Stereotactic navigation with the O-arm for placement of S-2 alar iliac screws in pelvic lumbar fixation

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1 J Neurosurg Spine 18: , 2013 AANS, 2013 Stereotactic navigation with the O-arm for placement of S-2 alar iliac screws in pelvic lumbar fixation Technical note Wilson Z. Ray, M.D., 1 Vijay M. Ravindra, M.D., 1 Meic H. Schmidt, M.D., 1,2 and Andrew T. Dailey, M.D. 1,2 1 Department of Neurosurgery, Clinical Neurosciences Center; and 2 Spinal Oncology Service, Huntsman Cancer Institute, University of Utah, Salt Lake City, Utah Object. Pelvic fixation is a crucial adjunct to many lumbar fusions to avoid L5 S1 pseudarthrosis. It is useful for treatment of kyphoscoliosis, high-grade spondylolisthesis, L5 S1 pseudarthrosis, sacral tumors, lumbosacral dislocations, and osteomyelitis. The most popular method, iliac fixation, has drawbacks including hardware prominence, extensive muscle dissection, and the need for connection devices. S-2 alar iliac fixation provides a useful primary or salvage alternative. The authors describe their techniques for using stereotactic navigation for screw placement. Methods. The O-arm Surgical Imaging System allowed for CT-quality multiplanar reconstructions of the pelvis, and registration to a StealthStation Treon provided intraoperative guidance. The authors describe their technique for performing computer-assisted S-2 alar iliac fixation for various indications in 18 patients during an 18-month period. Results. All patients underwent successful bilateral placement of screws mm in length. All placements were confirmed with a second multiplanar reconstruction. One screw was moved because of apparent anterior breach of the ilium. There were no immediate neurological or vascular complications due to screw placement. The screw length required additional instruments including a longer pedicle finder and tap. Conclusions. Stereotactic guidance to navigate the placement of distal pelvic fixation with bilateral S-2 alar iliac fixation can be safely performed in patients with a variety of pathological conditions. Crossing the sacroiliac joint, choosing trajectory, and ensuring adequate screw length can all be enhanced with 3D image guidance. Long-term outcome studies are underway, specifically evaluating the sacroiliac joint. ( Key Words S-2 alar iliac pelvic fixation image guidance technique Fusion to the sacrum is a crucial adjunct to many lumbar fusions requiring supplemental pelvic fixation. Biomechanically, inclusion of the sacrum in a long construct can provide a significant challenge. 12,20 Multiple authors have reported a high rate of pseudarthrosis and complications at the L5 S1 junction with scoliosis surgery. 8,12,13 The most popular method to supplement fusion to the sacrum, iliac fixation, has several drawbacks, including hardware prominence, the need for extensive muscle dissection, and the need for connection devices. 4,30 Despite these challenges, pelvic fixation can be useful in the treatment of kyphoscoliosis, high-grade spondylolisthesis, L5 S1 pseudarthrosis, sacral tumors, lumbosacral dislocations, and osteomyelitis, 9,11,23,24,30,31,33 leading many authors to explore alternative fixation techniques. S-2 alar iliac screws represent an attractive alternative to conventional iliac fixation. 5,22,26 These screws can be placed via a traditional midline incision, allowing alignment with rostral pedicle screw fixation. This alignment offsets the need for cross-connectors used with iliac bolts, allowing either open or minimally invasive surgical placement. In addition, they are lower profile than traditional iliac bolts, providing better fascial and muscular This article contains some figures that are displayed in color on line but in black-and-white in the print edition. 490

2 Stereotactic navigation for placement of S-2 alar iliac screws coverage on closure. Finally, they are reported to have greater pullout strength than traditional iliac screws. 32 Two-year follow-up data on this fixation technique in a pediatric patient population have demonstrated outcomes favorable to those obtained using traditional iliac fusion procedures, 26 yet similar data in an adult population are yet to be reported. Similar to iliac fixation, S-2 alar screws can be used for high-grade spondylolisthesis, for kyphoscoliotic deformities requiring caudal fixation, in revision of L5 S1 pseudarthrosis, and for adjacent-segment disease caudal to a long-segment fusion. 10 As a result, S-2 alar iliac fixation provides a useful primary or salvage alternative. 17 The placement of screws across the 2 cortical surfaces of the sacroiliac joint and into dense bone above the sciatic notch provides superior pullout strength. 32 We describe our techniques using stereotactic navigation in the placement of S-2 alar iliac screws. Methods Eighteen patients who underwent primary or revision lumbosacral fusion procedures were identified. The study was approved by the institutional review board. As detailed in Table 1, levels fused ranged from T3 ilium to L4 ilium. All patients underwent computer-assisted stereotactic placement of S-2 alar iliac screws to augment pelvic fixation. All S-2 alar iliac screws were placed using O-arm Surgical Imaging System/StealthStation image guidance (Medtronic, Inc.). A standard midline incision and open technique were used in all patients. The StealthStation reference arc was affixed to the most caudal solid bony structure, most commonly the S-2 spinous process, with the arc rotated as distally as possible, and the O-arm was then used to provide real-time CT scanning for intraoperative navigation. After image acquisition, the data were transferred to the StealthStation navigation system. Since the images are obtained following all soft-tissue dissection with exposure of bony anatomy, only a one-time instrumentation verification is needed prior to instrumentation placement. Obtaining the CT scan intraoperatively with the StealthStation reference arc affixed avoids any further landmark registration. To optimize stealth registration and intraoperative navigation, the StealthStation camera is typically positioned at the bottom of the bed (Fig. 1). As recently described in detail by Nottmeier et al. 21 for the placement of sacral alar screws, the O-arm Surgical Imaging System allows for CT-quality multiplanar reconstructions of the pelvis, providing verification for both an entry point and trajectory. Utilizing the navigation reduces radiation exposure to the operating room personnel, potentially limits radiation exposure to the patient, and allows for simultaneous multiplanar views of the screw trajectory. Our entry point was based on the S-1 foramen, typically up to 5 mm caudal and 2 3 mm lateral to the foramen. The dissection is not as extensive as required for iliac bolts; once the S-1 foramen is identified, a blunt instrument or finger palpation can be used to probe the alar ridge. The starting point can be identified by anatomical landmarks and verified with image guidance. If image Fig. 1. Photograph showing the setup in the operating room, with the stereotactic navigation workstation and detector at the foot of the bed and the 3D detector positioned above the patient to ensure maximal signal integrity. guidance is unavailable, traditional fluoroscopy can be used to visualize both the sciatic notch in the anteroposterior and lateral views and the ilium via oblique views; these techniques have been described elsewhere in detail. 5,10,26 A high-speed bur drill was then used to mark the starting point. The screw trajectory was from horizontal and caudal, aimed toward the greater trochanter, rostral to the sciatic notch; a trajectory is chosen that allows the screw to engage dense bone above the sciatic notch to improve screw pullout strength over traditional iliac screws (Fig. 2). The final trajectory and length ( mm) were then verified by image guidance. If using standard fluoroscopy, the anteroposterior view ensures adequate clearance of the sciatic notch. Once across the sacroiliac joint, the teardrop view is used to provide an ideal trajectory and determine optimal screw length. A standard short sharp pedicle finder (Lenke probe) is used to traverse the sacrum along the real-time ideal trajectory and to cross the sacroiliac joint. There is a palpable change in density as the probe crosses the sacroiliac joint (Fig. 3). Once across the sacroiliac joint, the custom pedicle finder, which is longer (markers up to 100 mm) (Fig. 4), is passed along dense cortical bone of the ilium above the sciatic notch. A custom longer tap (markers up to 100 mm) is then used across the sacroiliac joint (Fig. 4). This inclusion of the sacroiliac joint provided tricortical bony fixation for all screws (Fig. 5). All patients underwent placement of bilateral S-2 alar screws. After all instrumentation was placed in all patients, a second intraoperative CT scan with multiplanar reconstruction was obtained to confirm placement (Fig. 6). We collected and evaluated data on demographics, indications and comorbidities, fusion techniques, screw size, and complications (Table 1). Results Eleven women and 7 men were included, with a mean age of 60 ± 12 years (range years). Indications for surgery varied (Table 1). Nine patients had a diagnosis of 491

3 W. Z. Ray et al. Fig. 4. Photograph showing Lenke probe (lower), which is used during the initial probing down to and through the sacroiliac joint. The longer custom probe (upper) with depth markings up to 100 mm is used to traverse the ilium once the sacroiliac joint has been crossed. Fig. 2. Screen capture from the StealthStation image guidance computer workstation demonstrating the selected trajectory through the left ilium. The images on the left side show the width and length through the trajectory marked through the ilium. Cumulatively, these views show the relationship of this trajectory to the bony anatomy, specifically the sacroiliac joint. degenerative kyphoscoliosis, 3 patients had osteomyelitis, 2 had L5 S1 pseudarthrosis revision, 2 had adjacent-segment degeneration, 1 had a tumor, and 1 required salvage of failed iliac screws. Three patients had a diagnosis of rheumatoid arthritis. Twelve patients underwent fusion of the thoracic spine to the pelvis. Two patients underwent fusion of the high thoracic spine (T-3) to the pelvis, while 10 had fusion of the lower thoracic spine (T9 10) to the pelvis. Six patients underwent fixation of the lumbar spine to the pelvis. The gauge of the screws used for pelvic fixation ranged from 7.5 to 9.5 mm, and the length of the screws ranged from 80 to 100 mm. The most commonly used screw size was mm (6 patients) (Table 1). Twelve patients underwent L5 S1 interbody fusion (4 anterior and 8 posterior), and 4 patients had previously undergone a L5 S1 fusion procedure. In the other 3 patients, no additional L5 S1 interbody fusion was performed (2 patients with L5 S1 osteomyelitis and 1 patient treated for L1 3 chordoma). None of the patients suffered immediate vascular Fig. 3. Screen capture from StealthStation image guidance computer workstation demonstrating the selected trajectory through the left ilium. The lower left image demonstrates an ideal screw length with the trajectory clearly demarcated. 492 or neurological complications. As described in Methods, a second intraoperative CT scan was obtained in all patients to evaluate the S-2 alar iliac screw placement. One patient required repositioning of one of the alar iliac screws because of an apparent breach in the anterior cortex in the ilium with intraoperative confirmation of correct placement. Discussion We describe our procedure for and initial experience with placing S-2 alar screws using the O-arm and StealthStation navigation. In this series, all screws were placed without vascular or neurological complication. Use of StealthStation navigation allowed safe placement of all screws in line with S-1 pedicle screws while allowing us to choose a trajectory that maximized the greatest length and width available for osseous purchase (Figs. 4 and 5). Placing a screw across 2 cortical surfaces of the sacroiliac joint as well as the cortex of the ilium and the thick cortical bone above the sciatic notch should provide superior biomechanical fixation.32 Inclusion of the ilium in lumbosacral fusion procedures provides superior biomechanical fixation compared with sacral fixation alone.11,15,29 While the literature con- Fig. 5. The O-arm Surgical Imaging System display of axial images through the pelvis confirming screw placement through the sacroiliac joint.

4 Stereotactic navigation for placement of S-2 alar iliac screws correction of pelvic obliquity. In a technical note, Matteini et al. 17 described a patient with postlaminectomy degenerative scoliosis requiring spinopelvic fixation. This patient was treated with T10 pelvis fusion, including S-2 alar iliac screws. The authors reported that, at 1-year followup, the patient s Oswestry Disability Index had improved from 50 to 20, she had returned to independent activities, and she was no longer taking narcotics. Although multiple authors have described good results with sacroiliac fusion procedures, 1,17,18,34 one potential drawback of this technique is the inclusion of the sacroiliac joint in the fusion construct. As discussed by Chang et al., 5 fusion procedures that involve the sacroiliac joint may ultimately lead to lucency around the instrumentation, yet the clinical significance of this finding remains unclear. Tsuchiya et al. 30 reported the 5-year data of 67 patients treated with sacropelvic fixation, among whom there no reports of sacroiliac degeneration or osteoarthritis. Similarly, none of our patients reported sacroiliac pain, and there has not been any evidence of lucency on follow-up imaging. Because of the short follow-up period, however, this does remain a legitimate concern for future follow-up and warrants dedicated imaging to monitor the integrity of the sacroiliac joint. Intraoperative navigation is now an integral part of many routine intracranial procedures. 2,6,7,16,19,25,27,28 While we are not advocating the use of image guidance for all spinal instrumentation procedures, intraoperative navigation for the placement of alar iliac screws was useful in maximizing screw length and diameter and choosing a trajectory that could engage the dense cortical bone of the ilium. This is of added benefit in this region because the 3D anatomy of the pelvis is not as familiar to most neurosurgeons, and StealthStation CT navigation provides superior intraoperative imaging that cannot be provided by conventional uniplanar fluoroscopy. Fig. 6. Postoperative coronal CT image demonstrating screw placement through the sacroiliac joint. Trajectory maximized both screw length and width available for osseous purchase. tains a growing number of reports supporting this, the inclusion of iliac screws has several potential drawbacks, including significant soft-tissue dissection, the need for additional offset connectors, prominence of screws, and a high incidence of painful loosening, sometimes requiring instrumentation removal. 3,14,30 Despite these drawbacks, the high rate of pseudarthrosis that can occur with lumbosacral fixation alone 12,13 necessitates some additional fixation procedure. Recently, several authors have described the use of sacral alar screws and S-2 alar iliac screws in an attempt to minimize these complications. Sponseller and colleagues 26 reported the 2-year follow-up results for 26 pediatric patients treated for spinal deformity that included S-2 alar iliac fixation. The authors compared their results with the outcomes of 27 previously treated patients treated with conventional pelvic fixation. The authors reported no incidence of vascular or neurological injury. Compared with historical controls, patients treated with S-2 alar iliac fixation had a lower rate of deep infections and a greater Conclusions Placement of distal pelvic fixation with bilateral S-2 alar iliac fixation can be safely performed using stereotactic navigation. Further long-term follow-up data and formal biomechanical testing are needed to assess the long-term comparative results to other reported alternatives for sacroiliac fixation. The authors are cautiously optimistic that S-2 alar iliac screws represent a viable alternative to conventional iliac fixation in a variety of lumbosacral fusion procedures. Disclosure Dr. Dailey is a consultant for Biomet. Author contributions to the study and manuscript preparation include the following. Conception and design: Dailey. Acquisition of data: Ray, Ravindra. Analysis and interpretation of data: Ray, Ravindra. Drafting the article: Ray. 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: Dailey. Study supervision: Dailey. 493

5 W. Z. Ray et al. TABLE 1: Patients who underwent placement of S-2 alar iliac screws in pelvic-lumbar fixation* Case No. Age (yrs), Sex Indication for Op Procedure Screw Size (mm) L5 S1 Fusion Technique Approach to L5 S1 Interbody 1 59, F degenerative scoliosis T10 ilium PEEK cage posterior 2 56, M osteomyelitis L5 S1 L4 ilium & posterolat none 3 67, F osteomyelitis L5 S1 L2 ilium & posterolat none 4 67, F L5 S1 nonunion, degenerative scoliosis T3 ilium not reported posterolat none 5 34, M L1 3 chordoma T10 ilium posterolat none 6 66, M degenerative scoliosis T10 ilium PEEK cage anterior 7 57, M degenerative scoliosis T10 ilium PEEK cage posterior 8 66, F adjacent-segment disease T10 ilium PEEK cage posterior 9 66, F sagittal imbalance T3 ilium posterolat none 10 42, F failed L5 S1 fusion L2 ilium PEEK cage anterior 11 48, F L5 S1 nonunion L2 ilium & two PEEK cages posterior 12 65, F degenerative scoliosis T10 ilium PEEK cage posterior 13 78, M degenerative scoliosis L2 ilium PEEK cage posterior 14 55, F adjacent-segment disease T10 ilium posterolat none 15 75, M degenerative scoliosis T10 ilium PEEK cage posterior 16 73, F neuromuscular scoliosis T9 ilium PEEK cage anterior 17 43, F osteomyelitis L5 S1 L4 ilium PEEK cage anterior 18 61, M degenerative scoliosis T10 ilium & PEEK cage posterior * PEEK = polyetheretherketone. Cage sizes are in millimeters. Acknowledgment The authors thank Kristin Kraus, M.Sc., for assistance in manuscript preparation, editing, and submission. References 1. Arlet V, Marchesi D, Papin P, Aebi M: The MW sacropelvic construct: an enhanced fixation of the lumbosacral junction in neuromuscular pelvic obliquity. Eur Spine J 8: , Barnett GH, Miller DW, Weisenberger J: Frameless stereotaxy with scalp-applied fiducial markers for brain biopsy procedures: experience in 218 cases. J Neurosurg 91: , Bridwell KH: Utilization of iliac screws and structural interbody grafting for revision spondylolisthesis surgery. Spine (Phila Pa 1976) 30 (6 Suppl):S88 S96, Bridwell KH, Edwards CC II, Lenke LG: The pros and cons to saving the L5-S1 motion segment in a long scoliosis fusion construct. Spine (Phila Pa 1976) 28:S234 S242, Chang TL, Sponseller PD, Kebaish KM, Fishman EK: Low profile pelvic fixation: anatomic parameters for sacral alariliac fixation versus traditional iliac fixation. Spine (Phila Pa 1976) 34: , Greenfield JP, Cobb WS, Tsouris AJ, Schwartz TH: Stereotactic minimally invasive tubular retractor system for deep brain lesions. Neurosurgery 63 (4 Suppl 2): , Grunert P, Müller-Forell W, Darabi K, Reisch R, Busert C, Hopf N, et al: Basic principles and clinical applications of neu ronavigation and intraoperative computed tomography. Com put Aided Surg 3: , Harimaya K, Mishiro T, Lenke LG, Bridwell KH, Koester LA, Sides BA: Etiology and revision surgical strategies in failed lumbosacral fixation of adult spinal deformity constructs. Spine (Phila Pa 1976) 36: , Jackson RJ, Gokaslan ZL: Spinal-pelvic fixation in patients with lumbosacral neoplasms. J Neurosurg 92 (1 Suppl): 61 70, Kebaish KM: Sacropelvic fixation: techniques and complications. Spine (Phila Pa 1976) 35: , Kim JH, Horton W, Hamasaki T, Freedman B, Whitesides TE Jr, Hutton WC: Spinal instrumentation for sacral-pelvic fixation: a biomechanical comparison between constructs ending with either S2 bicortical, bitriangulated screws or iliac screws. J Spinal Disord Tech 23: , Kim YJ, Bridwell KH, Lenke LG, Cho KJ, Edwards CC II, Rinella AS: Pseudarthrosis in adult spinal deformity following multisegmental instrumentation and arthrodesis. J Bone Joint Surg Am 88: , Kim YJ, Bridwell KH, Lenke LG, Rhim S, Cheh G: Pseudarthrosis in long adult spinal deformity instrumentation and fusion to the sacrum: prevalence and risk factor analysis of 144 cases. Spine (Phila Pa 1976) 31: , Kuklo TR, Bridwell KH, Lewis SJ, Baldus C, Blanke K, Iffrig TM, et al: Minimum 2-year analysis of sacropelvic fixation and L5-S1 fusion using S1 and iliac screws. Spine (Phila Pa 1976) 26: , Mackinnon SE, Novak CB, Myckatyn TM, Tung TH: Results of reinnervation of the biceps and brachialis muscles with a double fascicular transfer for elbow flexion. J Hand Surg Am 30: , Mangano FT, Limbrick DD Jr., Leonard JR, Park TS, Smyth MD: Simultaneous image-guided and endoscopic navigation without rigid cranial fixation: application in infants: technical case report. Neurosurgery 58 (4 Suppl 2):ONS-E377, Matteini LE, Kebaish KM, Volk WR, Bergin PF, Yu WD, O Brien JR: An S-2 alar iliac pelvic fixation. Technical note. Neurosurg Focus 28(3):E13, Miladi LT, Ghanem IB, Draoui MM, Zeller RD, Dubousset JF: Iliosacral screw fixation for pelvic obliquity in neuromuscular scoliosis. A long-term follow-up study. Spine (Phila Pa 1976) 22: ,

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