Tethered cord syndrome is a disorder involving an. Spine-shortening osteotomy for patients with tethered cord syndrome caused by lipomyelomeningocele

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1 See the corresponding editorial in this issue, pp J Neurosurg Spine 15:21 27, 2011 Spine-shortening osteotomy for patients with tethered cord syndrome caused by lipomyelomeningocele Clinical article Shoichi Kokubun, M.D., Ph.D., 1 Hiroshi Ozawa, M.D., Ph.D., 1 Toshimi Aizawa, M.D., Ph.D., 1 Ngo Minh Ly, M.D., Ph.D., 1 and Yasuhisa Tanaka, M.D., Ph.D. 2 1 Department of Orthopaedic Surgery, Tohoku University School of Medicine, Sendai; and 2 Department of Orthopaedic Surgery, Tohoku Central Hospital, Yamagata, Japan Object. Tethered cord syndrome (TCS) is a disorder involving an abnormal stretching of the tethered spinal cord caused by several pathological conditions and presents with a variety of neurological symptoms. Untethering (tethered cord release) is the gold standard treatment for TCS. However, untethering carries risks of spinal cord injury and postoperative retethering. To avoid these potential risks, the authors applied spine-shortening osteotomy to adult patients with TCS, and report on the surgical procedure and treatment outcomes. Methods. Eight patients with TCS caused by a lipomyelomeningocele were surgically treated by the authors original procedure of spine-shortening osteotomy. Six patients were male and 2 were females; average age at the time of surgery was 31 years old. Spine-shortening osteotomy was performed at the level of L-1 in all but 2 patients, in whom it was performed at T-12, with spinal fusion between T-12 and L-2 or T-11 and L-1 using a pedicle screw rod system. The average follow-up period was 6.2 years and the patients pre- and postoperative conditions were evaluated clinically and radiologically. Results. Preoperatively, all patients displayed severe neurological deficits such as motor disturbance, muscle atrophy, and bladder dysfunction. Several months before surgery, all showed progressive symptoms. Those symptoms showed initial improvement in 6 patients and stabilized in 2 postoperatively, but the improved symptoms worsened again in 4 of the 6 patients. The osteotomized vertebrae were shortened by 21 mm on average, and all spines showed complete bone union without loss of correction. At the final follow-up evaluations, 6 patients showed stabilization as per the modified Japanese Orthopaedic Association score for thoracic myelopathy. Conclusions. Spine-shortening osteotomy successfully helps reduce the spinal cord tension without causing direct neural damage. At minimum, it stabilized the patients symptoms and/or helped delay neurological deterioration for a period of time. Spine-shortening osteotomy might be a feasible mode of treatment for adult TCS caused by a spinal lipoma. (DOI: / SPINE10114) Key Words tethered cord syndrome lipomyelomeningocele spine-shortening osteotomy Tethered cord syndrome is a disorder involving an abnormal stretching of the tethered spinal cord, the ascent of which is limited and is disproportionate to the growth of the spinal column. 23 Several pathological conditions including fatty and thickened filum terminale, lipomyelomeningocele, meningocele, split cord malformation, and postrepair myelomeningocele may be responsible for TCS. 3,23 At a later stage, stretching of the conus medullaris and nerve roots may induce back pain, leg weakness, foot deformity, scoliosis, sensory loss, and/ or bowel or bladder dysfunction. 3,5,18,19,22,23 It has been well established that children with TCS, whether symptomatic or otherwise, require untethering (tethered cord release) to be performed as soon as possible to prevent new or additional neurological deficits. 2,9,17,18 Abbreviations used in this paper: mjoa = modified Japanese Orthopaedic Association; TCS = tethered cord syndrome. Tethered cord syndrome in adults is a complex clinicopathological entity that remains poorly understood. Most adult patients with TCS experience normal daily living, or mild static neurological deficits in childhood, and therefore, TCS may not be diagnosed until symptoms or neurological abnormalities manifest or worsen in adulthood. 19,22 Adult-onset neurological deficits may develop not only due to static stress but also because of the intermittent or repeated stretching stress on the tethered spinal cord induced by the motion of the spine. 17 The results of untethering surgery for adult patients with TCS show great variation, possibly due to the varying causes and levels of the tethering, the preoperative duration and/or severity of symptoms, and the completeness of untethering. 2,3,19 Optimal surgical outcomes can be achieved only through complete untethering. 19 However, untethering is technically difficult to accomplish and carries the potential risk of injuring the spinal cord or essential nerves. In 21

2 S. Kokubun et al. addition, it is frequently followed by retethering of the spinal cord. 3,5,19 As an alternative to untethering, since 1995 we have performed spine-shortening osteotomy in patients with TCS caused by a lipomyelomeningocele because osteotomy is believed to reduce the tension in the spinal cord. 10,12,15,25 This concept of performing spineshortening osteotomy for surgical treatment of TCS is a pilot attempt in this field. In this paper we report our surgical technique and the associated outcomes in 8 patients treated at our hospital. Methods Patient Population The study group comprised 6 males and 2 females with TCS caused by a lipomyelomeningocele. The average age at the time of surgery was 31 years (range years) and the average postoperative follow-up period was 6.2 years (range years). The spinal cord was tethered at the level of L2 3 in 1 patient, L3 4 in 1 patient, L4 5 in 1 patient, and below S-1 in 5 patients. According to the Chapman classification for intraspinal lipomas, 1 the lesions were classified as transitional type in 4 patients, dorsal type in 3 patients, and caudal type in 1 patient. The average preoperative duration after the onset of symptoms was 12 years (range 3 24 years). According to the classification of Pang and Wilberger, 22 4 patients each were categorized as either normal in childhood and developing symptoms in adulthood, or having static neurological deficits or skeletal deformities diagnosed in childhood and developing new or progressive neurological deficits in adulthood. One patient had undergone untethering surgery at the age of 26 while 2 patients had undergone surgery for TCS during childhood (details unknown). All patients were adequately informed of the expected and adverse outcomes of the surgical trial and their written informed consent was obtained. Surgical Procedure Spine-shortening osteotomy was performed at the level of L-1 in all but 2 patients, in whom it was performed at T-12 because previous surgeries performed extended up to the level of L-2 or L-3. The surgical procedure performed at L-1 is described below (Fig. 1). The laminae from the T11 12 facets to the upper half of the L-2 lamina and the L-1 transverse processes were exposed. Pedicle screws were placed bilaterally at the T-12 and L-2 levels such that they were positioned parallel to the upper margin of the T-12 vertebral body and the lower margin of the L-1 body, respectively; in addition, it was ensured that the lengths of the pedicle screws at T-12 that were out of the vertebral body were equal to those of the L-2 screws, which was confirmed radiologically. This screw arrangement avoided sagittal translation and anterior or posterior opening between the T-12 and L-1 vertebral bodies after the spine-shortening procedure. The lower half of the T-12 lamina and the T12 L1 facet joints were excised (Fig. 1A1 and B1). The osteotomy line of the T-12 lamina was placed slightly cranial to the lower margin of the T-12 body, which facilitated the subsequent removal of the T12 L1 intervertebral disc. The upper three-quarters of the pedicles and transverse processes, and the upper one-third of the lamina of the L-1 vertebra were then removed using a rongeur and an air drill with a diamond bur. The upper half of the L-1 body was drilled away through the base of the partially removed pedicle on one side with a rod assembled with the pedicle screws on the opposite side, which prevented closure of the gap to be created. This was accomplished while still preserving the anterior longitudinal ligament. The osteotomy line was placed in a manner such that the T-12 upper and L-1 lower vertebral endplates would run parallel to each other after the spine-shortening procedure. The T12 L1 intervertebral disc was removed along with the posterior longitudinal ligament (Fig. 1A2 and B2). The same steps were accomplished on the other side with a rod assembled on the opposite side for the L-1 body and the T12 L1 intervertebral disc. The remaining part of the upper half of the vertebral body and posterior longitudinal ligament in front of the dural tube were resected using a Kerrison rongeur (Fig. 1A3 and B3). The gap created was closed by gradual compression applied to the pedicle screws at T-12 and L-2 using the rods as guides. The rods were then changed to shorter ones fit for the shortened spine (Fig. 1A4 and B4). Bone chips from the excised laminae and spinous processes were also placed over the T-12 and L-1 laminae for posterior fusion. Clinical Evaluation The medical and operative records, plain radiographs, and MR images were retrospectively reviewed. Gait ability, pain and numbness/tingling sensation, detailed motor and sensory functions, and the mjoa score 8 were obtained at the following 4 time points: immediately before surgery, 6 weeks to 3 months after surgery, 1 year after surgery, and at the final follow-up appointment. Walking ability was scored according to the following scale: 4 = normal, 3 = able to walk with no supports but a little clumsiness, 2 = able to walk with a cane, 1 = able to walk with crutches or double canes, and 0 = unable to walk by any means. Pain and numbness/tingling sensation was scored on the following scale: 3 = none, 2 = occasional and mild, 1 = frequently mild or occasionally severe, and 0 = frequently or continuously severe. The mjoa scale is an 11-point scale for scoring thoracic myelopathy based on lower-extremity motor function (4 points), lower-extremity and trunk sensory function (2 points each), and bladder function (3 points). The motor function was scored by adding the grade scores (0 5) obtained by manual muscle testing in the bilateral quadriceps, tibialis anterior, and gastrocnemius muscles (full score = 30). The sensory function was scored by considering the sum of the severity (none = 2, mild = 1, severe = 0) in the anterior and posterior trunks, bilateral anterior and posterior thighs, bilateral anterior and posterior lower legs, and the perineum (full score = 22). The operation duration, volume of intraoperative hemorrhage, and adverse events were also recorded. Image Evaluation The length of the spine shortening was measured on 22

3 Spine-shortening osteotomy for tethered cord syndrome Fig. 1. Illustrations of the surgical steps involved in spine-shortening osteotomy at the L-1 level. A1 and B1: The lower half of the T-12 lamina, the upper one-third of the L-1 lamina, and the T12 L1 facet joints are removed after pedicle screws are placed at T-12 and L-2. A2 and B2: The upper three-quarters of the L-1 pedicles are drilled away. The upper part of the L-1 vertebral body is removed through the partially resected pedicles. Then, the T12 L1 intervertebral disc is removed along with the posterior longitudinal ligament. A3 and B3: Representation of the spine after removal of the upper part of the L-1 vertebral body and T12 L1 intervertebral disc. The osteotomy line is placed in a manner such that the T-12 upper and L-1 lower vertebral endplates can run parallel to each other after the spine-shortening procedure. A4 and B4: The gap created is closed by gradual compression applied to the pedicle screws at T-12 and L-2 using the rods as guides. The rods are changed to shorter ones fit for the shortened spine. the pre- and postoperative plain lateral radiographs. The local kyphosis between the shortened and superjacent vertebrae (between T-12 and L-1 or between T-11 and T-12) was measured preoperatively, soon after surgery, and at the final follow-up appointment. Results Clinical Evaluation Symptoms of TCS progressed in all patients for several months before surgery. The progressed symptoms included muscle weakness in 4 patients, and back pain, numbness/tingling sensation, or bladder dysfunction in 2 patients each. Immediately prior to surgery, 6 patients were ambulatory without support but 5 of these patients reported some difficulties with walking. Another 2 patients each used 2 canes or crutches. Five patients complained of pain or numbness/tingling sensation in the back, buttocks, or legs. Five patients displayed both muscle atrophy of the lower extremities and foot deformities, and 2 had muscle atrophy alone. Seven patients showed muscle weakness in the lower extremities. Sensory disturbance was detected in the lower extremities or perineal region in all patients. Neurogenic bladder was observed in all patients and self urethral catheterization was necessary in 5 of these patients. The average operative duration was 6 hours and 40 minutes (range 5 hours and 40 minutes to 7 hours and 50 minutes). The intraoperative hemorrhage averaged 1290 ml (range ml), and < 900 ml hemorrhage occurred in 5 patients. Seven patients were transfused with autologous blood only, whereas 1 patient needed additional homogeneous blood transfusion. No obvious intraoperative adverse events (such as nerve root injury) or postoperative infections occurred in any patients. Postoperatively, the progressing symptoms showed improvement in 6 patients and stabilized over a period of 3 months to 3 years in 2 patients. However, at the final follow-up appointment, 4 patients showed worsening of the once-improved symptoms while 2 patients each continued to show improvement or stabilization of the preoperative progressing symptoms. The pre- and postoperative conditions are presented in Fig. 2. In terms of walking ability, 6 patients showed no significant change at the final follow-up appointments. One patient reported slightly increased spastic limping 3 years after surgery but was able to walk without supports at the final followup appointment 11 years after surgery. Another patient was unable to walk 10 months postoperatively (Fig. 2A). Concerning pain or numbness/tingling sensation, 3 patients showed improvement, 3 had stabilized, and 2 had worsened at the final follow-up appointment (Fig. 2B). One of the patients with worsened symptoms complained of buttock pain and leg numbness that improved until 3 months after surgery (although the pain score did not 23

4 S. Kokubun et al. Fig. 2. Line graphs representing the pre- and postoperative conditions of the patients. The evaluation points in the graphs were immediately before surgery (Pre-op), 6 weeks to 3 months (6 12W), 1 year after surgery (1Y), and the final follow-up appointment (Final F/U). A: Graph of walking ability. Six patients showed no significant changes while 2 patients showed worsening of walking ability. One patient became unable to walk 10 months after surgery. B: Graph of pain or numbness/tingling sensation. Three patients each showed improvements or stabilization, but 2 patients reported worsening of the symptoms at the final follow-up. C: Graph of motor function. Only 1 patient showed an increased score (by 2 points) at the final follow-up. On average, the score decreased by 6.5 points in 4 patients at the final follow-up, although it increased or stabilized for a certain period after surgery in 3 of these patients. D: Graph of sensory function. Two patients each showed improvement or stabilization at the final follow-up, whereas the other 4 patients showed a decreased score. E: Graph of mjoa score for thoracic myelopathy. This evaluation showed stabilization in 6 patients and worsening in 2 patients at the final follow-up. change), after which the patient experienced an increased numbness/tingling sensation. Another patient complained of thigh pain that stabilized until 4 years after surgery, but spread to the other thigh thereafter. In terms of motor function, only 1 patient showed an increase in score, by 2 points at the final follow-up appointment. On average, it was found that the score decreased by 6.5 points (range 1 15 points) in 4 patients at the final follow-up, although it increased or stabilized for a certain period after surgery in 3 of them (Fig. 2C). The sensory function was found to have improved or stabilized in 2 patients each at the final follow-up appointment. In contrast, it worsened in the other 4 patients but the decrease in the score was only 2 points compared with the preoperative score in 3 of the 4 patients (Fig. 2D). All patients showed no improvement in bladder function and 1 required urological surgery. The overall surgical outcome evaluated using the mjoa score showed stabilization in 6 patients and worsening in 2 patients at the final follow-up (Fig. 2E). Imaging Evaluation The average length of spine shortening as measured on radiographs was 21 mm (range mm). All of the operated levels showed complete bone union approximately 1 year after surgery. The average local curvature between the osteotomized and the superjacent vertebrae was 4 of kyphosis preoperatively (range -5 to 10 ), 2 of kyphosis immediately after surgery (range -4 to 8 ), and 2 of kyphosis at the final follow-up (range -5 to 8 ). No obvious loss of correction or vertebral compression fracture was noted (Fig. 3). Postoperative MR imaging examinations were performed in 5 patients. Of these, the MR images from 3 patients showed no significant relaxation, but a slight anterior shift of the spinal cord was evident (Fig. 4). One patient showed no significant changes while another demonstrated a swelling of the spinal cord as described below. Illustrative Case One patient showed aggressive neurological deterioration after surgery. For future understanding of the mechanism of neurological deterioration, this case is detailed here. This woman with a caudal type of intraspinal lipoma underwent a 22-mm spine-shortening osteotomy at the age of 54 (Fig. 5A and B). Soon after surgery, she reported improvement in buttock pain and leg numbness. However, her muscle strength started to decrease 4 days after surgery and sensory disturbance and leg numbness began worsening 3 months after surgery. Thereafter, her neurological deficits gradually increased and 10 months after surgery, she was unable to walk even with supports. Because a stainless steel pedicle screw rod system was used, MR imaging examination was performed after the completion of spinal union, and then the instruments were removed. Magnetic resonance images obtained 10 months after surgery depicted spinal cord swelling with intramedullary T2 high-intensity lesions at the osteotomy level. The subarachnoid space appeared to have been nar- 24

5 Spine-shortening osteotomy for tethered cord syndrome Fig. 3. Pre- and postoperative lateral plain radiographs obtained in a 45-year-old man. A: Preoperative image showing 7 of local kyphosis between T-12 and L-1. B: Image obtained just after an 18-mm spine-shortening osteotomy. The angle between the T-12 and L-1 vertebral bodies measures 1 of kyphosis. C: Image obtained 6 years after surgery shows a complete union with 1 of kyphosis between the T-12 and L-1 vertebral bodies. rowed between the lower half of the L-1 body and the L1 2 intervertebral disc (Fig. 5C and D). Intradural exploration performed 1 year after osteotomy revealed that the arachnoid was buckled over the dura mater and was found to be adhering to the spinal cord. The adhesion was released with care, but the patient s neurological condition did not improve. Discussion Previously reported results of untethering in cases with adult TCS have shown wide variations. 2,3,5,17 19,24 Generally, patients with a tight filum terminale and split cord malformation, or those who underwent surgery before 1 year of age, demonstrated excellent surgical prognoses while those with TCS caused by a lipomyelomeningocele or postrepair myelomeningocele showed poor prognoses. 2,9,19 Incomplete untethering might have also lead to unsatisfactory results. Complete untethering, however, is technique-sensitive and extremely difficult to accomplish without causing intraoperative damage to the neural tissues in patients with a lipomyelomeningocele or postrepair myelomeningocele. 3,5,19 Thus, we performed spine-shortening osteotomy in adult patients with TCS caused by these conditions because their spines had stopped growing. Spine-shortening osteotomy was principally performed on the L-1 vertebra for the following reasons: 1) possible damage to the L-1 or T-12 nerve roots during surgery would result in minor neurological deficits compared with deficits resulting from nerve root damage in the lower lumbar spine; 2) lipomyelomeningoceles seldom extend up to the L-1 spinal level; and 3) spinal fusion at the thoracolumbar junction reduces lesser spinal motion and exerts relatively lesser mechanical stress on the neighboring disc spaces compared with the situation when surgery is performed at the lower lumbar spine. 26 In the present study, the amount of shortening of the spine by osteotomy was approximately 20 mm at the level of L-1 or T-12. From our experience, the excision of 1 hemivertebra and its adjacent intervertebral discs is equivalent to shortening the spine by a normal half vertebral body and its adjacent intervertebral disc in the midline. The hemivertebra excision has never caused neurological deterioration previously. 13,14 This extent of shortening can also help preserve the continuity of the lamina with the vertebral body through the lower part of the pedicles, which is mechanically advantageous for posterior spinal fusion. Recent basic studies have suggested that spine-shortening operations in excess of twothirds of a vertebral segment might cause kinking of the spinal cord and consequently lead to neurological deterioration. 11,16 In addition, 20- to 25-mm shortening of the spine yielded significant tension reduction in the spinal cord at the thoracolumbar junction. 4 Spine shortening by approximately 20 mm, as performed in the present study, Fig. 4. Preoperative and postoperative sagittal T2-weighted MR images obtained in a 45-year-old man. Left: Preoperative image showing a dorsal type of spinal lipoma at L2 3 (arrows), which is attached to the spinal cord (arrowheads). Right: Postoperative image showing a slight shift forward of the spinal cord (arrowheads). The spinal lipoma (arrows) attached to the spinal cord appears to have a diminished volume, which might be caused by tension reduction in the spinal cord. 25

6 S. Kokubun et al. Fig. 5. Preoperative and postoperative images obtained in a patient with markedly deteriorated neurological conditions after surgery. A: Preoperative sagittal T2-weighted MR image showing a caudal type of spinal lipoma at the tip of the sacrum. B: Lateral radiograph obtained after a 22-mm spine-shortening osteotomy. C and D: Midline sagittal (C) and axial (D) T-2 weighted MR images of the L1 2 level obtained 10 months after surgery. The subarachnoid space is narrowed at the lower half of the L-1 body to the L1 2 intervertebral disc. The spinal cord shows an obvious swelling with T-2 high signal intensity lesions at the osteotomy level. is thus considered appropriate both in terms of safety and tension reduction in the spinal cord. More recently, studies regarding spine-shortening osteotomy for TCS involving 2 or 3 patients each have been reported by 2 institutes in which the authors kindly introduced our surgical procedure, previously described in Japanese. 6,7,20 These investigators reported good clinical outcomes in relatively young patients over a followup period of 15 months to 5 years. 6,20 This series, which included more patients with a longer follow-up period, indicated that spine-shortening osteotomy could successfully help prevent the initial progression of symptoms that worsened immediately prior to surgery, but the overall surgical results at the final follow-up appointments were similar to those reported in the previous studies on untethering, that is, they were not as good as expected. 1,5,18,19 Adult patients with symptoms lasting longer than 5 years less frequently showed improvements in neurological symptoms after untethering. 5 A tethered spinal cord indicates marked changes in oxidative metabolism that may cause structural damage to the neuronal perikarya and axons. 23,27 An increase in the severity of such damage may be proportional to the duration and strength of the tethering. The average age of the patients in the present study was 31 years and they displayed a long preoperative duration, spanning 12 years on average. Most of the patients showed muscle atrophy in their legs or foot deformities, which suggested that the tethered spinal cord had been stretched over a long period of time and that it had already undergone irreversible changes, at least to some extent. Another possible reason for unsatisfactory or temporary outcomes in some patients of the present study could be inadequate extent of spinal shortening to reduce spinal cord tension. Certainly, a previous study 4 had demonstrated some obvious effects of the spine-shortening osteotomy. However, effects similar to those observed in the cadaveric study, in which the spinal cord had a very short history of stretching, might not be achieved with the same amount of shortening in patients whose spinal cords had undergone stretching over a long period of time. Considering that little or no relaxation of the spinal cord was observed on postoperative MR images, a larger shortening using double osteotomy at skipped spinal levels, without disturbing the blood supply to the spinal cord, may possibly lead to better results. There have been patients who have shown an unpredictable pattern of deterioration of symptoms following untethering. 2,19 In the present study, 1 patient suffered gradual aggravation of the neurological condition after surgery, although the mechanism was unknown. The 22- mm spine shortening in this patient was almost equivalent to the average spine shortening in the present study. The adhesion between the arachnoid and the spinal cord possibly caused by buckling of the arachnoid was detected during the revision surgery. The adhesion might have blocked the circulation of the CSF and reduced the blood supply to the spinal cord. 21 Furthermore, the patient was the oldest in this study, which may indicate that the spinal cord had been stretched for the longest period and had undergone structural changes, thus rendering it vulnerable to the encroachment by the adhesion and to stretching by the motion of the spine. Consequently, the spinal cord might have been damaged gradually but severely. Conclusions Theoretically, spine-shortening osteotomy is an excellent surgical procedure that helps reduce spinal cord tension without causing direct neural damage. The present study confirmed that the procedure can successfully stabilize symptoms or at least delay neurological deterioration for a period of time. We thus believe that spine-shortening osteotomy may become one of the foremost choices for management of adult patients with TCS caused by a lipomyelomeningocele, especially the dorsal and transitional types, or a postrepair myelomeningocele. However, the inconsistency of the results in the present study raises questions regarding the indications for and suitable timing for performing the surgery as well as the optimal length of spine shortening. Further studies are required to satisfactorily address the queries. Disclosure The authors report no conflict of interest concerning the mate- 26

7 Spine-shortening osteotomy for tethered cord syndrome rials 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: Aizawa, Kokubun. Acquisition of data: Aizawa, Ly. Analysis and interpretation of data: Aizawa. Drafting the article: Aizawa, Kokubun. Critically revising the article: Aizawa, Kokubun. Reviewed final version of the manuscript and approved it for submission: all authors. Administrative/ technical/material support: Kokubun, Ozawa, Tanaka. Study supervision: Aizawa, Kokubun, Ozawa. References 1. Chapman PH: Congenital intraspinal lipomas: anatomic considerations and surgical treatment. Childs Brain 9:37 47, Cochrane DD: Cord untethering for lipomyelomeningocele: expectation after surgery. Neurosurg Focus 23(2):E9, Fehlings MG, Arvin B: Editorial. Recurrent tethered cord syn drome: a novel approach for a difficult surgical condition? 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White AA III, Panjabi MM: The basic kinematics of the human spine. A review of past and current knowledge. Spine (Phila Pa 1976) 3:12 20, Yamada S, Zinke DE, Sanders D: Pathophysiology of tethered cord syndrome. J Neurosurg 54: , 1981 Manuscript submitted February 2, Accepted February 28, Please include this information when citing this paper: published online April 15, 2011; DOI: / SPINE Address correspondence to: Toshimi Aizawa, M.D., Ph.D., De part ment of Orthopaedic Surgery, Tohoku University School of Med i cine, 1-1, Seiryo-machi, Aoba-ku, Sendai, , Japan. toshi-7@ra2.so-net.ne.jp. 27