convex to the right is known as dextroscoliosis, and convex to the left is known as levoscoliosis. Rarely is there just a single curve in scoliosis. The largest curve is called the “main” curve, which is typically structural (e.g., the curve does not correct completely on bending radiographs). Compensatory curves develop in response to the main structural curve and serve the purpose of maintaining spinal balance. Compensatory curves may or may not be structural.

Curve degree is measured via the Cobb method as illustrated in Figure 150-5 . Originally described by John Cobb in 1948, the technique involves selecting vertebrae maximally tilted into the curve (end vertebrae). Lines are drawn parallel to the superior end plate of the cephalad end vertebra and the inferior end plate of the caudal end vertebra. If the end plates are not clearly visualized, an alternative technique is to use the pedicle margins as the basis for these lines. Bisecting perpendicular lines from the end plate lines are drawn and the angle determined. The Cobb technique is classically thought to have an inherent error of 3 to 5 degrees; therefore, a change in angle between consecutive films has to be greater than 5 degrees to be considered a true change. Intraobserver error ranges between 1% and 5%, and interobserver error can be as high as 10 degrees. Modern Picture Archive and Communication Systems (PACS) workstations and image viewers have tools to measure Cobb angles digitally. To date, studies comparing Cobb angle measurements of primary and secondary curves on digital radiographs and traditional radiographs have shown no statistical difference in the intraobserver or interobserver variance between the two techniques. New systems that automatically measure the Cobb angle and determine rotation are currently under development and may be able to reduce intraobserver and interobserver variability. Proximal and distal neutral vertebrae (not rotated in the axial plane) as well as stable vertebrae (vertebrae above and below the end vertebra that is bisected by the central sacral vertical line [CSVL]) should also be determined because these are used to help select fusion levels in operative planning.

Cobb method used to determine the degree of scoliosis.

The Cobb method, although initially described for measurement of coronal deformity, is also used to measure degree of lordosis and kyphosis in the sagittal plane. Lumbar lordosis is the Cobb angle measured from the perpendicular lines drawn from the superior end plate of S1 to the superior end plate of L1. Thoracic kyphosis is measured from the inferior end plate of T12 to superior end plate of T4 (this can be also measured from T2 to T12). In patients with a normal spinal profile, thoracic kyphosis typically measures between 30 and 35 degrees and lumbar lordosis ranges between 35 and 50 degrees. Notably, approximately two thirds of the total lordosis of the lumbar spine occurs at L4-S1.

Global spinal alignment in the coronal and sagittal planes is determined through the use of vertical reference lines. The CSVL is a line drawn up through the center of the sacrum parallel to the edge of the film (e.g., perpendicular to the floor). The C7 plumb line (C7PL) on a frontal radiograph is a vertical line drawn down from the midportion of the C7 vertebral body (approximated by the spinous process, or more accurately through determination of the centroid). Coronal balance ( Fig. 150-6 ) is determined by measuring the distance between these reference lines. By convention, a difference with the C7PL to the left is considered negative and a difference to the right is positive.

Coronal balance as determined on a frontal radiograph by measuring the distance between the central sacral vertical line (CSVL), a line drawn up through the center of the sacrum parallel to the edge of the film, and the C7 plumb line (C7PL), a vertical line drawn down from the midportion of the C7 vertebral body. By convention, a difference with the C7PL to the left is considered negative and a difference toward the right is positive.

Sagittal balance ( Fig. 150-7 ) is determined on a lateral radiograph by measuring the distance between the C7PL, drawn from the centroid (geometric center of the vertebrae) of C7 down and the dorsal/rostral corner of the sacrum. This difference is known as the sagittal vertical axis (SVA). The SVA is considered positive if the C7PL falls ventral to this reference point and negative if it falls dorsal to it. The normal mean SVA offset is +0.5 cm and lies within a tight range between −1 and +1 cm.

Sagittal balance is determined on a lateral radiograph by measuring the distance between the C7 plumb line (C7PL), drawn from the centroid (geometric center of the vertebrae) of C7 down and the dorsal/rostral corner of the sacrum. This difference is known as the sagittal vertical axis (SVA) offset. The balance is considered positive if the C7PL falls ventral to this reference point and negative if it falls dorsal to it.

Axial rotation on plain radiographs can be determined using the Nash-Moe method. The Nash-Moe method categorizes vertebral rotation into five grades based on the location of the pedicle in relation to the lateral aspect of the vertebral body. CT measurement of rotation may be more reliable than standard radiographic techniques.

Overall sagittal balance of the patient is greatly affected by relationship of the spine to the pelvis. An adult patient with thoracic hyperkyphosis or loss of lumbar lordosis attempts to compensate for spinal sagittal imbalance through pelvic retroversion and hip extension, thereby attempting to bring his or her head back over the center of the pelvis. In the operative planning for deformity (including fusion to the sacrum or pelvis), spinopelvic relationships (including these compensatory mechanisms) should be taken into account. Figure 150-8 demonstrates the spinopelvic measurements, including pelvic tilt (PT), sacral slope, and pelvic incidence and T1 pelvic angle. Pelvic tilt (a measure of the degree of pelvic retroversion [e.g., compensation for sagittal imbalance]) is the angle determined between a vertical reference line drawn up from the center of the femoral heads (femoral head axis) and a line drawn from the femoral head axis to the midpoint of the sacral end plate. If femoral heads are not perfectly aligned on a lateral radiograph, the location of the femoral head axis is approximated by the midpoint of a line connecting the geometric center of each femoral head. Sacral slope simply is the angle of a line drawn along the sacral end plate and the horizontal. Pelvic incidence is measured by determining the angle between a line drawn perpendicular to the sacral end plate at its midpoint and a line from the femoral head axis to this point. The pelvic incidence is equal to the pelvic tilt plus the sacral slope.

Demonstration of spinopelvic parameters used in surgical decision making. A, Pelvic tilt is the angle determined between a vertical reference line drawn up from the center of the femoral heads (femoral head axis) and a line drawn from the femoral head axis to the midpoint of the sacral end plate. B, Sacral slope is the angle subtended by a line drawn along the sacral end plate and a horizontal reference line. C, Pelvic incidence is measured by determining the angle between a line drawn perpendicular to the sacral end plate at its midpoint and a line from the femoral head axis to this point.

The physiologic mechanisms of spinopelvic alignment to maintain upright posture were mainly defined by three pelvic parameters: pelvic incidence (fixed morphologic parameter), pelvic tilt, and sacral slope (dynamic parameters that changes with compensatory mechanisms). Using these parameters, lumbar lordosis and thoracic kyphosis can be predicted theoretically. It has been shown that pelvic retroversion (measured by PT) and SVA correlate with HRQOL scores :

Related posts:

Anatomy of Nerve Root Compression, Nerve Root Tethering, and Spinal Instability Ventral and Ventrolateral Subaxial Decompression Diffuse Idiopathic Skeletal Hyperostosis Spinal Dural and Extradural Vascular Malformations Minimally Invasive Spinal Decompression and Stabilization Techniques I: Philosophy and Rationale Explant Analysis of Wear, Degradation, and Fatigue in Motion Preserving Spinal Implants

Stay updated, free articles. Join our Telegram channel

Join