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Relationship between vertebral morphology and the potential risk of spinal cord injury by pedicle screw in adolescent idiopathic scoliosis
Accepted Manuscript Title: Relationship between vertebral morphology and the potential risk of spinal cord injury by pedicle screw in adolescent idiopathic scoliosis Authors: Masashi Miyazaki, Toshinobu Ishihara, Shozo Kanezaki, Naoki Notani, Tetsutaro Abe, Hiroshi Tsumura PII: DOI: Reference:
S0303-8467(18)30280-4 https://doi.org/10.1016/j.clineuro.2018.07.007 CLINEU 5096
To appear in:
Clinical Neurology and Neurosurgery
Received date: Revised date: Accepted date:
23-5-2018 4-7-2018 8-7-2018
Please cite this article as: Miyazaki M, Ishihara T, Kanezaki S, Notani N, Abe T, Tsumura H, Relationship between vertebral morphology and the potential risk of spinal cord injury by pedicle screw in adolescent idiopathic scoliosis, Clinical Neurology and Neurosurgery (2018), https://doi.org/10.1016/j.clineuro.2018.07.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Abstract Objective: We aimed to investigate the relative preoperative position of the spinal cord
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in AIS and explore the potential risk of spinal cord injury from placement of pedicle screws.
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Patients and Methods: Twenty-seven patients with a mean age of 15 ± 1.8 years
(range, 12–19 years) classified as having Lenke type 1 AIS (1A: 15 cases, 1B: 8 cases,
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1C: 4 cases) were analyzed. The mean Cobb angle of the main curve was 55.9 ± 14.4°.
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Axial CT myelography images were selected from the T4 to T12 vertebrae, and 243
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images were analyzed. Outer cortical pedicle width, inner cortical pedicle width, pedicle
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length, chord length, transverse pedicle angle, the angle of rotation (RAsag) of the vertebra, and the distance between the spinal cord and concave (Dc) and convex
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pedicles (Dv) were calculated from landmark locations.
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Results: The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12 (p<0.05) and smaller than
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the mean convex outer cortical pedicle width around the apex of the curve from T7 to T9 (p<0.05). The mean concave inner cortical pedicle width was larger than the mean convex inner cortical pedicle width at T4, T5, and T11 (p<0.05) and smaller than the
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mean convex inner cortical pedicle width around the apex of the curve at T7 and T8 (p<0.001). The mean Dc was smaller than the mean Dv around the apex of the curve
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from T6 to T11 (p<0.05). Dv was significantly correlated with the convex outer cortical pedicle width (R=0.286, p<0.001), convex inner cortical pedicle width (R=0.202,
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p=0.002), convex transverse pedicle angle (R=-0.286, p<0.001), and RAsag (R=0.277, p<0.001). Dc was significantly correlated with the concave outer (R=0.269, p<0.001)
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and inner cortical pedicle width (R=0.230, p<0.001).
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Conclusion: The distance from the spinal cord to the medial wall of the pedicle was
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significantly correlated with outer and inner cortical pedicle width, and the potential risk
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of spinal cord injury by pedicle screw is increased with insertion into a narrower
Highlights
Anatomic measurements revealed Dc was smaller than Dv around the apex of the curve.
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pedicle, especially on the concave side around the apex.
Dc was significantly correlated with the concave cortical pedicle width.
Dv was significantly correlated with the convex cortical pedicle width.
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Keywords: Scoliosis; Vertebral morphology; Computed tomography; Multiplanar reconstruction; Spinal cord injury; Thoracic spine
1. Introduction
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Adolescent idiopathic scoliosis (AIS) is a complex, three-dimensional deformity of the spine with lateral deviation in the coronal plane, alternation of
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kyphosis or lordosis in the sagittal plane, and rotation of the vertebrae in the axial plane. Pedicle screw fixation, a widely used surgical treatment for AIS, allows for segmental
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instrumentation in multiple vertebrae across a multilevel fusion area and provides strong pullout strength with the desired deformity correction [1-3]. Despite its advantages, its
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use at thoracic levels remains controversial owing to the relatively small pedicle
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dimensions and wide variation in morphologic features of the pedicles [4]. Screw
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misplacement may reduce pullout strength or lead to severe complications involving the nearby visceral, vascular, and neurologic structures [5-7]. Safe and reproducible
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placement of thoracic pedicle screws is generally dependent on a better understanding
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of abnormal anatomy in scoliosis. However, the incidence of screw misplacement has
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been reported to reach 43% [8, 9] when all the screws are evaluated by computed tomography (CT) postoperatively. Thoracic pedicle screw fixation is potentially risky
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because there is little space between the spinal cord and medial pedicle wall on the concave side of apex vertebrae. The incidence of screw-related neurologic complications during the treatment of spinal deformities with thoracic pedicle screws ranges from 0 to 0.9% [8, 9]. Mac-Thiong et al. [7] reported 9 cases of pedicle screws
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misplaced totally within the spinal canal during posterior surgery for AIS, with spinal canal intrusion ranging from 21–61%. They suggested that any pedicle screw misplaced
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within the spinal canal should be removed because of possible early or late neurological complications. Sarlak et al. [10] reported a 10.8% rate of medial pedicle screw
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misplacement in a study of 1797 screws in 148 scoliosis patients, with unacceptable
screw placement defined as medial violation greater than 2 mm. They suggested that the
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acceptability of medial pedicle breach may change at each level according to changes in
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canal width and cord shift.
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Although various anatomic studies on the unique characteristics of thoracic pedicles
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have been conducted [4,11,12], few have investigated the relationship between vertebral morphology and the distance from the spinal cord to the medial wall of the pedicle as a
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potential risk factor for spinal cord injury by a pedicle screw. The purpose of this study
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was to investigate the relative position of the spinal cord in AIS before surgery and explore the potential risk of spinal cord injury from the placement of pedicle screws.
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2. Materials and methods 2.1 Patient Demographic Data
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The present study included 27 patients with AIS classified as type 1 according to the Lenke classification [13]. The patients underwent pedicle screw fixation between
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August 2011 and November 2017. Criteria for patient selection were (1) undergoing careful screening to ensure that scoliosis was idiopathic and correctly classified as
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Lenke type 1 and (2) preoperative radiographs and CT myelography images were
available on file. Exclusion criteria were (1) proven or suspected congenital, muscular,
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neurologic, or hormonal causes of scoliosis and (2) clinical history of any condition that
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may have affected vertebral growth (e.g., history of cancer, vertebral abnormalities,
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muscular abnormalities, or neurologic conditions). We routinely obtained CT scans after myelography; all patients who underwent surgery during that period underwent CT
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myelography that was available for analysis. Twenty-seven patients (2 males and 25
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females) who met these criteria were treated during the period, with a mean age of 15 ±
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1.8 years (range, 12–19 years) classified as having Lenke type 1 AIS (1A: 15 cases, 1B: 8 cases, 1C: 4 cases). The mean Cobb angle of the main curve was 55.9 ± 14.4° (range,
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45°–103°). Patients’ mean height was 156.8 ± 6.5 cm (range, 145–171 cm), and their mean weight was 47.8 ± 7.3 kg (range, 31–63 kg).
2.2 Scanning Protocol
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Patients were placed in the prone position. After myelography, CT scans were obtained using a multislice scanner (Toshiba Aquilion 16, Toshiba Medical, Tochigi, Japan).
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Image data were obtained in 0.5-mm slices from the level of the occiput to S1. Each CT scan was opened with synchronized axial, coronal, and sagittal displays. The image
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contrast levels were standardized to enable clear soft tissue and bone demarcation at the vertebra. To measure the pedicle, the local axial viewing plane was adjusted to be
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parallel to the superior and inferior endplates of the vertebrae. When the superior and
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inferior endplate planes were not parallel owing to vertebral wedging, an orientation
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approximately halfway between (i.e., bisecting) the two endplate inclinations was selected. Vertebral morphology was measured on axial slices through the thinnest
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portion of the pedicle using reformatted slices closest to the middle of the pedicle in the
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craniocaudal dimension. Axial images were selected from the T4 to T12 vertebrae, and
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243 images were analyzed.
2.3 Measured parameters
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Measurements were performed using Picture Archiving and Communication System (PACS) software. Anatomic landmarks were identified and measured on each pedicle. The 3-D coordinates of 9 points (points A through I) were identified on each pedicle with the appropriate distances and angles between these 6
points (outer cortical pedicle width, inner cortical pedicle width, pedicle length, chord length, transverse pedicle angle) calculated from the landmark locations. Figure 1A
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showed the outer and inner cortical pedicle width. A CT myelography image of a thoracic vertebrae in the local axial plane shows the outer cortical pedicle width (AD)
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and the inner cortical pedicle width (BC), where A is the medial outer cortex margin, B is the medial inner cortex margin, C is the lateral inner cortex margin, and D is the
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lateral outer cortex margin. Figure 1B showed chord length, pedicle length, and
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transverse pedicle angle.A CT myelography image of a thoracic vertebra in the local
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axial plane shows chord length (EF), pedicle length (FG), and transverse pedicle angle (angle between EF and HI), where E is the anterior edge of the vertebral body along the
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pedicle axis, F is the posterior edge of the vertebra along the pedicle axis, G is a point in
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line with the posterior longitudinal ligament along the pedicle axis, H is the sagittal
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midvertebral line at the anterior aspect of the vertebral body, and I is the sagittal midvertebral line at the meeting of the laminae. These measurement definitions are
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similar to those described in the literature [4,11,12, 14]. Figure 2A showed the angle of rotation (RAsag). RAsag measured by using the angle between the junction of the laminae, the dorsal central aspect of the vertebral foramen, and the middle of the vertebral body and the sagittal plane. Figure 2B showed the distance from the spinal
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cord to the convex pedicle (Dv) and concave pedicle (Dc). Convex and concave refer to the axial direction of the pedicle on the convex and concave sides of the apex,
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respectively. Line a and line d are tangent to the medial walls of the pedicle. Lines b and c are tangent to the outer edge of the spinal cord. All lines are parallel to the pedicle
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direction. The vertical distance between lines a and b (Dv) and lines c and d (Dc)
represent the distance between the spinal cord and the medial wall of the pedicle on the
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convex and concave sides.
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Two independent observers measured each parameter. We investigated the reliability of
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the measurement techniques and observed good to excellent intra- and interobserver
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agreement for each parameter (kappa > 0.70). 2.4 Statistical analysis
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Between-group differences were evaluated using the Mann-Whitney U-test with a significance value of p < 0.05. Correlation coefficients were calculated to
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investigate the association between pedicle width and the distance between the spinal cord and medial wall of the pedicle. All analyses were performed using SPSS software
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(version 13; SPSS, Chicago, IL).
3. Results
3.1 Pedicle Anatomy
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The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12 (T4: 3.98 ± 0.64 mm vs. 2.67 ± 0.84 mm,
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p<0.001; T5: 3.87 ± 0.96 mm vs. 3.21 ± 0.91 mm, p = 0.012; T11: 6.90 ± 1.96 mm vs. 5.61 ± 1.36 mm, p<0.001; T12: 6.64 ± 1.53 mm vs. 5.88 ± 1.50 mm, p = 0.019). The
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mean concave outer cortical pedicle width was smaller than the mean convex outer
cortical pedicle width around the apex of the curve from T7 to T9 (T7: 3.20 ± 0.93 mm
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vs. 4.15 ± 0.75 mm, p <0.001; T8: 3.38± 0.90 mm vs. 4.22 ± 0.62 mm, p<0.001; T9:
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4.03 ± 0.91 mm vs. 4.36 ± 0.91 mm, p = 0.039) (Figure 3).
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The mean concave inner cortical pedicle width was larger than the mean convex inner
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cortical pedicle width at T4, T5, and T11 (T4: 1.83 ± 0.62 mm vs. 0.80 ± 0.43 mm, p <0.001; T5: 1.73 ± 0.80 mm vs. 1.09 ± 0.58 mm, p = 0.002; T11: 4.37 ± 1.97 mm vs.
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3.29 ± 1.43 mm, p = 0.009). The mean concave inner cortical pedicle width was smaller
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than the mean convex inner cortical pedicle width around the apex of the curve at T7 and T8 (T7: 1.15 ± 0.70 mm vs. 1.82 ± 0.63 mm, p <0.001; T8: 1.32 ± 0.59 mm vs. 1.89
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± 0.53 mm, p<0.001) (Figure 4).
The mean chord length on the concave side was larger than that on the convex side from T6 to T8 (T6: 37.30 ± 3.59 mm vs. 33.62 ± 3.42 mm, p<0.001; T7: 38.89 ± 3.51 mm vs. 34.89 ± 3.50 mm, p<0.001; T8: 38.97 ± 3.53 mm vs. 36.32 ± 3.34 mm, p<0.001) 9
(Figure 5). The mean pedicle length on the concave side was larger than the convex side at T6 and T7 (T6: 20.28 ± 2.14 mm vs. 17.84 ± 3.49 mm, p<0.001; T7: 20.82 ± 1.82
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mm vs. 17.78 ± 3.17 mm, p<0.001). The mean pedicle length on the concave side was smaller than that on the convex side at T10 and T11 (T10: 18.42 ± 2.09 mm vs. 20.10 ±
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2.18 mm, p<0.001; T11: 19.84 ± 2.49 mm vs. 21.34 ± 2.70 mm, p = 0.010) (Figure 6).
The mean concave transverse pedicle angle was larger than that on the convex side from
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T6 to T9 (T6: 11.37± 3.43° vs. 7.74 ± 3.47°, p<0.001; T7: 11.81 ± 3.88° vs. 7.89 ±
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3.79°, p<0.001; T8: 11.74 ± 3.46° vs. 7.63 ± 3.24°, p<0.001; T9: 10.89 ± 3.74° vs. 7.78°
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± 3.46°, p<0.001) (Table 1).
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3.2 The angle of rotation of the vertebra
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The mean RAsag increased in degree from T4 to T8 and decreased in degree from T8 to
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T12 (Table 2).
3.3 The distance between the spinal cord and concave and convex medial wall of
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pedicles
The mean Dc was smaller than the mean Dv around the apex of the curve from T6 to T11 (T6: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T7: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T8: 3.38± 0.90 mm vs. 4.22 ± 0.62 mm, p<0.001; T9: 4.03 ± 0.91
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mm vs. 4.36 ± 0.91 mm, p<0.001; T10: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T11: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p<0.001) (Figure 7).
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3.4 Correlation of Dc and Dv with outer and inner cortical pedicle width
Dv was significantly correlated with the convex outer cortical pedicle width (R=0.286,
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p<0.001), convex inner cortical pedicle width (R=0.202, p=0.002) (Figure 8A, 8B),
convex transverse pedicle angle (R=-0.286, p<0.001), and RAsag (R=0.277, p<0.001),
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but not with the chord length on the convex side or pedicle length on the convex side.
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Dc was significantly correlated with the concave outer cortical pedicle width (R=0.269,
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p<0.001) and concave inner cortical pedicle width (R=0.230, p<0.001) (Figure 9A, 9B),
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but not with the chord length on the concave side, pedicle length on the concave side, concave transverse pedicle angle, or RAsag.
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4. Discussion
Pedicle screw instrumentation has become a popular and widely accepted
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surgical technique for AIS patients. Although some newly developed navigation techniques may help surgeons to place pedicle screws more safely [15], it is still very important that spine surgeons have a clear knowledge of pedicle morphometric
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anatomy, especially in deformed spines, to optimize the starting point and trajectory of the screw path.
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Transverse pedicle width is an important factor that determines the diameter of screws that can be safely accommodated in a pedicle without breaching the medial or lateral
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medial cortex. Parent et al. examined the thoracic pedicle morphology of 325 cadaveric scoliotic vertebrae and found that pedicles located on the concavity of typical right
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thoracic curves were significantly thinner than those on the convex side, and the
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maximal mean difference was found around the apex [16]. Abul-Kasim et al. studied the
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pedicle width pattern in Lenke type 1 AIS patients and reported similar findings [17].
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Takeshita et al. also investigated the diameter, length, and direction of pedicle screws by multiplanar reconstruction of CT images from Japanese patients with various scoliotic
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spine conditions and found that a large proportion of thoracic pedicles were too small
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on the concave side to accept a 4-mm-diameter screw, with 37% of concave T3-T9 pedicles unable to hold a 4-mm-diameter screw even with 25% expansion [18]. In the
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present study, the narrowest mean inner and outer pedicle diameters were observed at T7 on the concave side, and the mean outer pedicle diameters from T4 to T8 on the concave side were less than 4 mm in diameter. These results indicate that the pedicles on the concave side of the main thoracic curve apex are substantially narrowed,
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highlighting the necessity of practicing caution when inserting pedicle screws on the concave side of the main thoracic curve.
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One possible solution may be to consider extrapedicular screw trajectories in the preoperative plan if pedicle screw insertion is judged to be difficult. Extrapedicular
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screw placement has been advocated as a safe and effective alternative to the thoracic transpedicular screw [19]. However, screws placed in an extrapedicular position have
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about 75% of the pullout failure load of those placed in a transpedicular position [20].
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Although the powerful corrective force and maintenance of segmental pedicle screws is
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attractive, surgeons should not always use pedicle screws.
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Chord length is another important measurement in preventing anterior cortex
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perforation. Inappropriate pedicle screw length can be more hazardous than
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inappropriate diameter. A concave screw advanced too anteriorly or too laterally poses a
potential risk of aorta injury. Vaccaro et al. analyzed nonscoliotic thoracic spine and
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found that the aorta and esophagus are at greatest risk of injury when a pedicle screw
penetrates the anterior cortex of a vertebral body [21]. Considering the lateral force
exerted during a correction maneuver, special attention should be given to screw
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placement on the concave side. The present study demonstrated that at the apical region
(T6-T8), chord length was significantly longer on the concave side than on the convex
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side, which suggests that the concave side at the apex region may be able to
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accommodate a slightly longer pedicle screw. Similar findings have been reported in
other studies of scoliotic spine [12,18], which may be explained by the fact that with
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rotation of the scoliotic spine, the vertebral body drifts toward the concavity in the
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transverse plane, resulting in the slightly longer chord length on the concave side over
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the apical region [22].
Transverse pedicle angle is also an important anatomical parameter related to
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pedicle screw insertion. In normal spine, decreased pedicle angles have been reported at
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the thoracolumbar junction [23]. In the present study, we found that the transverse pedicle angle was significantly increased on the concave side at the apical region (T6-
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T9), which could be due to the intravertebral deformation that develops with rotation of the scoliotic spine. There was also a decreased transverse pedicle angle at the thoracolumbar junction, and the smallest angulations were at the thoracolumbar junction (T11 and T12).
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In the present study, the distance from the spinal cord to the medial wall of the pedicle on the concave side was significantly smaller than that on the convex side from T6 to
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T11. There was a smaller space between the medial wall of the pedicle and the spinal cord on the concave side of the apical region. Therefore, a misplaced pedicle screw
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would be more dangerous to the spinal cord on the concave side than on the convex
side. Sarwahi et al. investigated pedicles in spines with AIS and normal cases and found
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a significantly higher prevalence of abnormal pedicles in patients with AIS, with most
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of the abnormal pedicles in the thoracic spine on the concave side and in the periapical
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and apical regions [24]. This risk was further increased by slimmer, more distorted, more sclerotic, and shorter pedicles in thoracic scoliosis [25]. On the other hand,
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pedicles on the convex side present advantages when compared with those on the
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concave side. One such advantage is the relatively increased safety zone for pedicle
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screw insertion on the convex side, which implies that a medial pedicle screw penetration on the convex side might be tolerated better, with less risk of injury to the
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spinal cord, than a penetration on the concave side of a scoliotic curve, especially at the apex of the curve. The present study also demonstrated that the distance from the spinal cord to the medial wall of the pedicle was significantly correlated with outer and inner cortical pedicle width, and the potential risk of spinal cord injury by pedicle screw is
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increased with insertion into a narrower pedicle, especially on the concave side around the apex. In this regard, other anchoring methods such as wires and polyethylene cables
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should be selected when preoperative evaluation reveals narrow pedicles that are not appropriate for pedicle screw placement.
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The strength of the present study is that it demonstrates the relationship
between the distance from the spinal cord to the medial wall of the pedicle and vertebral
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morphology in Lenke type 1 single thoracic curve scoliosis. However, the present study
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also has some limitations. The sample size was small, and the investigation was
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restricted to Lenke type 1 AIS. It would be ideal to increase the sample size and repeat
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the same measurements in AIS patients of all other Lenke types to observe whether these parameters differ among different Lenke curve types. Future studies will be
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needed to address this issue.
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5. Conclusion
The present study demonstrated that the distance from the spinal cord to the
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medial wall of the pedicle was related to vertebral morphology in Lenke type 1 AIS patients. This is the first morphological evidence to support that the potential risk of spinal cord injury by pedicle screw is increased with insertion into a narrower pedicle,
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especially on the concave side around the apex. Recognizing these patterns is critical to optimizing pedicle screw instrumentation. Pedicle screw placement on the concave side
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of the apex should be evaluated carefully before surgery. Based on the results of the present study, further studies are warranted to determine whether other features of
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vertebral morphology impact surgical complication risk in AIS
Conflict of interests
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None.
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Funding
This research did not receive any specific grant from funding agencies in the public,
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commercial, or not-for-profit sectors.
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None.
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Acknowledgement
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[22] Upendra B, Meena D, Kandwal P, Ahmed A, Chowdhury B, Jayaswal A: Pedicle
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morphometry in patients with adolescent idiopathic scoliosis. Indian J Orthop 2010;44:169-176.
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[23] Zindrick MR, Wiltse LL, Doornik A, Widell EH, Knight GW, Patwardhan AG et al: Analysis of the morphometric characteristics of the thoracic and lumbar
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pedicles. Spine (Phila Pa 1976) 1987;12:160-166.
[24] Sarwahi V, Sugarman EP, Wollowick AL, Amaral TD, Lo Y, Thornhill B:
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Prevalence, Distribution, and Surgical Relevance of Abnormal Pedicles in Spines with Adolescent Idiopathic Scoliosis vs. No Deformity: A CT-Based Study. J
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Bone Joint Surg Am 2014;96:e92.
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[25] Cui G, Watanabe K, Hosogane N, Tsuji T, Ishii K, Nakamura M et al:
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Morphologic evaluation of the thoracic vertebrae for safe free-hand pedicle screw
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placement in adolescent idiopathic scoliosis: a CT-based anatomical study. Surg
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Radiol Anat 2012;34:209-216.
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Figure legends
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Figure 1.
A: Illustration of a thoracic vertebra, showing the outer and inner cortical pedicle width
A CT myelography image of a thoracic vertebrae in the local axial plane shows the outer cortical pedicle width (AD) and the inner cortical pedicle width (BC), where A is
22
the medial outer cortex margin, B is the medial inner cortex margin, C is the lateral inner cortex margin, and D is the lateral outer cortex margin.
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B: Illustration of a thoracic vertebra, showing chord length, pedicle length, and transverse pedicle angle
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A CT myelography image of a thoracic vertebra in the local axial plane shows chord
length (EF), pedicle length (FG), and transverse pedicle angle (angle between EF and
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HI), where E is the anterior edge of the vertebral body along the pedicle axis, F is the
A
posterior edge of the vertebra along the pedicle axis, G is a point in line with the
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posterior longitudinal ligament along the pedicle axis, H is the sagittal midvertebral line
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at the anterior aspect of the vertebral body, and I is the sagittal midvertebral line at the
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Figure 2.
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meeting of the laminae.
A: Illustration of a thoracic vertebra, showing the angle of rotation (RAsag)
A
RAsag measured by using the angle between the junction of the laminae, the dorsal central aspect of the vertebral foramen, and the middle of the vertebral body and the sagittal plane.
23
B: Illustration of a thoracic vertebra, showing the distance from the spinal cord to the convex pedicle (Dv) and concave pedicle (Dc)
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Convex and concave refer to the axial direction of the pedicle on the convex and concave sides of the apex, respectively. Line a and line d are tangent to the medial walls
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of the pedicle. Lines b and c are tangent to the outer edge of the spinal cord. All lines
are parallel to the pedicle direction. The vertical distance between lines a and b (Dv) and
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A
the pedicle on the convex and concave sides.
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lines c and d (Dc) represent the distance between the spinal cord and the medial wall of
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Figure 3. The concave and convex outer pedicle widths
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The graph shows a comparison of the mean concave and convex outer pedicle widths at
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each anatomic vertebral level.
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The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12. The mean concave outer cortical pedicle
A
width was smaller than the mean convex outer cortical pedicle width around the apex of the curve from T7 to T9. *significance value of p < 0.05. **significance value of p < 0.001.
Figure 4. The concave and convex inner pedicle widths
24
The graph shows a comparison of the mean concave and convex inner pedicle widths at each anatomic vertebral level. The mean concave inner cortical pedicle width was larger
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than the mean convex inner cortical pedicle width at T4, T5, and T11. The mean concave inner cortical pedicle width was smaller than the mean convex inner cortical
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pedicle width around the apex of the curve at T7 and T8. *significance value of p < 0.05. **significance value of p < 0.001.
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Figure 5. The concave and convex chord lengths
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The graph shows a comparison of the mean concave and convex chord lengths at each
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anatomic vertebral level. The mean chord length on the concave side was larger than
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that on the convex side from T6 to T8. **significance value of p < 0.001.
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Figure 6. The concave and convex pedicle lengths
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The graph shows a comparison of the mean concave and convex pedicle lengths at each anatomic vertebral level. The mean pedicle length on the concave side was larger than
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the convex side at T6 and T7. The mean pedicle length on the concave side was smaller than that on the convex side at T10 and T11. *significance value of p < 0.05. **significance value of p < 0.001.
25
Figure 7. The distance from the spinal cord to the convex pedicle (Dv) and concave pedicle (Dc)
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The graph shows a comparison of the mean Dv and Dc at each anatomic vertebral level. The mean Dc was smaller than the mean Dv around the apex of the curve from T6 to
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T11. **significance value of p < 0.001.
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A: Correlation of convex outer pedicle width and Dv
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Figure 8. Correlation of convex pedicle width and Dv
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A
Convex outer pedicle width (R=0.286, p<0.001) was significantly correlated with Dv.
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B: Correlation of convex inner pedicle outer width and Dv
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Convex inner pedicle width (R=0.202, p=0.002) was significantly correlated with Dv.
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Figure 9. Correlation of concave pedicle width and Dc
A: Correlation of concave outer pedicle width and Dc
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Concave outer pedicle width (R=0.269, p<0.001) was significantly correlated with Dc.
B: Correlation of concave inner pedicle outer width and Dc
Concave inner pedicle width (R=0.230, p<0.001) was significantly correlated with Dc.
26
Compliance with Ethical Standards Conflicts of Interest: The authors report no conflict of interest concerning the materials or methods used in the present study or the findings specified in the present paper
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PT
ED
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A
N
U
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Informed Consent: For this type of study formal consent is not required.
27
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-1
28
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-2
29
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-3
30
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-4
31
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-5
32
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-6
33
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-7
34
A ED
PT
CC E
IP T
SC R
U
N
A
M
Figr-8
35
A ED
PT
CC E
IP T
SC R
U
N
A
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Figr-9
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Table 1. Comparison of concave and convex transverse pedicle angle Concave transverse pedicle angle (°)
Level
Convex transverse pedicle angle (°)
8.55 ± 2.31
8.74 ± 3.03
T5
10.78 ± 3.23
9.67 ± 5.07
T6
11.37 ± 3.43
T7
11.81 ± 3.88
T8
11.74 ± 3.46
T9
10.89 ± 3.74
7.78 ± 3.46**
T10
9.48 ± 3.49
8.22 ± 3.19
T11
8.85 ± 3.26
7.11 ± 2.87
8.04 ± 3.53
8.26 ± 3.78
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IP T
T4
A
N
T12
A
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ED
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**p<0.001
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7.74 ± 3.47** 7.89 ± 3.79** 7.63 ± 3.24**
Table 2. The angle of rotation (RAsag) Level
RAsag (°) 18.07 ± 3.97
T5
19.89 ± 3.12
T6
22.67 ± 4.81
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T4
T7
24.22 ± 3.80 25.59 ± 3.72
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T8 T9 T10
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T11
A
CC E
PT
ED
M
A
N
T12
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24.93 ± 3.84 24.15 ± 4.71 21.44 ± 3.47 20.96 ± 3.71
S0303-8467(18)30280-4 https://doi.org/10.1016/j.clineuro.2018.07.007 CLINEU 5096
To appear in:
Clinical Neurology and Neurosurgery
Received date: Revised date: Accepted date:
23-5-2018 4-7-2018 8-7-2018
Please cite this article as: Miyazaki M, Ishihara T, Kanezaki S, Notani N, Abe T, Tsumura H, Relationship between vertebral morphology and the potential risk of spinal cord injury by pedicle screw in adolescent idiopathic scoliosis, Clinical Neurology and Neurosurgery (2018), https://doi.org/10.1016/j.clineuro.2018.07.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Abstract Objective: We aimed to investigate the relative preoperative position of the spinal cord
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in AIS and explore the potential risk of spinal cord injury from placement of pedicle screws.
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Patients and Methods: Twenty-seven patients with a mean age of 15 ± 1.8 years
(range, 12–19 years) classified as having Lenke type 1 AIS (1A: 15 cases, 1B: 8 cases,
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1C: 4 cases) were analyzed. The mean Cobb angle of the main curve was 55.9 ± 14.4°.
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Axial CT myelography images were selected from the T4 to T12 vertebrae, and 243
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images were analyzed. Outer cortical pedicle width, inner cortical pedicle width, pedicle
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length, chord length, transverse pedicle angle, the angle of rotation (RAsag) of the vertebra, and the distance between the spinal cord and concave (Dc) and convex
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pedicles (Dv) were calculated from landmark locations.
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Results: The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12 (p<0.05) and smaller than
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the mean convex outer cortical pedicle width around the apex of the curve from T7 to T9 (p<0.05). The mean concave inner cortical pedicle width was larger than the mean convex inner cortical pedicle width at T4, T5, and T11 (p<0.05) and smaller than the
1
mean convex inner cortical pedicle width around the apex of the curve at T7 and T8 (p<0.001). The mean Dc was smaller than the mean Dv around the apex of the curve
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from T6 to T11 (p<0.05). Dv was significantly correlated with the convex outer cortical pedicle width (R=0.286, p<0.001), convex inner cortical pedicle width (R=0.202,
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p=0.002), convex transverse pedicle angle (R=-0.286, p<0.001), and RAsag (R=0.277, p<0.001). Dc was significantly correlated with the concave outer (R=0.269, p<0.001)
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and inner cortical pedicle width (R=0.230, p<0.001).
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Conclusion: The distance from the spinal cord to the medial wall of the pedicle was
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significantly correlated with outer and inner cortical pedicle width, and the potential risk
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of spinal cord injury by pedicle screw is increased with insertion into a narrower
Highlights
Anatomic measurements revealed Dc was smaller than Dv around the apex of the curve.
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pedicle, especially on the concave side around the apex.
Dc was significantly correlated with the concave cortical pedicle width.
Dv was significantly correlated with the convex cortical pedicle width.
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Keywords: Scoliosis; Vertebral morphology; Computed tomography; Multiplanar reconstruction; Spinal cord injury; Thoracic spine
1. Introduction
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Adolescent idiopathic scoliosis (AIS) is a complex, three-dimensional deformity of the spine with lateral deviation in the coronal plane, alternation of
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kyphosis or lordosis in the sagittal plane, and rotation of the vertebrae in the axial plane. Pedicle screw fixation, a widely used surgical treatment for AIS, allows for segmental
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instrumentation in multiple vertebrae across a multilevel fusion area and provides strong pullout strength with the desired deformity correction [1-3]. Despite its advantages, its
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use at thoracic levels remains controversial owing to the relatively small pedicle
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dimensions and wide variation in morphologic features of the pedicles [4]. Screw
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misplacement may reduce pullout strength or lead to severe complications involving the nearby visceral, vascular, and neurologic structures [5-7]. Safe and reproducible
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placement of thoracic pedicle screws is generally dependent on a better understanding
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of abnormal anatomy in scoliosis. However, the incidence of screw misplacement has
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been reported to reach 43% [8, 9] when all the screws are evaluated by computed tomography (CT) postoperatively. Thoracic pedicle screw fixation is potentially risky
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because there is little space between the spinal cord and medial pedicle wall on the concave side of apex vertebrae. The incidence of screw-related neurologic complications during the treatment of spinal deformities with thoracic pedicle screws ranges from 0 to 0.9% [8, 9]. Mac-Thiong et al. [7] reported 9 cases of pedicle screws
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misplaced totally within the spinal canal during posterior surgery for AIS, with spinal canal intrusion ranging from 21–61%. They suggested that any pedicle screw misplaced
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within the spinal canal should be removed because of possible early or late neurological complications. Sarlak et al. [10] reported a 10.8% rate of medial pedicle screw
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misplacement in a study of 1797 screws in 148 scoliosis patients, with unacceptable
screw placement defined as medial violation greater than 2 mm. They suggested that the
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acceptability of medial pedicle breach may change at each level according to changes in
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canal width and cord shift.
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Although various anatomic studies on the unique characteristics of thoracic pedicles
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have been conducted [4,11,12], few have investigated the relationship between vertebral morphology and the distance from the spinal cord to the medial wall of the pedicle as a
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potential risk factor for spinal cord injury by a pedicle screw. The purpose of this study
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was to investigate the relative position of the spinal cord in AIS before surgery and explore the potential risk of spinal cord injury from the placement of pedicle screws.
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2. Materials and methods 2.1 Patient Demographic Data
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The present study included 27 patients with AIS classified as type 1 according to the Lenke classification [13]. The patients underwent pedicle screw fixation between
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August 2011 and November 2017. Criteria for patient selection were (1) undergoing careful screening to ensure that scoliosis was idiopathic and correctly classified as
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Lenke type 1 and (2) preoperative radiographs and CT myelography images were
available on file. Exclusion criteria were (1) proven or suspected congenital, muscular,
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neurologic, or hormonal causes of scoliosis and (2) clinical history of any condition that
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may have affected vertebral growth (e.g., history of cancer, vertebral abnormalities,
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muscular abnormalities, or neurologic conditions). We routinely obtained CT scans after myelography; all patients who underwent surgery during that period underwent CT
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myelography that was available for analysis. Twenty-seven patients (2 males and 25
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females) who met these criteria were treated during the period, with a mean age of 15 ±
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1.8 years (range, 12–19 years) classified as having Lenke type 1 AIS (1A: 15 cases, 1B: 8 cases, 1C: 4 cases). The mean Cobb angle of the main curve was 55.9 ± 14.4° (range,
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45°–103°). Patients’ mean height was 156.8 ± 6.5 cm (range, 145–171 cm), and their mean weight was 47.8 ± 7.3 kg (range, 31–63 kg).
2.2 Scanning Protocol
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Patients were placed in the prone position. After myelography, CT scans were obtained using a multislice scanner (Toshiba Aquilion 16, Toshiba Medical, Tochigi, Japan).
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Image data were obtained in 0.5-mm slices from the level of the occiput to S1. Each CT scan was opened with synchronized axial, coronal, and sagittal displays. The image
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contrast levels were standardized to enable clear soft tissue and bone demarcation at the vertebra. To measure the pedicle, the local axial viewing plane was adjusted to be
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parallel to the superior and inferior endplates of the vertebrae. When the superior and
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inferior endplate planes were not parallel owing to vertebral wedging, an orientation
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approximately halfway between (i.e., bisecting) the two endplate inclinations was selected. Vertebral morphology was measured on axial slices through the thinnest
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portion of the pedicle using reformatted slices closest to the middle of the pedicle in the
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craniocaudal dimension. Axial images were selected from the T4 to T12 vertebrae, and
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243 images were analyzed.
2.3 Measured parameters
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Measurements were performed using Picture Archiving and Communication System (PACS) software. Anatomic landmarks were identified and measured on each pedicle. The 3-D coordinates of 9 points (points A through I) were identified on each pedicle with the appropriate distances and angles between these 6
points (outer cortical pedicle width, inner cortical pedicle width, pedicle length, chord length, transverse pedicle angle) calculated from the landmark locations. Figure 1A
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showed the outer and inner cortical pedicle width. A CT myelography image of a thoracic vertebrae in the local axial plane shows the outer cortical pedicle width (AD)
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and the inner cortical pedicle width (BC), where A is the medial outer cortex margin, B is the medial inner cortex margin, C is the lateral inner cortex margin, and D is the
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lateral outer cortex margin. Figure 1B showed chord length, pedicle length, and
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transverse pedicle angle.A CT myelography image of a thoracic vertebra in the local
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A
axial plane shows chord length (EF), pedicle length (FG), and transverse pedicle angle (angle between EF and HI), where E is the anterior edge of the vertebral body along the
ED
pedicle axis, F is the posterior edge of the vertebra along the pedicle axis, G is a point in
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line with the posterior longitudinal ligament along the pedicle axis, H is the sagittal
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midvertebral line at the anterior aspect of the vertebral body, and I is the sagittal midvertebral line at the meeting of the laminae. These measurement definitions are
A
similar to those described in the literature [4,11,12, 14]. Figure 2A showed the angle of rotation (RAsag). RAsag measured by using the angle between the junction of the laminae, the dorsal central aspect of the vertebral foramen, and the middle of the vertebral body and the sagittal plane. Figure 2B showed the distance from the spinal
7
cord to the convex pedicle (Dv) and concave pedicle (Dc). Convex and concave refer to the axial direction of the pedicle on the convex and concave sides of the apex,
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respectively. Line a and line d are tangent to the medial walls of the pedicle. Lines b and c are tangent to the outer edge of the spinal cord. All lines are parallel to the pedicle
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direction. The vertical distance between lines a and b (Dv) and lines c and d (Dc)
represent the distance between the spinal cord and the medial wall of the pedicle on the
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convex and concave sides.
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Two independent observers measured each parameter. We investigated the reliability of
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the measurement techniques and observed good to excellent intra- and interobserver
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agreement for each parameter (kappa > 0.70). 2.4 Statistical analysis
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Between-group differences were evaluated using the Mann-Whitney U-test with a significance value of p < 0.05. Correlation coefficients were calculated to
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investigate the association between pedicle width and the distance between the spinal cord and medial wall of the pedicle. All analyses were performed using SPSS software
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(version 13; SPSS, Chicago, IL).
3. Results
3.1 Pedicle Anatomy
8
The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12 (T4: 3.98 ± 0.64 mm vs. 2.67 ± 0.84 mm,
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p<0.001; T5: 3.87 ± 0.96 mm vs. 3.21 ± 0.91 mm, p = 0.012; T11: 6.90 ± 1.96 mm vs. 5.61 ± 1.36 mm, p<0.001; T12: 6.64 ± 1.53 mm vs. 5.88 ± 1.50 mm, p = 0.019). The
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mean concave outer cortical pedicle width was smaller than the mean convex outer
cortical pedicle width around the apex of the curve from T7 to T9 (T7: 3.20 ± 0.93 mm
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vs. 4.15 ± 0.75 mm, p <0.001; T8: 3.38± 0.90 mm vs. 4.22 ± 0.62 mm, p<0.001; T9:
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4.03 ± 0.91 mm vs. 4.36 ± 0.91 mm, p = 0.039) (Figure 3).
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The mean concave inner cortical pedicle width was larger than the mean convex inner
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cortical pedicle width at T4, T5, and T11 (T4: 1.83 ± 0.62 mm vs. 0.80 ± 0.43 mm, p <0.001; T5: 1.73 ± 0.80 mm vs. 1.09 ± 0.58 mm, p = 0.002; T11: 4.37 ± 1.97 mm vs.
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3.29 ± 1.43 mm, p = 0.009). The mean concave inner cortical pedicle width was smaller
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than the mean convex inner cortical pedicle width around the apex of the curve at T7 and T8 (T7: 1.15 ± 0.70 mm vs. 1.82 ± 0.63 mm, p <0.001; T8: 1.32 ± 0.59 mm vs. 1.89
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± 0.53 mm, p<0.001) (Figure 4).
The mean chord length on the concave side was larger than that on the convex side from T6 to T8 (T6: 37.30 ± 3.59 mm vs. 33.62 ± 3.42 mm, p<0.001; T7: 38.89 ± 3.51 mm vs. 34.89 ± 3.50 mm, p<0.001; T8: 38.97 ± 3.53 mm vs. 36.32 ± 3.34 mm, p<0.001) 9
(Figure 5). The mean pedicle length on the concave side was larger than the convex side at T6 and T7 (T6: 20.28 ± 2.14 mm vs. 17.84 ± 3.49 mm, p<0.001; T7: 20.82 ± 1.82
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mm vs. 17.78 ± 3.17 mm, p<0.001). The mean pedicle length on the concave side was smaller than that on the convex side at T10 and T11 (T10: 18.42 ± 2.09 mm vs. 20.10 ±
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2.18 mm, p<0.001; T11: 19.84 ± 2.49 mm vs. 21.34 ± 2.70 mm, p = 0.010) (Figure 6).
The mean concave transverse pedicle angle was larger than that on the convex side from
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T6 to T9 (T6: 11.37± 3.43° vs. 7.74 ± 3.47°, p<0.001; T7: 11.81 ± 3.88° vs. 7.89 ±
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3.79°, p<0.001; T8: 11.74 ± 3.46° vs. 7.63 ± 3.24°, p<0.001; T9: 10.89 ± 3.74° vs. 7.78°
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± 3.46°, p<0.001) (Table 1).
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3.2 The angle of rotation of the vertebra
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The mean RAsag increased in degree from T4 to T8 and decreased in degree from T8 to
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T12 (Table 2).
3.3 The distance between the spinal cord and concave and convex medial wall of
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pedicles
The mean Dc was smaller than the mean Dv around the apex of the curve from T6 to T11 (T6: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T7: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T8: 3.38± 0.90 mm vs. 4.22 ± 0.62 mm, p<0.001; T9: 4.03 ± 0.91
10
mm vs. 4.36 ± 0.91 mm, p<0.001; T10: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p <0.001; T11: 3.20 ± 0.93 mm vs. 4.15 ± 0.75 mm, p<0.001) (Figure 7).
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3.4 Correlation of Dc and Dv with outer and inner cortical pedicle width
Dv was significantly correlated with the convex outer cortical pedicle width (R=0.286,
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p<0.001), convex inner cortical pedicle width (R=0.202, p=0.002) (Figure 8A, 8B),
convex transverse pedicle angle (R=-0.286, p<0.001), and RAsag (R=0.277, p<0.001),
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but not with the chord length on the convex side or pedicle length on the convex side.
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Dc was significantly correlated with the concave outer cortical pedicle width (R=0.269,
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p<0.001) and concave inner cortical pedicle width (R=0.230, p<0.001) (Figure 9A, 9B),
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but not with the chord length on the concave side, pedicle length on the concave side, concave transverse pedicle angle, or RAsag.
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4. Discussion
Pedicle screw instrumentation has become a popular and widely accepted
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surgical technique for AIS patients. Although some newly developed navigation techniques may help surgeons to place pedicle screws more safely [15], it is still very important that spine surgeons have a clear knowledge of pedicle morphometric
11
anatomy, especially in deformed spines, to optimize the starting point and trajectory of the screw path.
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Transverse pedicle width is an important factor that determines the diameter of screws that can be safely accommodated in a pedicle without breaching the medial or lateral
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medial cortex. Parent et al. examined the thoracic pedicle morphology of 325 cadaveric scoliotic vertebrae and found that pedicles located on the concavity of typical right
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thoracic curves were significantly thinner than those on the convex side, and the
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maximal mean difference was found around the apex [16]. Abul-Kasim et al. studied the
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pedicle width pattern in Lenke type 1 AIS patients and reported similar findings [17].
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Takeshita et al. also investigated the diameter, length, and direction of pedicle screws by multiplanar reconstruction of CT images from Japanese patients with various scoliotic
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spine conditions and found that a large proportion of thoracic pedicles were too small
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on the concave side to accept a 4-mm-diameter screw, with 37% of concave T3-T9 pedicles unable to hold a 4-mm-diameter screw even with 25% expansion [18]. In the
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present study, the narrowest mean inner and outer pedicle diameters were observed at T7 on the concave side, and the mean outer pedicle diameters from T4 to T8 on the concave side were less than 4 mm in diameter. These results indicate that the pedicles on the concave side of the main thoracic curve apex are substantially narrowed,
12
highlighting the necessity of practicing caution when inserting pedicle screws on the concave side of the main thoracic curve.
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One possible solution may be to consider extrapedicular screw trajectories in the preoperative plan if pedicle screw insertion is judged to be difficult. Extrapedicular
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screw placement has been advocated as a safe and effective alternative to the thoracic transpedicular screw [19]. However, screws placed in an extrapedicular position have
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about 75% of the pullout failure load of those placed in a transpedicular position [20].
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Although the powerful corrective force and maintenance of segmental pedicle screws is
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attractive, surgeons should not always use pedicle screws.
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Chord length is another important measurement in preventing anterior cortex
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perforation. Inappropriate pedicle screw length can be more hazardous than
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inappropriate diameter. A concave screw advanced too anteriorly or too laterally poses a
potential risk of aorta injury. Vaccaro et al. analyzed nonscoliotic thoracic spine and
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found that the aorta and esophagus are at greatest risk of injury when a pedicle screw
penetrates the anterior cortex of a vertebral body [21]. Considering the lateral force
exerted during a correction maneuver, special attention should be given to screw
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placement on the concave side. The present study demonstrated that at the apical region
(T6-T8), chord length was significantly longer on the concave side than on the convex
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side, which suggests that the concave side at the apex region may be able to
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accommodate a slightly longer pedicle screw. Similar findings have been reported in
other studies of scoliotic spine [12,18], which may be explained by the fact that with
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rotation of the scoliotic spine, the vertebral body drifts toward the concavity in the
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transverse plane, resulting in the slightly longer chord length on the concave side over
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the apical region [22].
Transverse pedicle angle is also an important anatomical parameter related to
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pedicle screw insertion. In normal spine, decreased pedicle angles have been reported at
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the thoracolumbar junction [23]. In the present study, we found that the transverse pedicle angle was significantly increased on the concave side at the apical region (T6-
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T9), which could be due to the intravertebral deformation that develops with rotation of the scoliotic spine. There was also a decreased transverse pedicle angle at the thoracolumbar junction, and the smallest angulations were at the thoracolumbar junction (T11 and T12).
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In the present study, the distance from the spinal cord to the medial wall of the pedicle on the concave side was significantly smaller than that on the convex side from T6 to
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T11. There was a smaller space between the medial wall of the pedicle and the spinal cord on the concave side of the apical region. Therefore, a misplaced pedicle screw
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would be more dangerous to the spinal cord on the concave side than on the convex
side. Sarwahi et al. investigated pedicles in spines with AIS and normal cases and found
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a significantly higher prevalence of abnormal pedicles in patients with AIS, with most
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of the abnormal pedicles in the thoracic spine on the concave side and in the periapical
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and apical regions [24]. This risk was further increased by slimmer, more distorted, more sclerotic, and shorter pedicles in thoracic scoliosis [25]. On the other hand,
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pedicles on the convex side present advantages when compared with those on the
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concave side. One such advantage is the relatively increased safety zone for pedicle
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screw insertion on the convex side, which implies that a medial pedicle screw penetration on the convex side might be tolerated better, with less risk of injury to the
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spinal cord, than a penetration on the concave side of a scoliotic curve, especially at the apex of the curve. The present study also demonstrated that the distance from the spinal cord to the medial wall of the pedicle was significantly correlated with outer and inner cortical pedicle width, and the potential risk of spinal cord injury by pedicle screw is
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increased with insertion into a narrower pedicle, especially on the concave side around the apex. In this regard, other anchoring methods such as wires and polyethylene cables
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should be selected when preoperative evaluation reveals narrow pedicles that are not appropriate for pedicle screw placement.
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The strength of the present study is that it demonstrates the relationship
between the distance from the spinal cord to the medial wall of the pedicle and vertebral
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morphology in Lenke type 1 single thoracic curve scoliosis. However, the present study
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also has some limitations. The sample size was small, and the investigation was
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restricted to Lenke type 1 AIS. It would be ideal to increase the sample size and repeat
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the same measurements in AIS patients of all other Lenke types to observe whether these parameters differ among different Lenke curve types. Future studies will be
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needed to address this issue.
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5. Conclusion
The present study demonstrated that the distance from the spinal cord to the
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medial wall of the pedicle was related to vertebral morphology in Lenke type 1 AIS patients. This is the first morphological evidence to support that the potential risk of spinal cord injury by pedicle screw is increased with insertion into a narrower pedicle,
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especially on the concave side around the apex. Recognizing these patterns is critical to optimizing pedicle screw instrumentation. Pedicle screw placement on the concave side
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of the apex should be evaluated carefully before surgery. Based on the results of the present study, further studies are warranted to determine whether other features of
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vertebral morphology impact surgical complication risk in AIS
Conflict of interests
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None.
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Funding
This research did not receive any specific grant from funding agencies in the public,
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commercial, or not-for-profit sectors.
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None.
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Acknowledgement
References
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Figure legends
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Figure 1.
A: Illustration of a thoracic vertebra, showing the outer and inner cortical pedicle width
A CT myelography image of a thoracic vertebrae in the local axial plane shows the outer cortical pedicle width (AD) and the inner cortical pedicle width (BC), where A is
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the medial outer cortex margin, B is the medial inner cortex margin, C is the lateral inner cortex margin, and D is the lateral outer cortex margin.
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B: Illustration of a thoracic vertebra, showing chord length, pedicle length, and transverse pedicle angle
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A CT myelography image of a thoracic vertebra in the local axial plane shows chord
length (EF), pedicle length (FG), and transverse pedicle angle (angle between EF and
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HI), where E is the anterior edge of the vertebral body along the pedicle axis, F is the
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posterior edge of the vertebra along the pedicle axis, G is a point in line with the
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posterior longitudinal ligament along the pedicle axis, H is the sagittal midvertebral line
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at the anterior aspect of the vertebral body, and I is the sagittal midvertebral line at the
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Figure 2.
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meeting of the laminae.
A: Illustration of a thoracic vertebra, showing the angle of rotation (RAsag)
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RAsag measured by using the angle between the junction of the laminae, the dorsal central aspect of the vertebral foramen, and the middle of the vertebral body and the sagittal plane.
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B: Illustration of a thoracic vertebra, showing the distance from the spinal cord to the convex pedicle (Dv) and concave pedicle (Dc)
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Convex and concave refer to the axial direction of the pedicle on the convex and concave sides of the apex, respectively. Line a and line d are tangent to the medial walls
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of the pedicle. Lines b and c are tangent to the outer edge of the spinal cord. All lines
are parallel to the pedicle direction. The vertical distance between lines a and b (Dv) and
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the pedicle on the convex and concave sides.
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lines c and d (Dc) represent the distance between the spinal cord and the medial wall of
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Figure 3. The concave and convex outer pedicle widths
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The graph shows a comparison of the mean concave and convex outer pedicle widths at
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each anatomic vertebral level.
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The mean concave outer cortical pedicle width was larger than the mean convex outer cortical pedicle width at T4, T5, T11, and T12. The mean concave outer cortical pedicle
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width was smaller than the mean convex outer cortical pedicle width around the apex of the curve from T7 to T9. *significance value of p < 0.05. **significance value of p < 0.001.
Figure 4. The concave and convex inner pedicle widths
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The graph shows a comparison of the mean concave and convex inner pedicle widths at each anatomic vertebral level. The mean concave inner cortical pedicle width was larger
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than the mean convex inner cortical pedicle width at T4, T5, and T11. The mean concave inner cortical pedicle width was smaller than the mean convex inner cortical
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pedicle width around the apex of the curve at T7 and T8. *significance value of p < 0.05. **significance value of p < 0.001.
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Figure 5. The concave and convex chord lengths
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The graph shows a comparison of the mean concave and convex chord lengths at each
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anatomic vertebral level. The mean chord length on the concave side was larger than
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that on the convex side from T6 to T8. **significance value of p < 0.001.
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Figure 6. The concave and convex pedicle lengths
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The graph shows a comparison of the mean concave and convex pedicle lengths at each anatomic vertebral level. The mean pedicle length on the concave side was larger than
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the convex side at T6 and T7. The mean pedicle length on the concave side was smaller than that on the convex side at T10 and T11. *significance value of p < 0.05. **significance value of p < 0.001.
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Figure 7. The distance from the spinal cord to the convex pedicle (Dv) and concave pedicle (Dc)
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The graph shows a comparison of the mean Dv and Dc at each anatomic vertebral level. The mean Dc was smaller than the mean Dv around the apex of the curve from T6 to
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T11. **significance value of p < 0.001.
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A: Correlation of convex outer pedicle width and Dv
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Figure 8. Correlation of convex pedicle width and Dv
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Convex outer pedicle width (R=0.286, p<0.001) was significantly correlated with Dv.
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B: Correlation of convex inner pedicle outer width and Dv
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Convex inner pedicle width (R=0.202, p=0.002) was significantly correlated with Dv.
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Figure 9. Correlation of concave pedicle width and Dc
A: Correlation of concave outer pedicle width and Dc
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Concave outer pedicle width (R=0.269, p<0.001) was significantly correlated with Dc.
B: Correlation of concave inner pedicle outer width and Dc
Concave inner pedicle width (R=0.230, p<0.001) was significantly correlated with Dc.
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Compliance with Ethical Standards Conflicts of Interest: The authors report no conflict of interest concerning the materials or methods used in the present study or the findings specified in the present paper
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Informed Consent: For this type of study formal consent is not required.
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Table 1. Comparison of concave and convex transverse pedicle angle Concave transverse pedicle angle (°)
Level
Convex transverse pedicle angle (°)
8.55 ± 2.31
8.74 ± 3.03
T5
10.78 ± 3.23
9.67 ± 5.07
T6
11.37 ± 3.43
T7
11.81 ± 3.88
T8
11.74 ± 3.46
T9
10.89 ± 3.74
7.78 ± 3.46**
T10
9.48 ± 3.49
8.22 ± 3.19
T11
8.85 ± 3.26
7.11 ± 2.87
8.04 ± 3.53
8.26 ± 3.78
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T4
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T12
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**p<0.001
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7.74 ± 3.47** 7.89 ± 3.79** 7.63 ± 3.24**
Table 2. The angle of rotation (RAsag) Level
RAsag (°) 18.07 ± 3.97
T5
19.89 ± 3.12
T6
22.67 ± 4.81
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T4
T7
24.22 ± 3.80 25.59 ± 3.72
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T8 T9 T10
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T11
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T12
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24.93 ± 3.84 24.15 ± 4.71 21.44 ± 3.47 20.96 ± 3.71