x Spondylolysis and Spondylolisthesis JB.docx
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Spondylolysis and Spondylolisthesis
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Figure 1: Sschematic drawings of a lumbar vertebra, seen from above (axial view, on the left) and from the side (lateral view, right). VB: vertebral body; TP: transverse process (omitted from drawing on the right as it projects out of the plane); SP: spinous process. As seen from above, the central canal, behind the vertebral body, is surrounded by the pedicles (red), lamina (purple) and the superior and inferior articulating processes and the bone that connects them, the pars intraarticularis (gray).
Damage to the pars can incite an attempt at healing. The resultant fibrous tissue hypertrophy can cause foraminal stenosis with compression of the exiting nerve root.
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Figure 2: If there is a pars defect, the bodies above can slip forward relative to the body below.
In the vast majority of cases, spondylolysis occurs via the above mechanism of overuse resulting in stress and microfracture. In rare circumstances, however, a defect in the pars articularis can occur following an acute load or trauma.
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Figure 3: A pars defect is shown in the red circle; a normal pars is seen above in the green circle. (Case courtesy of Radswiki, Radiopaedia.org, rID: 11967)
The oblique film is the most important view in the assessment of spondylolysis and spondylolisthesis. From this angle, the vertebra in the lumbar spine demonstrate the classic “Scottie dog” appearance (Ffigure 4), consisting of the superior articular process (ears of the dog), transverse process (head), isthmus (neck), lamina and spinous process (body), and inferior articular processes (foreleg and hindleg). In spondylolysis, the fracture through the pars articularis appears as a neck collar on the Scottie dog.
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Figure 4: An oblique view of the lumbar spine, showing the “Scottie Dog”. The pars is represented by the dog’s collar, shown in faint red here. (Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org, rID: 7552)
In spondylolisthesis, there is anterior displacement of the superior articular process relative, allowing the entire body to slip forward (fFigure 5).
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Figure 5: At left is a lateral x-ray showing an anterolisthesis at the L5-S1 level. In the annotation at right, the sacrum is outlined in yellow and the lumbar vertebral bodies in purple. The faint red lines outline the correct anatomic position from which the lumbar bodies have slipped. (Modified from https://en.wikipedia.org/wiki/Spondylolisthesis#/media/File:SpondylolisthesisL5S1.jpg)
Spondylolisthesis can be graded based on magnitude of vertebral displacement. The superior endplate of the caudal vertebra is divided into 4 quarters. The grade is dependent on location of the poster-inferior corner of the vertebra above. Grade 1 represents a 0-25% displacement, grade 2: 25-50%, grade 3: 50-75%, and grade 4: > 75%. Grades 1 and 2 are considered ‘low grade’ and grades 3 and 4 are considered ‘high grade’, which will be important when developing an appropriate treatment strategy.
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