|Year : 2017 | Volume
| Issue : 1 | Page : 57-61
Safe zones of pin insertion in thoracic spine: a ‘cadaveric study’
Mohammed A Meselhy1, Osama F Ahmed2
1 Department of Orthopedics Surgery, Benha University, Benha, Egypt
2 Department of Anatomy and Embryology, Benha University, Benha, Egypt
|Date of Submission||15-Jan-2016|
|Date of Acceptance||24-Apr-2016|
|Date of Web Publication||6-Nov-2017|
Mohammed A Meselhy
Orthopedic Department, Benha University, Kafer el Gazar, Banha, Qalyubia 13511
Source of Support: None, Conflict of Interest: None
Purpose This experimental study was conducted to identify the safe zones and angles of half pin insertion in thoracic vertebrae.
Materials and methods Simple tools were used, power drill, 4, 5, and 6 mm half pins, goniometer, and portable radiographic apparatus. There were two bony specimens comprising complete lumbar thoracic and cervical spine with ribs and sternum attached, and three cadaveric specimens preserved in formalin, comprising the thorax and abdomen with the back muscle dissected to show the thoracic pedicle. The methodology comprised insertion of the Schanz screw in the thoracic pedicle from T1 to T12 using the free-hand technique in bony skeleton, and then this was repeated on the cadaveric specimens to evaluate the correct angle of insertion, checking the site of half pin insertion using radiography for the cadaveric specimen.
Results The suggested safe angulation of half pins was 20°–30° at T1, 15°–25° at T2, 10°–15° at T3, 10°–15° at T4, and from T5 to T9 the safe angle was from 5° to 15°. In T11 and T12, the safe angle was between 0° and 5°. The safe angle of half pin insertion in the pedicle of the thoracic spine in the sagittal plane in all vertebrae from T1 to T12 was between 10° and 15°.
Conclusion It was concluded that the application of the external fixator in the thoracic spine is safe, provided there is a better understanding of the anatomical properties of the thoracic spine.
Keywords: external fixator, safe zones, thoracic spine
|How to cite this article:|
Meselhy MA, Ahmed OF. Safe zones of pin insertion in thoracic spine: a ‘cadaveric study’. Egypt Orthop J 2017;52:57-61
| Introduction|| |
The vertebral column in the thoracic region appears cylindrical. The width of the vertebral body decreases from the first thoracic vertebrae to the fourth thoracic vertebrae, and then the width increases again until the sacrum, to accommodate the load progression from the head to the lower lumbar region ,,,.
In 1977, Magerl developed spinal skeletal external fixator for stabilization and fixation of the lumbar and lower thoracic vertebrae; it consisted of two pairs of half pins attached to the vertebral bodies through the pedicles with an adjustable device .
The clinical application of this fixator was in the field of spinal infection, trauma, and instability; it allows compression, distraction, and scaffolding for neutral fixation .
The purpose of this experimental study was to identify the safe zones and angles of half pin insertion in thoracic vertebrae.
| Materials and methods|| |
In this cadaveric study we evaluated the pedicle as a safe zone of half pin insertion in the thoracic spine and determined the safe angle for half pin fixation. The study was approaved by ethical committee of banha university and were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
This study was conducted in our Faculty of Medicine in the Anatomy and Embryology Department, Laboratory Section, in cooperation with the Orthopedic Surgery Department.
Simple tools such as power drill and half pins of 4, 5, and 6 mm in diameter were used; in addition, a portable radiographic apparatus was used.
In this cadaveric study, there were two bony vertebral columns with ribs at the thoracic region and three cadaveric specimens preserved in formalin, comprising the thorax and abdomen with the back muscle dissected to show the pedicle and ribs ([Figure 1] and [Figure 2]).
|Figure 1 Bony specimen. (a) Clinical photos show angles of half pin insertion in pedicles of thoracic vertebrae in the sagittal plane; (b) clinical photos show angles of half pin insertion in pedicles of thoracic vertebrae in the transverse plane.|
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|Figure 2 Cadaveric specimen. (a) Clinical photos show angles of half pin insertion in pedicles of thoracic vertebrae in the sagittal plane; (b) clinical photos show angles of half pin insertion in pedicles of the thoracic vertebrae in the transverse plane.|
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The methodology of this study comprised the insertion of the Schanz screw in the thoracic pedicle from T1 to T12 using the free-hand technique in bony skeleton, and then this was repeated on the cadaveric specimen to evaluate the correct angle of insertion. In addition, the site of half pin insertion was checked using radiography for the cadaveric specimen ([Figure 3]).
|Figure 3 Radiography: anteroposterior and lateral views for the half pins in the cadaveric specimen.|
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The entry point is present at the junction of a vertical line along the lateral side of the pars interarticularis and a transverse line bisecting the transverse process in half. On moving cranially toward the mid thoracic spine, the entry point moves medially. At T7–T9, the starting point lies most medial, located along a vertical line just lateral to the midpoint of the superior articular process at a transverse location along the superior border of the transverse process. On moving more proximally, the entry point shifted more laterally. At T1–T2, the entry point is found at the point of intersection of a vertical line along the lateral border of the pars interarticularis and a transverse line bisecting the transverse process.
| Results|| |
The half pins in the thoracic pedicle were inserted using the free-hand technique in both bony skeleton and the cadaveric specimens.
The data in this experimental study were collected by means of direct observation and using a goniometer to measure the correct angle of half pin insertion in the pedicle.
In this current study, on analysis of the angles of half pin insertion on both bony skeleton and cadaveric specimens, it was found that the safe angle of half pin insertion in the pedicle in the transverse plane using the free-hand technique in the thoracic spine is as follows:
- T1: 20°–30°.
- T2: 15°–25°.
- T3: 10°–15°.
- T4: 10°–15°.
From T5 to T9 the safe angle was from 5° to 15°.
In T11 and T12, the safe angle of half pin insertion was between 0° and 5° ([Figure 4]).
|Figure 4 A chart showing safe angles of half pin insertion in the transverse plane.|
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However, it was found that the safe angle of half pin insertion in the pedicle of thoracic spine in the sagittal plane in all vertebrae from T1 to T12 ranges from 10° to 15° ([Figure 5]).
|Figure 5 A chart showing safe angles of half pin insertion in the sagittal plane.|
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| Discussion|| |
The thoracic region consists of 12 vertebrae, which increase in size from cranial to caudal to accommodate the load transmission; the dorsal spine is characterized by the presence of costovertebral complex .
According to anatomic classification, thoracic vertebrae are divided into typical and atypical vertebrae. Typical vertebrae have anteroposterior diameter more than the transverse diameter, whereas the second group, which comprises five vertebrae, are different ,,,.
The first dorsal vertebra considered as transitional site, while the seventh cervical vertebra, in addition the ninth thoracic vertebrae has only superior costal articulation, also the 11th looks like the lumber vertebrae as it has large body and small transverse process because it lies between the mobile cervical vertebrae and slightly mobile thoracic vertebrae ,,.
Thoracic aorta is in close relation with the vertebrae; it is the continuation of the arch of aorta at the level of T4, and it ends at the level of lower border of T12. Proximally, it is to the left of the vertebral column, but at the level of diaphragm it becomes central ,,,,,.
Intercostal arteries are of great relationship to this study purpose. They are located in the center of the vertebral bodies. They originate from the thoracic aorta, except for the upper two vessels, which arise from the subclavian artery ,,,,,,,,,.
The intercostal arteries divide into two branches: the anterior and posterior branches. The posterior branches are important during surgery, as they enter the intervertebral foramen and then divide into muscular and spinal branches. The spinal branch supplies the entire spinal cord ,,,,,,,,,.
Pedicle has a important relationship as the dura and the spinal cord lie medial to it and the roots of the spinal nerves pass below, and hence nerve root injury can occur ,.
To avoid injury of important structures that lie close to the pedicle, during percutaneous pedicle screw application, the accessory process is fractured by some surgeons to facilitate accurate screw insertion .
Preoperative planning allows accurate pedicular screw application. This is best when carried out using routine roentgenograms, transaxial computed tomography scan, and MRI to determine all features of the pedicle ,,,,,,.
The transverse angle of the pedicle is changeable; it decreases from 30° convergent at T1 to 0° or 5° divergent at T12. The pedicle axis of the lower thoracic spine (T10, T11, and T12) becomes neutral to little divergent because of the presence of the rib head ,,.
Pedicle screw should be introduced parallel to the upper surface of the vertebra with inclination to the sagittal plane of about 10°, which increases while descending caudally to 15°–20° at L5 ,,.
In this current study, the angles of half pin insertion in the thoracic pedicles in both transverse and sagittal planes are corresponding to the anatomical consideration and anatomical angles of vertebral and lamina inclination.
In the study by Weinsteind et al.  it was concluded that thoracic pedicles are inclined in the transverse plane by different angles with craniocaudal location, being less than 10° in the thoracic spine. The pedicles also show different angles in the sagittal plane. The pedicles are directed ∼15° to 17° cephalad for the majority of the thoracic spine .
From our observation, the results of suggested safe angulation and zones of introduction of half pins were 20°–30° at T1 and 15°–25° at T2.
In T3 the safe angle was 10°–15°, and at T4 it was 10°–15°. From T5 to T9 the safe angle was from 5° to 15°.
In T11 and T12, the safe angle of half pin insertion was between 0° and 5°.
Moreover, it was found that the safe angle of half pin insertion in the pedicle of the thoracic spine in the sagittal plane in all vertebrae from T1 to T12 ranges from 10° to 15°.
It was concluded that the lower thoracic pedicles allow a strong site of the pedicle screw from a lateral entry point and 10°–15° convergent angle ,,.
One of the important factors that allows safe screw insertion is the transverse pedicle diameter, as the lower three dorsal spines have the biggest pedicle diameter .
As regards the technique, an image intensifier is always necessary for correct insertion of the half pins, as they must enter the vertebral body through the pedicles and should not to penetrate its anterior wall. With the image intensifier, the position of the pedicle is demarcated with the patient in prone position, and the image intensifier is placed perpendicular to the patient. The table is then tilted until the long axis of the pedicle corresponds with the center beam. The pedicle will then appear as a sharply defined oval. The half pin is inserted at the center of this oval through the long axis of the pedicle. This procedure is followed in the application of external skeletal spine fixator as well ,.
Magerl  discussed the direction and point of entry of the Schanz screws. The direction of the Schanz screws is 10°–20° convergent toward the sagittal plane. The point of entry is in the central axis of the pedicular tube, indicated by the intersection of the two lines. The vertical line touches the lateral border of the superior articular process; the horizontal line bisects the base of the transverse process .
Olerudi et al.  has described the use of image intensification when introducing 5 mm Schanz screws placed percutaneously into the pedicles. The Schanz screws are then connected to an external fixator .
The most important advantage of advanced imaging technology in the field of thoracic pedicle fixation is identifying the location of the aorta in relationship to the thoracic pedicle .
There were some limitations in this cadaveric study: the limited number of bony and cadaveric specimens, and the method of cadaveric specimen preservation, as the formalin changes the color of the soft tissues.
| Conclusion|| |
From our study we concluded that the application of an external fixator in the dorsal spine is safe, provided there is good understanding of thoracic spine anatomy and the inclination of the pedicle, in addition to the facilities of advanced imaging techniques that allow better and accurate half pin introduction into thoracic pedicles.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]