Spondylolysis, or fracture, of the pars interarticularis is a cause of spondylolisthesis distinct from degenerative changes. The fracture, also known as a pars defect, is usually oriented perpendicular to the articular process.
Bilateral pars defects lead to a discontinuity between the anterior and posterior portions of the ring of a vertebra, and anterior translational movement of the vertebral body is no longer impeded by the facet joints. Spondylolysis is the most common cause of spondylolisthesis in patients younger than 50 yr of age and is believed to result from repetitive stresses and minor trauma with formation of fatigue fractures.
Sagittal images taken through the medial aspect of the pedicles and facet joints best demonstrate the normal anatomy. The superior facet has a triangular shape, and the remainder of the pars interarticularis can be seen extending in an inferoposterior direction to the inferior facet joint. A single sagittal image, however, cannot image all lumbar facet joints owing to the increasing diameter of the spinal canal inferiorly. More laterally, only facet joints may be seen without the more medially located pars. This should not be mistaken for a pars defect. On axial images, the pars interarticularis is seen slightly superior to the level of the intervertebral foramina. If the images are taken
perpendicular to the posterior vertebral body, the entire vertebral arch may be demonstrated (28).
Imaging spondylolysis is best performed with sagittal images. Discontinuity in the cortical bone is better demonstrated on T1 and proton density weighted images than with T2-weighting, as these allow a greater discrimination between cortical and medullary bone. The defect is seen as a perpendicular lesion with respect to the orientation of the articular processes (28). T2-weighted images may show increased signal intensity from the region of the pedicle and superior articular process, signifying marrow edema in the acute setting (25). In addition, type II marrow changes may be found in roughly 40% of pedicles next to a pars defect (25). Midsagittal images illustrate the degree of spondylolisthesis (25). Axial views may also demonstrate the defect, but facet joints may be mistaken for a pars defect (28).
Anterior spondylolisthesis with spondylolysis may complicate a bilateral pars defect. The width of the pars defect in this case is typically >5 mm and the central canal may actually increase in anteroposterior dimension due to the immobility of the posterior fragment and anterior subluxation of the vertebral body (28,29).
In addition to spondylolisthesis, sequelae of spondylolysis include acceleration of degenerative changes and stenosis. Intervertebral foraminal stenosis at adjacent levels is a fairly common finding but is usually more severe at the inferior level (28). With L5-S1 spondylolytic anterolisthesis, lateral disc herniations are a frequent finding, and the L5 spinal nerves in the intervertebral foramen may show signs of impingement from both foraminal stenosis and the herniation (25).
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