Eur Spine J (1997) 6:19-24 © Springer-Verlag 1997 M. M. Panjabi J. D. O'Holleran J. J. Crisco III R. Kothe Complexity of the thoracic spine pedicle anatomy Received: 8 January 1996 Revised: 14 April 1996 Accepted: 24 April 1996 M. M. Panjabi ([~) Biomechanics Laboratory, Department of Orthopedics and Rehabilitation, Yale University School of Medicine, P.O. Box 208071, New Haven, CT 06520-8071, USA J. D. O'Holleran Yale University School of Medicine, Department of Orthopedics and Rehabilitation, New Haven, CT 06520-8071, USA J. J. Crisco Rhode Island Hospital, Orthopedic Research, Providence, Rhode Isiand, USA R. Kothe Department of Orthopedics, Heinrich-Heine University, Dfisseldorf, Germany Abstract Transpedicular screw fixa- tion provides rigid stabilization of the thoracolumbar spine. For accu- rate insertion of screws into the pedi- cles and to avoid pedicle cortex per- forations, more precise knowledge of the anatomy of the pedicles is neces- sary. This study was designed to vi- sualize graphically the surface anatomy and internal architecture of the pedicles of the thoracic spine. Fifteen vertebrae distributed equally among the upper, middle, and lower thoracic regions were used. For the purpose of mapping surface anatomy, each pedicle was cleaned, spray- painted white, and marked with more than 100 fine points. Using an opto- electronic digitizer, three-dimensional coordinates of the marked points and three additonal points, representing a coordiate system, were digitized. A solid modeling computer program was used to create three-dimensional surface images of the pedicle. To ob- tain cross-sectional information, each pedicle was sectioned with a thin di- amond-blade saw to obtain four slices, 1 mm in thcikness and 0.5 mm apart. The pedicle slices were X-rayed and projected onto a digi- tizer. The internal and external con- tours were digitized and converted into graphs by a computer. The pedi- cles exhibited significant variability in their shape and orientation, not only from region to region within the thoracic spine, but also within the same region and even within the same pedicle. These variations are extremely significant in light of cur- rent techniques utilized in transpe- dicular screw fixation in the thoracic spine. Information documenting the three-dimensional complexity of pedicle anatomy should be valuable for surgeons and investigators inter- ested in spinal instrumentation. Key words Anatomy • Pedicles • Thoracic spine • Pedicle instrumentation • Biomechanics Introduction The current use and popularity of transpedicular screw fixation in rigid stabilization of the thoracolumbar spine have been well documented. Many studies have described techniques of pedicle screw fixation [4, 7, 21, 24] as well as the complex anatomy of the pedicle from a variety of perspectives (see below). Controversy continues, how- ever, regarding the benefits and performance of this tech- nique [12]. Unacceptable rates of pedicle cortex perfora- tions have been described in both in vivo and in vitro studies [5, 7, 23]. Clearly, opinions vary regarding the safety and efficacy of transpedicular screws in spinal sta- bilization, and more information is needed to assess accu- rately the correct specific approach to the procedure as well as the overall value of pedicle screw fixation. The anatomy of the pedicle is complex and varies from region to region in the thoracolumbar spine [8, 15, 25]. There is much information in the literature concerning the morphometry of the thoracolumbar spine [15, 20] as well as the specific anatomy of the pedicle (height, width, axis
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`20 angles, and landmarks) [2, 3, 9-11, 14, 22, 25]. These studies have served as an anatomical foundation for the development of more sophisticated surgical instrumenta- tion. To our knowledge, however, there is a void in the liter- ature regarding the true complexity of the pedicle as a variable three-dimensional structure. Studies have described the pedicle as ovoid in shape, with relatively constant di- mensions for each spinal level [17, 18, 25]. Our experi- ence has shown otherwise. For adequate utilization of a screw that traverses the extent of the pedicle, in light of the pitfalls of such instrumentation, we feel that accurate information about the pedicle as a non-homogeneous, three-dimensional structure is necessary. The purpose of this study was to provide an anatomical analysis of the three-dimensional structure of the pedicle. Two methodologies were used: a computer-generated three-dimensional surface model, and a set of cross-sec- tional contours. Hopefully this graphical study will spark further investigations into instrumentation and surgical approaches in the treatment of the thoracic spine. Materials and methods Fifteen isolated thoracic vertebrae were harvested from 13 fresh- frozen, human spinal specimens. The mean age of the spines was 57 years (range 33-82 years), with a male to female ratio of 6 : 8. Five specimens were chosen from each of the three thoracic transi- tional zones, as described by Panjabi et al. (upper: five T2; middle three T6 and two T7; and lower: three T10 and two T11) [15]. Specimens with documented abnormalities in bone quality or gross pathology as determined by radiographs were excluded from the study. Each vertebra was dissected out and all soft tissues removed by sharp dissection. The specimens were subsequently immersed in a 1 : 1 hypochloride bleach solution for 12 h to remove all re- maining soft tissue. Three-dimensional surface model The cleaned vertebrae were spray-painted with a white flat latex paint for ease of visualization of surface points marked with a fine- tip marker (Fig. 1). The right and left pedicles of each vertebra were marked with a number of points, ranging from 96 in the smaller specimens to 120 in the larger specimens, arranged in a symmetric and even distribution. To establish a standardized coor- dinate system for the pedicles relative to their vertebral bodies, three small steel balls were glued to the endplates: one on the su- perior and two on the inferior endplate, in the mid-sagittal plane. Fig. 1 A, B Photographs of a cleaned vertebra, spray-painted white and marked with more than 100 fine points on its pedicle surface. The points were digitized, documenting the three-dimensional sur- face of the pedicle. A Right-side view; B top view Y Anterior L ft Right Posterior Fig. 2 A schematic representation of a typical vertebra, demon- strating the orientation of the pedicle cross-sections. Slice 1 repre- sents the posterior end of the pedicle at the junction with the infe- rior articular process, and slice 4 represents the anterior end of the pedicle at the junction with the vertebral body
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`21 The two inferior endplate balls were placed at the interesection of the mid-sagittal plane with the anterior-most and posterior-most boundaries of the inferior endplate. Similarly, the superior ball was placed at the posterior-most aspect of the superior endplate. These three "coordinate points" enabled us later to convert all contour data to one standardized coordinate system for inter-specimen comparison. To digitize the points, a three-dimensional digitizing pointer (Optotrak, Northern Digital, Waterloo, Canada) was used. The speci- men was rigidly held in space using a clasp stand. The specimen was oriented to facilitate reaching the whole pedicle with the pointer. The pointer was gently placed on the mark of interest and the point was recorded by a computer upon depression of a foot pedal connected to the Optotrak system. Points were recorded in a systematic fashion around the extent of the pedicle. The data for each specimen were converted to a standardized three-dimensional coordinate system using the three reference balls, as described above. Special software was written for the co- ordinate transformation of the array of points on the pedicle into a series of cross sections, and subsequently into a three-dimensional solid model. Cross-sectional contours The vertebral specimens were then placed in a specially designed fixation device for rigid attachment to the thin sectioning machine (Hamco Machines, Rochester, N.Y.) such that the 0.5-mm dia- mond-blade was perpendicular to the axis of one of the pedicles. Although the orientation of the pedicle axis was not quanitified, lateral and transverse radiographs were used to align the pedicle axis in the sectioning machine, so that the cuts were made perpen- dicular to the pedicle axis. Starting from a point just anterior to the inferior articular facet and progressing anteriorly, 1.0-mm slices were cut. The thin sections were then arranged in anatomical order on a plastic mount, together with a radiographic scale, and X- rayed. The first (no. 1) and last (no. 4) slice were used to describe the anatomical orientation of the slices (Fig. 3). Slice 1 represented the posterior end of the pedicle, at the junction with the inferior ar- ticular process, and slice 4 represented the anterior end of the pedi- cle, at the junction with the vertebral body. The 35-mm photo- graphic slides of the contact radiographs were rear-projected on to a digitizer (magnification x 10) and the internal and external borders of the pedicle were digitized. These coordinate data were then trans- formed, using specially written software, into smooth cross-sec- tional surface contours and presented graphically for visualization. Fig. 3 A-D Photographs showing the outer surface three-dimensional images of the right pedicle as generated by solid-model computer software. The coordinate system defined in Fig. 2 is shown in the inserts. A Postero-ante- rior view, B antero-posterior view, C inferior- superior view, D superior-inferior view
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`22 Results The data for this study are visual in nature, depicting the three-dimensional complexity of the thoracic pedicle. Three-dimensional surface model The computer-generated surface model of the pedicles clearly documents the complexity of the shape of the tho- racic pedicles (Fig. 3). The cross-sectional shape and size vary along the pedicle axis, and the centers of the cross sections are not aligned along a straight line. Cross-sectional contours The external and internal contours of the cross sections of the pedicles are described separately for each of the three regions of the thoracic spine. Upper thoracic spine In the upper thoracic spine (Fig. 4), the pedicle (level T2) changes in shape from a teardrop to a kidney shape as the slices progress postero-anteriorly (slice 1 to slice 4). Specimens T2-244, T2-413, and T2-450 all display this progression, with the concavity directed laterally and the medial cortical wall being much thicker than the lateral cortical wall. In specimen T2-207, the superior aspect of the posterior slices is more sharply pointed than in the other specimens, and the anterior slices are more squarely shaped. Specimen T2-402 shows all four slices as being relatively teardrop shaped. Middle thoracic spine In the middle thoracic spine (Fig. 5), the pedicle (levels T6 and T7) changes in shape from a tall, narrow teardrop to an "inverted" teardrop (wider convexity at the cuperior pole) as the slices progress postero-anteriorly (slice 1 to slice 4). Variations exist among specimens, yet this pat- tern is basically seen in all examples. Specimen T6-215 produced only three slices from the left pedicle, due to the length of the pedicle itself. Again, concavity is directed laterally, and the medial cortical wall is substantially thicker than the lateral cortical wall. Lower thoracic spine In the lower thoracic spine (Fig. 6), the pedicle (levels T10 and Tll) changes in shape in a similar fashion to speci- mens of levels T6 and T7. All specimens display hetero- UPPER THORACIC PEDICLES Left Right 1 2 3 4 Slices 4 3 2 1 E • -,o:Ob o 000 99 0000 @ 0000 Fig.4 Upper thoracic pedicles. External and internal contours of the four slices of the left and right pedicles of five vertebrae MIDDLE THORACIC PEDICLES Left Right 1 2 3 4 Slices 4 3 2 1 -~ E ~- Fig. 5 Middle thoracic pedicles. External and internal contours of the four slices of the left and right pedicles of five vertebrae
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`23 T10 - 515 Tll -101 LOWER THORACIC PEDICLES Left Right 1 2 3 4 Slices 4 3 2 1 T10"101 ~(~ ~ ~ ~ ~ ~@ - ,~ ~" Tll-226 Fig. 6 Lower thoracic pedicles. External and internal contours of the four slices of the left and right pedicles of five vertebrae geneity in shape near the vertebral body (slices 3 and 4), yet the overall pattern of teardrop to inverted teardrop is seen. Specimen T10-515 is much narrower in the trans- verse plane, and T11-226 left is markedly wider than the other specimens. Discussion In the present anatomical study we have attempted to demonstrate the complex three-dimensional structure of the thoracic pedicle. This was best achieved visually, us- ing sequential cross sections to elaborate the changes in pedicle shape as one moves postero-anteriorly. Among various shapes, we found the majority of the pedicles to be teardrop or kidney shaped, with a laterally directed concavity and a medial convexity. The pedicle exhibited variations in shape and orientation from region to region in the thoracic spine, as well as within the same region and even within the same pedicle. These variations are ex- tremely significant in light of current techniques and equipment utilized in transpedicular screw fixation. Since early descriptions of the pedicle as a simple cylinder [18, 19], most surgeons have presumably visual- ized the pedicle as a homogeneous structure with an oval shape. Similarly, anatomical studies have been limited to the description of the height, width, and orientation of the pedicle in the transverse and sagittal planes [3, 9-11, 14, 22, 25]. Although some authors have described the pedi- cle to be teardrop shaped [10], our study is, to our knowl- edge, the first systematic, three-dimensional anatomical study of the thoracic pedicle. The use of the Optotrak three-dimensional opto-elec- tronic camera system enabled us to reconstruct the pedicle surface with high accuracy, as is demonstrated by the three-dimensional image. More difficult was the interpre- tation of the pedicle cross sections in an analytical man- ner. It was not possible to quantify the pedicles' complex shapes; we therefore chose to present our data in a quali- tative visual manner. The internal structure of these differ- ent pedicle specimens, specifically the relationship be- tween the cortical and cancellous bone thicknesses, has been investigated quantitatively in a separate study [8]. The fact that the pedicle structure is significantly more complex than that of a simple cylinder should be of im- portance for those using pedicle screws for spinal instru- mentation. For a kidney-shaped pedicle that curves along its postero-anterior axis, the effective diameter permitting a given screw size is much less than one would expect on the basis of simple measurements of external pedicle width and height. This is especially important for the mid- to lower thoracic spine, where the diameter of the avail- able pedicle screws often exceeds the diameter of the pedicle. The increasing use of more sophisticated techniques in spinal surgery, e.g., computer-assisted pedicle screw placement as described only recently [1, 13], requires a three-dimensional understanding of the anatomical struc- tures involved. We hope that this study will provide valu- able information for surgeons and other investigators in- terested in spinal instrumentation, and will spark further study into the three-dimensional complexity of the human spine. Acknowledgement Support was provided in part by National In- stitutes of Health Grant AR39209 and by the AIOD, Germany. References 1. Amiot L-P, Labelle H, DeGuise JA, Sati M, Brodeur P, Rivard C-H (1995) Computer-assisted pedicle screw fixa- tion. A feasibility study. Spine 20 : 1208-1212 2. Bernard TN, Seibert CE (1992) Pedicle diameter determined by computed tomography. Its relevance to pedicle screw fixation in the lumbar spine. Spine 17 : S160-S163 3. Berry JL, Moran JM, Berg WS, Steffee AD (1987) A morphometric study of human lumbar and selected thoracic vertebrae. Spine 12:362-367
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`24 4. Dickman CA, Yahiro MA, Lu HTC, Melkerson MN (1994) Surgical treat- ment alternatives for fixation of unsta- ble fractures of the thoracic and lumbar spine. A meta-analysis. Spine 19: $2266-$2273 5. George DC, Krag MH, Johnson CC, Van Hal ME, Haugh ID, Grobler LJ (1991) Hole preparation techniques for transpedicular screws: effect on pull- out strength from human cadaveric vertebrae. Spine 16:181-184 6. Gertzbein SD (1992) Scoliosis Re- search Society multicenter spine frac- ture study. Spine 17:528-540 7. Gertzbein SD, Robbins SE (1990) Ac- curacy of pedicle screw placement in vivo. Spine i5:11-14 8. Kothe R, O'Holleran JD, Liu W, Pan- jabi MM (1996) Intemal architecture of the thoracic pedicle. An anatomical study. Spine 21:264-270 9. Krag M, Weaver DL, Beynnon BD, Hangh LD (1988) Morphometry of the thoracic and lumbar spine related to transpedicular screw placement for sur- gical spinal fixation, spine 13 : 27-32 10. Misenheimer GR, Peek RD, Wiltse LL, Rothman SLG, Widell EH (1989) Anatomic analysis of pedicle cortical and cancellous diameter as related to screw size. Spine 14 : 367-372 11. Moran JM, Berg WS, Berry JL, Geiger JM, Steffee AD (1989) Transpedicular screw fixation. J Orthop Res 7 : 107- 114 12. Mulholland RC (1994) PedicIe screw fixation in the spine (editorial). J Bone Joint Surg [Br] 76 : 517-519 13. Nolte LP, Zamorano LJ, Jiang Z, Wang Q, Langlotz F, Berlemann U (1995) Image-guided insertion of transpedicular screws. A laboratory set-up. Spine 20 : 497-500 14. Olsewski JM, Simmons EH, Kallen FC, Mendel FC, Severin CM, Berens DL (1990) Morphometry of the lumbar spine: anatomical perspectives related to transpedicular fixation. J Bone Joint Surg [Am] 72:541-549 15.Panjabi MM, Takata K, Goel V, Fed- erico D, Oxland T, Duranceau J (1991) Thoracic human vertebrae: quantitative three-dimensional anatomy. Spine 16: 888-901 16. Panjabi MM, Oxland TR, Takata K, Goel V, Duranceau J, Krag M, Price M (1992) Human lumbar vertebrae: quan- titative three-dimensional anatomy. Spine 17 : 299-306 17. Phillips JH, Kling TF, Cohen MD (1994) The radiographic anatomy of the thoracic pedicle. Spine 19 : 446- 449 18. Roy-Camille R, Saillant G, Mazel C (1986) Plating of thoracic, thoracolum- bar, and lumbar injuries with pedicle screw plates. Orthop Clin Orth Am 17 : 147-159 19. Saillant G (1976) Etude anatomique des pedicules vertebraux. Application chirugicale. Rev Chir Orthop 62: 151- 160 20. Scoles PV, Linton AE, Latimer B, Levy ME, Digiovanni BF (1988) Ver- tebral body and posterior element mor- phology. The normal spine in middle life. Spine 13 : 1082-1086 21. Steinmann JC, Herkowitz HN, E1- Kommos H, Wesolowski DP (1993) Spinal pedicle fixation. Confirmation of an image-based technique for screw placement. Spine 18:1856-1861 22. Vaccaro AR, Rizzolo S J, Allardyce TJ, Ramsey M, Salvo J, Balderston RA, Cotler JM (1995) Placement of pedicle screws in the thoracic spine. J Bone Joint Surg [Am] 77 : 1193-1199 23. Weinstein JN, Spratt KF, Spengler D, Brick C, Reid S (1988) Spinal pedicle fixation: reliability and validity of roentgenogram-based assessment and surgical factors on successful screw placement. Spine 13 : 1012-1018 24. Yuan HA, Garfin SR, Dickman CA, Mardjetko SM (1994) A historical co- hort study of pedicle screw fixation in thoracic, lumbar, and sacral spinal fu- sions. Spine 19 : $2279-$2296 25. Zindrick MR, Wiltse LL, Doornik A, Widell EH, Knight GW, Patwardhan AG, Thomas JC, Rothman SL, Fields BT (1987) Analysis of the morphomet- ric characteristics of the thoracic and lumbar pedicles. Spine 12:160-166
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