SPINE Volume 32, Number 14, pp 1555–1560 ©2007, Lippincott Williams & Wilkins, Inc. Does Lumbar Facet Fluid Detected on Magnetic Resonance Imaging Correlate With Radiographic Instability in Patients With Degenerative Lumbar Disease? Jeffrey A. Rihn, MD,* Joon Y. Lee, MD,* Mustafa Khan, MD,* James A. Ulibarri, MD,* Chadi Tannoury, MD,† William F. Donaldson, III, MD,* and James D. Kang, MD* Study Design. Retrospective radiographic/imaging study. Objective. The purpose of this study was to analyze the association between lumbar facet fluid on MRI and sagittal instability on flexion lumbar radiographs in pa- tients with degenerative disease at L4 –L5. We hypothe- sized that the amount of facet fluid on MRI correlates with instability on the flexion radiograph. Summary of Background Data. Although never for- mally studied, it has been suggested that lumbar facet fluid detected on MRI is indicative of spinal segment in- stability. Methods. Patients who underwent laminectomy or lam- inectomy and fusion for the treatment of degenerative lum- bar disease at L4 –L5 at our institution between 2002 and 2004 and who had preoperative lumbar MRI and anteropos- terior and flexion-extension radiographs available for re- view were study eligible. Axial T2 MRI images through the L4 –L5 facets were analyzed for facet fluid. The facet fluid index was calculated, i.e., the ratio of the sum of the width of fluid in each facet (bilateral) to the sum of the width of each facet (bilateral). Instability on the flexion radiograph was measured as percent anterior slip at L4 –L5. Results. Fifty-one patients were included in the study, 28 (55%) of whom had facet fluid noted on MRI. Of those patients who did have facet fluid on the MRI, 23 of 28 had instability on the flexion lumbar radiograph and 5 of 28 had no instability. The mean facet fluid index and percent anterior slip for the 28 patients with MRI facet fluid was 0.12% and 11.1%, respectively. There was a positive linear association between these values (Pearson correlation coefficient of 0.90, P  0.001). The positive predictive value of L4 –L5 facet fluid on MRI as an indicator of radio- graphic instability was 82%. Conclusions. There is a close linear association be- tween the facet fluid index and the amount of radio- graphic instability at L4 –L5. Facet fluid on MRI should raise high suspicion of lumbar instability. Key words: degenerative, lumbar, facet, fluid, MRI, instability. Spine 2007;32:1555–1560 Whether an arthrodesis is performed in addition to a laminectomy for symptomatic lumbar stenosis may de- pend on the stability of the involved spinal segments. Several studies support arthrodesis at the time of lami- nectomy in those patients with evidence of degenerative spondylolisthesis. 1–3 For this reason, it is important to accurately identify those patients who have lumbar spine segmental instability in addition to spinal stenosis. Tra- ditionally, spinal instability has been evaluated using weight-bearing lateral flexion-extension radiographs, where anteroposterior (AP) translation is suggestive of lumbar spine instability. 4–7 Although contradictions exist in the literature, several studies support the use of these dynamic radiographs. 4 – 6,8 Other imaging methods, includ- ing traction-compression radiographs, 9 dynamic magnetic resonance imaging (MRI), 10 –12 and three-dimensional dy- namic computed tomography, 13 have been reported as use- ful means of evaluating the lumbar spine for instability. There is currently no consensus in the literature, however, on the most reliable means of accurately diagnosing lumbar instability. Several studies have found that degeneration of the lumbar facet joints is coupled to intervertebral disc de- generation, both of which are important contributors to instability. 14 –22 It has been suggested that fluid collec- tion within the lumbar facet detected on MRI is indica- tive of segmental instability. 23 Mailleux et al 23 reported 2 cases of degenerative lumbar spondylolisthesis with stenosis, which was not initially detected using MRI due to reduction of the spondylolisthesis in the supine posi- tion. Facet fluid was detected on the MRI at the involved lumbar segments. Although this case report suggests that facet fluid detected on MRI should raise suspicion of lumbar instability, this issue has not, to our knowledge, been formally studied. MRI is commonly used to evaluate patients with de- generative lumbar disease and lumbar spinal stenosis. The signs of lumbar intervertebral disc degeneration on MRI are well described in the literature. 24,25 Several studies have attempted to characterize the association between intervertebral disc degeneration on MRI and lumbar spinal segmental instability with inconsistent re- sults. 21,26,27 Less has been published regarding the MRI evaluation of lumbar facet degeneration. 21,28 Recently, Fujiwara et al studied the association between interver- tebral disc and facet joint degeneration and developed an MRI-based grading system for the severity facet joint degeneration. 22 These authors found that both interver- From the *Department of Orthopaedic Surgery, University of Pitts- burgh School of Medicine, Pittsburgh, PA; and the †Department of Orthopaedic Surgery, Thomas Jefferson University, Philadelphia, PA. Acknowledgment date: June 8, 2006. First revision date: September 6, 2006. Second revision date: November 15, 2006. Third revision date: December 4, 2006. Acceptance date: December 5, 2006. The manuscript submitted does not contain information about medical device(s)/drug(s). No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. Address correspondence and reprint requests to Joon Y. Lee, MD, University of Pittsburgh School of Medicine, Department of Orthopae- dic Surgery, 3741 Fifth Avenue, Suite 1010, Pittsburgh, PA 15213; E-mail: jrihno16@yahoo.com 1555