Unilateral lumbar spondylolysis on radiography and MRI: emphasis on morphologic differences according to involved segment.

OBJECTIVE: The objective of our study was to retrospectively compare the radiography and MRI findings of unilateral spondylolysis in the upper lumbar segment and in the lower lumbar segment and to consider how these radiologic findings can be applied in the diagnosis of unilateral spondylolysis.

MATERIALS AND METHODS: Thirty patients with unilateral lumbar spondylolysis were categorized into one of two groups according to the lumbar levels involved with pars interarticularis defects: group A (L1, L2, and L3) or group B (L4 and L5). On radiographs, we evaluated contour bulging of the affected pars interarticularis, reactive sclerosis in the contralateral pedicle, anterolisthesis of the involved vertebra, and deviation of the spinous process. On MRI, we assessed pseudoarticulation of the pars interarticularis defect, uneven distribution of posterior epidural fat, the interspinous distance between adjacent segments, facet and disk degeneration in adjacent segments, and other anomalous changes.

RESULTS: Among the 63 patients with unilateral spondylolysis, the upper lumbar segment was involved in 29 and the lower lumbar segment, in 34. Group A often displayed contour bulging of the affected pars interarticularis, reactive sclerosis of the contralateral pedicle, and contralateral deviation of the spinous process, all of which were easily detectable on radiography. Group B frequently showed anterolisthesis, pseudoarticulation of the pars interarticularis defect, adjacent facet-disk degeneration, and other anomalous changes that were well observed on MRI.

CONCLUSION: Unilateral lumbar spondylolysis displayed radiologic differences in morphology of the isthmic defect itself and in ancillary findings of the adjacent structures based on the segment involved. Recognition of different ancillary features of unilateral spondylolysis with the use of a feasible diagnostic tool can be helpful for the diagnosis of cases in which a direct sign of isthmic defect is equivocal.