ORIGINAL ARTICLE An Interspinous Process Distractor (X STOP) for Lumbar Spinal Stenosis in Elderly Patients Preliminary Experiences in 10 Consecutive Cases Jangbo Lee, MD*, Kazutoshi Hida, MD*, Toshitaka Seki, MD*, Yoshinobu Iwasaki, MD*, and Akino Minoru, MD Background: Lumbar spinal stenosis (LSS) is often a positiondependent condition that is aggravated in extension and relieved in flexion. Methods: Ten consecutive elderly patients with LSS were assessed postoperatively by magnetic resonance imaging and the Swiss Spinal Stenosis Questionnaire. Cross-sectional areas of the dural sac and intervertebral foramina at the stenotic level were measured postoperatively and compared with the preoperative values. Results: Postoperatively the cross-sectional area of the dural sac increased 16.6 mm 2 or 22.3% and intervertebral foramina increased 22 mm 2 or 36.5%. The intervertebral angle and the posterior disc height changed significantly. Seventy percent of the patients were satisfied with the surgical outcome. Conclusion: This new surgical method is effective in elderly LSS patients. Key Words: lumbar spinal stenosis, dural sac, elderly patients, spinal canal, intervertebral foramen (J Spinal Disord Tech 2004;17:72 77) Japan is currently experiencing an unprecedented era of an aged society. As the life expectancy of the population increases, the Japanese physician is faced with an increasing number of elderly patients who are suffering from disabling pain from lumbar spinal stenosis (LSS). Many of these patients are candidates for decompressive surgery, but many of them are unable to undergo major surgery due to co-morbid conditions and health risks. This dilemma poses a serious concern to Received for publication June 9, 2003; accepted September 4, 2003. From the *Department of Neurosurgery, Graduate School of Medicine, Hokkaido University and Asabu Neurosurgical Hospital (Dr Minoru), Sapporo, Japan. Reprints: Jangbo Lee, MD, Department of Neurosurgery, Graduate School of Medicine, Hokkaido University, N-15 W-7, Kita-Ku, Sapporo, 060-8638, Japan (e-mail: Jangbo@med.hokudai.ac.jp). Copyright 2004 by Lippincott Williams & Wilkins today s physician. Many patients are forced to continue with ineffective conservative, nonoperative therapies and endure the restricted function and painful symptoms of LSS. A treatment that is as risk-free as conservative treatment and as effective as surgery is needed. The X STOP interspinous spacer (St. Francis Medical Technologies, Concord, CA) was developed to prevent extension at the stenotic level and increase the dimensions of the spinal canal and the intervertebral foramina. Ten cases of this new treatment method for LSS were performed in a prospective study from July 2001 to April 2002. The purpose of this study is to report the safety and efficacy of the X STOP in these LSS patients by 1) reviewing the surgical and postoperative complications, 2) examining the results of a condition-specific outcome measure, and 3) analyzing the dimensions of the dural sac and intervertebral foramina in a prospective manner. MATERIAL AND METHODS Patient Population Ten consecutive patients underwent X STOP surgery for LSS between July 2001 and April 2002. There were seven men and three women, with a mean age of 71 years (range 61 79 years) at the time of surgery (Table 1). All patients had preoperative leg pain with or without back pain that was relieved in flexed positions such as bending forward and sitting or lying down and aggravated in extension. There was spinal claudication in seven patients (see Table 2). The dynamic nature of the condition was verified using a dynamic magnetic resonance imaging (MRI) system; the spinal canal area decreased, and the dural sac became compressed in extension and improved in flexion. Inclusion criteria required that each patient 1) be older than 60 years, 2) have mild to moderate stenotic symptoms, 3) have pain that is relieved when flexed and aggravated when extended, and 4) have dural sac compression in extension and relief in flexion as verified on dynamic MRI. The exclusion criteria included 1) unremitting pain in any position, 2) fixed motor deficit, 3) severe 72 J Spinal Disord Tech Volume 17, Number 1, February 2004
J Spinal Disord Tech Volume 17, Number 1, February 2004 Interspinous Process Distractor (X STOP) TABLE 1. Study Group Patient Characteristics (n = 10) Sex Male = 7 Female = 3 Age (y) Mean = 71 Range = 61 79 Duration of follow-up (mo) Range = 9 18 Mean = 11. of levels treated Single level = 9 Two level = 1 symptomatic LSS at three or more levels, and 4) significant spinal instability. Instrumentation The titanium X STOP implant has two components: a main body assembly that includes the spacer and a universal wing assembly (Fig. 1). There are four sizes available: 6, 8, 10, and 12 mm. Surgical Procedure Standard general anesthesia was administered in each case, and surgery was performed with the patient in the prone (knee chest) position with the lumbar spine flexed as much as TABLE 2. Average Score of SSS Questionnaire and Radiologic Parameters Before and After the Operation Average Score of SSS Questionnaire. Age (y)/sex Symptoms Follow-up (mo) Symptom Physical Function Pre Post Pre Post Satisfaction OP Level 1 69/M LBP & R 14 3.43 2.86 2.60 1.80 2.00 L4 L5 2 79/M LBP & R, SC 14 4.43 1.86 3.00 2.60 2.83 3 75/F LBP & R, SC 18 2.14 1.86 1.60 1.40 1.17 L4 L5 4 77/M LBP 9 3.14 2.57 2.80 2.80 3.33 L3 L4 5 72/M LBP & R, SC 9 2.43 1.71 1.80 1.60 1.67 L4 L5 6 62/M LBP & R, SC 10 3.29 3.29 2.80 3.00 2.00 L4 L5 7 61/M LBP 10 2.00 1.86 2.00 2.00 2.83 L4 L5 8 63/F LBP & R, SC 9 1.57 1.57 1.80 1.60 1.67 L4 L5 9 74/M LBP, SC 10 2.29 2.29 2.20 2.20 1.17 L4 L5 9 L5 S1 10 77/F LBP & R, SC 10 2.71 2.71 2.71 2.20 2.20 L3 L4 DSD (mm 2 ) (n=9) IVF (mm 2 ) (n=9) Angle ( ) (n = 11) PDH (mm) (n = 11) ISP (mm) (n = 10). Pre Post Pre Post Pre Post Pre Post Pre Post 1 61.1 ± 1.02 74.3 ± 3.02 51.7 ± 3.50 74.1 ± 1.25 9.0 8.0 7.14 8.14 1.53 8.33 2 NP NP NP NP 11.0 8.00 7.67 8.05 2.31 8.57 3 60.2 ± 3.02 84.8 ± 3.76 50.1 ± 4.57 87.5 ± 1.69 15.5 14.4 4.65 6.48 3.73 9.16 4 40.5 ± 0.58 55.4 ± 1.94 68.5 ± 3.16 80.9 ± 3.08 4.10 2.40 7.62 8.69 5.34 10.6 5 62.5 ± 5.07 61.5 ± 1.70 68.3 ± 3.61 92.2 ± 3.73 3.70 3.05 4.28 7.74 3.83 7.92 6 NP NP NP NP 10.9 5.77 7.05 8.34 3.61 7.27 56.2 ± 1.59 89.2 ± 1.43 80.6 ± 0.81 92.2 ± 7.93 12.1 10.5 9.79 9.95 5.67 12.3 8 24.9 ± 2.98 59.4 ± 5.15 34.9 ± 2.74 82.2 ± 3.23 12.0 11.0 5.57 7.83 2.91 8.64 9 124.4 ± 4.15 132.9 ± 2.57 59.1 ± 6.61 69.1 ± 2.43 13.4 11.9 4.19 9.56 4.29 9.95 9 106.1 ± 1.74 110.9 ± 2.37 51.6 ± 0.78 71.5 ± 1.71 18.7 13.8 2.04 3.26 NP NP 10 126.2 ± 3.74 143.5 ± 2.72 78.1 ± 2.00 91.3 ± 6.08 11.8 10.7 5.18 6.43 2.6 7.94 Patients 1, 2, 4, and 5 had a significant change (0.5) in symptom severity, and patient 1 showed a significant improvement (0.5) in physical function. Patients 1, 3, 5, 6, 8, 9, and 10 were satisfied with the surgical result. The values for dural sac and intervertebral foramen dimension are means ± SD. Patients 2 and 6 were excluded from radiologic study because MRI was performed at other institutions. The reduction of the values for intervertebral angle after surgery implies that the implant decreased lordosis. OP, operative; DSD, dural sac dimension; IVF, intervertebral foramen; PDH, posterior disc height; ISP, interspinous process; LBP, low back pain; R, radicular pain; SC, spinal claudication; NP, not performed. 2004 Lippincott Williams & Wilkins 73
Lee et al J Spinal Disord Tech Volume 17, Number 1, February 2004 process distances were determined at the narrowest point between the adjacent projections of the spinous process. These postoperative MR images were all taken within a month after surgery. FIGURE 1. The X STOP system is composed of two parts. Right, X STOP after uniting together; left, the main body and the universal wing possible. A midline skin incision of approximately 5 cm was made above the spinous processes of the stenotic level. The paraspinal muscle was elevated from both sides of the spinous processes to the level of the facets and laminae. The supraspinous ligament was preserved, and a curved dilator was used to pierce the interspinous ligament and locate the space between the spinous processes. The operative level was verified with fluoroscopy. The interspinous space was gently sized with a sizing distractor. The correct implant size was determined by opening the sizing distractor until significant resistance was encountered. The main body of X STOP was inserted from the right side as close to the laminae as possible. The universal wing was attached and locked in position by tightening the nut. Radiographic Outcome Measurements MRI was performed in all patients preoperatively and postoperatively (1.5 T, Signa Horizon; GE, St. Louis, MO). All patients were examined with sagittal and axial T2-weighted images of the lumbar spine with 4-mm slices. Measurement of the cross-sectional area of the dural sac at the site of maximal compression was calculated on axial view T2-wighted images. The dural sac had the smallest area at the disc level. The cross-sectional area of the intervertebral foramen was measured with the parasagittal MR images at the section that showed the narrowest cross-sectional area of the foramen, which usually corresponded to the middle of the pedicle. After the right and left sides of the cross-sectional area of the foramen were calculated, the narrower side was selected. Three parameters were measured on the midsagittal MR images, including the intervertebral angle (n = 11), the posterior disc height (n = 11), and the interspinous process distance (n = 10). Under the threefold magnification, all the five parameters were calculated using digital cursor and digitizing software (RADWORKS version 5.0; Applicare Medical Imaging). All measurements were repeated three times. The intervertebral angle was measured from the superior endplate of the vertebra above to the superior endplate below the vertebral body (Fig. 2). 1 Positive values reflected lordosis, and negative values reflected kyphosis. Posterior disc height was measured according to the Dabbs method (see Fig. 2). 2,3 The interspinous Clinical Outcome Measurement Clinical outcomes were measured using the Swiss Spinal Stenosis (SSS) Questionnaire (Table 3). 4,5 The final questionnaires were performed between 9 and 18 months postoperatively in 10 patients. The mean period of follow-up was 11 months after surgery. Statistical Analyses Statistical significance was determined with Stat View software (Abacus Concepts, Berkeley, CA) using paired t test. The confidence level for significance was P < 0.05. RESULTS There were 11 levels treated in the 10 patients in the study; there were 9 single-level cases and 1 double level. The X STOP was placed at L4 L5 in seven cases, L3 L4 in two, and L5 S1 in one. An 8-mm implant was used in five cases, 10 mm in five cases, and 12 mm in one. The average operative time was 20 minutes per level (range 15 30 minutes), and the blood loss did not exceed 100 ml. There were no intraoperative complications or site-related postoperative complications such as implant failure, bony failure, or infection. The most common medical co-morbidities were hypertension and car- FIGURE 2. Three parameters (intervertebral angles, posterior disc height, and interspinous distance) measured in midsagittal MR image of the lumbar spine. Dotted cross-lines are used to determine corners of the vertebral body. To investigate intervertebral angles, a straight line (dotted lines) was drawn on the upper margin of each vertebra from L4 to L5. The posterior disc height (arrow) was determined as the distance from the most inferior corner of the upper vertebra to the most superior corner of the lower vertebra. The interspinous process distances (dotted lines) were determined at the narrowest point between the adjacent projections of the spinous process. After the operation (right), the compressed dural sac at the L4 L5 level was relieved (case 8). 74 2004 Lippincott Williams & Wilkins
J Spinal Disord Tech Volume 17, Number 1, February 2004 Interspinous Process Distractor (X STOP) diac disease, but others included diabetes mellitus, cerebral stroke, and peripheral vascular disease (Table 4). Outcome data were obtained at a minimum follow-up period of 9 months. The patient satisfaction domain of the SSS demonstrated that 70% of the patients were at least somewhat satisfied with the outcome of their surgery (5/10 very satisfied and 2/10 somewhat satisfied) (see Table 2). Four patients had a significant change (postoperative average score preoperative average score 0.5) in symptom severity, and only one patient showed a significant improvement in physical function. The mean dural sac area changed from 73.6 mm 2 preoperatively to 90.2 mm 2 a 23% increase (P < 0.002) (see Table 2). Likewise, the intervertebral foraminal area increased from 60.3 to 82.3 mm 2 a 36% increase (P < 0.0005). Pre- and postoperative intervertebral angle changed from 11.1 to 9.05 (P = 0.016). The posterior disc height increased from 5.93 to 7.68 mm (P = 0.031), and the interspinous process distance increased from 3.58 to 9.07 mm (P < 0.0001) (see Table 2). DISCUSSION The current treatment options for elderly lumbar stenosis patients able to undergo surgery include a facetectomy or laminectomy with or without fusion. 6,7 Elderly patients, however, typically have medical co-morbidities that may prevent them from undergoing elective spinal surgery. The principle of the X STOP is to prevent extension at the stenotic level and increase the area of the spinal canal and neural foramina in a normal symptomatic position such as standing. The outcomes of our study were measured with the SSS Questionnaire, which proved to be the most reliable and valid one among many other methods of evaluation for LSS in a recent study. 4 The normal values for the dural sac are 150 200 mm 2, and the critical size of the dural sac in LSS is <100 mm. 2,8,9 The current results demonstrate that the dural sac area significantly increased by 23% following X STOP surgery. The mechanism of this increase is supported by previous reports that demonstrate bony dimensions of the spinal canal change FIGURE 3. Preoperative (left) and postoperative (right) axial T1-weighted images of the lumbar spine at the disc level of L4 L5 reveal that the area dural sac was increased by decreasing the thickness of the ligamentum flavum (case 2). TABLE 3. SSS Questionnaire 4,5 In the past month, how would you describe 1. The pain you have had on the average including pain in your back and buttocks as well as pain that goes down the legs? Very severe 2. How often have you had back, buttock, or leg pain? Less than once a week At least once a week Every day, for at least a few minutes Every day, for most of the day Every minute of the day 3. The pain in your back or buttocks? Very severe 4. The pain in your legs or feet? 5. Numbness or tingling in your legs or feet? 6. Weakness in your legs or feet? Very severe 7. Problems with your balance?, I ve had no problem with balance. Yes, sometimes I feel my balance is off or that I am not sure-footed. Yes, often I feel my balance is off or that I am not sure-footed. In the past month, in a typical day 8. How far have you been able to walk? More than 2 miles More than 2 blocks, but less than 2 miles More than 50 feet, but less than 2 blocks Less than 50 feet 9. Have you taken walks outdoors or around the shops for pleasure? 2004 Lippincott Williams & Wilkins 75
Lee et al J Spinal Disord Tech Volume 17, Number 1, February 2004 TABLE 3. (continued) SSS Questionnaire 4,5 10. Have you been shopping for groceries or other items? 11. Have you walked around the different rooms in your house or apartment? 12. Have you walked from your bedroom to the bathroom? If you have had treatment for your back or legs recently, how satisfied are you with 13. The overall result of your back operation? 14. Relief of pain after your operation? 15. Your ability to walk after your operation? 16. Your ability to do housework, yard work, or job after your operation? 17. Your strength in your thighs, legs, and feet? 18. Your balance, or steadiness, on your feet? *The score increases with worsening disability. Subsections of the scale include the Symptom Severity Scale (questions 1 7), subdivided into a pain domain (questions 1 4) and a neuroischemic domain (questions 5 7); the Physical Function Scale (questions 8 12); and the Satisfaction (with Treatment) Scale (questions 13 18). significantly during flexion and extension. 8,10 12 Inufusa et al 13 reported findings based on the computed tomography data that showed flexion increased the size of the central canal 24 mm 2 or 11% and extension decreased the size of the canal 26 mm 2 or 11%. In another article, Schönströmetal 8 reported that the cross-sectional area of the spinal canal is reduced by 40 mm 2 and the midsagittal diameter of the canal is reduced by 2 mm when the lumbar spine moves from flexion to extension. Penning and Wilmink 12 also reported on the dynamic phenomenon of the spinal canal in which the spinal canal narrows in extension and widens in flexion, resulting in a relief of nerve root compression. The ligamentum flavum plays a significant role in this finding. The mean maximum thickness differs by >2 mm between upright flexion and upright extension. The ligamentum flavum buckles during extension, presumably because of a decrease of vertical distance between the adjacent lamina. The intervertebral disc is another structure with a position-dependent morphology; even minor disc bulging during extension may decrease the cross-sectional area of the spinal canal significantly. With regard to foraminal size, the current results are supported by previous studies that showed the neural foramen dimensions were shown to be position dependent. 8,10,14 17 The current results demonstrated a mean change of the foraminal cross-sectional area of 36%. The intervertebral foramen is shaped like an inverted teardrop, and its height and cross-sectional area vary from 11 to 19 mm and from 40 to 160 mm 2, respectively. 18 The intervertebral foramen is bounded superiorly and inferiorly by the pedicles of the adjacent vertebrae. The anterior boundary is formed by the posterior margin of the vertebral bodies and the intervertebral disc. The posterior boundary comprises the pars interarticularis, ligamentum flavum, and superior articular process. 19,20 Panjabi et al 11 found that the area of the intervertebral foramen decreased about 20% in extension in a cadaver study. Fujiwara et al 21 reported that flexion opened the foramen by 11.3%, whereas extension closed it by 12% in biomechanical and anatomic studies. Also Schmid et al 22 used an open-configuration MRI system with the individuals in an up- TABLE 4. Co-existing Co-morbidities (n = 10) Coronary artery disease: 4 cases (40%) Previous percutaneous transluminal coronary angioplasty: 1 case (10%) Previous coronary artery bypass graft: 1 case (10%) Previous myocardial infarction: 2 cases (20%) Arrhythmia: 2 cases (20%) Hypertension: 4 cases (40%) Diabetes mellitus: 2 cases (20%) Cerebral stroke: 2 cases (20%) Peripheral vascular disease: 1 case (10%) 76 2004 Lippincott Williams & Wilkins
J Spinal Disord Tech Volume 17, Number 1, February 2004 Interspinous Process Distractor (X STOP) right position in vivo to confirm the changes in foraminal area during flexion and extension. They found a 23.2% decrease in the cross-sectional foraminal area from the upright neutral to the upright extended position and a 19.2% increase from the upright neutral to the upright flexed position. Interestingly, degenerated segments showed more changes in the foraminal area during flexion and extension. 11,14 Takahashi et al 23 assessed the relationship between epidural pressure and lumbar posture in patients with LSS. The pressure was increased with extension but decreased with flexion. In addition to these commonly reported anatomic changes associated with LSS, the segmental angle, posterior disc height and the interspinous process distance all changed significantly. Finally, the patient satisfaction domain of the SSS showed that 70% of these patients were satisfied with the overall result of this operation. Patient satisfaction is a more definitive outcome of surgery for LSS than the degree of radiographic findings. 24 26 Arayanpur et al 24 suggested that complete decompression may not be necessary to achieve symptomatic relief. Also Thomas et al 26 reported a statistically significant increase in dural sac size after laminotomy or laminectomy but found no statistical relationship between the extension of decompression and clinical outcome. It may only be necessary to bring the patients below a symptomatic threshold to achieve a good clinical result. This minimally invasive surgical technique is very favorable when compared with decompressive surgery: fewer complications, shorter recoveries, and almost no risk of significant complications. 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