Pedicle

Deepak Awasthi, MD and Najeeb Thomas, MD

Department of Neurosurgery; Louisiana State University Health Sciences Center; New Orleans, Louisiana; USA

Introduction:
Use of pedicle screw systems for spinal stabilization has become increasingly common in spine surgery. A variety of pedicle screw systems have been described and new systems are being developed everyday. The technique and principle of screw placement as well as anatomical landmarks of screw placement, however, are common to all systems. It is not the purpose of this article to define the advantages and disadvantages of the pedicle screw systems, but to outline the background, anatomy, indications, advantages and technique of pedicle screw placement. The potential risks and complications of pedicle screw placement will not be the focus of this article. For more information, the reader is referred to the chapter by Halliday A, et al in the book Spine Surgery (ed. Ed Benzel, Churchill Livingstone, 1999, pp 1053-1064).

Brief History:
The first descriptions of the use of bone screws to obtain internal spinal fixation at the time of fusion were by Tourney in 1943 and King in 1944. Boucher in 1959 was the first to use pedicle screws. Roy-Camille was the first to use pedicle screws connected to a dorsal plate (Cin Orthop 203:7-17, 1986). The first system using both screws and hooks connecting them with rods or plates (i.e universal spinal instrumentation) was introduced by Cotrel and Dubousset (Clin Orthop 227:10-23, 1988).

Over the years, pedicle fixation systems proved to be biomechanically superior for segmental fixation. Thus, numerous systems and variations have been developed and continue to be developed by several spine companies in the US and abroad.

Advantages of Pedicle Screw Fixation:
Since pedicle screws traverse all three columns of the vertebrae, they can rigidly stabilize both the ventral and dorsal aspects of the spine. The pedicle also represents the strongest point of attachment of the spine and thus significant forces can be applied to the spine without failure of the bone-metal junction. Furthermore, the rigidity of pedicle fixation allows for the incorporation of fewer normal motion segments in order to achieve stabilization of an abnormal level.

Pedicle screw fixation does not require intact dorsal elements. Thus, it can be used after a laminectomy or traumatic disruption of laminae, spinous processes and/or facets. Additional advantages include less requirements for postoperative bracing and improvements in fusion rates.

Disadvantages of Pedicle Screw Fixation:
1. Steep learning curve.
2. Caudal or medial penetration of the pedicle cortex can result in dural or neural injury.
3. Implantation of pedicle screws requires extensive tissue dissection to expose the entry points and to provide the required lateral to medial orientation for optimal screw trajectory.
4. Lengthy operative time with potential for significant blood loss and increased risk of infection.
5. Postoperative imaging studies (especially MRI) are, in part, obscured by the implants.
6. Rigid fixation can accelerate adjacent motion segment degeneration.
7. Costly procedures.

Principal Indications for Pedicle Fixation:
1. Existing painful spinal instability:
* post-laminectomy spondylolisthesis
* painful pseudoarthrosis
2. Potential instability:
* spinal stenosis
* degenerative scoliosis
3. Unstable fractures.
4. Augmenting anterior strut grafting:
* tumor
* infection
5. Stabilizing spinal osteotomies.

Contraindications for Pedicle Fixation:
1. Recent infection.
2. Laminectomies that will not cause instability
3. Fusions which are normally successful without fixation.

Pedicle Anatomy (Figure 1):

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Figure 1: Diagram ilustrating the pedicle width (A), the tranverse or coronal pedicle angle (B),
and the sagittal pedicle angle (C). Taken from Spine Surgery (ed: Benzel; p1054).

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Figure 2:The width of the pedicle increases in the lower lumbar spine and is variable in the thoracic spine. Taken from Zindrick MR, et al; Clin Orthop 203:99, 1986).


pedicle: strong, cylindrical, anatomic bridge between the dorsal spinal elements and the vertebral body; consists of a strong shell of cortical bone and a core of cancellous bone.

It is important to remember that the pedicle size and angulation varies throughout the spinal column. The transverse pedicle width is narrower than the sagittal pedicle width (height) except in the lower lumbar spine. The pedicle width is more important than pedicle height for pedicle screw placement (Figure 1A and 2). Most of the pedicles below T10 are greater than 7mm in transverse diameter and most below 1 are greater than 8 mm in diameter. In addition, the transverse pedicle width increases from L1 to S1.

Tranverse pedicle angle or coronal angulation (Figure 1B) decreases as one descends caudally in the spine until the lumbar region. The angle increases as the lumbar spine is descended.

Sagittal pedicle angle (Figure 1C) is steep throughout the midthoracic spine and in the upper lumbar spine.

Intrathecal nerve roots course along the medial aspect of the pedicle. At T12, the dural sac is 0.2 to 0.3 mm away from the pedicle. Below L1, the medial side of the pedicle is almost touching the cauda equina. Nerve root occupies the ventral and rostral one-third of the foramen. Thus, violation of medial or caudal cortex of the pedicle risks injury to the nerve root.

Pedicle Screw Entry Sites and Trajectory:
General Remarks:
1. Preoperative planning using plain radiographs and CT scan is important in deciding the bone quality, pedicle transverse diameter and screw trajectory.
2. Sagittal pedicle angle increases in the thoracic spine from an average of 0 degs at T1 to 10 degs at T8 and then decreases to 0 degs at T12.
3. Usually the L4 sagittal pedicle angle is 0 degs and subsequent rostral and caudal levels are associated with progressively greater sagittal angles.
4. Lordotic curve of the lumbar spine produces a rostral angulation for upper lumbar screws.
5. L5 pedicle screw is 5 degs to 10 degs caudally inclined.
6. Coronal plane angulation (how medial?) at T1 is 10 degs to 15 degs and at T12 is 5 degs.
7. At L1 the medial angulation of 5 degs to 10 degs is satisfactory.
* a wider angle in the coronal plane is necessary to avoid lateral penetration of the pedicle in
the lower lumbar spine.
* the coronal plane angle increases approximately 5 degs per level from L1 to the sacrum (Figure 3).

Figure 3: Schematic diagram showing that the coronal (medial) angle increase approximately 5 degs per level from L1 to the sacrum.



Thoracic Spine:
In the thoracic spine, transverse process commonly does not align with the pedicle in the axial plane. Thus, the anatomic landmarks that are used for lumbar pedicle screw insertion cannot be reliably used in the thoracic spine. The transverse process is rostral to the pedicle in the upper thoracic spine and caudal to the pedicle in the lower thoracic spine (crossover occurs at T6-7). Because of this variability, fluoroscopic guidance or direct vision and palpation of the pedicle via a laminotomy is highly recommended for insertion of thoracic pedicle screws. At T1-3, 4.5 mm diameter screws that are 25-30 mm in length are usually recommended. At T4-T10, screws are usually 4.5 mm in diameter and 30-35 mm in length.

Lumbar Spine:
Conventional entry site for pedicle screw placement in the lumbar spine is at the junction of
the lateral facet and the transverse process (Figure 4A) or bisection of a vertical line through the facet joints and a horizontal line through the transverse process (Figure 4B).

A.B.

Figure 4: A: The conventional entry point for pedicle screw placement in the lumbar spine is at the junction of the lateral facet and the transverse process (arrow)- taken from Spine Surgery (ed: Benzel; p 1056). B. Bisection of a vertical line through the facet joints and a horizontal line through the transverse process can also serve as a useful landmark for lumbar pedicle entry site.


Nuances in pedicle screw insertion in lumbar spine:
1. Although the midline of the transverse process corresponds to the location of the pedicle at L4,
this relationship does vary at different lumbar levels. Above L4, the midline of the transverse
process is rostral to the pedicle. At L5, it is an average of 1.5 mm caudal to the pedicles.
2. Muscle dissection is performed as lateral as possible to allow palpation of the transverse
process. You have to be persistent and patient- have some good retractors.
3. Lateral aspect of the pedicles is palpated with a nerve hook over the transverse process.
4. Small laminotomy may be performed (if no decompression is being performed) to palpate the
medial aspect of the pedicle and its rostral/ caudal borders.
5. Palpation of the pedicle helps to guide accurate placement of the screw into the pedicle.
6. Some of the superior facet at the transverse process has to be decorticated for a good entry site
of the screw.

Steps in Screw Insertion (Figure 5):

A.B.

Figure 5: A: After decorticating the pedicle entry site with a burr and penetrating the site with an awl, a curved or straight pedicle probe is used to develop a path for the screw through the cancellous bone of the pedicle into the vertebral body as seen in this schematic diagram. See text for details.
B. After cannulation and confirmation of the pedicle as well as the appropriate trajectory, the largest possible pedicle screw is placed as shown in this intraoperative picture.


Step 1: Entry site is decorticated using a burr and a high-speed drill or a rongeur.
Step 2: Burr or awl is used to penetrate the dorsal cortex of the pedicle.
Step 3: Curved or straight pedicle probe is used to develop a path for the screw through the
cancellous bone of the pedicle into the vertebral body.
* advancement of the probe should be smooth and consistent.
* sudden plunge suggests breaking out of the pedicle laterally.
* an increase in resistance indicates abutment against the pedicle or vertebral body
cortex.
Step 4: After cannulation, the pedicle sounding probe is placed into the pedicle that is then palpated from within to make sure there is not a medial, lateral, rostral or caudal disruption in the
cortex of the pedicles.
* sound should also be used to determine that there is bone at the bottom of the pilot
hole verifying that penetration of the ventral cortex of the vertebral body has
not occurred.
Step 5: After pedicles have been probed, one can either:
a. place Steinman pins or K-wires bilaterally or unilaterally into the pedicles to confirm the
the trajectory and entry site and then go to (b) below:
b. tap the pedicle screw path if non-self tapping screws are used; then go to c. below:
c. place the permanent screws with the longest diameter that will not fracture the pedicle.
* length of the screw can be determined by measuring the length of he Steinman pin/
K-wire/pedicle probe from the pedicle entry site to a depth of 50-80% of the
vertebral body.
* the screws in the lumbar spine usually have a 4.5 to 7 mm diameter and a 35-50 mm
length.
Step 6: After pedicle screw placement, the transverse process and the lateral aspects of the facet joints
are decorticated, screws are connected to a longitudinal construct, usually a rod (can be a
plate)- these may need to be bent to conform to the proper curvature of the spine; screws
are secured (top-loading or side-loading); bone graf is then placed on the previously fusion
bed.

Intraoperative Verification of the Screw Trajectory and Placement:
1. Lateral and an AP radiograph or fluoroscopic image
* this does not guarantee accurate screw placement.
* accuracy can be improved with a slightly oblique AP view- a pin located in the middle of the
pedicle has a characteristic "target sign".
* direct AP views demonstrate the lateral to medial orientation of the screws.
- excessive medial orientation of the screws seen on AP films raises the concern of medial
penetration of the pedicle by the screw.
* lateral imaging is useful to view the depth of penetration into the vertebral body and sagittal
angulation of the trajectory (Figure 6).
- ventral screw penetration is usually between 50 and 80% of the AP diameter of the vertebral
body; penetration >80% of the vertebral body on lateral plain x-ray raises the concern
of ventral penetration of the vertebral body cortex.

Figure 6: Plain lateral lumbosacral x-ray showing adequate penetration of the pedicle screw into the vertebral body (between 50% to 80%). Greater than 80% penetration raises concern of ventral penetration of the vertebral body cortex.


2. Screw placement can be checked electrophysiologically with direct stimulation of the pedicle
probe or screw producing an electromyogram (EMG) response peripherally; if this response
occurs below the threshold expected for intact cortical bone, the screws may be redirected or
removed.
* For more information on this topic please refer to Clements et al in Spine 21:600-604, 1996.
3. Interactive frameless stereotaxy systems (including fluoronavigation) can be help guide in he proper placement of the screws. In the future, such technology will lead to percutaneous pedicle screw placement- Stay tuned!!