Lumbar Spondylolysis without Spondylolisthesis: Recognition of Isolated Posterior Element Subluxation on Sagittal MR John L. Ulmer, Vincent P. Mathews, Allen D. Elster, and James C. King PURPOSE: To document the occurrence of isolated dorsal subluxation of posterior elements in cases of lumbar spondylolysis without spondylolisthesis both quantitatively (using spinal canal measurements) and qualitatively (by visual inspection) on sagittal MR images. METHODS: Ret- rospective analysis identified 63 patients with lumbar spondylolysis (confirmed by CT or conven- tional radiography) who had undergone MR imaging. From these we identified 12 patients with pars interarticularis defects but no evidence of spondylolisthesis. Measurements of anteroposterior spinal canal diameters were performed in these 12 patients to ascertain whether the sagittal canal diameter at the level of the spondylolysis exceeded the normal range as determined from 100 control subjects. RESULTS: In 9 of 12 patients the spinal canal was abnormally widened at the level of the spondylolysis because of dorsal subluxation of posterior elements. In 5 of these patients, the subluxation was readily visible on midline sagittal MR images. In 4 patients, spinal canal measurements were necessary to document this phenomenon. CONCLUSION: In the ma- jority of patients with spondylolysis but without spondylolisthesis, sagittal MR images can show isolated dorsal subluxation of posterior spinal elements. Index terms: Spine, dislocation; Spine, magnetic resonance AJNR Am J Neuroradiol 16:1393–1398, August 1995 Lumbar spondylolysis is present in more than nous process step-off sign” on conventional ra- 14 million people in the United States, 25% of diographs (10) or as a “wide-canal sign” on whom will eventually have significant low back sagittal MR images (5). Recently, we noticed pain or sciatica (1, 2). Although magnetic res- that in some patients with spondylolysis but onance (MR) imaging is widely used to evaluate without spondylolisthesis, dorsal subluxation of such patients, it remains inferior to computed posterior elements may occur as an isolated tomography (CT) for displaying defects in the sign of the open-arch defect (Fig 1). The goal of pars interarticularis. The sensitivity of MR for this study was to analyze and quantify this phe- detecting pars defects is even further reduced nomenon, thereby determining the use of mid- when there is coexisting facet joint degenera- sagittal MR images for revealing such a struc- tion, pedicle sclerosis, or spondylolysis without tural change in the posterior spinal column. spondylolisthesis (2–5). When both spondylolysis and spondylolisthe- sis are present, the anteroposterior diameter of Subject and Methods the spinal canal becomes widened (2, 5–10). This phenomenon may be recognized as a “spi- Reviewing medical records and radiology reports gen- erated over a 4-year period, we identified 63 patients with lumbar spondylolysis who also had undergone MR imag- Received September 13, 1994; accepted after revision February 8, ing for lower back or radicular pain at our institution. From 1995. this group, 12 patients were found to have spondylolysis From the Department of Radiology, Bowman Gray School of Medicine but no evidence of spondylolisthesis (ie, vertebral body of Wake Forest University, Winston-Salem, NC. subluxation) on sagittal MR images (Table). There were 9 Address reprint requests to Vincent P. Mathews, MD, Department of male and 3 female patients, ranging from 17 to 63 years Radiology, Bowman Gray School of Medicine, Medical Center Boulevard, old (mean age, 36.3 years). Eight patients had bilateral Wiston-Salem, NC 27157-1088. pars defects at L-5, 2 patients had unilateral defects at L-5, AJNR 16:1393–1398, Aug 1995 0195-6108/95/1607–1393 1 patient had bilateral defects at L-3, and 1 patient had q American Society of Neuroradiology bilateral defects at L-2. Defects in the pars interarticularis 1393 1394 ULMER AJNR: 16, August 1995 Fig 1. Diagram illustrates two forms of vertebral subluxation that may occur asso- ciated with spondylolysis: ventral subluxa- tion of the involved vertebral body and iso- lated dorsal subluxation of posterior elements. were confirmed by conventional radiography with oblique perpendicular distance between these two lines (Figs 2 views in all patients, supplemented by CT in 7 patients. and 3). All MR studies were performed using surface coils on a To normalize these measurements according to patient high-field (1.5-T) unit. Midline T1-weighted sagittal im- size, a sagittal canal ratio was defined as the midsagittal ages (600/15/2 [repetition time/echo time/excitations]) canal diameter at the level of spondylolysis divided by that were used exclusively for measurements in this investiga- at L-1. Based on analysis of data from 100 control subjects tion. Other imaging parameters included section thick- without spondylolysis (5), the normal mean values ness, 4 mm; intersection gap, 1 mm; field-of-view, 30 cm; (6standard deviation) of the sagittal canal ratios as a and imaging matrix, 192 3 256. function of vertebral level have been determined and are Sagittal canal diameters at the level of spondylolysis as follows: L-2/L-1, 0.95 (60.05); L-3/L-1, 0.93 (60.07); and at L-1 were measured as previously described (5). At L-4/L-1, 0.96 (60.08); L-5/L-1, 0.99 (60.10). The range each level, a reference line was first constructed tangent to of values at each level is as follows: L-2/L-1, 0.87 to 1.06; and parallel with the posterior border of the vertebral body L-3/L-1, 0.77 to 1.14; L-4/L-1, 0.83 to 1.24; L-5/L-1, 0.78 at its midportion. A second line was then drawn, parallel to to 1.25. The control subjects from this prior study (5) were the first, at the most anterior aspect of the lamina of the selected from young adult patients referred for MR imaging same vertebra at the spinolaminar junction. The midsag- to evaluate lower back pain whose MR images were inter- ittal canal diameter at this level was defined to be the preted as unequivocally normal by two experienced neu- roradiologists and whose conventional radiographs (with obliques) showed no evidence for lumbar spondylolysis. Sagittal canal ratios at involved vertebral levels Using magnified midline sagittal T1-weighted images on an independent console, one observer measured the ca- Age, Level of Sagittal Canal Patient nals of each patient three times at each level. The mean of y/ Sex Spondylolysis Ratio the three measurements was taken as the anteroposterior 1 17/M L-5 1.39† canal diameter that was used to calculate the sagittal canal 2 18/M L-2 1.27† ratios relative to L-1 in each control subject at the other 4 3 20/M L-3 1.00 lumbar levels. 4 24/M L-5 1.27† The upper-limit sagittal canal ratio for each level in the 5 25/ F L-5 1.17 100 control subjects of the prior study (5) was selected as 6 31/M L-5 1.50†‡ a threshold value to define normal from abnormal in the 7 37/M L-5* 1.05 8 41/ F L-5 1.39†‡ current study. At each level, the upper-limit sagittal canal 9 51/ F L-5 1.75†‡ ratio value chosen was greater than 2 standard deviations 10 54/M L-5* 1.29† above the mean and, therefore, above the 95% confidence 11 54/M L-5 1.71†‡ limit established from our control subjects with intact neu- 12 63/M L-5 1.48†‡ ral arches. When the sagittal canal ratio exceeds the upper *Unilateral pars defect only. limits of normal (L-2, 1.06; L-3, 1.14; L-4, 1.24; L-5, † Exceeds upper limits of normal by level (L-2 5 1.06, L-3 5 1.14, 1.25), an “abnormally widened” anteroposterior diameter L-4 5 1.24, L-5 5 1.25). of the spinal canal on midsagittal MR images can be diag- ‡ Canal measurements were not necessary, because posterior nosed, suggesting an open-arch defect (5). element subluxation was visible. AJNR: 16, August 1995 LUMBAR SPONDYLOLYSIS 1395 Fig 2. Thirty-one-year-old man with bilateral L-5 spondylolysis (patient 6 in the Table). A, Midsagittal T1-weighted MR image shows isolated posterior element subluxation, resulting in an increased sagittal canal diameter at the level of spondylolysis (sagittal canal ratio, 1.50). Axial T1-weighted MR image (B) and CT (C) confirm the spondylolysis and demonstrate increased canal diameter (arrow). Results Discussion In 9 (75%) of the 12 patients, abnormal wid- Lumbar spondylolysis is characterized by ening of the spinal canal at the level of the bone defects in the pars interarticularis (11–14). spondylolysis because of dorsal subluxation of These defects are thought to result from re- posterior elements was demonstrated (Table). peated microfractures and elongation of a con- In 5 of these patients, posterior displacement of genitally weakened pars, usually first becoming the spinolaminar line and spinous process at the radiographically visible in late childhood or ad- involved level was visually apparent (Fig 2, pa- olescence. Typically, the pars defects remain tient 6 in the Table). In the other 4 patients, the bridged by fibrocartilage but may form a abnormally widened canal (and dorsal posterior pseudojoint. Occasionally, healing and bone element displacement) was not obvious to vi- union occurs, a phenomenon likely accounting sual inspection but could be diagnosed by mea- for many of the 10% to 15% of cases with uni- surement of the sagittal canal ratios (Fig 3, pa- lateral defects (2, 11–15). The L-5 level is in- tient 10 in the Table). (One of these 4 patients volved in 90% to 95% of patients.