Spondylolysis is a defect in the pars interarticularis, commonly involving the L5 and occasionally the L4 vertebrae. Most spondylolysis is thought to be produced by repetitive stress. The gravitational and muscular loads acting across the steep incline of the upper surface of the sacrum can be resolved into a shearing component, which tends to displace the L5 vertebral body forward on S1, and a compressive component at right angles to the superior surface of S1 (Fig. 6-63). In accordance with Newton’s third law, S1 will exert an equal and opposite force against the inferior aspect of the L5 vertebral body. The tendency of L5 to be displaced forward on S1 is primarily resisted by the impaction of the inferior articular processes of L5 on the superior articular processes of S1. Again, Newton’s third law dictates that there will be an equal and opposite force exerted against the inferior articular process of L5. The upward and forward force of the sacral body on the L5 body and the upward and backward force of the superior articular process of the sacrum on the inferior articular process of L5 cause shearing stresses to be concentrated on the pars interarticularis, and this can produce a stress fracture.
FIGURE 6-63. The gravitational load (G) is applied across the lumbosacral junction. The equal and opposite forces acting on the inferior aspect of the L5 body and the anterior aspect of the inferior articular
FIGURE 6-64. Spondylolysis and spondylolisthesis. An oblique radiograph demonstrates a spondylolytic defect in the pars interarticularis of L4 (arrow). Note the intact neck in the “Scotty dog” outline in the L3 vertebra.
Spondylolisthesis is an anterior subluxation of one vertebral body on another. It can occur at any vertebral level, but the mechanics of the lumbosacral junction cause a higher incidence at this level. The most common cause at this level is spondylolysis, where the impaction of the inferior articular process of L5 or L4 will no longer be able to resist forward displacement of the vertebral body. Whether or not a spondylolisthesis follows a spondylolysis is largely determined by the resistance of the other supporting structures of the lumbosacral junction, which include the intervertebral disc, the anterior longitudinal ligament, and the iliolumbar ligaments. When they fail, the lysis becomes a listhesis.
Other causes of spondylolisthesis include degenerative changes in the facet joints and disc that produce joint instability, fractures, dysplasia of the upper sacrum or the neural arch of L5, generalized pathology such as Paget’s disease, or iatrogenically induced laminectomy or facetectomy (62).
On oblique plain films, spondylolysis is visualized as a break in the neck of the “Scotty-dog” outline, which is produced by the ipsilateral transverse process forming a nose; the ipsilateral pedicle, an eye; the pars interarticularis, a neck; the ipsilateral inferior articular process, a forelimb; the lamina, a body; the contralateral inferior articular process, a hind limb; and the spinous process, a tail (Fig. 6-64). When spondylolysis is suspected clinically or on plain film studies, a volumetric CT with sagittal reconstruction, MR, or plain film examination with nuclear medicine scintigraphy can be useful for diagnosis (Fig. 6-65A,B) (70).
Spondylolisthesis is graded by the amount of subluxation, with grade I being a forward displacement of less than 25%; grade II, a forward displacement of 25% to 50%; grade III, a displacement of 50% to 75%; and grade IV, a displacement of greater than 75%. Grading the spondylolisthesis is usually accomplished by lateral plain films or sagittal reconstructed images in CT (Fig. 6-65C,D).
FIGURE 6-65. Spondylolysis and spondylolisthesis. A: Right parasagittal reconstruction of an MDCT demonstrates a spondylolytic defect in the pars interarticularis of L5 (PD arrow ). B: Parasagittal reconstructed image form a volumetric CT demonstrating the pars defect in a patient with stage II spondylolisthesis (PD arrow ). C: Midplane sagittal reconstruction demonstrates the grade II spondylolisthesis. D: Volume rendering, oblique posterior view demonstrating the pars defect (PD) and a normal pars (NP) above.
On CT, the defect of spondylolysis is differentiated from the facet joint interval by its location at the axial level of the pedicles rather than at the level of the neural foramen, as well as by the defect’s irregular margins and adjacent sclerosis. By MRI, the defect in the pars is visualized as a low–signalintensity zone within the high–signal-intensity marrow of the pars.