Proceedings of the NASS 17th Annual Meeting / The Spine Journal 2 (2002) 47S–128S 89S

tion into 2.5-mm axial slices and 3-mm sagital and coronal slices. UsingCT evaluation, 130 of the pedicle screws were contained, whereas 64screws were malpositioned. Using open dissection, 149 of the screws werecontained, whereas 45 were malpositioned. Assuming that open dissectionis the gold standard, computed tomography was 45% sensitive and 88%specific in diagnosing a malpositioned screw (55% positive predictivevalue, 88% negative predictive value). CT was also 78% sensitive and 64%specific in diagnosing a contained screw (88% positive predictive value,55% negative predictive value). The overall accuracy of CT was 73% inthis study.Relationship between findings and existing knowledge: The use of pedi-cle screw instrumentation in the thoracic spine has become increasinglypopular. Thoracic pedicle screws offer many advantages in the treatment ofspinal pathology. However, potential for complications exist. Surgeonsneed reliable means to evaluate the placement of these screws. Likewise,they need to understand the limitations of any given technique of evalua-tion. Based on the results of this investigation, CT scan was found to berelatively insensitive in the diagnosis of malpositioned screws.Overall significance of findings: CT alone may not be sufficiently accu-rate to serve as the sole method of evaluation of thoracic pedicle screw po-sition. In the face of postoperative neurologic deficits, surgical explorationand hardware removal should remain as the “gold standard” of care.Disclosures: Device or drug: pedicle screws. Status: investigational.Conflict of interest: No conflicts.

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2:26Safety of thoracic pedicle screw insertion with and without computer assistance: a cadaveric studyGregory Wiggins, MD1, Christopher Shaffrey, MD1, Sohail Mirza, MD1,Carol Bellabarba, MD1, Jens Chapman, MD1; 1University of Washington,Seattle, WA, USA

Purpose of study: The placement of pedicle screws is technically demand-ing and poses potential hazards related mainly to penetration of the antero-lateral vertebral body or medial pedicle. To investigate safety, we under-took an in vitro analysis of thoracic pedicle screw placement by two seniorspine surgeons using standard fluoroscopy, virtual fluoroscopy and com-puted tomography image guidance.Methods used: In a simulated OR environment, cadaveric thoracic spineswith the posterolateral rib cage intact were mounted in foam. Pediclescrews were placed from T1 up to T11 in 20 cadaveric thoracic spines us-ing standard anatomic landmarks and fluoroscopic guidance (Group 1),FluoroNav with one reference (Group 2) or multiple reference points(Group 3), and Stealth (Group 4). Radiation exposure to the surgeon’storso, hands and specimen were measured. The specimens were then dis-sected, sagittally split and examined for evidence of pedicle or vertebralbody cortical screw perforations. Any cortical violation except the costo-vertebral junction was considered out. The cortical perforations were thengraded by 2-mm increments. Statistical analysis was performed.Summary of findings: There were 94, 99, 70 and 74 screws placed in thefour groups. There was significantly more average radiation exposure tothe specimen in Group 3 (27 mREM) and less in Group 4 (0 mREM) thanin Group 1 (7.8 mREM). Radiation exposure to the surgeon was minimal,measuring less than 1 mREM to the surgeon’s torso per specimen in allgroups except 4 (0 mREM). Average time per screw was significantlylonger in Group 4 (6.8 minutes) compared with Group 1 (2.4 minutes).Cortical violations were significantly less in Group 3 (11.4%) and Group 4(5.4%) compared with Group 1 (20.2%). The only medial cortical viola-tions were in Group 1, and none were greater than 2 mm.Relationship between findings and existing knowledge: Anatomic land-marks and standard fluoroscopy have been shown to have approximately20% thoracic screw malpositioning. The current study confirms this anddemonstrates greater accuracy with image guidance.Overall significance of findings: Although the time for screw placementwas longer with Stealth and FluoroNav, the percentage of screws with cor-

tical violations was significantly less. Computer-assisted thoracic pediclescrew placement may provide more accurate placement and less risk tocritical structures.Disclosures: Device or drug: pedicle screws. Status: approved. Device ordrug: FluoroNav. Status: approved. Device or drug: Stealth. Status: approved.Conflict of interest: Christopher Shaffrey, grant research support; JensChapman, grant research support Syntles USAAO/ASIF International So-famor Danek; Jens Chapman, CHAIR, Surgical Dynamics.

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2:32Intraoperative monitoring during interbody fusionJeffrey Balzer, PhD1, William Welch, MD1, Elizabeth Tyler-Kabara, MD,PhD1, Peter Gerszten, MD2, Robert Sclabassi, MD, PhD2; 1University ofPittsburgh, Pittsburgh, PA, USA; 2University of Pittsburgh, Pittsburgh,PA, USA

Purpose of study: A reported 5% to 10% neurologic complication rate hasbeen reported after interbody fusion (both anterior [ALIF] and posterior[PLIF]). Iatrogenic injuries during these procedures have been reported tooccur in response to excessive nerve root retraction and thecal sac manipu-lation. Somatosensory evoked potential (SSEP) and spontaneous elec-tromyographic (EMG) recordings may detect and potentially prevent nerveroot and thecal sac injury during ALIF and PLIF procedures. We describethe feasibility and utility of recording SSEPs and spontaneous electromyo-graphy EMG during ALIF and PLIF procedures.Methods used: A total of 165 patients underwent surgical procedures in-volving decompression and stabilization of the lumbar spine using eitherALIF (n�90) or PLIF (n�75) procedures. Pedicle screw fixation was usedin 65 of the PLIF cases. Spinal procedures were performed by one team ofsurgeons, and standardized SSEP and EMG was performed sequentially onall patients undergoing interbody fusion procedures. Detection of nerveroot compression, manipulation, stretching and/or permanent injury wasbased on changes from baseline EMG recordings. The severity of nerveroot compromise was based on the type of spontaneous discharges thatwere recorded.Summary of findings: In a total of 12 cases (all PLIF), spontaneous EMGactivity was reported to the surgeon. In no cases were significant SSEPchanges observed. Five patients awoke with new or an exacerbation of apreexisting neurological deficit. In those cases where EMG recording wasperformed, only two new transient neurological deficits were observedwith cage placement. In both of these cases, multiple periods of sustainedEMG activity were recorded during placement of instrumentation/retrac-tion of nerve roots.Relationship between findings and existing knowledge: Thecal sac andnerve root injury may or may not be detected depending on the neurophys-iologic modality that is being monitored. The present report demonstratesthe increased sensitivity and specificity of combined neurophysiologic mo-dality testing during instrumented procedures in predicting postoperativemorbidity. The data in this report describe a powerful conjunction of sim-ple tools that provide immediate “early-warning” feedback to the surgeonconcerning the state of sensory and motor function during lumbar inter-body fusion procedures.Overall significance of findings: Spontaneous EMG activity appears to bea sensitive indicator of nerve root retraction and predictor of potential ia-trogenic nerve root compromise during PLIF procedures. Our data suggestthat EMG recording during lumbar interbody fusion procedures is a sen-sitive and specific indicator of nerve root compromise. Multimodalitymonitoring, an assessment of both sensory and motor function, is the bestapproach for the prevention of iatrogenic injury incurred during decom-pression and complex fusion of the lumbosacral spine.Disclosures: Device or drug: interbody fusion cages. Status: approved. De-vice or drug: pedicle screws. Status: approved.Conflict of interest: William Welch, consultant, Sulzer Spine Tech.

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