Journal ArticleObject. To validate computerized tomography (CT) scanning as a tool to assess the accuracy of thoracic pedicle screw placement, the authors compared its accuracy with that of direct visualization in instrumented cadaveric spine specimens.   Methods. A grading scale was devised to score the placement of the pedicle screw. The grades ranged from 0 to 3 depending on the extent to which the pedicle had been violated. One hundred fifty-five pedicles were fitted with instrumentation in eight cadaveric spines. A single observer graded the appearance of the screw based on CT scans (3-mm axial sections with 1-mm overlap) and direct visualization of the specimen. The authors arrived at a Kappa value of 0.51, which suggested only moderate agreement between the two measurement techniques. Whereas CT had a positive predictive value of 95%, it had a negative predictive value of 62%.   Conclusions. The authors thus conclude that although CT scanning is the most valid tool to assess the accuracy of  thoracic pedicle screw placement, it tends to overestimate the number of misplaced screws

Dailey, Andrew T.

Rao, Ganesh

English

The University of Utah: J. Willard Marriott Digital Library

HE indications for pedicle screw fixation in the tho-racic spine continue to expand the more familiarsurgeons become with the anatomy and techniquesfor pedicle fixation.1,3,7,13,17,18,21 By providing rigid fixation,pedicle screws allow for greater correction of deformity18and greater pullout strength than traditional hook/rod con-structs.12 Whereas the authors of initial reports suggestedscrew misplacement rates of up to 41%,20 those of morerecent studies indicate rates of approximately 10%, withan incidence of neural complications of less than 1%.18,25Stereotactic navigation for placement of thoracic pediclescrews has been shown to decrease the occurrence of cortical perforation compared with the use of anatomi-cal landmarks and standard fluoroscopy.25 There is no con-sistent method, however, with which to evaluate screwplacement accuracy in the literature concerning thoracicpedicle screws.The accuracy of pedicle screw placement is importantfor structural support, fusion, and ultimate surgical cor-rection. This is particularly true in the thoracic spine whereanatomical landmarks are small and where misplacedscrews threaten vital thoracic structures.5,20,25 Computerizedtomography scanning is often used both clinically and inthe laboratory setting to assess the accuracy of pediclescrew positioning.12,16,18,20,25 The presence of artifact andflare from metallic implants, however, makes accurate CTassessments of screw position difficult.15,24 Other investiga-tors have evaluated plain radiography to assess successfulpedicle screw placement.19 In one previous study the au-thors examined the accuracy of CT scanning to identifypedicle screw placement in the lumbar spine.24 The authorsdid not consider the unique anatomy of the thoracic spine,nor did they use a grading system to differentiate the se-verity of pedicle screw misplacement. To our knowledge,there have been no other studies conducted to evaluate thereliability of CT scanning as a modality for assessing theaccuracy of thoracic pedicle screw placement. To validateCT scanning in this endeavor, we compared pedicle screwplacement demonstrated on CT scans with direct visualiza-tion in a cadaveric thoracic spine.Materials and MethodsEight thoracic spines were harvested from fresh-frozen adultcadavers for use in our analysis. In these eight specimens, 155 pedi-cles were instrumented. Preinstrumentation CT scans were obtainedto determine the diameter of the thoracic pedicles. Pedicles less than4.5 mm were not instrumented. Pedicles 4.5 to 5.5 mm were instru-mented using 4.5-mm screws (Danek M8 screws; Sofamor Danek,Memphis, TN). The remaining pedicles were instrumented with5.5-mm screws (Dynalok screws; Sofamor Danek). Titanium pedi-cle screws were placed using either a freehand technique or stereo-tactic or virtual fluoroscopy. Regardless of whether the pedicle wasinstrumented with the assistance of stereotaxy, the same basic tech-nique was performed.4 After the spines had been instrumented, we performed CT scanning (CT/I scanner; General Electric, Mil-waukee, WI). The protocol for imaging each spine consisted of 3-mm axial cuts with 1-mm overlap. Window settings were adjust-ed for optimum visualization of the screws and bone. Each pedicle was assigned a grade from Grade 0 to 3. The grad-ing scale is as follows: 0 reflected no perforation of the pedicle, J. Neurosurg: Spine / Volume 97 / September, 2002J Neurosurg (Spine 2) 97:223–226, 2002Comparison of computerized tomography and directvisualization in thoracic pedicle screw placementGANESH RAO, M.D., DARREL S. BRODKE, M.D., MATTHEW RONDINA, B.S., AND ANDREW T. DAILEY, M.D.Departments of Neurosurgery, Orthopedics, and the College of Medicine, University of Utah, Salt Lake City, UtahObject. To validate computerized tomography (CT) scanning as a tool to assess the accuracy of thoracic pediclescrew placement, the authors compared its accuracy with that of direct visualization in instrumented cadaveric spinespecimens. Methods. A grading scale was devised to score the placement of the pedicle screw. The grades ranged from 0 to 3depending on the extent to which the pedicle had been violated. One hundred fifty-five pedicles were fitted with instru-mentation in eight cadaveric spines. A single observer graded the appearance of the screw based on CT scans (3-mmaxial sections with 1-mm overlap) and direct visualization of the specimen. The authors arrived at a Kappa value of0.51, which suggested only moderate agreement between the two measurement techniques. Whereas CT had a posi-tive predictive value of 95%, it had a negative predictive value of 62%. Conclusions. The authors thus conclude that although CT scanning is the most valid tool to assess the accuracy ofthoracic pedicle screw placement, it tends to overestimate the number of misplaced screws. KEY WORDS • computerized tomography • pedicle • thoracic spineT223Abbreviation used in this paper: CT = computerized tomography.1 indicated less than 2 mm of perforation of the pedicle with onescrew thread out of the pedicle, 2 represented 2 to 4 mm of perfora-tion of the pedicle, and Grade 3 reflected greater than 4 mm. Medialand lateral perforations were grouped together for final analysisbecause of the small number of medial perforations in the study (seeResults). Examples of each CT-visualized grade are shown in Fig.1. Each spine was then directly visualized and assigned a grade aswell. Figure 2 provides examples of Grades 1 and 2. Grading, bothon CT scans and direct inspection, was performed by the seniorauthor (A.T.D.), who was blinded to which CT scans correlatedwith each specimen. ResultsThe positioning of 155 pedicle screws were grad-ed based on CT scans and on direct visual inspection ofeach specimen. Figure 3 shows the distribution of gradesbetween the two methods of measurement. Based onresults of CT evaluation Grade 0 was assigned to 110pedicle screws, whereas Grade 0 was assigned to 122screws based on direct visualization. There were 32 Grade1 screws based on CT examination compared with 23based on direct visualization. There were 13 Grade 2scores based on CT studies versus 10 based on direct visu-alization. There were no Grade 3 screws determined usingeither technique. Only six screws appeared to be mediallyplaced on CT examination, whereas four appeared to bemedially placed on visual inspection. Because of the smallnumber of medial perforations compared with the totalnumber of perforations, all screw perforations weregrouped together for statistical analysis.We then calculated a Kappa value (performed withSPSS for Windows; SPSS, Chicago, IL), which is a relia-bility test used to determine observed agreement betweentechniques of measure.11 In other words, how do these twomethods for assessing the accuracy of pedicle screwplacement in the thoracic spine compare? A Kappa valuecloser to 1 suggests a perfect association of agreement(that is, that CT measurement is just as accurate as directvisualization), whereas a value closer to 0 suggests anyobserved agreement is likely to be just chance. We founda Kappa value of 0.507, which suggests a moderate degreeof agreement.19We then calculated sensitivity, specificity, positive pre-dictive value, and negative predictive value of CT scan-ning as a measure of the accuracy of pedicle screw place-ment (also performed with SPSS). We assumed that directvisual inspection of the instrumented specimen is the goldstandard for determining such accuracy because the spec-imen can be viewed in all dimensions. We found a sensi-tivity of 86%, a specificity of 85%, a positive predictivevalue of 95%, and a negative predictive value of 62%. DiscussionPlacement of pedicle screws for spinal stabilization is acommon procedure. In the lumbar spine, screw placementis made easier by the larger pedicles and less dangerousbecause of the lack of major vital structures.4,6,15,22,26 In-strumentation for thoracic fixation has more traditionallybeen of pedicle and laminar hooks because of the smallerdiameters of thoracic pedicles and the proximity of impor-tant neural and vascular structures.5,9,20 Investigators ofpedicle morphometry have revealed that the superoinferi-or diameter of the thoracic pedicle is much greater thanthe mediolateral diameter, with the narrowest pedicles oc-curring at T-5 or T-6 and measuring less than 4.5 mm onaverage.2,5,10 Additionally, the medial wall is significantlythicker than the lateral wall and may explain why mostpedicle fractures after screw placement occur laterally.9Accurate placement of screws is paramount, because thedural sac lies immediately adjacent to the pedicle through-out the thoracic spine.5 Numerous techniques have beenconducted to make instrumentation-related fixation of thethoracic spine easier;4,8,13,16,20,23 however, transpedicular fix-ation in the thoracic spine is generally limited to thosecases in which the clinical indication is clear.13,20Recently, a number of articles have appeared in whichthe authors describe pedicle screw fixation in the thoracicG. Rao, et al.224 J. Neurosurg: Spine / Volume 97 / September, 2002FIG. 1. Computerized tomography studies depicting Grades 0(A), 1 (B), 2 (C), and 3 (D).spine.4,7,12,13,17,18,20 Surgeons have successfully used thoracicpedicle screw fixation for a variety of indications includ-ing idiopathic scoliosis, traumatic injury, and tumor. Theability to achieve greater correction of a deformity andmaintain that correction has led to wider application oftranspedicular fixation in the thoracic spine. In manyreports the authors have outlined the accuracy of varioustechniques in vivo by using postoperative CT examinationto determine the accuracy of screw positioning.1,4,7,12,13,17,18,20,25 Perforation rates (medial and lateral) have been re-ported to range from 1 to 43% when these methods areperformed.1 To our knowledge, there has not been a criti-cal examination of the ability of CT scanning to assess theaccuracy of pedicle screw placement in the thoracic spine.We therefore undertook this study to validate this modali-ty for assessing the accuracy of pedicle screw placement,particularly in the thoracic spine where the pedicles aresmaller and misplaced screws can be less forgiving. Wefound that CT scanning has a moderate measure of agree-ment compared with a standard of direct visual inspectionof the specimen (Kappa value 0.51). This suggests that CTscanning provides an adequate assessment of screw accu-racy in vitro or in vivo.Using direct visual inspection of the pedicle as the goldstandard of pedicle screw placement, we also determinedthe sensitivity, specificity, and positive and negative pre-dictive values of CT-documented accuracy. With a speci-ficity and sensitivity of 86 and 85%, respectively, CTevaluation is comparable with the gold standard (directvisual inspection) as a test of proper screw placement.These values are consistent with others reported in the lit-erature with respect to evaluation of screws in the lumbarspine.24 With a positive predictive value of 95%, when CTscanning demonstrated that a pedicle screw was in place,it was in fact most likely in place. We found, however, thatCT scanning has a negative predictive value of 62%,which suggests that when CT scanning demonstrated thata screw was misplaced, it was only correct 62% of thetime. In essence, CT scanning seems to overestimatescrew misplacement. The reason for the appearance ofscrew breakout on CT scans, despite apparent pedicle in-tegrity with direct visualization, probably lies in the phe-nomenon of plastic deformation of bone.14,15 This occursonce the endosteal diameter of the pedicle is exceeded bythe screw size or within 80% of the outer cortical diame-ter.14 Deformation may occur without pedicle fracture orscrew cutout. In the study by Sjostrom, et al.,15 the authorsshowed, in an in vivo setting, that pedicle screws increasea pedicle’s width, giving credence to the principle of plas-tic deformation. Surgeons should therefore take plastic de-formation of the pedicle into consideration when CT scan-ning demonstrates a titanium pedicle screw that appears toperforate the pedicle minimally. We have shown that CT scanning can be used reliablyto assess the accuracy of thoracic pedicle screw place-ment. This validates it as a tool both clinically and in thelaboratory. Yoo, et al.,24 found a significantly lower sensi-tivity and specificity for CT scanning when they evaluat-ed cobalt-chrome screws compared with titanium screwsin the lumbar spine. They concluded that in the setting ofchrome-cobalt screws, CT scanning could not be relied onalone to provide accurate information after screw place-ment. Analysis of our results suggests that while CT scan-ning has good sensitivity and specificity for evaluation oftitanium screw placement in the thoracic spine, it tends toexaggerate a minimal violation of the pedicle by a screw.Proper placement of pedicle screws is important for struc-tural stability and avoidance of vital structures. Screw sta-bility is dependent on the purchase it obtains within theJ. Neurosurg: Spine / Volume 97 / September, 2002Comparison of CT and direct visualization225FIG. 2. Photographs. A: Specimen representing a Grade 1 pedicle perforation. B: Specimen representing a Grade 2 pedicle perforation.FIG. 3. Graph demonstrating grades obtained for CT scanningand direct visual inspection.cancellous portion of the pedicle,18 and the ability to dem-onstrate this on CT scans has obvious importance. Weconclude from this study that CT scanning is a useful andreliable tool for assessing the accuracy of thoracic pediclescrew placement. Because of plastic deformation of thepedicles following screw placement, however, CT scan-ning tends to be overly sensitive to screws that appear tobe placed adjacent to the pedicle yet remain within thecortical margins. DisclosureThe authors acknowledge the contribution of the cadaveric spec-imens and instruments from Sofamor Danek. References1. Belmont PJ Jr, Klemme WR, Dhawan A, et al: In vivo accura-cy of thoracic pedicle screws. Spine 26:2340–2346, 20012. Berry JL, Moran JM, Berg WS, et al: A morphometric study ofhuman lumbar and selected thoracic vertebrae. Spine 12:362–367, 19873. Chang KW: Oligosegmental correction of post-traumatic thora-columbar angular kyphosis. Spine 18:1909–1915, 19934. Cinotti G, Gumina S, Ripani M, et al: Pedicle instrumentationin the thoracic spine. A morphometric and cadaveric study forplacement of screws. Spine 24:114–119, 19995. Ebraheim NA, Jabaly G, Xu R, et al: Anatomic relations of thethoracic pedicle to the adjacent neural structures. Spine 22:1553–1557, 19976. Ebraheim NA, Xu R, Ahmad M, et al: Projection of the thorac-ic pedicle and its morphometric analysis. Spine 22:233–238,19977. Esses SI, Botsford DJ, Wright T: Operative treatment of spinalfractures with the AO internal fixator. Spine 16:S146–S150,19918. Gertzbein SD, Robbins SE: Accuracy of pedicular screw place-ment in vivo. Spine 15:11–14, 19909. Kothe R, O’Holleran JD, Liu W, et al: Internal architecture ofthe thoracic pedicle. An anatomic study. Spine 21:264–270,199610. Krag MH, Weaver DL, Beynnon BD, et al: Morphometry of thethoracic and lumbar spine related to transpedicular screw place-ment for surgical spinal fixation. Spine 13:27–32, 198811. Landis JR, Koch GG: The measurement of observer agreementfor categorical data. Biometrics 33:159–174, 197712. Liljenqvist U, Hackenberg L, Link T, et al: Pullout strength ofpedicle screws versus pedicle and laminar hooks in the thoracicspine. Acta Orthop Belg 67:157–163, 200113. Liljenqvist UR, Halm HF, Link TM: Pedicle screw instrumen-tation of the thoracic spine in idiopathic scoliosis. Spine 22:2239–2245, 199714. Misenhimer GR, Peek RD, Wiltse LL, et al: Anatomic analysisof pedicle cortical and cancellous diameter as related to screwsize. Spine 14:367–372, 198915. Sjostrom L, Jacobsson O, Karlstrom G, et al: CT analysis ofpedicles and screw tracts after implant removal in thoracolum-bar fractures. J Spinal Disord 6:225–231, 199316. Steinmann JC, Herkowitz HN, el-Kommos H, et al: Spinal pedicle fixation. Confirmation of an image-based technique forscrew placement. Spine 18:1856–1861, 199317. Suk S, Lee CK, Kim WJ, et al: Segmental pedicle screw fixa-tion in the treatment of thoracic idiopathic scoliosis. Spine 20:1399–1405, 199518. Suk SI, Kim WJ, Lee SM, et al: Thoracic pedicle screw fixationin spinal deformities: are they really safe? Spine 26:2049–2057,200119. Szklo M, Nieto FJ: Epidemiology: Beyond the Basics. Gai-thersburg, MD: Aspen, 2000, pp 375–38020. Vaccaro AR, Rizzolo SJ, Balderston RA, et al: Placement ofpedicle screws in the thoracic spine. Part II: An Anatomical and radiographic assessment. J Bone Joint Surg (Am) 77:1200–1206, 199521. Wu SS, Hwa SY, Lin LC: Management of rigid post-traumatickyphosis. Spine 21:2260–2267, 199622. Xu R, Ebraheim NA, Ou Y, et al: Anatomic considerations ofpedicle screw placement in the thoracic spine. Roy-Camilletechnique versus open-lamina technique. Spine 23:1065–1068,199823. Xu R, Ebraheim NA, Shepherd ME, et al: Thoracic pediclescrew placement guided by computed tomographic measure-ments. J Spinal Disord 12:222–226, 199924. Yoo JU, Ghanayem A, Petersilge C, et al: Accuracy of usingcomputed tomography to identify pedicle screw placement incadaveric human lumbar spine. Spine 22:2669–2671, 199725. Youkilis, AS, Quint DJ, McGillicuddy JE, et al: Stereotac-tic navigation for placement of pedicle screws in the thoracicspine. Neurosurgery 48:771–779, 200126. Yuan HA, Garfin SR, Dickman CA, et al: A historical cohortstudy of pedicle screw fixation in thoracic, lumbar, and sacralspinal fusions. Spine 19:S2279–S2296, 1994 Manuscript received November 28, 2001.Accepted in final form May 14, 2002.Address reprint requests to: Andrew T. Dailey, M.D., Departmentof Neurosurgery, University of Utah, 30 North 1900 East, Suite3B409 SOM, Salt Lake City, Utah 84132. email: andrew.dailey@hsc.utah.edu.G. Rao, et al.226 J. Neurosurg: Spine / Volume 97 / September, 2002

Comparison of computerized tomography and direct visualization in thoracic pedicle screw placement

https://collections.lib.utah.edu/dl_files/b4/48/b448db827361bfbb3ac4a1a192be87d82dc83a46.pdf

Comparison of computerized tomography and direct visualization in thoracic pedicle screw placement

Abstract

Similar works

Full text

Available Versions

The University of Utah: J. Willard Marriott Digital Library