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The Journal of Bone & Joint Surgery, Volume 84, Issue 3

Scientific Article | March 01, 2002

Analysis of Vertebral Morphology in Idiopathic Scoliosis with Use of Magnetic Resonance Imaging and Multiplanar Reconstruction

Ulf R. Liljenqvist, MD; Thomas Allkemper, MD; Lars Hackenberg, MD; Thomas M. Link, MD; Jörn Steinbeck, MD; Henry F.H. Halm, MD

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Investigation performed at the Department of Orthopaedics, Universitätsklinikum Münster, Germany

Ulf R. Liljenqvist, MD
Thomas Allkemper, MD
Lars Hackenberg, MD
Jörn Steinbeck, MD
Departments of Orthopaedics (U.R.L., L.H., and J.S.) and Clinical Radiology (T.A.), Universitätsklinikum Münster, Albert-Schweitzer-Strasse 33, 48149 Münster, Germany. E-mail address for U.R. Liljen- qvist: liljenqv@uni-muenster.de

Thomas M. Link, MD
Department of Radiology, Technische Universität München, Ismaninger Strasse 22, 81675 München, Germany

Henry F.H. Halm, MD
Department of Spinal Surgery, Klinikum Neustadt, Am Kiebitzberg 10, 23730 Neustadt, Germany

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

The Journal of Bone & Joint Surgery. 2002; 84:359-368

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Abstract

Background: Several studies have provided data on the vertebral morphology of normal spines, but there is a paucity of data on the vertebral morphology in patients with idiopathic scoliosis.

Methods: The morphology of the pedicles and bodies of 307 vertebrae as well as the distance between the pedicles and the dural sac (the epidural space) in twenty-six patients with right-sided thoracic idiopathic scoliosis were analyzed with use of magnetic resonance imaging and multiplanar reconstruction.

Results: A distinct vertebral asymmetry was found at the apical region of the thoracic curves, with significantly thinner pedicles on the concave side than on the convex side (p < 0.05). The degree of intravertebral deformity diminished farther away from the apex, with vertebral symmetry restored at the neutral level. In the thoracic spine, the transverse endosteal width of the apical pedicles measured between 2.3 mm and 3.2 mm on the concave side and between 3.9 mm and 4.4 mm on the convex side (p < 0.05). In the lumbar spine, the pedicle width measured between 4.6 mm at the cephalad part of the curve and 7.9 mm at the caudad part of the curve. The chord length and the pedicle length gradually increased from 34 mm and 18 mm, respectively, at the fourth thoracic vertebra to 51 mm and 25 mm, respectively, at the third lumbar vertebra. The transverse pedicle angle measured 15Â° in the cephalad aspect of the thoracic spine, decreased to 7Â° at the twelfth thoracic vertebra, and increased again to 16Â° at the fourth lumbar vertebra. The width of the epidural space was <1 mm at the thoracic apical vertebral levels and averaged 1 mm at the lumbar apical vertebral levels on the concave side, whereas it was between 3 mm and 5 mm on the convex side (p < 0.05).

Conclusion: Idiopathic scoliosis is associated with distinctive intravertebral deformity, with smaller pedicles on the concave side and a shift of the dural sac toward the concavity.

Clinical Relevance: Care must be exercised during pedicle-screw instrumentation, especially in the apical region of the concavity of thoracic curves, because of the small pedicle width and the limited epidural safe zone in this area. Surgeons should be aware of these altered conditions when considering pedicle-screw instrumentation for patients with thoracic scoliosis.

Figures in this Article

Introduction

Pedicle screws have become widely accepted as an invaluable part of spinal instrumentation. Because of their superior biomechanical properties and their position outside the spinal canal, pedicle screws have gained popularity even for the surgical treatment of scoliosis^1-6. Several clinical studies have demonstrated better curve correction, less loss of correction, and a shorter fusion length when pedicle screws were used instead of hooks or wires in the treatment of idiopathic scoliosis^4,7-12. However, a detailed knowledge of the vertebral morphology is mandatory to ensure safe pedicle-screw instrumentation. Numerous investigators have analyzed the vertebral morphology of normal spines in both cadaver and clinical studies and have provided clinically relevant data with respect to pedicle-screw instrumentation^13-19. However, there is a paucity of reports on the vertebral morphology in patients with idiopathic scoliosis. Cadaver studies on a few single apical vertebrae from scoliotic spines have demonstrated intravertebral deformity and asymmetrical growth of scoliotic vertebrae but have not provided exact morphometric data^20,21. In a previous study in which postoperative computed tomographic scans were used to analyze 337 pedicles in twenty-nine patients with thoracic idiopathic scoliosis, we found a substantial intravertebral deformity, with significantly smaller pedicles on the concave side of the curve (p < 0.05)²². However, computed axial tomography allows an alignment of the gantry only in the sagittal plane and not in the frontal plane. Thus, all vertebrae except for the apical ones are transected obliquely to a different extent in the axial plane, which may impair the accuracy of the measurements. The aim of the present study was to analyze the vertebral morphology in patients with idiopathic scoliosis with use of magnetic resonance imaging with true transverse and frontal plane reconstruction.

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Fig. 1-A:: Individually adjusted planes of reconstruction were used to allow a true transverse and frontal section of each vertebra. Figs. 1-A and 1-B Transverse plane reconstruction.

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Fig. 1-B:: Individually adjusted planes of reconstruction were used to allow a true transverse and frontal section of each vertebra. Figs. 1-A and 1-B Transverse plane reconstruction.

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Fig. 1-C:: Frontal plane reconstruction.

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Fig. 2:: Illustration of a thoracic (left) and a lumbar (right) vertebra, demonstrating the measurements of the chord length (AC), the pedicle length (AB), the pedicle width (DE), and the transverse pedicle angle (F).

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Fig. 3:: Illustration of the measurement of the width of the pedicle-rib unit (AB).

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Fig. 4:: Illustration of the measurements of the width of the epidural space on the convex (aa’) and concave (bb’) sides of the curve and the distance between the vertebral body and the aorta (cc’). The gray area is the dural sac. A = aorta.

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Fig. 5:: Illustration of different pedicle shapes: round (A), oval (B), kidney-shaped (C), teardrop-shaped (D), and reverse teardrop-shaped (E).

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Fig. 6-A:: True transverse section through an apical thoracic vertebra in a patient with a King type-II curve. The pedicles are asymmetrical, with a much thinner pedicle on the concave side, and the dural sac is shifted toward the concavity. A = aorta, and D = dural sac.

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Fig. 6-B:: True transverse section through an apical thoracic vertebra in a patient with a King type-II curve. The pedicles are asymmetrical, with a much thinner pedicle on the concave side, and the dural sac is shifted toward the concavity. A = aorta, and D = dural sac.

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Fig. 7:: Illustration of the width of the epidural space on the concave and the convex sides of the curve as measured in the transverse plane. Significant differences were noted between the concave and convex sides at all levels (p < 0.05) except the fourth and fifth thoracic and fourth lumbar levels (T4, T5, and L4).

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Fig. 8-A:: Frontal section through the apex of a thoracic curve, demonstrating the asymmetrical pedicles and the shift of the dural sac toward the concavity. Note the extremely thin pedicles on the concave side. R = ribs, P = pedicles, and S = subarachnoidal space.

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Fig. 8-B:: Frontal section through the apex of a thoracic curve, demonstrating the asymmetrical pedicles and the shift of the dural sac toward the concavity. Note the extremely thin pedicles on the concave side. R = ribs, P = pedicles, and S = subarachnoidal space.

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TABLE I: Average Vertebral Rotation and Distribution of Apical Vertebrae

*The values are given in degrees, according to the system of Perdriolle and Vidal²⁵, with the range in parentheses.

Level	Average Rotation*	No. of Apical Vertebrae
T4	?0	?0
T5	?2.1 (0-8)	?0
T6	10.1 (0-28)	?0
T7	20.3 (5-35)	?3
T8	24.0 (8-35)	14
T9	21.9 (0-38)	?9
T10	16.0 (0-32)	?0
T11	?8.6 (0-30)	?0
T12	10.3 (0-40)	?0
L1	13.2 (0-50)	?3
L2	17.3 (0-50)	14
L3	12.9 (0-38)	?9
L4	?4.8 (0-20)	?0

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TABLE I: Average Vertebral Rotation and Distribution of Apical Vertebrae

*The values are given in degrees, according to the system of Perdriolle and Vidal²⁵, with the range in parentheses.

Level	Average Rotation*	No. of Apical Vertebrae
T4	?0	?0
T5	?2.1 (0-8)	?0
T6	10.1 (0-28)	?0
T7	20.3 (5-35)	?3
T8	24.0 (8-35)	14
T9	21.9 (0-38)	?9
T10	16.0 (0-32)	?0
T11	?8.6 (0-30)	?0
T12	10.3 (0-40)	?0
L1	13.2 (0-50)	?3
L2	17.3 (0-50)	14
L3	12.9 (0-38)	?9
L4	?4.8 (0-20)	?0

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TABLE II: Compiled Data on Pedicle Width

*The measurements were made in female subjects. †The measurements were made in twelve to seventeen-year-old subjects. ‡The measurements were made with use of circular sounds of graduated diameter. §The measurements were made on the convex side of the curve in patients with idiopathic scoliosis.

Study	Vaccaro et al.¹⁸	Cinotti et al.¹⁴	Scoles et al.¹⁷	Ebraheim et al.^15,33	Zindrick et al.¹⁹	Zindrick et al.³⁸	Banta et al.³⁷	Liljenqvist et al.²²	Present Study
Pedicle width evaluated (outer or inner cortical width)	Outer Cortical Width	Outer Cortical Width	Outer Cortical Width*	Outer Cortical Width*	Not specified	Not specified†	Inner Cortical Width	Inner Cortical Width§	Inner Cortical Width§
Measurement technique	Calipers	Calipers	Calipers	Calipers	Computed tomography	Computerized video analysis	Manual‡	Computed tomography	Magnetic resonance imaging
Pedicle width (mm)
T4	4.5	4.8		?3.8	?4.7	4.1			2.5
T5	4.4	4.8		?4.0	?4.5	3.9		3.7	3.0
T6	4.6	4.3	3.0	?4.0	?5.2	4.1	4.8		3.6
T7	4.9	4.7		?4.6	?5.3	4.3	4.8	4.1	4.0
T8	5.1	4.7		?4.6	?5.9	4.5	4.9	4.2	4.2
T9	5.8	5.6	4.1	?5.5	?6.1	5.0	5.0	4.2	4.4
T10	6.7	6.0		?6.0	?6.3	5.9	5.2	5.0	5.1
T11	8.0	7.2		?8.8	?7.8	7.0	5.4	5.7	6.2
T12	7.8	7.8	7.2	?9.4	?7.1	7.2	5.5	5.9	6.0
L1			6.5	?7.9	?8.7	5.9	5.5	5.1	4.9
L2				?7.5	?8.9	6.1	5.6	4.8	4.9
L3			7.9	?9.0	10.3	7.5	5.7	7.0	6.6
L4				10.6	12.9	9.7	5.9	9.4	8.1

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TABLE II: Compiled Data on Pedicle Width

Study	Vaccaro et al.¹⁸	Cinotti et al.¹⁴	Scoles et al.¹⁷	Ebraheim et al.^15,33	Zindrick et al.¹⁹	Zindrick et al.³⁸	Banta et al.³⁷	Liljenqvist et al.²²	Present Study
Pedicle width evaluated (outer or inner cortical width)	Outer Cortical Width	Outer Cortical Width	Outer Cortical Width*	Outer Cortical Width*	Not specified	Not specified†	Inner Cortical Width	Inner Cortical Width§	Inner Cortical Width§
Measurement technique	Calipers	Calipers	Calipers	Calipers	Computed tomography	Computerized video analysis	Manual‡	Computed tomography	Magnetic resonance imaging
Pedicle width (mm)
T4	4.5	4.8		?3.8	?4.7	4.1			2.5
T5	4.4	4.8		?4.0	?4.5	3.9		3.7	3.0
T6	4.6	4.3	3.0	?4.0	?5.2	4.1	4.8		3.6
T7	4.9	4.7		?4.6	?5.3	4.3	4.8	4.1	4.0
T8	5.1	4.7		?4.6	?5.9	4.5	4.9	4.2	4.2
T9	5.8	5.6	4.1	?5.5	?6.1	5.0	5.0	4.2	4.4
T10	6.7	6.0		?6.0	?6.3	5.9	5.2	5.0	5.1
T11	8.0	7.2		?8.8	?7.8	7.0	5.4	5.7	6.2
T12	7.8	7.8	7.2	?9.4	?7.1	7.2	5.5	5.9	6.0
L1			6.5	?7.9	?8.7	5.9	5.5	5.1	4.9
L2				?7.5	?8.9	6.1	5.6	4.8	4.9
L3			7.9	?9.0	10.3	7.5	5.7	7.0	6.6
L4				10.6	12.9	9.7	5.9	9.4	8.1

Materials and Methods

Prior to the clinical study, the appropriate imaging parameters were established on the basis of an evaluation of six healthy volunteers. Acquisition time was kept to a minimum to minimize motion artifacts. Magnetic resonance imaging examinations were per-formed with use of a 1.5-tesla Magnetom Vision unit (Siemens AG, Erlangen, Germany) and a three-dimensional gradient-echo sequence (echo time, 4 msec; repetition time, 40 msec; flip angle, 40Â°). A sagittal slab with an examination volume of 450 250 80 mm was acquired in seventeen minutes and forty-eight seconds, and seventy-four slices with a thickness of 1.08 mm were reconstructed. An effective voxel size of 0.88 0.69 1.08 mm resulted from an image matrix of 512 360 pixels. These imaging parameters enabled optimal spatial resolution and high contrast between bone and soft tissue.

To optimize the examination, acquisition volumes were adapted to the individual spinal curvature. A sagittally oriented saturation pulse with a thickness of 100 mm was used to reduce respiration and motion artifacts. Multiplanar reconstructions (Figs. 1-A, 1-B, and 1-C) were calculated from these three-dimensional data sets on an online workstation with use of special analysis software (EasyVision; Philips Medical Systems, Hamburg, Germany) that allowed digital measurements of distances and angles with a precision of 0.1 mm and 0.1Â°, respectively.

Twenty-six consecutive patients with right-sided thoracic idiopathic scoliosis who were scheduled for surgical correction and arthrodesis between December 1998 and January 2000 were included in the study. Sixteen curves were classified as type II; nine, as type III; and one, as type IV, according to the system of King et al.²³. Patients with double-major (type-I), thoracolumbar, or lumbar curves were excluded in order to achieve a homogenous study population of comparable primary thoracic curves, since previous studies have shown that intravertebral deformity is maximal at the apex of the scoliotic curve^20,22,24. The average vertebral rotation according to the system of Perdriolle and Vidal25 and the distribution of the apical vertebrae are listed in Table I .

A total of 307 vertebrae (614 pedicles) with proper reconstruction on magnetic resonance imaging were investigated. The most frequent end-vertebrae of the thoracic curves were the fifth and the eleventh thoracic vertebrae, and the most frequent end-vertebrae of the secondary lumbar curves were the twelfth thoracic and the fourth lumbar vertebrae. The average Cobb angle26 was 66Â° (range, 50 to 108Â°) for the thoracic curve and 46Â° (range, 28 to 86Â°) for the secondary lumbar curve. Twenty-one female and five male patients with an average age of 15.4 years (range, twelve to twenty-one years) were examined (Appendix).

The measurements in the transverse plane included chord length, transverse endosteal pedicle width, transverse pedicle angle, and pedicle length as described in the study by Vaccaro et al.18 (Fig. 2). The chord length was measured as the distance between the posterior cortical entry point of the pedicle and the anterior vertebral cortex in line with the axis of the pedicle. The transverse pedicle width was measured as the endosteal width at the narrowest part of the pedicle. The transverse pedicle angle was measured as the angle between a line perpendicular to the transverse isthmus and a sagittal midvertebral line. The pedicle length was measured as the distance from the posterior cortical entry point of the pedicle to the posterior longitudinal ligament along the axis of the pedicle.

The width of the pedicle-rib unit was measured as the distance between the medial pedicle wall and the lateral cortex of the rib head (Fig. 3). Furthermore, the width of the epidural space on the concave and convex sides of the curve, the closest distance from the aorta to the vertebral body, and the topographic relations were recorded at every level (Fig. 4). In the frontal plane, the height and width as well as the shape of the pedicle were analyzed. The pedicle shape was classified as round, oval, kidney-shaped, teardrop-shaped, or reverse teardrop-shaped (Fig. 5).

Statistical analysis was performed with use of StatView 5.0 software (SAS Institute, Cary, North Carolina). The parameters on the concave and convex sides of each curve were separately analyzed (i.e., the parameters on the convex side of the thoracic curves are always right-sided, and the parameters on the convex side of the lumbar curves are always left-sided). The Wilcoxon matched-pairs signed-rank test was applied to compare the parameters on the concave and convex sides. The Pearson correlation coefficient was used to evaluate the relationship between the degree of intravertebral deformity and the Cobb angle and that between the degree of deformity and vertebral rotation. All statistical tests were performed at a 5% level of significance.

Results

Transverse Plane

The measurements of the transverse pedicle width, the transverse pedicle angle, the chord length, and the pedicle length are summarized in the Appendix. On the convex side, the transverse pedicle width increased gradually from 2.3 mm at the fourth thoracic vertebra to 7.9 mm at the fourth lumbar vertebra. No significant differences in pedicle width were found between the convex and concave sides of the secondary lumbar curves, with the numbers available. Separate analysis of the lumbar region did not reveal any significant differences among the King type-II, type-III, and type-IV curves (p > 0.05). In the apical region of the thoracic spine, however, the pedicles on the concave side (between 2.3 mm and 3.2 mm wide) were significantly thinner than those on the convex side (between 3.9 mm and 4.4 mm wide) (p < 0.05) (Figs. 6-A and 6-B), with symmetry being restored at the level of the end-vertebrae. Correlation analysis revealed no significant relationship between pedicle asymmetry (calculated as the difference between the pedicle widths on the concave and convex sides of the curve) and either the Cobb angle or vertebral rotation (p > 0.05). The transverse pedicle angle decreased from 15Â° at the cephalad aspect of the thoracic spine to 7Â° at the twelfth thoracic vertebra but increased again to 16Â° at the fourth lumbar vertebra. In the apical region of the thoracic spine, the transverse pedicle angles on the concave side were significantly greater than those on the convex side (p < 0.05). Both chord length and pedicle length increased gradually from the cephalad aspect of the thoracic spine (34 mm and 18 mm, respectively) to the caudad aspect of the lumbar spine (51 mm and 25 mm, respectively). At the apical region of both the thoracic and the lumbar spine, no significant differences in pedicle length or chord length were detected between the concave and convex sides of the curve (p > 0.05).

The average width of the pedicle-rib unit ranged from 11 mm in the cephalad aspect to 14 mm in the caudad aspect of the thoracic spine (Appendix). In the apical region, the pedicle-rib unit on the concave side was significantly wider than that on the convex side (p < 0.05). The width of the epidural space was <1 mm at the thoracic apical vertebral levels and an average of 1 mm at the lumbar levels on the concave side, whereas it was between 3 mm and 5 mm on the convex side (Fig. 7). Because of the shift of the dural sac toward the concavity (Figs. 6-A, 6-B, 8-A, and 8-B), the width of the epidural space on the concave side was significantly smaller than that on the convex side at all levels (p < 0.05) except the fourth and fifth thoracic and fourth lumbar levels.

The closest distance between the thoracic aorta and the vertebral body was an average of 6 mm at the fourth, fifth, and sixth thoracic vertebrae; 7 mm at the seventh, eighth, and ninth thoracic vertebrae; and 4 mm at the tenth thoracic vertebra. At the eleventh thoracic vertebra, the closest distance measured an average of 3 mm, and at the twelfth thoracic vertebra (and throughout the lumbar spine) it measured an average of 2 mm. Between the fourth and the eighth thoracic vertebrae, the thoracic aorta runs at the three o’clock position in reference to the axis of the vertebral body. It changes to the two o’clock position at the ninth thoracic vertebra, the one o’clock position at the eleventh thoracic vertebra, and the twelve o’clock position at the twelfth thoracic vertebra. In the lumbar spine, the aorta runs between the eleven and twelve o’clock positions.

Frontal Plane

The frontal plane measurements of pedicle width and height are summarized in the Appendix. In the apical region of the thoracic curve, the pedicles on the concave side were significantly thinner and lower than those on the convex side (p < 0.05) (Figs. 8-A and 8-B). The pedicles were predominantly oval or kidney-shaped with a medially directed convexity in the thoracic spine and cephalad aspect of the lumbar spine. Of a total of 410 thoracic pedicles, twenty-five had a teardrop shape and eight had a reverse teardrop shape. The pedicles of the first lumbar vertebra were predominantly oval, and the pedicles of the middle and caudad parts of the lumbar spine were round.

Intraobserver Variability

The intraobserver variability was calculated by means of five repeated measurements of pedicle width, pedicle length, chord length, and pedicle angle in the transverse plane as well as pedicle width and height in the frontal plane in one patient at two to three-month intervals. The intraobserver error (and 95% confidence limits) measured 0.3 mm (0.1 to 0.6 mm) for pedicle width in the transverse plane, 0.9 mm (0.3 to 1.7 mm) for pedicle length, 1.1 mm (0.5 to 2.0 mm) for chord length, 1 (0.4 to 1.9Â°) for the pedicle angle, 0.3 mm (0.1 to 0.6 mm) for pedicle width in the frontal plane, and 0.7 mm (0.3 to 1.3 mm) for pedicle height.

Discussion

Athorough knowledge of vertebral morphology is mandatory to ensure safe pedicle-screw placement. Screw misplacement is associated with a risk of injury to neural as well as vascular and visceral structures27-30, and it affects the screw-bone interface strength30-32. Several studies on the vertebral morphology of normal spines have provided surgically relevant data15,17-19,33. In the present study, magnetic resonance imaging was used to study the vertebral morphology in patients with idiopathic scoliosis.

A substantial intravertebral deformity, which was greatest in the apical region of the thoracic curves, was found in the present study (Figs. 6-A and 6-B). Both the frontal and horizontal measurements demonstrated significantly thinner pedicles (average endosteal width, <3 mm), and the frontal sections demonstrated significantly lower pedicles on the concave side of the thoracic curves (Figs. 8-A and 8-B). The degree of intravertebral deformity diminished farther away from the apex, with symmetrical vertebrae at the neutral level. This observation is similar to that described in other studies on the vertebral morphology in patients with scoliosis21,34, on single apical vertebrae in scoliotic skeletons20,24,35, and on animals with experimentally induced scoliosis^20,36. The authors of those studies reported a consistent pattern of intravertebral deformity, with considerably thinner pedicles on the concave side than on the convex side, but they did not provide exact morphometric data.

In the lumbar spine, the differences between concave and convex sides were minimal, which may reflect the secondary nature of the lumbar curves that were included in the present study. Separate analysis of the lumbar curves did not reveal any morphometric differences between the patients with type-II scoliosis and those with type-III and IV scoliosis. In the thoracolumbar and lumbar spine, the smallest pedicle width was found at the first lumbar level.

Our measurements of the endosteal pedicle width on the convex side of the curve were consistently smaller than the measurements of pedicle width that have been reported in previous studies on the morphology of normal spines^{14,15,17-19,33,37,38}, with differences of 1 to 2 mm in the thoracic region and of 2 to 3 mm in the lumbar region (Table II). Some investigators have measured the outer cortical pedicle width^14,15,17,18, and Zindrick et al.¹⁹ did not differentiate between inner and outer cortical width. Other investigators have measured the endosteal pedicle width as the effective pedicle diameter^22,37. In the present study, we measured the endosteal pedicle width because we believe that it corresponds more accurately to the effective pedicle diameter with respect to pedicle-screw instrumentation than the outer cortical width of the pedicle does^22,37,39,40. The thickness of the cortex of the thoracic pedicles measures approximately 0.5 to 1.5 mm, with the medial wall being two to three times thicker than the lateral wall^41,42.

The accuracy of magnetic resonance imaging for measurements of pedicle width may be influenced by the increased susceptibility effect between cortical and cancellous bone, which results in a less clear depiction of the cortical structures compared with that seen on computed tomography. This phenomenon can lead to a slight underestimation of the endosteal pedicle width on magnetic resonance imaging.

The data on pedicle width in the present study compare well with those from our previous study, in which computed tomography scans were used to analyze vertebral morphology in patients with scoliosis who had had an operation²² (Table II). In the study by O’Brien et al.⁴³, preoperative computed tomography scans of the chest were used to analyze 512 thoracic pedicles in twenty-nine patients with idiopathic scoliosis. The outer cortical width of the pedicle ranged from 4.6 mm to 8.5 mm, without any significant differences between the concave and convex sides. However, the curves were investigated preoperatively and therefore all vertebrae except for the apical ones were transected obliquely, since the gantry can only be aligned in the sagittal plane with the patient supine. Xiong et al.²⁴ found that a vertebral tilt of 10Â° in the frontal plane substantially influenced the accuracy of measurements of the pedicle width in scoliotic vertebrae.

As has been demonstrated in previous studies^15,18,19, the transverse pedicle angle increases in the cephalad and middle parts of the thoracic spine, decreases toward the thoracolumbar junction, and increases again in the middle and caudad aspects of the lumbar spine. In the apical region of the thoracic curves, the pedicles on the concave side were found to be significantly more angulated than those on the convex side, which can be attributed to the intravertebral rotation of the scoliotic vertebrae.

Penetration of the medial wall of the pedicle by a screw does not always result in neurological symptoms. Gertzbein and Robbins⁴⁴ analyzed the accuracy of pedicle-screw placement between the tenth thoracic and the fourth lumbar vertebrae in forty consecutive patients and found no neurological symptoms even when a screw had penetrated the medial border of the pedicle by as much as 4 mm. They described the region between 0 and 4 mm as an epidural safe zone. Other investigators⁴⁵ have reported an epidural safe zone of 2 to 3 mm. In patients with idiopathic scoliosis, however, the dural sac is shifted toward the concavity and the contained neural structures are in direct proximity to the medial wall of the pedicles on that side. Thus, any violation of the medial pedicular wall by a misplaced screw might lead to neurological complications.

Penetration of the anterior vertebral cortex can endanger the aorta. In the present study, the closest distance between the aorta and the vertebral body measured 6 to 7 mm between the fourth and ninth thoracic vertebrae and <5 mm between the tenth thoracic and the fourth lumbar vertebrae. Vaccaro et al.²⁸ reported that the distance between the thoracic aorta and the vertebral body was <5 mm between the sixth and twelfth thoracic vertebrae. Our data on the topographic relationship of the thoracic aorta to the vertebral body compare well with those reported by Vaccaro et al.²⁸. The aorta runs anterolaterally (to the left) in the cephalad and middle aspects of the thorax, changes to a vertebral midline position in the caudad part of the thorax, and remains there throughout the lumbar region. Therefore, any left-sided anterolateral penetration of the cortex by a pedicle screw in the cephalad and middle aspects of the thoracic spine and any anterior penetration of the cortex by a pedicle screw in the caudad aspect of the thoracic spine and in the lumbar spine mi7ght endanger the aorta.

In conclusion, the findings in the present study indicate that the vertebral morphology in patients with thoracic idiopathic scoliosis is substantially different from that in patients with normal spines. Specifically, patients with thoracic idiopathic scoliosis have a distinct asymmetrical intravertebral deformity that is maximal at the apical region of the curve. Care should be exercised during pedicle-screw instrumentation in the apical region of the concavity of thoracic curves, as the spinal cord may be jeopardized by the small endosteal pedicle width and the shift of the dural sac toward the concavity. Surgeons should be aware of these altered conditions when considering pedicle-screw instrumentation for the treatment of thoracic scoliosis.

Note: The authors thank Randal R. Betz, MD, Shriners Hospitals for Children, Philadelphia, Pennsylvania, for his support and critical analysis of the manuscript. They also thank Dr. Dieter Rosenbaum, PhD, for his help with the statistical analysis.

Appendix

Tables listing the measurements in each patient, the morphometric measurements of the vertebrae in the transverse and frontal planes, the width of the pedicle-rib units, and the compiled data on pedicle width are available with the electronic versions of this article, on our web site at www.jbjs.org (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM).

References

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Fig. 1-A:: Individually adjusted planes of reconstruction were used to allow a true transverse and frontal section of each vertebra. Figs. 1-A and 1-B Transverse plane reconstruction.

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Fig. 1-B:: Individually adjusted planes of reconstruction were used to allow a true transverse and frontal section of each vertebra. Figs. 1-A and 1-B Transverse plane reconstruction.

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Fig. 1-C:: Frontal plane reconstruction.

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Fig. 3:: Illustration of the measurement of the width of the pedicle-rib unit (AB).

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Fig. 5:: Illustration of different pedicle shapes: round (A), oval (B), kidney-shaped (C), teardrop-shaped (D), and reverse teardrop-shaped (E).

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TABLE I: Average Vertebral Rotation and Distribution of Apical Vertebrae

*The values are given in degrees, according to the system of Perdriolle and Vidal²⁵, with the range in parentheses.

Level	Average Rotation*	No. of Apical Vertebrae
T4	?0	?0
T5	?2.1 (0-8)	?0
T6	10.1 (0-28)	?0
T7	20.3 (5-35)	?3
T8	24.0 (8-35)	14
T9	21.9 (0-38)	?9
T10	16.0 (0-32)	?0
T11	?8.6 (0-30)	?0
T12	10.3 (0-40)	?0
L1	13.2 (0-50)	?3
L2	17.3 (0-50)	14
L3	12.9 (0-38)	?9
L4	?4.8 (0-20)	?0

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TABLE I: Average Vertebral Rotation and Distribution of Apical Vertebrae

*The values are given in degrees, according to the system of Perdriolle and Vidal²⁵, with the range in parentheses.

Level	Average Rotation*	No. of Apical Vertebrae
T4	?0	?0
T5	?2.1 (0-8)	?0
T6	10.1 (0-28)	?0
T7	20.3 (5-35)	?3
T8	24.0 (8-35)	14
T9	21.9 (0-38)	?9
T10	16.0 (0-32)	?0
T11	?8.6 (0-30)	?0
T12	10.3 (0-40)	?0
L1	13.2 (0-50)	?3
L2	17.3 (0-50)	14
L3	12.9 (0-38)	?9
L4	?4.8 (0-20)	?0

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TABLE II: Compiled Data on Pedicle Width

Study	Vaccaro et al.¹⁸	Cinotti et al.¹⁴	Scoles et al.¹⁷	Ebraheim et al.^15,33	Zindrick et al.¹⁹	Zindrick et al.³⁸	Banta et al.³⁷	Liljenqvist et al.²²	Present Study
Pedicle width evaluated (outer or inner cortical width)	Outer Cortical Width	Outer Cortical Width	Outer Cortical Width*	Outer Cortical Width*	Not specified	Not specified†	Inner Cortical Width	Inner Cortical Width§	Inner Cortical Width§
Measurement technique	Calipers	Calipers	Calipers	Calipers	Computed tomography	Computerized video analysis	Manual‡	Computed tomography	Magnetic resonance imaging
Pedicle width (mm)
T4	4.5	4.8		?3.8	?4.7	4.1			2.5
T5	4.4	4.8		?4.0	?4.5	3.9		3.7	3.0
T6	4.6	4.3	3.0	?4.0	?5.2	4.1	4.8		3.6
T7	4.9	4.7		?4.6	?5.3	4.3	4.8	4.1	4.0
T8	5.1	4.7		?4.6	?5.9	4.5	4.9	4.2	4.2
T9	5.8	5.6	4.1	?5.5	?6.1	5.0	5.0	4.2	4.4
T10	6.7	6.0		?6.0	?6.3	5.9	5.2	5.0	5.1
T11	8.0	7.2		?8.8	?7.8	7.0	5.4	5.7	6.2
T12	7.8	7.8	7.2	?9.4	?7.1	7.2	5.5	5.9	6.0
L1			6.5	?7.9	?8.7	5.9	5.5	5.1	4.9
L2				?7.5	?8.9	6.1	5.6	4.8	4.9
L3			7.9	?9.0	10.3	7.5	5.7	7.0	6.6
L4				10.6	12.9	9.7	5.9	9.4	8.1

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TABLE II: Compiled Data on Pedicle Width

Study	Vaccaro et al.¹⁸	Cinotti et al.¹⁴	Scoles et al.¹⁷	Ebraheim et al.^15,33	Zindrick et al.¹⁹	Zindrick et al.³⁸	Banta et al.³⁷	Liljenqvist et al.²²	Present Study
Pedicle width evaluated (outer or inner cortical width)	Outer Cortical Width	Outer Cortical Width	Outer Cortical Width*	Outer Cortical Width*	Not specified	Not specified†	Inner Cortical Width	Inner Cortical Width§	Inner Cortical Width§
Measurement technique	Calipers	Calipers	Calipers	Calipers	Computed tomography	Computerized video analysis	Manual‡	Computed tomography	Magnetic resonance imaging
Pedicle width (mm)
T4	4.5	4.8		?3.8	?4.7	4.1			2.5
T5	4.4	4.8		?4.0	?4.5	3.9		3.7	3.0
T6	4.6	4.3	3.0	?4.0	?5.2	4.1	4.8		3.6
T7	4.9	4.7		?4.6	?5.3	4.3	4.8	4.1	4.0
T8	5.1	4.7		?4.6	?5.9	4.5	4.9	4.2	4.2
T9	5.8	5.6	4.1	?5.5	?6.1	5.0	5.0	4.2	4.4
T10	6.7	6.0		?6.0	?6.3	5.9	5.2	5.0	5.1
T11	8.0	7.2		?8.8	?7.8	7.0	5.4	5.7	6.2
T12	7.8	7.8	7.2	?9.4	?7.1	7.2	5.5	5.9	6.0
L1			6.5	?7.9	?8.7	5.9	5.5	5.1	4.9
L2				?7.5	?8.9	6.1	5.6	4.8	4.9
L3			7.9	?9.0	10.3	7.5	5.7	7.0	6.6
L4				10.6	12.9	9.7	5.9	9.4	8.1