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Isthmic Spondylolisthesis
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Electro-radiological Diagnosis

Plain radiographs remain an excellent initial step in the evaluation of patients with spondylolysis and spondylolisthesis. All patients should be studied with conventional standing lateral, anteroposterior, and 30 oblique cranial tilt views, the latter being more reliable than conventional oblique films for detecting spondylolysis. Although plain radiographs may initially be negative in young patients who later develop isthmic spondylolisthesis, partial cracks and sclerosis are suspicious findings and serial radiographs may be necessary throughout the adolescent growth spurt.

A familiarity with standardized methods of documenting anterior displacement, sagittal rotation, sacral inclination, and lumbar index, however, is important in the routine evaluation of patients with spondylolisthesis. Anterior displacement or translation is often referred to as "slip" or "olisthesis" and is measured either as a percentage of the inferior vertebra or by the Meyerding grading system that divides the inferior vertebra into four equal parts. Although the percentage system is more precise, interobserver error and minor changes in positioning can fabricate changes greater than 10 percent; therefore, the Meyerding grading system of comparison may be more accurate and useful. Sagittal rotation or "roll," also referred to as "slip angle", "lumbosacral kyphosis," describes the angular relationship between the two vertebral bodies. Sacral inclination or "tilt" describes the vertical position of the sacrum, while the lumbar index describes the wedging of the listhetic vertebra. Although some authors think that radiographs in positions of lumbar flexion and extension with the patient standing and lying can add additional information, this is not universally practiced.

Additional imaging modalities are always needed to evaluate patients with radicular pain or neurological deficits. Magnetic resonance images are essential by providing better soft tissue definition and multiplanar images that track the nerve root well beyond the foramen. Scintigraphy, which is rarely in use, can be performed to exclude acute fractures and malignant disease. Finally, discography and facet arthrography may be helpful in some instances but have not been adopted universally

Preferred Examination

Lateral and anteroposterior plain radiographs of the lumbar spine should be obtained in patients with complaints of back pain. The lateral view is useful in detecting spondylolisthesis. The lateral view may demonstrate the pars defect; bilateral oblique views are especially useful to visualize the pars interarticularis defect, which has the appearance of a Scottie dog with a collar. An elongated pars also may be seen. Straight lateral radiograph of the L4-S1 level of the spine shows a lucency at the pars area. Bilateral pars defects must be present to visualize this in a lateral projection. Grade 1 spondylolisthesis is associated with an appearance resembling a Scottie dog with a collar. (The collar is the pars defect.)
Oblique projection radiograph shows the presence of bilateral pars defects, with an appearance resembling a Scottie dog
with a collar. (The collar is the pars defect.
Plain radiographs also may demonstrate congenital types and the changes of spondylosis. In the setting of trauma, fractures may be apparent. Note that other causes of the patient's symptoms may be demonstrated, such as an osteoid osteoma, Paget disease, and osteolytic lesions. The grade of spondylolisthesis can be measured by using the lateral view. Bilateral pars defects must be present to visualize this in a lateral projection.
Cross-sectional imaging should be considered next.
In patients with back pain and no clinical findings of nerve root involvement, CT scan of the lumbar spine yields information regarding spondylolisthesis and its cause, along with other possible conditions, such as disk disease, disk herniations, spondylosis, and spinal canal stenosis. Other associations, such as spina bifida, may be seen.
In patients with radiculopathy, CT myelography can yield information regarding nerve-root impingement and its etiology, such as disk herniation, abscess, or neoplasm.
CT of the spine can be performed with or without intrathecal contrast enhancement. Axial images are obtained in a plane parallel to the disk spaces at each level imaged. Sagittal reconstruction images are obtained by using post-acquisition processing software. Bone window (e.g., 1500/300 HU) and soft-tissue window (e.g., 300/30 HU) settings are used.
Section-thickness selection is important to avoid problems, such as volume averaging, and thin sections should be used. Contiguous images also reduce such problems. Indeed, if the sections are too thick and if a gap is present between sections, spondylolysis can be missed. In such cases, sagittal reconstructions may be of help. With spondylolysis, CT is performed as close as possible to 90 to the level of interest. CT scans typically demonstrate a horizontally oriented defect in the pars, which interrupts the normally complete bony ring of the posterior elements. Sagittal reconstruction images also show the spondylolysis.
Axial CT image shows bilateral spondylolysis. Note elongation of the spinal canal at this level.
Sagittal CT reconstruction image shows the pars defect along with grade
1 spondylolisthesis. Sagittal CT reconstruction image shows the pars defect along with grade 1 spondylolisthesis.
Spondylolisthesis is evaluated best on lateral topogram but can be suggested in patients with spinal stenosis in the absence of disk pathology, posterior hypertrophic changes, or a congenitally narrow spinal canal. One typically looks for an elongated spinal canal.
CT scans can also demonstrate findings of congenital/dysplastic and degenerative types of spondylolisthesis.
Abnormalities of the vertebral body or articular processes may be present.
Changes of spondylosis deformans (degenerative changes) are apparent on CT scans. Degenerative disease of the spine has a characteristic appearance involving a loss of disk space height with or without the presence of vacuum phenomenon, narrowing of the facet joint space, subchondral sclerosis, osteophyte formation, and subchondral cysts. Some or all of the changes may be present and cause altered alignment of the facet-joint articular surface, leading to slippage. Spinal canal and/or intervertebral neural foraminal stenosis may be present.
In traumatic spondylolisthesis, findings may include jumped facets and fractures of the articular processes and/or lamina that result in spondylolisthesis.
MRI has the distinct advantage of being able to image the spine in any plane without exposure to radiation. Typically, the axial and sagittal planes are used, but images in the coronal plane can also be acquired easily, if needed.
Spin-echo and fast spin-echo sequences are used for image acquisition in these planes. A fat-saturation technique can be applied to minimize signal from fat and to bring out signal from fluid structures (eg, bone edema). Gradient-echo sequences can also be used and have the advantage of faster image acquisition, limiting problems related to motion. In a postoperative patient, consideration should be given to gadolinium enhancement with T1-weighted spin-echo sequences in the sagittal and axial planes.
MRIs should be scrutinized for the presence of a spondylolisthesis and for any abnormality of the pars interarticularis, pedicles, or facet joints. Nervous structures, including exiting neural foramina and spinal canal, should be evaluated.
A spondylolisthesis is best assessed on median sagittal images of the spine. The levels involved and the grade can be seen.
A spondylolysis pathologically can be a fibrous bridge or a pseudoarthrosis, both of which have corticated/sclerotic margins in the adjacent portions of the bony ring. The bony sclerosis and fibrous tissue appear as an area of low signal intensity in the region of the pars interarticularis on images obtained with all sequences. This finding may not be easily seen and is a limitation of the use of MRI for spondylolysis.
Similar signal-intensity changes in these areas may be seen with bony sclerosis, volume averaging with adjacent osteoarthritic facet joints, osteoblastic metastases, and even involvement of the pedicles with Paget disease. Even if one sees normal bone signal extending from vertebral body to pedicle into the lamina, it is not possible to exclude a spondylolysis because there may be minimal sclerosis in the bone and because its signal intensity is similar to that of posterior element bone. This is a limitation of MRI in detecting spondylolysis.
High signal intensity may be seen in the pars interarticularis with T2-weighted sequences. This finding indicates the presence of fluid, a pseudoarthrosis, or bone edema from infection.
Degenerative disease can also be seen. Narrowed disk space, with disk desiccation (low T2 signal intensity), should be sought. This disk narrowing allows for superoinferior subluxation at the facet joint at the level of disease, which allows for anterolisthesis or retrolisthesis. Reactive marrow changes should also be sought; such changes may occur in portions of vertebral body adjacent to disks and also in marrow adjacent to facet joints, resulting in abnormal signal intensity in the pars interarticularis.
Neoplastic disease involving the pars interarticularis or other parts of the vertebra typically yields low marrow signal intensity with all sequences. Infection may be evidenced by fluid signal intensity (appearing bright on T2-weighted MRIs) from bone edema. Both disease processes show enhancement with a gadolinium-based contrast agent. Other diseases causing a sclerotic response (eg, Paget disease) result in low signal intensity with all sequences.
MRI is not appropriate in every patient, as the presence of metal hardware and claustrophobia may preclude its use. In addition, some pediatric patients may need sedation to undergo MRI, which poses some risk.

Limitation of Techniques

If present, spondylolisthesis usually is detected on plain radiographs. A spondylolysis may not always be visible.
CT scanning is more sensitive for detecting spondylolysis, but occasionally this can be missed, since scanning occurs in the plane of the spondylolysis or from volume averaging. Sagittal reconstruction images are of help in patients with these findings.
MRI reveals spondylolisthesis on sagittal views. Spondylolysis may not be readily apparent on MRIs, especially if there is a mild degree of bony sclerosis. Other sclerotic lesions, such as osteoblastic metastases, in the pars interarticularis may give similar appearances.

Differential Diagnoses

Other Problems to Be Considered if spondylolisthesis is confirmed using lateral radiographs, the differential diagnosis involves ascertaining the etiology of the spondylolisthesis. In this regard, the patient's age and all imaging information is considered.
A pars defect has a typical appearance, but other causes of bone lysis, such as neoplasm and infection, need to be entertained on rare occasions.
Degenerative disease of the spine has a characteristic appearance, including loss of disk space height with or without the presence of vacuum phenomenon, narrowing of the facet joint space, subchondral sclerosis, osteophyte formation, and subchondral cysts. Some or all of these changes may be present and cause altered alignment of the facet-joint articular margins, leading to slippage. Spinal-canal or neural foraminal stenosis also may be present and are viewed best on CT (with sagittal reconstructions); MRI can also display spinal stenoses and neural foraminal narrowing.



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