Mark Dumonski, Thomas Stanley, Michael J. Lee, Bart Wojewnik, and Kern Singh
ANATOMY
Pedicle morphology is detailed in Table 1.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Standing posteroanterior and lateral radiographs should be obtained whenever possible.
Additional flexion–extension views may provide insight into subtle instabilities (FIG 1).
Full-length posteroanterior and lateral radiographs are obtained in cases of spinal deformity to assess for global balance (coronal or sagittal).
Lateral bending views can help determine the flexibility of the curve and levels for fusion.
Axial computed tomography (CT) images can provide invaluable information about pedicle morphology, particularly in the setting of deformity.
SURGICAL MANAGEMENT
Indications
Degenerative
Spondylolisthesis
Iatrogenic instability
Discogenic back pain
Pseudarthroses
Adult deformity
Curve progression
Neurologic deficit
Back pain refractory to nonoperative care
Pulmonary compromise secondary to deformity
Coronal or sagittal imbalance
Pediatric deformit.
Progressive scoliosis more than 50 degrees
Kyphosis more than 75 degrees
Curve progression despite bracing in a skeletally immature individual
Isthmic spondylolisthesis more than 50%
Preoperative Planning
Pedicle anatomy can be best assessed on CT (FIG 2).
A general assessment as to whether a pedicle is instrumentable can be gained by examining its size on an anteroposterior radiograph of the pedicle.
Pedicle width and length and starting points can be determined from the axial image.
Positioning
Patients should be placed in the prone position on a radiolucent table (FIG 3).
Care is taken to ensure that the neck is in a neutral position and is not hyperextended.
The arms are positioned at 90 degrees or less of abduction to minimize the likelihood of rotator cuff impingement. The arms are allowed to hang down slightly in a forward-flexed position about 10 degrees. The axilla should be clear from any padding to prevent brachial plexus palsy.
FIG 1 • Flexion and extension lumbar lateral spine radiographs can show evidence of spondylolisthesis as seen here at the L4–5 level.
FIG 2 • Pedicle anatomy for screw placement can be assessed with CT scan.
FIG 3 • The patient is positioned prone on the Jackson frame.
Elbow pads are placed along the medial epicondyle to protect the ulnar nerve.
The chest pad is placed just proximal to the level of the xiphoid process and distal to the axilla. In women, care is taken to tuck the breasts and ensure that the nipples are pressure-free.
The iliac pads are placed two fingerbreadths distal to the anterior superior iliac spine, allowing the abdomen to hang free and reducing any unnecessary epidural bleeding.
Proper placement of the chest and iliac pads allows for optimal restoration of sagittal alignment via gravity.
Approach
Two approaches are used: the midline approach and the paraspinal approach.
The midline approach is used for most spinal procedures as it allows direct access to the spinal canal.
The paraspinal approach, also known as the Wiltse approach, was initially described for spondylolisthesis but is also used for far-lateral discectomies and minimally invasive muscle-sparing techniques (eg, minimally invasive pedicle screw instrumentation or transforaminal lumbar interbody fusion).
Specific screw entry points are detailed in Table 2.
TECHNIQUES
THORACOLUMBAR PEDICLE SCREW PLACEMENT
Pedicle Start Point
Once the bony anatomy of the dorsal elements is meticulously exposed, the proper position of the pedicle entry point is defined. Anatomic landmarks include the lateral edge of the facet joint, the pars interarticularis, and the transverse processes (TECH FIG 1A).
The actual pedicle starting point may vary significantly from the commonly quoted “norms” in many patients. What follows are general guidelines. Preoperative imaging studies (such as CT scan, or even the relationship between the pedicle and the lateral aspect of the pars on an anteroposterior radiograph) can provide clues about anatomic variations in a given patient or level.
In both the lower (T10–12) and upper (T1–3) thoracic spine, the entry point is at the junction of the bisected transverse process and the lateral edge of the pars interarticularis.
In the midthoracic region (T5–9), the starting point is more medial and cephalad. Here, it is at the junction of the superior margin of the transverse process and the lateral third of the superior articular process (TECH FIG 1B).
In the lumbar spine, the entry point is at the midpoint of the transverse process and 2 mm lateral to the pars interarticularis.
The sacral entry point is at the inferolateral aspect of the L5–S1 facet joint.
Using a 4-mm high-speed burr, the posterior cortex is breached to a depth of about 5 mm (TECH FIG 1C).
Alternatively, fluoroscopic imaging may be used with the bull’s-eye technique to identify the correct starting point, particularly when patient anatomy is distorted (TECH FIG 1D).
Cannulating the Pedicle
A 3.2-mm hand drill is placed into the starting hole and advanced along the axis of the pedicle (TECH FIG 2A,B). The drill is advanced under fluoroscopic guidance into the vertebral body to an ultimate depth of 35 to 40 mm in the lumbosacral spine, 25 to 30 mm in the lower and upper thoracic spine, and 30 to 35 mm in the midthoracic spine.
Measurements of pedicle length can be made on axial CT or MRI scans and used to guide screw length.
The advantage of using a hand drill is that cortical violations are lessened. When resistance is met (cortex), the drill fails to advance, and consequently the angle is adjusted.
Alternatively, a “gearshift” type of device can be used to sound the pedicle. The gearshift should be rotated or wiggled as it is advanced with only gentle pressure. This technique allows the instrument to seek the proper path within cancellous bone rather than being pushed forcefully through a cortical wall. The process is analogous to feeding a guidewire into a vein during central line placement: the idea is to provide guidance, not force, to the instrument as it navigates a path within the cortical margins of the bone.
For the S1 pedicle, the drill is directed 25 degrees medially and 10 degrees inferiorly toward the sacral promontory. A lateral fluoroscopic image is used to identify the sacral promontory (TECH FIG 2C).
Ideally, the screw tip should achieve tricortical purchase (engaging the anterior and posterior cortex and superior endplate of S1) (TECH FIG 2D).
A flexible ball-tipped probe is then advanced down the pedicle tract. Bone should be encountered at the base of the tract as well as along all four walls of the pedicle. Medial and lateral cortical breaching is most common as the pedicle is narrowest in this plane.
A medial pedicle breach is most likely to occur at a depth between 15 and 20 mm ventral to the transverse process, which is the depth at which the spinal canal is encountered in most levels.
TECH FIG 1 • A. Posterior anatomy of the lumbar spine. B. Starting points for pedicle screws in the thoracic spine. C. The posterior cortex is breached with a 4-mm burr. D. The “bull’s-eye” technique with fluoroscopy can be used to correctly identify the starting point.
TECH FIG 2 • A. The hand drill is advanced into the pedicle. B. Path for the tricortical sacral pedicle screw. C. L5–S1 instrumentation with tricortical sacral fixation.
If a proper start site is selected, lateral breaches are more likely to occur deeper than 20 mm due to failure to medialize and follow the proper trajectory as the pedicle transitions into the vertebral body. However, if the start site is too lateral, a lateral breach may occur more superficially.
Pedicle Screw Sizing
With the ball-tipped probe advanced along the length of the pedicle tract, the surgeon measures the tract depth using a hemostat and a ruler (TECH FIG 3A).
In general, pedicles are tapped 1 mm smaller than the diameter of the screw to be used to optimize screw purchase. If the pedicle is sclerotic, “line-to-line” tapping should be performed. If the patient is osteoporotic, tapping is not necessary.
After tapping, the ball-tipped probe is again advanced through the pedicle tract to confirm that the pedicle cortices and anterior vertebral body are intact.
A Kirschner wire is then placed into the pedicle while the remaining pedicle tracts are cannulated.
All Kirschner wires are confirmed to be positioned properly via fluoroscopy. At this point, fusion bed preparation may occur (TECH FIG 3B,C).
Fusion Bed Preparation
The wound is copiously irrigated before decortication to preserve the local bone graft generated with high-speed burring.
Using a high-speed burr, the transverse process, the pars interarticularis, and the lateral wall of the facet joint of each level to be fused are decorticated.
Bone graft is placed over the decorticated areas. The fusion bed can be prepared with any combination of autogenous iliac crest bone graft, autogenous local bone graft (from the spinous processes and lamina), allograft, demineralized bone matrix, or bone morphogenic protein (TECH FIG 4).
TECH FIG 3 • A. With the ball-tipped probe inserted in the pedicle path, a hemostat marks the length probe inserted. B,C. Pedicle marker positions are confirmed with fluoroscopy.
TECH FIG 4 • After decortication, bone graft is placed over the decorticated areas.
Decorticating and bone-grafting the intertransverse, lateral pars, and lateral facet regions are performed before placing the screws to optimally prepare the fusion bed without the instrumentation getting in the way.
Once the bone graft has been placed, the Kirschner wires serve as identifying landmarks for pedicle screw cannulation. Care is taken to advance the screw slowly in the same angulation noted with the Kirschner wire in place.
PELVIC FIXATION
Sacropelvic fixation can be used in the setting of longdeformity reconstructions and tumor and in traumatic settings involving the lower lumbosacral spine.
Modern pelvic fixation is most easily accomplished via modular iliac screw placement.
After dissection of the posterior superior iliac spine, a starting point is identified 1.5 cm distal to the tubercle.
A burr or rongeur is used to create a recessed defect such that the iliac screw head will lie recessed within the posterior superior iliac spine.
A gearshift is then inserted into the starting point and advanced between the inner and outer tables of the pelvis, with the medial point of the probe scraping along the medial wall.
The trajectory should generally aim toward the hip joint.
The cortex of the medial wall is thicker than the lateral, and thus lateral violations are more likely than medial violations.
A ball-tipped probe is used to assess the inner and outer tables.
Depth is measured and the screw is inserted. The screw is typically 7.5 to 8.5 mm in diameter and roughly 60 to 80 mm long (TECH FIG 5).
TECH FIG 5 • Fusion to pelvis with iliac screws.
CROSS-CONNECTORS
Cross-connectors can significantly increase the rotational and bending stiffness of a multilevel construct.
One, two, or three cross-links can be used, depending on the length of the construct. If multiple cross-connectors are used, they should be spread as far apart as possible from each other for maximal construct rigidity.
HOOK INSERTION
Hooks can be placed about the pedicle, transverse process, or lamina.
Fixation is increased with a claw configuration.
A claw figuration is composed of two hooks directed toward each other, separated by one or two levels. Claws are primarily used at the ends of a construct (TECH FIG 6A).
Pedicle hooks provide the strongest fixation of all hook constructs. Always oriented cephalad, the pedicle hook is placed between the lamina of the superior vertebra and the superior articular process of the inferior vertebra (TECH FIG 6B). The U-shaped tip fits around the pedicle and allows for increased stability.
The inferior facet of the vertebra can be removed with an osteotome. It is helpful to resect enough of the facet so that the lateral edge of the spinal canal is identified so that it can be avoided during implant placement. The cartilage of the superior facet is removed with a curette. A pedicle hook developer is placed within the facet to develop the plane before placing the hook itself (TECH FIG 6C).
TECH FIG 6 • A. Thoracic hooks oriented in the claw configuration. B. Placement of a thoracic pedicle hook. C. A pedicle hook developer developing a plane for the pedicle hook. D. Placement of an upgoing laminar hook. E. Upgoing and downgoing transverse process hooks.
Laminar hooks can be placed on the superior (downgoing) or inferior (upgoing) laminae (TECH FIG 6D). They should be used with caution as a portion of the implant is placed within the spinal canal. Generally, placing two laminar hooks into the canal at the same level (eg, two downgoing hooks or two upgoing hooks on the same lamina) should be avoided to minimize implant volume in the canal unless canal volume is capacious.
The ligamentum flavum is dissected off the lamina, and the laminar surface receiving the hook is prepared with a Kerrison rongeur so the hook will be flush against the bone.
Transverse process hooks can be used when sublaminar or pedicle hooks are not possible (TECH FIG 6E). They can be oriented either cephalad or (more commonly) caudad. A transverse process hook developer is used to create a plane for the implant. Although weaker than sublaminar or pedicle hooks, they avoid violation of the spinal canal.
POSTOPERATIVE CARE
With secure multilevel pedicle screw fixation, it is probably not necessary to brace patients postoperatively, although that decision should be individualized based on the patient’s pathology.
COMPLICATIONS
Infection
The incidence of infection for posterior spine surgery is increased with the addition of an instrumented fusion.
A 1% infection rate has been noted for discectomies, a 6% infection rate for discectomies and fusion.
Although there is a wide range reported for instrumented posterior fusions, the overall infection rate appears to be around 5% to 6%.
Pseudarthrosis (nonunion rates, particularly crossing the lumbosacral junction)
The incidence of nonunion after posterior lateral intertransverse fusion ranges from 3% to 25%.
Smoking has been shown to be a risk factor for nonunion.
A wide range of fusion rates across the lumbosacral junction has been reported (22% to 89%).
A 92.5% fusion rate is reported across the L5–S1 junction when using iliac screws.
Neurologic and vascular injur.
Although there is potential for severe vascular injury with pedicle screws in the thoracolumbar spine, vascular complications are rare, outside of a few reports.
The risk of nerve root irritation has been reported to be very low (0.2%) from pedicle screw instrumentation.
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