DISTINCTION BETWEEN DEGENERATIVE AND ISTHMIC SPONDYLOLISTHESIS ON SAGITTAL MR-IMAGES - IMPORTANCE OF INCREASED ANTEROPOSTERIOR DIAMETER OF THE SPINAL-CANAL (WIDE CANAL SIGN)

OBJECTIVE. The purpose of this study was to determine whether a visual ly apparent increase in the anteroposterior diameter of the spinal can al (''wide canal sign'') can be used reliably to differentiate degener ative from isthmic spondylolisthesis on midline sagittal MR images. We hypothesized that the wide canal sign would be present only in isthmi c spondylolisthesis, where the vertebral ring is disrupted by defects in the pars interarticularis. MATERIALS AND METHODS. The midline sagit tal MR images of 100 control subjects without spondylolysis or spondyl olisthesis were analyzed to establish the normal range of sagittal can al diameters at the various lumbar levels. Midline sagittal MR images of the lumbar spine were reviewed in 53 patients in whom either isthmi c (n = 35) or degenerative (n = 18) spondylolisthesis was confirmed wi th conventional radiography and/or CT. For each spinal level, the sagi ttal canal ratio, defined as the maximum anteroposterior diameter of t he canal at that level divided by the diameter of the canal at L1, was calculated. From our analysis of the control subjects, a normal sagit tal canal ratio was determined to be less than 1.25. A sagittal canal ratio of 1.25 or greater at the level of a spondylolisthesis was consi dered to represent an abnormally increased sagittal canal diameter (wi de canal sign), indicating the presence of bilateral pars interarticul aris defects. Using this sign alone, two neuroradiologists who had no knowledge of the true diagnosis classified the type of spondylolisthes is in a randomized subgroup of 34 age-matched adults (all more than 40 years old and with grade I isthmic or degenerative spondylolisthesis) . RESULTS. In the 100 subjects without spondylolisthesis and in the 18 patients with degenerative spondylolisthesis, the sagittal canal rati o did not exceed 1.25 (mean values, 0.93-0.99) at any level of the lum bar spine. Conversely, the sagittal canal ratio at the level of isthmi c spondylolisthesis exceeded 1.25 (mean value, 1.56) in 34 of 35 patie nts. In patients more than 40 years old, in wham degenerative spondylo listhesis is prevalent and misdiagnosis of isthmic spondylolisthesis i s potentially more likely, both neuroradiologists were 100% accurate i n correctly assigning the type of spondylolisthesis when using the wid e canal sign alone. CONCLUSION. The wide canal sign on midline sagitta l MR images (corresponding to an sagittal canal ratio greater than or equal to 1.25) is a reliable predictor of the presence of defects of t he pars interarticularis at the level of a spondylolisthesis. This sig n could be useful for distinguishing degenerative from isthmic spondyl olisthesis when degenerative sclerosis in the pars interarticularis mi mics spondylolysis, when direct axial imaging of the pars interarticul aris has not been done, or when imaging is technically inadequate.