Spondylolysis refers to a cleft or break in the pars interarticularis of the vertebra. It is found in about 6% of adults, mostly in males, 93-95% occur at L5, and most are bilateral. The etiology is uncertain, but the current theory is that it represents a stress fracture from repeated trauma to the spine. Endnote The pars defect is demonstrated best in parasagittal images and is easier to see if the bone has a generous component of marrow or if soft tissue is interposed between the bone fragments. With subluxation, there is often a step-off at the pars defect. On axial views, the key observation is a horizontal line (an extra joint) between adjacent facets joints on consecutive images.


      Spondylolisthesis refers to forward displacement of one vertebra over another, usually of the fifth lumbar over the body of the sacrum, or of the fourth lumbar over the fifth. Spondylolisthesis is graded according to how far the vertebral body moves forward on the one below (Grade 1 = 25%, Grade 2 = 50%, Grade 3 = 75%). There are two types of spondylolisthesis, isthmic (open-arch type), associated with spondylolysis, and degenerative (closed-arch type).

      With isthmic spondylolisthesis, the pars defect divides the vertebra into an anterior part (vertebral body, pedicles, transverse processes, and superior articular facet) and a posterior part (inferior facet, laminae, and spinous process). The anterior part slips forward, leaving the posterior part behind. As a result, the spinal canal elongates in its anteroposterior dimension, so that spinal canal stenosis is uncommon with isthmic spondylolisthesis. Grade I spondylolisthesis is often asymptomatic, but with progressive anterior subluxation, the intervertebral disk and the posterior-superior aspect of the vertebral body below encroach on the superior portion of the neural foramen. Endnote The foramen is also elongated in a horizontal direction and may have a bilobed configuration. Exuberant fibrocartilage at the pars pseudarthrosis can further compromise the neural foramen and cause nerve root compression.


      Degenerative spondylolisthesis occurs in an older age group, usually over 60 years old, and it is more common in women at the level of L4-L5. It develops when there are severe degenerative changes and excess motion of the facet joints. Subluxation at the facet joints allows forward or posterior movement of one vertebra over another. A degenerative spondylolisthesis narrows the spinal canal, and symptoms of spinal stenosis are common. Hypertrophic facet arthrosis is a frequent cause of foraminal narrowing.

      The sagittal plane is best for displaying the abnormal anatomy of spondylo-listhesis, T2-weighted images for the canal and T1-weighted images for the pars interarticularis and neural foramina. The sagittal view clearly shows the degree of subluxation and the relationship of the intervertebral disk to the adjacent vertebral bodies and the spinal canal. Parasagittal images are good for showing encroachment on the foramina by disk or hypertrophic bone. Loss of the normal fat signal cushioning the nerve root is a sign for significant foraminal stenosis.

      Ulmer and colleagues Endnote proposed the "wide canal sign" to distinguish between isthmic and degenerative spondylolisthesis. Using a midline sagittal section, they noted that the sagittal canal ratio (maximum anteroposterior diameter at any level divided by the diameter of the canal at L1) did not exceed 1.25 in normal controls and in subjects with degenerative spondylolisthesis. In patients with spondylolysis, the measurement always exceeded 1.25.  

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