[Spondylolysis and -listhesis. A computed tomographic differential diagnosis of lumbar discopathy].

Spinal CT scans of 680 patients with suspected disc herniation were reviewed to detect lumbar spondylolysis. In this group of patients, 3.23% (n = 22) had pars interarticularis defects at L4 or L5. Disc herniation at the interspace of the pars defect was seen in 27.27% (6/22) of patients with spondylolysis. Using Meyerding's method, which measures the degree of vertebral forward dislocation, grade 2 and grade 3 spondylolisthesis were most often seen. While moderate and severe spondylolisthesis can be already detected via lateral localiser image (scout view), pars defects with only little forward dislocation can be easily underdiagnosed. Sources of diagnostic error are: 1. an atypically posterior disc margin which extends beyond only one vertebral body and presents the appearance of disc herniation, 2. pars defects simulating the adjacent facet joints involve the risk of overlooking spondylolysis, 3. CT sections made through the intervertebral discs and facet joints only, may fail to show up the pars defect 10 or 15 mm above the disc plane.