Percutaneous or mini-open pedicle screws are increasingly

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1 J Neurosurg Spine 18: , 2013 AANS, 2013 Navigated guide tube for the placement of mini-open pedicle screws using stereotactic 3D navigation without the use of K-wires Technical note Benjamin J. Shin, M.D., 2 Innocent U. Njoku, B.S., 2 A. John Tsiouris, M.D., 1 and Roger Härtl, M.D. 2 1 Department of Radiology, and 2 Brain and Spine Center, NewYork-Presbyterian Hospital, Weill Cornell Medical College, New York, New York Object. Three-dimensional spinal navigation increases screw accuracy, but its implementation in clinical practice has been difficult, mainly because of surgeons concerns about increased operative times, disturbance of workflow, and safety. The authors present a custom-designed navigated guide that addresses some of these concerns by allowing for drilling, tapping, and placing the final screw via a minimally invasive approach without the need for K-wires. In this paper, the authors goal was to describe the technical aspects of the navigated guide tube as well as pedicle screw accuracy. Methods. The authors present the technical details of a navigated guide that allows drilling, tapping, and the placement of the final screw without the need for K-wires. The first 10 patients who received minimally invasive mini-open spinal pedicle screws are presented. The case series focuses on the immediate postoperative outcomes, pedicle screw accuracy, and pedicle screw related complications. An independent board-certified neuroradiologist determined pedicle screw accuracy according to a 4-tiered grading system. Results. The navigated guide allowed successful placement of mini-open pedicle screws as part of posterior fixation from L-1 to S-1 without the use of K-wires. Only 7-mm-diameter screws were placed, and 72% of screws were completely contained within the pedicle. Breaches less than 2 mm were seen in 23% of cases, and these were all lateral except for one screw. Breaches were related to the lateral to medial trajectory chosen to avoid the superior facet joint. There were no complications related to pedicle screw insertion. Conclusions. A novel customized navigated guide tube is presented that facilitates the workflow and allows accurate placement of mini-open pedicle screws without the need for K-wires. ( Key Words neuronavigation instrumentation spine pedicle screw implant surgical technique Percutaneous or mini-open pedicle screws are increasingly being used in spinal surgery, especially for minimally invasive lumbar fusion. 32 However, accurate placement can be technically challenging, and the incidence of screw misplacements can vary from anywhere between 6% and 40%. 4,9,17,30 Misplacement can lead to iatrogenic damage to neural, vascular, or visceral tissues. 1,6,7,24,30 Three-dimensional image guidance or computer-assisted navigation allows for a multiplanar visualization of the spinal anatomy to facilitate the tracking of surgical instruments in real time With the use Abbreviations used in this paper: MIS = minimally invasive surgery; PEEK = polyetheretherketone. of computer-assisted techniques introduced in the early 1990s, the surgeon can use pedicle screw placement with increased accuracy. 26,27,30 A recent meta-analysis found that 3D navigation reduces the misplacement rate of pedicle screws from 15% to 6%. 27 Currently, lumbar pedicle screw placement through MIS relies primarily on fluoroscopy and the use of K- wires over which cannulated pedicle screws are introduced. The current techniques carry several potential risks or disadvantages. The use of live fluoroscopy can be associated with significant radiation exposure to the surgeon, the patient, and the surgical staff. 2,18 Visualization of pedicle anatomy can also be limited, especially in obese or osteoporotic patients. 15 The use of K-wires 178 J Neurosurg: Spine / Volume 18 / February 2013

2 Navigated guide tube for mini-open pedicle screws can be harmful to the patient as the wires can break or bend during the procedure and pose the risk of visceral or vascular injury. In addition, the surgical workflow using K-wires is complex and requires the use of multiple instruments that pass back and forth between the surgeon and the scrub nurse. Systems that use 3D neuronavigation for percutaneous screw placement also work via K-wires and require the separate navigation of multiple instruments, such as the drill guide, awl, tap, and, finally, the screw. 5,19,25,28 This theoretically results in a complicated and disrupted workflow in the operating room and is time consuming. We present here a navigated guide tube that allows all steps from the planning of the incision to placement of the final screw to be performed using only one navigated instrument while obviating the need for K-wires. We describe the technical aspects of the navigated guide tube as well as pedicle screw accuracy in a series of 10 patients. Methods Guide Tube Description A navigated guide tube was designed by the senior author (R.H.) and was manufactured in close collaboration with an industrial partner (Synthes). The navigated guide tube (Synthes) is a 170-mm-long metal tube with a 10-mm outer diameter, 8.3-mm inner cannulation, and handle (Fig. 1). An interface for attachment to a Universal Instrument Adapter Array, Starlink (Brainlab, Inc.) is positioned on a 270 rotatable collar on the proximal end to allow for flexible positioning of the guide tube with respect to the navigation camera. The guide tube has distal teeth to stabilize the instrument on bone. Hudson connectors allow optional connection to a stability arm, the MIS Support System Table Mount (Synthes). The guide tube was designed to include a suite of compatible instruments. An outer PEEK sleeve can be used for soft-tissue protection. A blunt PEEK trocar can be used for soft-tissue protection during insertion. A 3.5-mm drill bit, pedicle probe, and screwdriver with holding sleeve for headless screw insertion were designed to center inside the tube for navigation of those instruments via guide tube trajectory. The guide tube and tap have a length of 35 mm beyond the tip of the navigated guide tube, which prevents insertion of these instruments beyond the anterior border of the vertebral body. A longer tap is available for the S-1 level or situations in which the anterior cortex needs to be reached. This tap can be navigated separately or inserted under fluoroscopy guidance. All instruments have 1-, 2-, and 3-cm markings to monitor insertion depth. A custommade long ball-tip can be used through the guide tube to test the drilled and tapped screw hole. Intraoperative stimulation of the inserted pedicle screw was performed through the outer sleeve PEEK tube. Patient Selection The first 10 consecutive patients who received miniopen spinal pedicle screws between May 2011 and July 2011 at Weill Cornell Medical College, NewYork-Presbyterian Hospital, New York, using this integrated guide J Neurosurg: Spine / Volume 18 / February 2013 tube were included in this series (mean age 54 years). All operations were performed by the senior author (R.H.). In addition, all patients except one (who presented with an unstable fracture after a motor vehicle accident [Case 9]) underwent surgery for various types of degenerative diseases of the lumbar spine, including spondylolisthesis, degenerative scoliosis, and stenosis associated with instability. Demographics, procedures, and operative details are listed in Table 1. Other variables recorded included personal data, preoperative diagnosis, fixation levels, and immediate/postoperative complications. Surgical Technique The cases presented involved either a transpsoas approach for discectomy and fusion or a transforaminal lumbar interbody fusion technique through 22-mm tubular retractors followed by the placement of pedicle screws in the lumbar spine. Neurophysiological monitoring was used in all cases. Below L-3, a reference array was placed into the iliac crest. For L-3 and above, the reference array was placed on the spinous process. An intraoperative fluoro-ct scan (ARCADIS Orbic 3D, Siemens) of the spinal anatomy was obtained and used for navigation (Brainlab). The skin incision was determined using the offset function in the navigation system, which allows the surgeon to project the probe trajectory deeper into the anatomy. A skin incision was then made, and the muscle fascia was incised sharply. The accuracy of the navigation was confirmed by pointing out the tip of the transverse process at each level. The navigated guide tube attached to a Brainlab reference array was then used to determine the ideal pedicle trajectory. An appropriate-sized pedicle screw was simulated using the navigation software. The navigated guide tube was then gently impacted so that the teeth would hold on to the bony anatomy. At this stage, the guide tube can be secured in place via the Hudson connectors and the stability arm. A hand drill was then used, and a 35- mm hole was prepared (the drill and tap cannot advance deeper than 35 mm). A custom-made long ball-tip was used through the guide tube to test the drilled and tapped screw hole. The hole was then tapped, and an appropriate pedicle screw without a screw head was inserted. We used a MATRIX or MIRS pedicle screw system (both Synthes) with detachable screw heads. At this stage, all screws were electrically stimulated through the outer PEEK sleeve for neurophysiological verification. After placement of all screws, a fluoro-ct spin or postoperative CT scan (Fig. 2) was obtained to confirm accurate placement of instrumentation. No screws were modified after the confirmatory spin or after neurostimulation. Next, the screw heads and extension sleeves were attached using the application tool, and the appropriate rod length was determined using the point-to-point measurement function in the navigation software (Fig. 3). Patient Evaluation Pedicle screw accuracy grading was determined by a board-certified neuroradiologist (A.J.T.) according to a 4-tiered grading system: Grade 0, no cortical breach; 179

3 B. J. Shin et al. Fig. 1. A: The navigated guide tube comprises a 170-mm-long tube with a 10-mm outer diameter and 8.3-mm cannulation and handle. An interface for attachment to an infrared reference array positioned on a 270 rotatable collar on the proximal end to allow flexible positioning of the guide tube with respect to the navigation camera. B D: A drill, tap, and finally a pedicle screw without screw head can be inserted through this guide tube. E and F: A tissue protection sleeve and a trocar are made of PEEK material and facilitate percutaneous placement of the guide tube. TABLE 1: Patient demographics and pedicle data* Case No. Age (yrs), Sex Procedure Neuronavigated Levels for Pedicle Screws No. of Pedicle Screws Inserted w/ Guide No. of Pedicle Breaches (>Grade 1) 1 75, M L3 4, L4 5 ELIF, L5 S1 TLIF, L-3, L-4, L-5, S pedicle screws at L3 S1 2 59, M L3 4 redo laminectomy, TLIF L-3, L , F L5 S1 TLIF L-5, S , F L4 5 TLIF L-4, L , M L4 5 ELIF w/ pedicle screws L-4, L , M L4 5 TLIF L-4, L , F L4 5 ELIF w/ pedicle screws L-4, L , F L3 5 ELIF w/ pedicle screws L-3, L4, L , M L1 3 pedicle screw fixation for L-2 L-1, L chance fracture 10 67, M L3 5 ELIF w/ pedicle screws L-3, L-4, L * ELIF = extreme lateral (transpsoas) interbody fusion; TLIF = transforaminal lumber interbody fusion. Greater than 2 mm cortical breach. 180 J Neurosurg: Spine / Volume 18 / February 2013

4 Navigated guide tube for mini-open pedicle screws TABLE 2: Pedicle screw assessments Pedicle Screw Grade No. of Pedicle Screws (%) No. of Medial/Lat Breaches 0 34 (72) 0/ (23) 1/ (4) 0/ /0 [10%]) (Table 1). The levels of pedicle screw insertions included L-3 to S-1. All patients received mini-open pedicle screws as part of posterior fixation using the navigated guide tube. No K-wires were used during screw insertion. A total of 47 screws were inserted. Only 7-mm-diameter screws were placed. The mean (± SD) estimated blood loss and total length of operation were 151 ± 87 ml and ± 99 minutes, respectively. There were 6 singlelevel and 4 multilevel procedures. Fig. 2. Case 2. Axial CT image showing placement of a 7-mm pedicle screw in the right L-3 pedicle for an L3 4 transforaminal lumbar interbody fusion. Lateral breaches were occasionally seen due to the large screw diameter and due to the more lateral to medial trajectory that was chosen to avoid the superior adjacent facet joint (in this case, L2 3 [arrow]). Grade 1, cortical breach less than 2 mm; Grade 2, cortical breach greater than 2 mm; and Grade 3, completely outside the pedicle. 20 Operative data were obtained through a chart review. Statistical Analysis Descriptive statistics were used for the patient demographics and resultant data. The significance level was defined to be less than Statistical analyses were conducted in SPSS (PASW) version 18.0 (SPSS, Inc.). Results Preoperative and Intraoperative Data The various indications for spinal surgery included degenerative lumbar disease (90%) and fracture (Case 9 Fig. 3. Measurement of the rod length from the pedicle screw (left) to the adjacent pedicle screw (right) using the neuronavigated drill interface. J Neurosurg: Spine / Volume 18 / February 2013 Pedicle Screw Accuracy All pedicle screws (total 47) were graded for each patient by a board-certified neuroradiologist (A.J.T.) according to the accuracy scale described previously. The number of Grade 0, 1, 2, and 3 pedicle screw placements was 34 (72%), 11 (23%), 2 (4%), and 0 (0%), respectively (Table 2). Therefore, the percentage of completely intrapedicular screw placement was 72%, and the percentage of pedicle screws resulting in less than 2 mm of cortical perforation or completely within the pedicle was 95%. There were no completely extrapedicular screws placed, and only 2% resulted in cortical perforation greater than 2 mm. Only 1 of 13 of the cortical breaches was medial; all other cortical breaches were lateral (92.3% [12 of 13]). There were no pedicle screw violations of the cranial adjacent facet joints. Complications There were no early complications in this study. Discussion Three-dimensional navigation increases screw accuracy, 27 but its implementation into clinical practice has been difficult, mainly because of surgeons concerns about increased operative times, disturbance of workflow, and safety. 11 The underlying goal in designing this guide tube was to facilitate the workflow in the operating room by reducing the number of instruments that need to be navigated and to reduce the potential risks associated with current techniques for the insertion of mini-open pedicle screws by eliminating the need for K-wires. Precluding the Use of K-Wires K-wires have been used for directing pedicle screw insertion after fluoroscopic verification. 21 Their use is not without risk; they can bend or break and be inadvertently advanced beyond the bony anatomy. The K-wires can perforate the anterior vertebral body, which may 181

5 B. J. Shin et al. cause severe vascular or visceral complications. There have been published cases of K-wire migration and infection. 3,10,29 In addition, the K-wire s sharp, exposed end can puncture the skin or eye of the surgeon, assistant, or operating room staff. One of the principal advantages of our 3D-navigated guide tube is that it eliminates the need for K-wires. Our navigated drill allows the surgeon to insert pedicle screws using stereotactic navigation instead of K- wire based fluoroscopic verification. K-wires can still be used with our technique to mark the pedicle trajectory when obtaining a control spin and before inserting the final pedicle screw; however, in our case series this was not necessary. Other Advantages of the Guide Tube The navigated guide facilitates the workflow for pedicle screw insertions. Conventional navigation requires the registration of multiple instruments that have to be handed back and forth between the surgeon and the scrub nurse throughout the procedure. The one-step approach with a navigated guide tube is easier, faster, and, we believe, also safer when compared with conventional fluoroscopy or navigation-assisted instrumentation. 31,33 This technique facilitates the screw and rod placement after pedicle screw insertion. For instance, the screw without the screw head is placed through the guide tube itself immediately after the hole has been drilled and tapped. Thereby the screw provides an immediate seal, and bleeding is minimized. After all the screws are placed, we attach the screw heads with the extension sleeves. The avoidance of K-wires as well as the practice of attaching screw posts at the end, when all screws have already been inserted, avoids interference of the screw posts or K-wires with planning and placement of the next screw, a problem frequently encountered with conventional MIS techniques in very lordotic and/or scoliotic spines. Nottmeier and Fenton 22 published their technique of 3D image-guided placement of percutaneous pedicle screws without the use of biplanar fluoroscopy or K- wires. In their technique, they place an image-guided awl through the incision down onto the pedicle screw entry point and use the intraoperative planning function on the image guidance system to determine the proper trajectory, length, and diameter of the screw. The awl is then removed, and the pedicle is probed and tapped using imageguided instruments. The pedicle screw is placed using an image-guided screwdriver. These authors described excellent results with this technique in 15 patients. In our experience, however, the need to use multiple navigated instruments adds to the complexity and time of surgery. The hole has to be re-found with navigation, which can sometimes be difficult. Also, there may be significant bleeding from the screw hole between steps. Pedicle Screw Accuracy Pedicle screw insertion with the 3D-navigated guide tube resulted in complete intrapedicular placement in 72% of the screws and less than 2 mm cortical breach in 95%. All breaches but one were lateral and therefore clinically insignificant (Fig. 2). Recent literature on minimally invasive pedicle screw insertion has reported a significant incidence of adjacent-level facet joint violation with percutaneous screw placement. 23 This and our own previous experience prompted us to use 3D navigation to avoid adjacent facet violation by optimizing the entry point and trajectory of the pedicle screw. This made us choose a more lateral to medial trajectory of the cranial pedicle screw in an attempt to avoid injury to the cranial (superior) facet joint. Minor lateral breaches associated with this trajectory were detected intraoperatively and accepted. In an attempt to maximize the biomechanical pullout strength of the screws, we used only 7-mm-diameter screws and this also sometimes resulted in minor lateral breaches. However, in our opinion the advantages of these modifications in technique outweigh the potential downsides. In a previous study, 8 we found that 3D neuronavigation for percutaneous screw insertion in lumbar fusion resulted in 91% of pedicle screws being placed without cortical breaches. A meta-analysis likewise revealed that neuronavigated pedicle screw insertion is less likely to breach the cortex when compared with conventional insertion techniques. 27 A meta-analysis performed by Kosmopoulos and Schizas 16 found that median placement accuracy was 95.2% for neuronavigation. These accuracy rates were similar to the rate found in the present study. However, only a total of 47 pedicle screws were included in this technical note, which describes our early experience with the 3Dnavigated guide tube. Therefore, a comprehensive cohort study with more patients is needed to better determine pedicle screw insertion accuracy for this guide tube. Conclusions This newly designed navigated guide tube facilitates accurate placement of pedicle screws through MIS. It improves the surgical workflow and eliminates the use of K-wires. We observed no complications related to pedicle screw placement. Further comparative studies are required to determine relative accuracy compared with conventional fluoroscopic and navigated pedicle screw insertion. We are currently working on a guide tube that allows insertion of a pedicle screw with a screw head, which will further facilitate the workflow. Disclosure The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper. Dr. Härtl is a consultant for Synthes and Brainlab. Author contributions to the study and manuscript preparation include the following. Conception and design: Härtl. Acquisition of data: Shin, Njoku, Tsiouris. Analysis and interpretation of data: all authors. Drafting the article: Shin, Njoku, Härtl. 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: Shin. Study supervision: Härtl. References 1. 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Spine (Phila Pa 1976) 34: , 2009 Manuscript submitted June 5, Accepted October 25, Please include this information when citing this paper: published online November 30, 2012; DOI: / SPINE Address correspondence to: Roger Härtl, M.D., Department of Neurological Surgery, Weill Cornell Medical College, NewYork- Presbyterian Hospital, 525 East 68th Street, Box 99, New York, New York roh9005@med.cornell.edu. J Neurosurg: Spine / Volume 18 / February