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Figure 5a. Grade IV spondylolisthesis in a 17-year-old boy with chronic low back pain and no neurologic dysfunction. (a) Sagittal midline nonenhanced dual-echo MR image (3,500/16, 98) shows grade IV spondylolisthesis of the L5 vertebra with respect to the S1 vertebra, with almost complete canal stenosis in the center. Note the overriding of the L5 vertebra anterior to the S1 vertebra. Note also the degenerated disk, two components (arrows) of which are adhering to the parent L5 and S1 vertebral bodies, without extrusion into the canal. (b) Sagittal nonenhanced MR image (3,500/16, 98) far left of the midline shows the complete obliteration (arrows) of the L5-S1 foramen due to the anatomic deformity produced by the spondylolisthesis; despite this, no discrete neural compromise of the L5 nerve root was encountered clinically.