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00007632-200309010-0002900007632_2003_28_e351_sagi_electromagnetic_17report< 80_0_8_1 >Spine© 2003 Lippincott Williams & Wilkins, Inc.Volume 28(17)1 September 2003pp E351-E354Electromagnetic Field-Based Image-Guided Spine Surgery Part Two: Results of a Cadaveric Study Evaluating Thoracic Pedicle Screw Placement[Case Report]Sagi, H. C. MD*; Manos, R. MD2; Park, S.-C. MD3; Von Jako, R. MD4; Ordway, N. R. MS, PE5; Connolly, P. J. MD5From *University of California San Francisco-Fresno, Fresno, CA;2Naval Medical Center, San Diego, CA;3Kwandong University, Korea;4GE Surgical Navigation, Lawrence, MA; and5SUNY Upstate Medical University, Syracuse, NY.Acknowledgment date: November 19, 2002.Revision date: February 21, 2003.Acceptance date: May 1, 2003.Address correspondence to H. Claude Sagi, MD, University Medical Center, Fourth Floor, Department of Orthopaedics, 445 South Cedar Avenue, Fresno, CA 93702; e-mail: eversosaggy@cs.comPartial funding and assistance was provided by Visualization Technology, Inc. and Synthes, USA.AbstractStudy Design. Human cadaveric.Objectives. Compare the accuracy of electromagnetic field (EMF)-based image-guided thoracic pedicle screw insertion to conventional techniques using anatomic landmarks and fluoroscopy.Background. Image-guided surgical systems that aid in spinal instrumentation seek to minimize radiation exposure and improve accuracy. EMF image guidance was developed as an alternative to optical tracking to eliminate potential line of sight issues.Materials and Methods. Four fresh-frozen human cadavers were randomly allocated into two groups. Pedicle screws were inserted from T1 to T12 using anatomic landmarks and fluoroscopy in group 1 and image guidance in group 2. Insertion and fluoroscopy time were recorded. Anatomic dissections were performed to assess screw placement.Results. Image guidance placed 92% of thoracic pedicle screws safely, and conventional fluoroscopy placed 90% safely. The average degree of perforation was 2.4 mm with conventional fluoroscopy and 1.7 mm with image guidance (P = 0.055). Fluoroscopic time per screw was 5.9 seconds for conventional fluoroscopy and 3.6 seconds for image guidance (P = 0.045). Insertion time per screw was 4.35 minutes for conventional fluoroscopy and 2.98 minutes for image guidance (P = 0.007). However, when set-up time and image capture time were taken into account, the total insertion time per screw was not significantly different between the two groups.Conclusions. Our study has shown that EMF image-guided thoracic pedicle screw placement results in a similar incidence of safely placed screws as does conventional fluoroscopy. When set-up time and image-capture time were factored in for image guidance, the average time to insert a pedicle screw was equal for both techniques. The use of EMF image guidance significantly reduced fluoroscopic time and thus radiation exposure per screw compared with conventional fluoroscopic techniques.The advent of image-guided surgery has helped to decrease the need for continuous or repeated radiographic images in spinal surgery. The current standard for image-guided spinal surgery uses optical technology for tracking of instruments and the patient’s anatomy. Many studies have shown that this technology is very accurate and in many instances can improve on the accuracy of pedicle screw placement in the thoracic spine when compared with conventional fluoroscopy. 1–6In a previous study, we introduced electromagnetic field (EMF) technology in tracking and navigating spinal anatomy for placement of lumbar pedicle screws. 7 Although there are no specific reports in the literature describing line of sight as a major technical problem with existing systems, EMF tracking was developed as an alternative to optical tracking to avoid this potential problem. This technology allows instruments to be taken into the body and construction of intraoperative three-dimensional “CAT scanning” with the conventional C-arm. We found that accuracy was improved over conventional technique for the application of lumbar pedicle screws. The major setback with the use of this technology remains increased time for pedicle screw insertion and distortion of the EMF by any substance that can carry a current (i.e., 400-series stainless steel instruments). 7The purpose of this study is to compare EMF tracking with conventional fluoroscopy in the placement of thoracic pedicle screws in a human cadaveric model. Accuracy, fluoroscopic exposure time, and screw insertion time will be evaluated.Materials and MethodsFour fresh-frozen human cadavers were obtained, thawed, and randomly allocated into one of two groups. None of the specimens had significant deformity as a result of scoliosis, spondylolisthesis, or fracture. The posterior aspect of the thoracic spine was exposed in the usual fashion that would allow localization of landmarks for placement of thoracic pedicle screws and performance of an intertransverse fusion. Two spine surgeons who were at the end of their fellowship training year placed the pedicle screws. Screws were applied bilaterally at each level from T1 to T12 in all specimens. An awl was used to perforate the posterior cortex, and then a blunt pedicle finder was used to locate the pedicle. Five-millimeter pedicle screws (Universal Spine System; Synthes, Paoli, PA) were used in all cases.Group 1 pedicle screws were inserted with conventional fluoroscopic (OEC 9800 image intensifier; GE/OEC, Salt Lake City, UT) technique using anteroposterior, lateral, and oblique views as necessary. There were two specimens in this group and two screws were inserted at each thoracic level for a total of 48 screws.Group 2 pedicle screws were inserted using the EMF based image guidance system (Insta-Trak 3500; Visualization Technology, Lawrence, MA) alone. This system uses intraoperatively acquired anteroposterior and lateral fluoroscopic images to navigate the spinal anatomy. There were two specimens in this group and two screws were placed at each level for a total of 48 screws. The technical aspects of using EMF guidance have been outlined previously. 7The time to insert pedicle screws and the total image-intensification exposure time were recorded for each specimen in each group. In addition, the time required for set-up of the image-guidance system (placement of transmitter) and time for the computer to capture an appropriate image for navigation were recorded in group 2. This was later factored into pedicle screw insertion time for this group.Once all pedicle screws were placed, the specimens were dissected en bloc. Individual levels were then dissected to assess the accuracy of screw placement. The number of cortical perforations was noted for each specimen and level. In addition, the direction and extent of perforation (in millimeters) were measured with a flexible ruler and noted. Any perforation that was through the medial or inferior aspect of the pedicle or the anterior cortex of the vertebral body was termed a critical perforation, implying that neurovascular structures were potentially at risk. A lateral perforation with the screw between the rib and pedicle was not considered a critical perforation as long as no screw thread was exposed outside of pedicle or rib. 8The data were then analyzed to compare accuracy, insertion time, and fluoroscopic time between each of the groups. Statistical analysis was performed using one-way ANOVA and chi squared analysis. Data were considered significant if P < 0.05.ResultsA total of 48 screws were placed in groups 1 and 2. Results are summarized in Table 1. The average insertion time per pedicle screw differed significantly between the two groups. Group 1 averaged 261 seconds per pedicle screw and group 2 averaged 179 seconds per pedicle screw (P = 0.04). However, when image acquisition and set up time (i.e., application of bone pin and transmitter) were factored in, insertion time averaged 293 seconds per pedicle screw for group 2. This difference was no longer statistically significant.Table 1. Thoracic Pedicle Screw Insertion1 Safe screw = perforation less than 5 mm and only lateral or superior cortex, or between rib head and pedicle.2 Accurate screw = no perforation through any cortex.3 Critical perforation = medial or inferior pedicle cortex, anterior vertebral cortex.IGS = image-guided surgery.Total fluoroscopic time for group 1 was 270 seconds, and for group 2 it was 162 seconds. The average amount of fluoroscopic time per pedicle screw was 5.9 seconds for group 1 and 3.6 seconds for group 2. This difference was statistically significant (P = 0.04).Groups 1 and 2, respectively, placed 90% and 92% of pedicle screws safely. This difference was not statistically significant. Safe screws were those screws positioned without placing vital structures such as spinal cord, nerve root, vascular conduit, or lung at risk. The difference between accuracy (those screws placed completely within the confines of the pedicular cortex) was also not significantly different between the groups (Table 1),Critical perforations were seen in 10% in group 1 and 8% in group 2. This difference was not statistically significant. The average extent of break through or perforation was 2.36 mm for group 1 and 1.71 mm for group 2 (P = 0.055).DiscussionMany spine surgeons are familiar with the application and technique for placement of lumbar pedicle screws. In quality bone, it is an extremely powerful method of segmental fixation in the spine for many spinal deformities. 9,10 There are an increasing number of reports in the literature regarding the safety and efficacy of this technique applied in the thoracic spine. 8,11,12 Advantages over conventional laminar and pedicle hook fixation include not violating the canal and a sounder mechanical construct with superior control of the spine for achieving and maintaining correction of deformity. 13–15 However, highly variable pedicle anatomy coupled with the proximity of vital neurologic, vascular, and pulmonary structures makes the application of this fixation technique in the thoracic spine less inviting to the casual spine surgeon. 16–18 Thus, accurate localization of pedicle anatomy is key in avoiding complications, and many techniques have been devised to aid in optimizing screw placement. The more recent of these include aiming devices and computer navigation with image guidance using either preoperative CT scans or intraoperative fluoroscopy. 1–6,19,20As in the lumbar spine, the issues are accurate placement in a timely fashion with minimal exposure of the surgeon and patient to fluoroscopic radiation. Although technically more demanding, attention to detail, preoperative imaging, and thorough knowledge of anatomic structures have resulted in safe placement of thoracic pedicle screws in a high percentage of patients. 8,12,13 The current concern with thoracic pedicle screws apart from safe placement, however, continues to be the excessive use of fluoroscopy, which is often more than that required for lumbar pedicle screw placement.Thus, with the concerns of accurate placement in difficult hidden anatomy and excessive radiation exposure to both patient and operating room personnel, image-guided spinal surgery has enjoyed a rapid gain in popularity.Initial in vitro studies using optical tracking and “preoperative” CT scans provided the benchmark for image-guided thoracic pedicle screw placement. Assaker et al. showed that the accuracy was equal for both computer-assisted and conventional fluoroscopic technique (97.5% and 95%, respectively). Because of time required to perform surface matching, however, insertion time per pedicle screw was much longer with image guidance (13 min vs. 4 min). 2 Kothe et al. showed that computer generated images (virtual images) represented “reality” (in this case radiographs) with remarkable accuracy (±1–2 mm). None of the 54 screws breached the pedicle cortex. 4 Kim et al. further validated this technology with additional cadaver work that produced a 7.5% major perforation rate (neurovascular structures potentially at risk). They also highlighted the significant learning curve involved in using computer navigation that other reports have touched on. 3Clinical reports of using optical image guidance for placement of thoracic pedicle screws with preoperative CT scan data also show encouraging results. Arand et al. reported an 80% accuracy rate (completely within the pedicle) for image guided spinal surgery (IGSS). No malpositioned screws resulted in any neurovascular complications. 19 Youkolis et al. demonstrated an 8.5% cortical penetration rate in 224 thoracic screws placed with IGSS. Only 2.2% were felt to be structurally significant; however, no neurovascular complications were encountered in any of the misplacements. 6One clinical report exists using EMF tracking coupled with preoperative CT scan data. Amiot et al. compared a prospectively followed group of patients treated with IGSS (294 screws) with an historical cohort of patients treated with conventional fluoroscopic technique (544 screws). They found that accuracy improved from 85% to 95% with IGSS, and that the extent of perforation was much less with IGSS (no screws >2 mm out with IGSS; 15 screws >2 mm out with fluoroscopic techniques). 1Our study validates two technologies in an in vitro setting of thoracic pedicle screw placement: EMF tracking with image guidance and intraoperatively obtained fluoroscopic data to create the virtual imagery. IGSS placement of thoracic pedicle screws had an accuracy of 58% (completely within pedicle); however, 92% of screws were considered safely positioned (no neurovascular structures at risk). This is in agreement with reports of other existing image-guidance systems and conventional fluoroscopic technique. We were also able to demonstrate a significant reduction (40% less with IGSS) in the amount of radiation exposure related to fluoroscope utilization time.As with other tracking technologies, inherent technical problems exist with EMF IGSS. Although surface matching with preoperative CT data are not required, extra time is incurred with setting up of the system, placing the transmitter, and acquiring images that are suitable for computer navigation. When all of these variables are factored in, EMF IGSS does not decrease the time required to apply thoracic pedicle screws. In this study, total time to insert pedicle screws was the same for both IGSS and standard fluoroscopic technique. In addition, interference from ferritic (i.e., 400-series stainless) instruments with the EMF continues to be a problem that can affect tracking and accuracy.ConclusionEMF tracking technology coupled with intraoperative fluoroscopic imaging provides an alternative to optical tracking for placement of thoracic pedicle screws. We have demonstrated the potential for accurate insertion of screws with a substantial reduction in radiation exposure without increasing operative time.Key Points * EMF in image-guided application of thoracic pedicle screws results in a decrease in fluoroscopic radiation exposure and degree of perforation compared with conventional techniques using anatomic landmarks and fluoroscopy * Line-of-sight restrictions are eliminated; however, problems can arise with field distortion when the transmitter is moved or ferromagnetic substances approach the electromagnetic field.References1. Amiot LP, Lang K, Putzier M, et al. Comparative results between conventional and computer-assisted pedicle screw installation in the thoracic, lumbar, and sacral spine. Spine. 2000; 25 ( 5): 606–614. [CrossRef] [Full Text] [Medline Link] [Context Link]2. Assaker R, Reyns N, Vinchon M, et al. Transpedicular screw placement: image-guided versus lateral-view fluoroscopy: in vitro simulation. Spine. 2001; 26 ( 19): 2160–2164. [CrossRef] [Full Text] [Medline Link] [Context Link]3. Kim KD, Patrick Johnson J, Bloch O, et al. Computer-assisted thoracic pedicle screw placement: an in vitro feasibility study. Spine. 2001; 26 ( 4): 360–364. [CrossRef] [Full Text] [Medline Link] [Context Link]4. Kothe R, Matthias Strauss J, Deuretzbacher G, et al. Computer navigation of parapedicular screw fixation in the thoracic spine: a cadaver study. Spine. 2001; 26 ( 21): E496–E501. [Context Link]5. Schwend RM, Dewire PJ, Kowalski TM. Accuracy of fluoroscopically assisted laser targeting of the cadaveric thoracic and lumbar spine to place transpedicular screws. J Spinal Disord. 2000; 13 ( 5): 412–418. [CrossRef] [Full Text] [Medline Link] [Context Link]6. Youkilis AS, Quint DJ, McGillicuddy JE, et al. Stereotactic navigation for placement of pedicle screws in the thoracic spine. Neurosurgery. 2001; 48 ( 4): 771–779. [CrossRef] [Full Text] [Medline Link] [Context Link]7. Sagi HC, Manos RE, Benz R, et al. Electromagnetic Field-Based Image-Guided Spine Surgery Part One: Results of a Cadaveric Study Evaluating Lumbar Pedicle Screw Placement. Spine. 2003;28:xxx–xxx. [Context Link]8. Belmont PJ Jr, Klemme WR, Dhawan A, et al. In vivo accuracy of thoracic pedicle screws. Spine. 2001; 26 ( 21): 2340–2346. [CrossRef] [Full Text] [Medline Link] [Context Link]9. Gayet LE, Hamcha H, Charbonneau A, et al. Biomechanical study and digital modeling of traction resistance in posterior thoracic implants. Rev Chir Orthop Reparatrice Appar Mot. 2001; 87 ( 5): 459–468. [Medline Link] [Context Link]10. Liljenqvist U, Hackenberg L, Link T, et al. Pullout strength of pedicle screws versus pedicle and laminar hooks in the thoracic spine. Acta Orthop Belg. 2001; 67 ( 2): 157–163. [Medline Link] [Context Link]11. Liljenqvist UR, Halm HF, Link TM. Pedicle screw instrumentation of the thoracic spine in idiopathic scoliosis. Spine. 19971;22(19):2239–2245. [Context Link]12. 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Link]|00007632-200309010-00029#xpointer(id(R18-29))|11065213||ovftdb|00002517-199906000-00010SL0000251719991222211065213P74[CrossRef]|00007632-200309010-00029#xpointer(id(R18-29))|11065404||ovftdb|00002517-199906000-00010SL0000251719991222211065404P74[Full Text]|00007632-200309010-00029#xpointer(id(R18-29))|11065405||ovftdb|00002517-199906000-00010SL0000251719991222211065405P74[Medline Link]|00007632-200309010-00029#xpointer(id(R19-29))|11065213||ovftdb|SL000075472001104107611065213P75[CrossRef]|00007632-200309010-00029#xpointer(id(R19-29))|11065405||ovftdb|SL000075472001104107611065405P75[Medline Link]|00007632-200309010-00029#xpointer(id(R20-29))|11065213||ovftdb|SL0000508820019432811065213P76[CrossRef]|00007632-200309010-00029#xpointer(id(R20-29))|11065405||ovftdb|SL0000508820019432811065405P76[Medline Link]11302643Electromagnetic Field-Based Image-Guided Spine Surgery Part Two: Results of a Cadaveric Study Evaluating Thoracic Pedicle Screw PlacementSagi, H. C. MD; Manos, R. MD; Park, S.-C. MD; Von Jako, R. MD; Ordway, N. R. MS, PE; Connolly, P. J. MDCase Report1728