Satisfactory surgical results for CSM have recently

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1 J Neurosurg Spine 18: , 2013 AANS, 2013 A less-invasive cervical laminoplasty for spondylotic myelopathy that preserves the semispinalis cervicis muscles and nuchal ligament Clinical article Masayuki Umeda, M.D., Ph.D., 1 Kunihiko Sasai, M.D., Ph.D., 2 Taketoshi Kushida, M.D., Ph.D., 1 Ei Wakabayashi, M.D., 2 Tokun Maruyama, M.D., 3 Atsushi Ikeura, M.D., 1 and Hirokazu Iida, M.D., Ph.D. 1 1 Department of Orthopedic Surgery, Kansai Medical University, Hirakata City, Osaka; 2 Spine Center, Kishiwada-Eishinkai Hospital, Kishiwada City, Osaka; and 3 Department of Orthopedic Surgery, Social Insurance Shiga Hospital, Otsu City, Shiga, Japan Object. Modified cervical laminoplasty techniques have been developed to reduce postoperative axial neck pain and preserve function in patients with cervical spondylotic myelopathy (CSM). However, the previous studies demonstrating satisfactory surgical outcomes had a retrospective design. Here, the authors aimed to prospectively evaluate the 2-year outcomes of a modified cervical laminoplasty technique for CSM that preserves the paravertebral muscles. Methods. Outcomes were analyzed for 40 patients (22 men and 18 women; mean age, 66.6 years; age range years) with CSM who underwent C4 6 laminoplasty with C-3 and C-7 partial laminectomies or C-3 total and C-7 partial laminectomies and received hydroxyapatite spacers. Neurological, pain severity, and spinal radiographic evaluations were performed preoperatively and at 3, 6, 12, 18, and 24 months postoperatively. Plain radiography and MRI of the cervical spine were performed to evaluate the range of motion (ROM), sagittal alignment, and crosssectional areas of the deep extensor muscles. The extent of bone spacer bonding and bony union at the gutter was assessed by CT. Results. The mean preoperative Japanese Orthopaedic Association CSM score was 10.2, but it increased to 14.4 by 24 months after surgery. Eleven patients had axial neck pain preoperatively, but only 3 reported mild pain at 24 months, and in all 3 cases the pain was mild. The mean angle of lordosis was 11.7 preoperatively and years postoperatively. Although the ROM at the C2 7 levels was significantly reduced 3 months postoperatively, an increasing trend was observed up to 12 months, and 86% of the preoperative ROM was achieved by 2 years postoperatively. The mean paravertebral muscle cross-sectional areas were 833 ± 215 mm 2 preoperatively and 763 ± 197 mm 2 24 months postoperatively, but the difference was not statistically significant. The rates of bone spacer bonding and bony union at the gutter were low during the early stages but increased to 90% and 93%, respectively, by 2 years after surgery. Conclusions. The modified laminoplasty technique used in this study ensured very good neurological status and ROM after 2 years and was associated with low incidences of axial neck pain and serious complications. This simple and easy operative method could benefit future laminoplasty protocols. ( Key Words laminoplasty paravertebral muscles preservation postoperative axial pain cervical spondylotic myelopathy Abbreviations used in this paper: CSM = cervical spondylotic myelopathy; HA = hydroxyapatite; JOA = Japanese Orthopaedic Association; Oc = occiput; ROM = range of motion. J Neurosurg: Spine / Volume 18 / June 2013 Satisfactory surgical results for CSM have recently been reported. 23,40 In particular, laminoplasty, which causes fewer complications than anterior cervical fusion, has been widely applied. The popular laminoplasty methods include spinous process splitting laminoplasty 29 (double-door laminoplasty) and unilateral expansive open-door 9 laminoplasty (unilateral open-door laminoplasty). Decreased excursive ROM, 2,9,25,39,40,52 cervical malalignment, 3,5,16,18,37 late exacerbation of myelopathy symptoms, 19,25,40 and axial neck pain are postoperative problems associated with laminoplasty. 2,5,22,23,25,37,39,43,46 Axial neck pain greatly reduces patient satisfaction with the procedure. 13,24,45 Moreover, decreased lordotic curvature and kyphosis, which involve the semispinalis cervi- 545

2 M. Umeda et al. cis and nuchal ligament, are unfavorable clinical prognostic factors. Preservation of these posterior supporting tissues ensures better postoperative results. 7,11,42,45,47,48,52 In particular, the semispinalis muscles at the C-2 and C-7 levels are important dynamic stabilizers and should be preserved to the fullest extent possible. 3,8,11,17,31,41,44,45,48,51 Preservation of the paravertebral muscles by modified laminoplasty techniques reportedly reduces the incidence of adverse effects of cervical laminoplasty. 11,42,45,47 50,52 However, the advantages of these methods have previously only been investigated retrospectively by using medical records and questionnaires; no prospective studies to address the time-dependent variations in the clinical results have been performed to date. Here, we aimed to prospectively evaluate the 2-year outcomes of a modified cervical laminoplasty technique for CSM that preserves the paravertebral muscles. Methods This prospective study was approved by the institutional review board of Kansai Medical University, Hirakata, Japan. Sixty-two patients (35 men and 27 women; mean age [SD] 64.3 ± 12.1 years; age range years) who underwent C4 6 or C4 7 laminoplasty with or without C-3 partial or total laminectomy for the treatment of CSM between January 2006 and June 2008 provided informed consent for participation in the study. Patients with ossification of the posterior longitudinal ligament or destructive spondyloarthropathy were not included. Among the 62 patients who provided informed consent, 10 who underwent C-7 laminoplasty for severe spinal cord compression at the C6 7 and/or lower levels were also excluded. Of the remaining 52 patients, 2 were lost to follow-up, 5 died during the follow-up period, and 5 developed significant pathological conditions unrelated to CSM or the surgery. Data obtained in the remaining 40 patients (22 men and 18 women; mean age 66.6 ± 12.0 years, age range years) were analyzed. The mean operative time was 151 minutes (range minutes) and the mean blood loss volume was 125 ml (range ml). No concomitant pathological conditions, such as infection or paralysis, were observed. All 40 patients underwent neurological, pain severity, and spinal radiographic evaluations before surgery and at the 3-, 6-, 12-, 18-, and 24-month follow-up visits. Surgical Procedures A skin incision of approximately 8 cm and a medial incision in the nuchal ligament were made. The continuity of the nuchal ligament at the C-7 spinous processes was completely conserved. The patients underwent ablation of the paravertebral muscles at the C4 6 levels, and the C4 6 spinous processes were removed. Bur hole positions were marked as far inside the myeloid width, measured by preoperative myelography, as possible. Then, full-thickness bur holes and contralateral partialthickness gutters (as the hinge side) were created. C-7 dome decompressions were performed on the proximal ventral side of the arcus vertebrae. In the absence of stenosis at the C2 3 level, the distal side of the C-3 arcus vertebrae was excised; otherwise, C-3 total laminectomy was performed (Figs. 1 and 2). C-3 total laminectomy and C-7 dome decompression enabled the expansion of mild spinal canal stenosis at the C2 3 and C6 7 levels, respectively (Fig. 3). The insertions of the paravertebral muscles, including the semispinalis, into the C-2 spinous processes were easily conserved without the need for C-3 laminoplasty. 45 The arcus vertebrae were lifted with one hand and the ligamenta flava were cut at the bur holes. Because partial laminectomy was performed at the C-3 and C-7 levels, the ligamenta flava between the C3 4 and the C6 7 arcus vertebrae could easily be excised. Hydroxyapatite spacers (Boneceram-P, Olympus Corp.) were then placed between the opened lamina and the lateral mass and fixed with silk thread (Figs. 1 and 2). The opening of the lamina faced the more affected side, but the left lamina was selected if the right and left sides were not different. Bone grafting was not performed. All of the patients wore a soft collar for 1 or 2 weeks. Neurological Evaluation Neurological status was evaluated according to the CSM scoring system of the JOA. 36,53 The average percentage of recovery after laminoplasty was calculated by using the Hirabayashi 9 formula: (postoperative JOA score - preoperative JOA score) 100/(17 - preoperative JOA score), where 17 is the highest JOA score attainable. Evaluation of Axial Neck Pain Pain around the posterior neck or suprascapular areas was classified into 4 grades, in accordance with previous reports: 12,13,36 severe (pain medication or local injection regularly needed), moderate (physiotherapy or compression regularly needed), mild (no treatment needed), or none. Spinal Radiographic Evaluation Plain radiography and MRI of the cervical spine were performed to evaluate the ROM and sagittal alignment changes over the course of the study. The sagittal align- Fig. 1. Images obtained in a 72-year-old man with CSM. Sagittal plain radiograph (A) and sagittal CT image (B) obtained 2 years after partial laminectomy on the caudal side of the C-3 vertebra and rostral side of the C-7 vertebra. 546 J Neurosurg: Spine / Volume 18 / June 2013

3 Less-invasive cervical laminoplasty Fig. 3. Magnetic resonance images obtained in a 61-year-old woman with CSM. A: Preoperative sagittal T2-weighted MR image demonstrating spinal canal compression at the C2 3 and C6 7 levels. B: Postoperative sagittal T2-weighted MR image showing decompression at the C2 3 and C6 7 levels by C-3 total and C-7 partial laminectomies, respectively. Fig. 2. Illustration of open-door C4 6 laminoplasty with HA spacers. C-3 total laminectomy and C-7 dome decompressions on the proximal ventral side of the arcus vertebrae were performed. The insertions of the paravertebral muscles, including the semispinalis, into the C-2 spinous processes and the continuity of the nuchal ligament and paravertebral muscles at the C-7 spinous processes were completely conserved. ment was measured as the angle formed by 2 lines extending from the inferior border of the C-2 vertebral body and the superior border of the C-6 or C-7 vertebral body. The occiput C2 (Oc C2) angle was measured between the McGregor line and the inferior border of the C-2 vertebral body. 28,47 The lines were measured on lateral radiographs obtained in flexion and extension. The total ROM was calculated by subtracting the maximum flexion angle from the maximum extension angle. The %ROM was then calculated for each patient by using the following formula: %ROM = (postoperative ROM/preoperative ROM)/100. Evaluation of Paravertebral Muscle Damage Axial T2-weighted MRI was performed to determine the cross-sectional areas of the deep extensor muscles at the C5 6 level. To identify paravertebral muscle damage, the muscle atrophy rate was calculated for each patient from the preoperative and postoperative cross-sectional areas of the deep extensor muscles. The same 3 examiners measured these areas by using a picture archiving and communication system (Care Stream PACS version 11.0, Care Stream Health Corp.). Mean values of 2 intraobserver measurements and, then, 3 interobserver measurements were calculated. Evaluation of Bone Spacer Bonding and Bony Union The HA spacers were synthesized by sintering at J Neurosurg: Spine / Volume 18 / June C, which produces a pore size of mm and porosity of 35% 48%. These spacers were used at the C4 6 level in en bloc laminoplasty. They (120 implants) were examined at 3, 6, 12, 18, and 24 months after the surgery by using CT to determine fusion. Bone spacer bonding was defined as the absence of a clear space between the bone and the spacer. Bony union at the gutters was considered to have occurred when both cortices were observed. Statistical Analysis Statistical analyses were accomplished by using the Mann-Whitney U-test. Differences at p < 0.05 were considered significant. Results Neurological Status The mean preoperative JOA score was 10.2 ± 2.6, but it increased significantly (p < 0.05) to 13.2 ± 2.3, 13.6 ± 2.3, 13.9 ± 2.1, 14.1 ± 2.0, and 14.4 ± 2.0 at the 3-, 6-, 12-, 18-, and 24-month follow-up visits, respectively (Fig. 4). The final improvement rate was 61.4%. None of the patients had a JOA score worse than their preoperative values. Severity of Axial Neck Pain Before the surgery, 28% of the patients reported axial neck pain (mild in 9 cases and moderate in 2); no cases of severe pain were noted. At the 3-, 6-, 12-, 18-, and 24-month follow-up visits, respectively, 23% (7 with mild pain and 2 with severe pain), 20% (6 with mild pain and 2 with moderate pain), 13% (5 with mild pain), 18% (6 with mild pain and 1 with moderate pain), and 7.5% (3 with mild pain) of the patients reported axial neck pain, showing a tendency toward gradual improvement. None of the 547

4 M. Umeda et al. Fig. 4. Changes in the neurological status evaluated preoperatively and postoperatively according to the CSM scoring system of the JOA. The bars indicate standard deviations. *p < 0.05 compared with the preoperative JOA scores by the Mann-Whitney U-test. patients had severe axial neck pain at the 2-year follow-up visit (Fig. 5). Sagittal Alignment Table 1 shows the sagittal alignment at the C2 7 levels. The mean angle of lordosis was 11.7 ± 10.1 before the surgery, and it changed to 8.9 ± 10.5, 11.1 ± 10.7, 12.1 ± 10.8, 11.5 ± 8.5, and 12.0 ± 10.0 at 3, 6, 12, 18, and 24 months, respectively, after the surgery. Although the sagittal alignment tended to decrease by 3 months, it improved to the preoperative level by 6 months, and no significant differences were observed at 2 years. At the final evaluation, the mean angle of lordosis was 103% of the preoperative mean. Range of Motion Table 1 shows the flexion angles, extension angles, and ROM at the C2 7 levels. The ROM and flexion angles were significantly reduced at 3 months postoperatively, compared with the preoperative measurements, but they showed an increasing trend from 3 to 12 months. The flexion angles, extension angles, and ROM at 24 months were 88%, 83%, and 86% of the preoperative levels, respectively. Table 2 shows the sagittal alignment at the Oc C2 levels. In comparison with the preoperative measurements, no significant differences in the flexion Fig. 5. Incidences of axial neck pain (AP) preoperatively and at 3, 6, 12, 18, and 24 months postoperatively. Pain was classified into 4 grades: severe (pain medication or local injection regularly needed), moderate (physiotherapy or compression regularly needed), mild (no treatment needed), and none. angles, extension angles, and ROM at both the C2 7 and the Oc C2 levels were observed during the 2-year followup visit. Paravertebral Muscle Damage The mean paravertebral muscle cross-sectional area was 833 ± 215 mm 2 before the surgery and measured 785 ± 212, 773 ± 196, 770 ± 194, 761 ± 187, and 763 ± 197 mm 2 at 3, 6, 12, 18, and 24 months, respectively, after the surgery (Fig. 6). No statistically significant difference was observed between the preoperative and the 2-year postoperative measurements. At the final evaluation, the mean paravertebral muscle cross-sectional area was 92% of the preoperative level. Bone Spacer Bonding and Bony Union The mean bone spacer bonding rates at 3, 6, 12, 18, and 24 months postoperatively were 49%, 67%, 80%, 88%, and 90%, respectively, and the mean bony union rates were 5%, 28%, 81%, 91%, and 93%, respectively TABLE 1: Cervical spine sagittal alignment and ROM at C2 6 or C2 7* Time of Measurement Lordosis Angle ( ) Flexion Angle ( ) Extension Angle ( ) ROM ( ) preop 11.7 ± ± ± ± mos postop 8.9 ± ± ± ± mos postop 11.1 ± ± ± ± mos postop 12.1 ± ± ± ± mos postop 11.5 ± ± ± ± mos postop 12.0 ± ± ± ± 9.6 * Data are presented as means ± SDs. Significantly different from the preoperative value (p < 0.05, Mann-Whitney U-test). 548 J Neurosurg: Spine / Volume 18 / June 2013

5 Less-invasive cervical laminoplasty TABLE 2: Cervical spine sagittal alignment and ROM at Oc C2* Time of Measurement Lordosis Angle ( ) Flexion Angle ( ) Extension Angle ( ) ROM ( ) preop 20.5 ± ± ± ± mos postop 20.4 ± ± ± ± mos postop 19.8 ± ± ± ± mos postop 19.3 ± ± ± ± mos postop 18.8 ± ± ± ± mos postop 18.8 ± ± ± ± 7.9 * Data are presented as means ± SDs. (Fig. 7). The rates of bonding increased in each region over time, especially from 3 to 6 and from 6 to 12 months after the surgery. The bony union rates were low during the early stages, but increased from 3 to 6 and 6 to 12 months after the surgery. The bony union rate was remarkably elevated at 12 months. Discussion In the present study, we assessed several prognostic factors at routine intervals to verify whether our modified laminoplasty technique reduced the incidence of complications associated with cervical laminoplasty. To avoid false results due to differences among CSM, ossification of the posterior longitudinal ligament, and herniated nucleus pulposus, 40 patients with only CSM were prospectively observed for 2 years. Neurological Status We evaluated postoperative neurological symptoms by using the JOA scoring system. Although it is limited by the influence of age-related changes, this scoring method is widely used by cervical spine surgeons for analyzing neurological status. In the present study, improved JOA scores were noted at the 3-month recall visit, and the symptoms gradually improved over the 2-year follow-up period. The improvement rate at the final evaluation was 61.4%, which is in agreement with previous results. 23,39,40 Even when C-2 and C-7 were preserved, the improvement in neurological symptoms was similar to that obtained with conventional operative methods. Other complications, such as hematoma with infection and paralysis, were not seen in our study. Axial Neck Pain Postoperative neck pain can disturb the patient s quality of life. 13,23,42 Axial neck pain reportedly occurs in 27% 90% of patients with CSM, and 60% 82% of patients report axial neck pain after conventional surgical techniques. 13,30 Although the causes of this pain remain uncertain, many studies have shown that when the C-2 and C-7 spinous processes are preserved, the frequency of persistent postoperative axial neck pain is reduced. 10,11,32,36,42,45,47,48 This finding can be explained by the fact that the muscles inserting into the C-7 spinous process affect the nuchal and shoulder suspensory Fig. 6. Changes in the cross-sectional areas of the cervical posterior deep extensor paravertebral muscles (PVM) calculated from preoperative and postoperative axial T2-weighted MR images. The bars indicate standard deviations. *p < 0.05 compared with the preoperative measurements by the Mann-Whitney U-test. J Neurosurg: Spine / Volume 18 / June 2013 Fig. 7. Postoperative changes in bone spacer bonding and bony union at the gutters. Bone spacer bonding was defined as the absence of a clear space between the bone and the spacer. Bony union at the gutters was considered to have occurred when both cortices were observed. 549

6 M. Umeda et al. muscles, 32,36 and the paravertebral muscles inserting into the C-2 spinous processes act as important extensors of the cervical spine. 3,4,31,51 The frequency of axial neck pain after newer surgical techniques has been reported to be 2% 47% at the final follow-up examination. 11,42,45 In the present study, 36% of the patients had axial neck pain before the surgery. Although 15% 27% of the patients complained of neck pain up to 18 months postoperatively, only 8% had pain (mild pain) at 2 years. One reason for the relative infrequency of axial neck pain in our patients was reported by Takeuchi et al.; 45 they found that axial neck pain can be prevented by using operative methods that preserve the semispinalis insertion into C-2, such as C-3 laminectomy or dome decompression. With reconstruction of the semispinalis cervicis, impaired regeneration reportedly occurs in 18% of the cases. 18 Therefore, it is important to select an operative method that preserves the insertion of the semispinalis muscle into C-2, as does the technique used for our study. Moreover, the paravertebral muscles at the C-7 level can also be preserved by dome decompression of the arcus vertebrae, another factor influencing axial neck pain. Moreover, the length of time that patients are instructed to wear a cervical collar in the early postoperative period should be shortened. 16 In the present study, the brace therapy period was limited to 2 weeks, and active sitting, standing, and ambulation were started on postoperative Day 1 or 2. However, this study has one limitation in this regard: we did not assess neck pain with the neck disability index or a visual analog scale, which are the standard methods for evaluating the outcomes of modified cervical laminoplasty. To demonstrate the decrease in axial neck pain following the use of our technique, the data should be compared with those shown in previous reports. 12,13,30 For this reason, we used the same evaluation methods as already described. 12,13 Sagittal Alignment and ROM Both lordotic curvature and ROM have been noted to decrease after conventional surgical techniques. 33,34,37,39 These problems, which both inhibit postoperative activities of daily living, remain under discussion. 47 Hukuda et al. 14 reported worsened lordotic curvature following laminoplasty in 28% of cases. The C2 7 angle can reach after conventional surgery. 1,23,37 In addition, cervical ROM is reportedly reduced to 30% 70% of the preoperative value following laminoplasty. 2,6,27 Several authors have recommended that the insertion of the semispinalis into C-2 should be preserved during laminoplasty. 10,42,45 Biomechanical analyses have shown that the semispinalis muscle and the C-2 lamina act as the main dynamic stabilizers of the cervical spine. 31 The use of recent surgical methods that preserve the paravertebral muscles, including the semispinalis, result in increased lordotic angle and maintained ROM. Nevertheless, Yukawa et al. 53 compared a laminoplasty that preserved the semispinalis cervicis with skip laminectomy and reported no significant differences in the lordotic angle and extension ROM between these techniques. Range of motion and sagittal alignment can reportedly be maintained by using a laminoplasty technique that preserves the semispinalis insertion into C-2 rather than one that reattaches the muscle to C Sakaura et al. 36 reported that the lordotic angle of C2 C7 increased significantly from 16.8 to 21.1 at the final evaluation after a laminoplasty technique that preserved the bilateral muscle insertions into the C-2 spinous processes. In the present study, the lordotic angle tended to decrease at 3 months postoperatively, but it continued to improve and returned to the preoperative level by 6 months. The average cervical ROM was also significantly reduced at 3 months, but gradually increased thereafter and reached a plateau at 12 months; however, it decreased from a preoperative value of 36.5 to 31.5 at 24 months. One study showed that a decrease in the ROM at the C2 7 levels is related to both Oc C2 and C-1 compression postoperatively. 47 However, the ROM at the C2 7 and Oc C2 levels was well preserved at the end of our study. Cross-Sectional Areas of the Paravertebral Muscles The excellent postoperative results with respect to axial neck pain, cervical spine alignment, and ROM in our study could be attributed to the less-invasive method used, possibly because failure of the lever arm mechanism was greatly suppressed by preserving the paravertebral muscles at the C-2 and C-7 levels. To identify muscle damage, the cross-sectional areas of the paravertebral muscles after laminoplasty have been analyzed by CT or MRI. 5,42,45 However, only a few reports have compared the preoperative and postoperative states of the deep extensor muscles. 5,42,45 Shiraishi et al. 42 and Fujimura and Nishi 5 reported 60% atrophy of the paravertebral muscles after conventional laminoplasty. On the other hand, Takeuchi et al. 45 reported that the entire cervical posterior muscular volume could be well maintained by using a modification of laminoplasty that preserves the semispinalis insertion into C-2. In the present study, the cross-sectional areas of the deep extensor muscles, as observed on MRI, were reduced 3 months postoperatively but were preserved comparatively well at 3 24 months after surgery. The ratio between the preoperative and the postoperative states of the deep extensor muscles at the 2-year follow-up visit was 92%, suggesting good preservation of these muscles, similar to that found in previous studies. 42,45 Bone Spacer Bonding and Bony Union In laminoplasty, HA spacers and titanium plates are commonly used to maintain alignment and preserve the spinal canal. 15,21,35 However, delayed spinal cord compression by dislodged HA spacers has been reported. 21,26 Retrospective studies have shown bony union rates of 61% to 77% achieved by using HA spacers after doubledoor laminoplasty. 15,21,38 In our prospective study, the bony union rate after unilateral open-door laminoplasty was 90%. The higher bony union rate in our study can be explained by the fact that double-door laminoplasty creates a quadrangular spinal canal, whereas unilateral open-door laminoplasty produces a dynamically stronger triangular spinal canal. 20 Conclusions In our study, the patients with CSM who underwent 550 J Neurosurg: Spine / Volume 18 / June 2013

7 Less-invasive cervical laminoplasty C4 6 laminoplasty with C-3 and C-7 partial laminectomies or C-3 total and C-7 partial laminectomies had improved neurological status, reduced severity of axial neck pain, and very good spinal radiographic results at 24 months postoperatively. The lordotic angle and ROM were comparatively well preserved, and atrophy of the deep extensor muscles was slight. Bony union was achieved between most HA spacers and vertebral arches. Although our technique is not applicable in the absence of spinal canal stenosis at the C-2, C-7, or lower levels, it reduces various complications of laminoplasty. This unilateral open-door laminoplasty method with HA spacers can completely conserve the C-2 and C-7 spinous processes and inserted muscles. The simple and easy surgical technique, associated with low incidences of axial neck pain and serious complications, could benefit future laminoplasty protocols. Disclosure The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper. Author contributions to the study and manuscript preparation include the following. Conception and design: Umeda, Sasai, Wakabayashi, Iida. Acquisition of data: Umeda, Sasai, Kushida, Wakabayashi, Ikeura, Iida. Analysis and interpretation of data: Umeda, Sasai, Kushida, Wakabayashi, Ikeura, Iida. Drafting the article: Umeda, Sasai, Ikeura, Iida. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Umeda. Statistical analysis: Umeda, Sasai, Kushida, Wakabayashi, Maruyama, Iida. Administrative/technical/material support: Umeda, Sasai. Study supervision: Umeda, Sasai, Wakabayashi. 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Spine (Phila Pa 1976) 32: , 2007 Manuscript submitted May 17, Accepted February 25, Portions of this work were presented in poster form at EuroSpine 2010, Vienna, Austria, September 15 17, 2010, and EuroSpine 2007, Brussels, Belgium, October 25 28, Please include this information when citing this paper: published online March 29, 2013; DOI: / SPINE Address correspondence to: Masayuki Umeda, M.D., Ph.D., Department of Orthopedic Surgery, Kansai Medical University, Shinmachi, Hirakata, Osaka , Japan J Neurosurg: Spine / Volume 18 / June 2013