Imaging Studies
Imaging studies are necessary for an accurate assessment and diagnosis of spondylolisthesis. Consensus is lacking on the optimal imaging protocol for spondylolysis/spondylolisthesis, in part because of the issue of radiation exposure to the pediatric spine, the many pros and cons for each modality, and developing technology, potentially making magnetic resonance imaging (MRI) the most sensitive option. [18] Imaging is typically pursued in the following order:
-
Radiography
-
Single-photon emission computed tomography (SPECT) scanning
-
Computed tomography (CT) scanning
-
MRI
Radiography
Standing lateral radiographs are the preferred method of evaluating slippage of the vertebrae in persons with spondylolisthesis, and they are an excellent means of monitoring for progression of the condition. [19, 20, 21] The standing lateral view is best because the translation occurs in the sagittal plane and is often accentuated during standing (due to the oblique orientation of the lower lumbosacral intervertebral disc spaces).
Standing flexion/extension films should be obtained to assess the degree of instability of the involved vertebrae. These radiographs are also useful in detecting an occult spondylolisthesis.
The anteroposterior view offers limited information in mild cases of spondylolisthesis; however, in cases of severe slips, this view may reveal the so-called reverse Napoleon hat sign because the L5 vertebra is viewed end-on through the sacrum, giving rise to the appearance of an upside-down Napoleon hat.
Oblique films are best for evaluating the integrity of the pars interarticularis. A defect is seen as a collar on the neck of the Scotty dog.
Single-photon emission computed tomography (SPECT) scanning
SPECT scanning is an extremely sensitive means of evaluating the integrity of the pars interarticularis, and this bone scanning modality is useful in determining the acuteness of slippage in those with spondylolisthesis. The scan results are positive at the pars interarticularis in those with acute spondylolysis, and then the findings revert to normal once the condition is chronic, even though healing has not occurred.
As spondylolisthesis develops and progresses, findings become positive more anteriorly and diffusely. Therefore, SPECT scanning helps the physician predict who would benefit from a spinal fusion. Those patients with positive findings should benefit because the slippage is acute.
Computed tomography (CT) scanning
CT scans are the best test for defining bony detail, especially in surgical planning. [22, 23] CT is an excellent imaging modality for evaluating the pars interarticularis. The size of the defects in type IIA lesions can be measured.
CT scans may help distinguish between type IIA and IIC lesions by identifying the presence of cortication of the defects. These studies are also helpful in identifying fibrocartilaginous tissue at the defects, which may cause nerve root compression, leading to radicular symptoms
Sagittal reformations may help clinicians in assessing foraminal stenosis
Magnetic resonance imaging (MRI)
The value of MRI in the diagnosis of spondylolysis is highly dependent on sequencing. It is useful in the diagnosis of early active spondylolysis. [24]
MRI should be ordered in cases that are associated with neurologic deficits. This imaging modality is an excellent means of observing compression of the dural sac, such as in degenerative cases in which the posterior arch is left behind and increased compression results from advancing slip.
Grading spondylolisthesis
The following methods are used to grade spondylolisthesis:
-
Meyerding technique: This technique involves dividing the superior aspect of the vertebra below the slip into 4 equal divisions, as is observed on a lateral radiograph. Assess where the posterior arch of the slipped vertebral body lies with respect to these 4 quadrants.
Grade 1: Less than 25% slippage
Grade 2: Between 25% and 50% slippage
Grade 3: Between 50% and 75% slippage
Grade 4: Between 75% and 100% slippage
Grade 5: Greater than 100% slippage (also called spondyloptosis)
-
Taillard method: This method describes the degree of slippage as a percentage of the anteroposterior diameter of the top of the first sacral or fifth lumbar vertebra (ie, %slip = displacement of (L5 on S1/width of S1) X 100). The result is an exact percentage of the slip.
-
Sacral inclination: This inclination is the angle formed between the posterior sacral border and a vertical line perpendicular to the floor and, thus, it is another means of measuring the extent of slippage. The sacrum tends to a more vertical position with increasing slips.
-
Slip angle: This angle is the relationship determined by a line along the posterior border of S1 and the inferior endplate of L5.
The slip angle and percentage slip may predict the risk of future slip progression.
Laboratory Studies and Other Diagnostic Tests
If alternative diagnoses are being considered, order appropriate laboratory tests (eg, laboratory workup for malignancy).
An electromyogram (EMG) may be helpful for detecting subtle radiculopathy, especially in the setting of a negative neurologic examination.
