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00007632-200512010-0001300007632_2005_30_2677_siddiqui_interspinous_23miscellaneous-article< 87_0_8_7 >Spine© 2005 Lippincott Williams & Wilkins, Inc.Volume 30(23)1 December 2005pp 2677-2682The Positional Magnetic Resonance Imaging Changes in the Lumbar Spine Following Insertion of a Novel Interspinous Process Distraction Device[Diagnostics]Siddiqui, Manal FRCS†; Nicol, Malcolm MRCS†; Karadimas, Efthimios MD†; Smith, Frank MD*; Wardlaw, Douglas FRCS (Ortho)†From the Departments of *Radiology and †Orthopaedics, Woodend Hospital, Aberdeen, Scotland.Acknowledgment date: November 10, 2004. First revision date: July 8, 2005. Second revision date: July 22, 2005. Acceptance date: July 23, 2005.Grants have been received from St. Francis Medical Technologies Incorporated to fund the MRI scans.The device(s)/drug(s) that is/are the subject of this manuscript is/are not FDA- approved for this indication and is/are not commercially available in the United States.Corporate/Industry funds were received in support of this work. Although one or more of the author(s) has/have received or will receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this manuscript, benefits will be directed solely to a research fund, foundation, educational institution, or other nonprofit organization which the author(s) has/have been associated.Address correspondence and reprint requests to Douglas Wardlaw, FRCS (Ortho), Consultant Orthopaedics, Woodend Hospital, Eday Road, Aberdeen AB2 4NA, Scotland; E-mail: Design. Patients with symptomatic lumbar spinal stenosis underwent magnetic resonance imaging to study changes in the lumbar spine in various postures before and after implantation of the X STOP Interspinous Process Distraction Device (St. Francis Medical Technologies, Concord, CA).Objective. To visualize the effect of the device in vivo.Summary of Background Data. Previous studies have shown vertebral canal and exit foraminal area to reduce from flexion to extension. Recently, reports on improved kinematics in vitro at the implantation level of the X STOP device have also been published.Methods. Using positional magnetic resonance imaging, patients were scanned before and 6 months after surgery. Images were taken with the patient in sitting flexed, extended, neutral, and standing positions. The total range of motion of the lumbar spine and individual segments were measured, along with changes in disc height, areas of the exit foramens, and dural sac.Results. In 12 patients with 17 distracted levels, the area of the dural sac at these levels increased from 77.8 to 93.4 mm2 after surgery in the standing position (P = 0.006), with increase in the exit foramens, but no change in lumbar posture.Conclusions. This study shows that the X STOP device increases the cross-sectional area of the dural sac and exit foramens without causing changes in posture.Vertebral canal stenosis is a frequently seen condition in the outpatient setting, occurring most commonly in the lower lumbar spine.1 The etiopathogenesis is most frequently a result of a cascade of degenerative processes.2,3 Disc space narrowing, disc herniation, hypertrophy of the facet joints, and thickening of the ligamentum flavum may all contribute to the stenosis.4A century has passed since this condition was first diagnosed,5 but it was Verbiest6 who first described the symptoms of neurogenic claudication, and classified spinal stenosis into bony and nonbony categories. The classic presentation is sciatica with claudication and/or weakness, the relief of which is posture-dependent.7,8 Once the diagnosis is established, the treatment options vary from nonoperative treatment with nonsteroidal anti-inflammatory drugs, physiotherapy, and caudal epidurals for mild symptoms or those unfit for surgery, to decompression and/or fusion in patients in whom nonoperative treatment has failed. Surgical options have significant risks and morbidity.9,10The X STOP (St. Francis Medical Technologies, Concord, CA) device (Figure 1 available for viewing on ArticlePlus only) is an oval titanium spacer, which is placed between adjacent spinous processes (Figure 2) of the stenotic level. It flexes and distracts the affected level, stretching the soft tissue encroaching the canal, thereby increasing the cross-sectional lumbar vertebral canal and intervertebral foramen dimensions (Figure 3). The design of this product is based on the documented evidence of relief of symptoms with flexion and worsening of symptoms with extension. The aim of the manufacturers of the X STOP is to provide a low-risk alternative to the current options available for the treatment of lumbar spinal stenosis (LSS), particularly in patients who are not satisfied with the results of nonoperative therapy, and are unwilling or unable to undergo a major surgery (i.e., decompressive laminectomy with or without fusion).11Figure 2. A and B, Lateral lumbar radiographs before and after the double-level X STOP procedure.Figure 3. A and B, Anteroposterior lumbar radiographs before and after double-level X STOP procedure.The process of implantation is minimally invasive, performed with the patient under local anesthesia in the lateral position, without extensive stripping of paraspinal muscles, and without removal of any tissue. Zucherman et al12 showed that implantation of the device improved the symptoms of neurogenic intermittent claudication in patients with LSS in a percentage similar to those published for decompressive laminectomy but with considerably less morbidity. The concept of an interspinous implant has been studied before13,21 for various indications excluding neurogenic claudication. Results of controlled studies are awaited. Our study looks at the changes in the vertebral canal and intervertebral foraminal dimensions in vivo of patients with symptomatic LSS, before and after X STOP implantation using positional magnetic resonance imaging (MRI).Materials and MethodsThere were 12 patients with LSS and neurogenic intermittent claudication (confirmed on supine MRI) who had not responded to nonoperative treatment, such as bed-rest, physiotherapy, and antiinflammatory/analgesic medication, enrolled in the study provided they met all the inclusion and none of the exclusion criteria. The inclusion criteria were: older than 50 years; and leg, buttock, or groin pain with or without back pain while standing or walking. Rest must relieve the leg pain when the spine is flexed, such as when sitting; the back pain, if present, must be at least partially relieved during resting in a flexed position. The patient must be able to sit comfortably for at least 50 minutes (duration of positional MRI). In addition, narrowing of the spinal canal and/or neural foramina, at 1 or 2 level should have been demonstrated by conventional supine MRI imaging. The exclusion criteria were: not able to sit for 50 minutes without pain; unremitting spinal pain in any position; cauda equina syndrome, defined as neurocompression causing bowel or bladder incontinence or retention; pathologic fractures of the vertebrae; severe osteoporosis of the spine; body mass index >40 kg m−2; presence of active infection; Paget disease at the involved segments or spinal metastases; and spinal anatomy, such as ankylosing spondylitis or fusion at the affected level.Positional MRI is a relatively new imaging tool (Figure 4). The first commercially available upright positional MRI scanner (Fonar “Upright”; Fonar Corp., Melville, NY) has only been available since October 2000. Rather than using magnets arranged around a bore, the positional MRI scanner has an open configuration, with the magnetic field generated between the vertically mounted poles of a resistive magnet. This process gives enough space for the scanning table to rotate from 15° head down to vertical (standing), and to move vertically and horizontally enough to place any part of the body in the iso-center of the magnet with the patient in any position. With positional MRI, it is possible to compare the relative positions of the lumbar vertebrae throughout the full range of movement. By performing this before and after the placement of an X STOP implant, the changes that the implant causes can be measured. These changes are not just in the angles of the vertebral bodies but also in the dimensions of the dural sac and of the exit foramens. By using positional MRI, the patient can be studied in the very position that exacerbates the symptoms (i.e., standing) and then compare it with the position that relieves the symptoms (i.e., sitting).Figure 4. The 0.6T Upright Fonar scanner.Once enrolled, patients had a preoperative positional MRI. Each subject had T2 parasagittal and transverse sequences through the 5 lumbar discs in positions of erect (standing), neutral sitting, sitting in flexion, sitting in extension, and supine. The sequence parameters are detailed later (Table 1). There were 4.5-mm slices taken for the axial and sagittal views. The same radiographer who also positioned the patients scanned the patients. For the erect scan, the patients were actually leaning back against a rest at 5° from the vertical. This procedure was necessary because we have found, from previous studies, that no subject was able to stand absolutely still for the time needed for the study. By having the patient leaning against an almost vertical surface, this problem was eliminated. For the positions in which the patient sat in flexion and extension, support rests were placed once the patient had taken up the posture. Patients were asked to flex and extend only to the degree that they found comfortable for the duration of the scan.Table 1. Preoperative and Postoperative Positional MRI MeasurementsThe devices were implanted by or under the direct supervision of a single surgeon. The patients underwent the procedure either under local anesthetic with or without sedation, or under a light general anesthetic. After surgery, patients were mobilized immediately once they had recovered from the effects of any anesthetic or sedation and discharged within 2 days.Patients were reviewed in clinic at 6 and 12 weeks, and had a second positional MRI scan at 6 months. A single researcher using the Osiris 4.19 program (University of Geneva, Switzerland) made the measurements of the positional MRI. On the midline sagittal images (Figure 5 available for viewing on ArticlePlus only), distance cursors were used to measure anterior and posterior disc heights to the nearest pixel at all instrumented levels. Distances were measured between the most anterior or posterior points on each vertebral body, excluding osteophytes. Disc height measurements were performed with the patient in the erect posture to determine the true extent of kyphosis at the operated level and of the lumbar spine. Angles (Figure 6) were measured using angle calipers placed over lines drawn through the endplates from anterosuperior or anteroinferior corner to posterosuperior or postero-inferior corner, excluding osteophytes. The angles measured were the angles between the superior endplate of L1 and the superior endplate of S1 (L1−S1 angle) (Figure 7), and the angles between adjacent vertebral bodies at the instrumented levels. Endplate angles and foraminal surface areas were measured on the seated flexion and seated extension images.Figure 6. Midsagittal MRI showing endplate angle measurement in extension at L4–L5 before (A) and after (B) the X STOP procedure.Figure 7. Midsagittal MRI image showing L1–S1 angle measurement in flexion before (A) and after (B) X STOP.On the parasagittal images (Figures 8 and 9 available for viewing on ArticlePlus only), the surface area of the exit foramen at instrumented levels was measured using the region of interest cursor. This cursor gives a surface area within a drawn boundary. The same tool was used on the axial images to measure the surface area of the dural sac.On axial images (Figure 10 available for viewing on ArticlePlus only), the dural sac surface area at the instrumented levels was measured using the region of interest cursor where the canal is at the narrowest. The measurement included the extradural fat pad, but excluded disc, ligament flavum, and facet joint. Laterally, the measurement was taken up to the subarticular diameter. Dural sac surface area was measured on the standing and all seated images.All image measurements were made 5 times, both on the same day and on a later date to reduce error. The measurements were verified with another observer in the same manner. The highest and lowest value of each image measurement was disregarded, and mean was calculated from the remaining 3 values. SPSS software (version 8; SPSS, Inc., Chicago, IL) was used to analyze the data with the Wilcoxon sign rank test to determine significance of change in the values after surgery. The interobserver and intraobserver correlation was calculated using the Cohen kappa value.ResultsTwelve 12 patients (17 levels) were measured. There were 13 males and 4 females. The age ranged from 57 to 99 years. There were 6 L3–L4 and 11 L4–L5 levels. Five patients had 2 levels operated on. Measurements are shown in Table 1. Before surgery, from flexion to extension, the left exit foramens decreased by 31.2% and the right exit foramens by 18.3%. The dural sac area decreased by 26.7% from flexion to standing, 20.3% from flexion to extension, and 16.5% from neutral to standing.After surgery, from flexion to extension, the left exit foramens decreased by 14.1% and the right exit foramens by 9.4%. The dural sac area decreased by 18.1% from flexion to standing, 5.9% from flexion to extension, and 13.8% from neutral to standing. Therefore, the X STOP reduces the degree of encroachment on the exit foramens and dural sac.Comparing areas before and after surgery, in extension, the left and right exit foramens increased by 34.2% and 25.4%, respectively. Similarly, the increase in dural sac area by 20% in standing, 16.3% in neutral, and 26.9% in standing was statistically significant. This area also increased by 7.5% in flexion but was not statistically significant. Furthermore, the net increase of spinal canal area after the insertion of the X STOP was maintained when the patient went from sitting in neutral to standing in extension. The reduction in the endplate angles was statistically insignificant, and the range of lumbar flexion-extension was maintained.It is also noteworthy that the area of the canal measured before surgery while the patient is sitting in neutral (position of symptom relief) was very similar to the area of the canal measured while standing (the position of maximum extension) (93.14 vs. 93.39 mm2, respectively) after X STOP insertion, which may explain the mechanism of action of the device. The Cohen kappa coefficient was used to analyze the data between the 2 observers. The Kappa values calculated for interobserver correlation were: endplate angles (0.9132), disc heights (0.9209), L1S1 angles (0.9849), exit foramens (0.7137), and dural sac (0.7923). A linear regression analysis showed r2 = 0.99 and a slope of 1.18. A scatter graph was plotted (Figure 11). The Kappa values for intraobserver correlation were: endplate angles (0.9728), disc heights (0.9981), L1S1 angles (0.9990), exit foramens (0.9218), and dural sac area (0.9024).Figure 11. Linear regression analysis of interobserver measurements.Placement of the X STOP did not appear to have a significant impact on the alignment of the spine of the studied subjects. Overall, the total lumbar angles in flexion and extension were not significantly altered, and the small changes in the alignment of the endplates were not statistically significant. The overall range of lumbar spine movements was preserved.DiscussionLSS typically presents with low back pain, with radiculopathy and neurogenic claudication, which is worse in extension and easier in flexion.14 Based on this result, a distraction device has been developed to limit encroachment of the vertebral canal and exit foramens by soft tissue and bone. Previous radiologic15,16 and cadaveric18,22 biomechanical studies have shown changes in the kinematics of the spine with posture. Although our study concurs with the findings of Schmid et al,17 our results are not in agreement with the previous study of the X STOP,19 showing significant reduction in the overall range of motion from 25.8° to 20.8° after surgery. Unlike the previous studies, this research was based on the in vivo finding of the X STOP in patients with spinal stenosis.The posterior disc height did not increase after surgery, as we would have expected. Perhaps the reason for this result is that the distracted segment hinges at the facet joint level. There may also be, 6 months after surgery, at the second scan, some degree of soft tissue stress relaxation at the level of the posterior anulus. This result may explain why canal and foraminal dimensions increased, but not the posterior disc height.Metaanalysis of 74 published studies of surgery for LSS showed a 26% to 100% good-to-excellent outcome.20 High morbidity and mortality is associated with spinal decompressions with or without fusion.23 Clinical results with the X STOP24 show improvement in Zurich Claudication Questionnaire outcomes by 45.4% in symptom severity (control 7.4%) and 44.3% in physical function (control 0.6%).To the authors’ knowledge, no study exists in the literature that measures vertebral and foraminal canal dimensions before and after X STOP implantation in patients with LSS. Our preliminary results suggest that the X STOP improves the anatomic cross-sectional areas of the lumbar spinal canal at the implanted level(s) and, thus, may have a clinical role in the treatment of LSS as an alternative to other nonoperative or surgical options. This is an ongoing study, and the clinical benefit is being investigated with relevant questionnaires comparing preoperative and postoperative scores.Key Points * To our knowledge, the first in vivo study documenting changes in kinematics in patients with spinal stenosis before and after the X STOP. * Use of positional MRI. * Supports previous clinical studies on the X STOP.AcknowledgmentThe authors thank Drs. D. Villegas and S. Yerby, and St. Francis Medical Technologies, Inc., for statistical support.References1. Amundsen TH, Weber F. Lumbar spinal stenosis. Clinical and radiologic features. Spine 1995;20:1178–86. [CrossRef] [Full Text] [Medline Link] [Context Link]2. Arnoldi CG, Brodsky AE, Cauchoix J, et al. Lumbar spinal stenosis and nerve root entrapment syndromes. Definition and classification. Clin Orthop 1976;115:4–5. [CrossRef] [Full Text] [Medline Link] [Context Link]3. Johnssson KE, Rosen I, Uden A. The natural course of lumbar spinal stenosis. Clin Orthop 1992;279:82–6. 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The implant significantly improved the vertebral canal and exit foramens area without affecting overall movement of the lumbar spine.The Positional Magnetic Resonance Imaging Changes in the Lumbar Spine Following Insertion of a Novel Interspinous Process Distraction DeviceSiddiqui, Manal FRCS; Nicol, Malcolm MRCS; Karadimas, Efthimios MD; Smith, Frank MD; Wardlaw, Douglas FRCS (Ortho)Diagnostics2330