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A retrospective radiographic study involving analysis of abdominal and pelvic computed tomography (CT) scans obtained on patients presenting with clinical conditions other than back pain.To determine the incidence of spondylolysis and spondylolisthesis in patients requiring inpatient or emergency department CT evaluation for unrelated abdominal and pelvic conditions.Spondylolysis and spondylolisthesis are part of a disease process that is thought to be resultant from biomechanical stresses related to bipedal locomotion. The incidence is estimated to be 3% to 10% in the general population. Many of these cases occur without associated symptoms. To our knowledge, there is a relative paucity of data on the use of CT to evaluate the prevalence of these 2 entities in patients seeking medical attention for unrelated conditions.Five hundred ten consecutive abdominal and pelvic multi-detector CT scans obtained on a single scanner (Philips MX8000; Eindhoven, The Netherlands) were reviewed. These patients presented with such complaints as abdominal pain and fever, or were imaged as part of their inpatient evaluation for conditions unrelated to lumbar spine pathology. A board certificated radiologist and a radiology resident retrospectively evaluated CT scans for lumbar spondylolysis, spondylolisthesis, and associated degenerative changes. A neuroradiologist confirmed all positive cases.Of the 510 cases examined, there were 29 cases of spondylolysis at L5, corresponding to a prevalence of 5.7%. Twenty-three of the cases demonstrated bilateral spondylolysis and 6 unilateral. Sixteen of the 23 cases of bilateral spondylolysis also had spondylolisthesis, 13 of which were grade I, and 3 of which were grade II. In patients 45 years old and younger who did not have spondylolysis or spondylolisthesis, we observed a 32.2% incidence of sclerosis involving the L5 lumbar pedicles.This study demonstrates a 5.7% prevalence of spondylolysis and a 3.1% prevalence of spondylolisthesis in patients undergoing CT scans of the abdomen and pelvis for unrelated reasons, corresponding to the rate of spondylolysis and spondylolisthesis detected in prospective plain radiographic studies. We observed a 1.2% incidence of unilateral spondylolysis, and approximately 67% of these demonstrated contralateral sclerosis. It is suggested in the literature that sclerosis of the contralateral pedicle seen in cases of unilateral spondylolysis may be a compensatory response to mechanical stresses on an unstable lumbar vertebral body.