Placement of Thoracic Pedicle Screws
Kevin W. Wilson, MD
Ronald A. Lehman Jr, MD
Lawrence G. Lenke, MD
Dr. Lehman or an immediate family member serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons, the North American Spine Society, the Cervical Spine Research Society, and the Scoliosis Research Society. Dr. Lenke or an immediate family member has received royalties from Medtronic; has received research or institutional support from DePuy and Axial Biotech; and serves as a board member, owner, officer, or committee member of the Scoliosis Research Society. Neither Dr. Wilson nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this chapter.
PATIENT SELECTION
Indications
Thoracic pedicle screw fixation in spinal disorders has gained an increasingly larger acceptance because of its well-recognized advantages in achieving stable three-column fixation and improved control of three-dimensional deformities. Thoracic pedicle screw fixation is commonly and safely used for instrumented fusion constructs in the treatment of degenerative disorders, traumatic injuries, and deformities, but a thorough knowledge of the thoracic spine anatomy and surrounding structures is required.1,2,3,4 Thoracic pedicle screws are indicated for the stabilization of spinal segments after neural decompression for fracture, tumor, or infection or after correction of kyphotic or scoliotic deformities.
Contraindications
Contraindications to elective pedicle screw fixation are similar to those for other instrumentation procedures of the spine. As with any surgical procedure involving implants, the patient should be free from signs and symptoms of infection. Metal allergies can be a consideration. The ability to place pedicle screws and salvage options should be considered when preoperative images are reviewed for severely deformed, hypoplastic, or absent pedicles at the level of the planned instrumentation. Relatively few contraindications exist to surgical stabilization of unstable thoracic spine trauma; however, medically unstable or underresuscitated patients should be stabilized before undergoing a prolonged procedure in the prone position with expected blood loss. Severe osteoporosis is a relative contraindication. Inadequate screw purchase as a result of low bone mineral density may be addressed with screw augmentation techniques, such as polymethylmethacrylate screw augmentation.5
PREOPERATIVE IMAGING
Preoperative AP and lateral radiographs are imperative for surgical planning (Figure 1). CT can be obtained to evaluate the pedicle size and the anatomy of deformities. Pedicle dimensions can be evaluated on AP and lateral radiographs if the surgeon is mindful of the plane of the X-ray beam. The most accurate assessment of pedicles is made in the true frontal plane. Assessment of pedicles of rotated levels may be nearly impossible on plain radiographs. If, at any level, the pedicle size appears to be too small to accept a 5-mm screw, or the plain radiography examination is insufficient, CT may be used to provide accurate evaluation of the levels to be instrumented. It is our experience, however, that CT scans occasionally are fraught with inadequacies when used to estimate the true size of a pedicle. A detailed understanding of the three-dimensional anatomy of the vertebral body/pedicle is of the utmost importance. A thorough knowledge of the curve magnitude, length of fixation construct, and pedicle size will optimize surgical time. Pedicles in the midthoracic spine have the smallest width in patients of all ages, and the pedicles at the apices of the concavity of a scoliotic curve typically are smaller than those on the convex side. In one report of 30 patients with adolescent idiopathic scoliosis, thoracic pedicle size varied from 4.0 to 8.2 mm.6 Typically, screws 80% to 115% of the size of the outer pedicle diameter can be inserted safely through gradual plastic deformation—a technique known as pediculoplasty—with a probe and bone tap. Pediatric pedicles possess greater inherent viscoelastic potential for pedicle expansion than do pedicles in adults.
PROCEDURE
Room Setup/Patient Positioning
The patient is placed prone on a radiolucent four-poster frame or OSI table (Orthopedic Systems) with a spine top, which consists of an open-frame design with positioning pads to support the prone patient, to facilitate intraoperative imaging, maintain adequate sagittal alignment, and minimize pressure on the anterior thorax or abdomen. During prone positioning, it is important to ensure that the patient is moved down toward the foot of the
operating table as much as possible. This allows the arm boards to be positioned as close to the head of the operating table as possible; otherwise, the surgeon has difficulty standing while gaining access to place the screws in the proximal thoracic spine. We typically place the legs in a sling to promote systemic venous return. Hyperkyphosis of the proximal thoracic segments can make it difficult to gain the correct sagittal trajectory when placing screws in the proximal thoracic spine. Additionally, the skull and paraspinal muscles in the lateral aspect of the wound can interfere with probe trajectory, which is convergent in the upper thoracic spine. Halo skull traction can be considered, if necessary, to facilitate flexing the patient’s neck to reduce the kyphotic angle for easier screw placement (Figure 2).
operating table as much as possible. This allows the arm boards to be positioned as close to the head of the operating table as possible; otherwise, the surgeon has difficulty standing while gaining access to place the screws in the proximal thoracic spine. We typically place the legs in a sling to promote systemic venous return. Hyperkyphosis of the proximal thoracic segments can make it difficult to gain the correct sagittal trajectory when placing screws in the proximal thoracic spine. Additionally, the skull and paraspinal muscles in the lateral aspect of the wound can interfere with probe trajectory, which is convergent in the upper thoracic spine. Halo skull traction can be considered, if necessary, to facilitate flexing the patient’s neck to reduce the kyphotic angle for easier screw placement (Figure 2).
 FIGURE 1 Preoperative AP, side-bending, and lateral radiographs show a patient with a Lenke type 1AN curve with only 34% side-bending flexibility. (Adapted with permission from Lehman RA, Lenke LG, Keeler KA, et al: Operative treatment of adolescent idiopathic scoliosis with posterior pedicle screw-only constructs: Minimum three-year follow-up of one hundred fourteen cases. Spine [Phila Pa 1976] 2008;33[4]:1598-1604.) |
 FIGURE 2 Photograph depicts a halo skull traction setup with the patient on the spine table. |
Neuromonitoring, including somatosensory-evoked potentials and motor-evoked potentials, should be used to monitor the spinal cord during screw insertion and correction maneuvers. Real-time spontaneous electromyography (EMG) monitoring of thoracic nerve roots T6 through T12 through the rectus abdominis musculature adds a layer of safety, and triggered EMG can be used to confirm screw placement.
Special Instruments/Equipment/Implants
Fluoroscopic and computer-generated image-guided techniques have been developed to improve pedicle screw placement accuracy7; however, their use typically requires additional resources and may require longer surgery time, theoretically increasing blood loss and the
opportunity for infection. In time, advancements in technology may improve placement accuracy and patient outcomes, but details of these techniques are beyond the scope of this chapter. The foundations of pedicle screw placement are understanding the anatomy and developing the skill and tactile sense to navigate pedicle tracts, which can be done with techniques and instruments available to most spine surgeons. Freehand insertion of screws, without the use of intraoperative radiographic guidance or tracking, appears to be safe and reliable and is described in this chapter.8 The thoracic gearshift probe and flexible ball-tipped pedicle-sounding device are critical components of the pedicle screw instrumentation set.
opportunity for infection. In time, advancements in technology may improve placement accuracy and patient outcomes, but details of these techniques are beyond the scope of this chapter. The foundations of pedicle screw placement are understanding the anatomy and developing the skill and tactile sense to navigate pedicle tracts, which can be done with techniques and instruments available to most spine surgeons. Freehand insertion of screws, without the use of intraoperative radiographic guidance or tracking, appears to be safe and reliable and is described in this chapter.8 The thoracic gearshift probe and flexible ball-tipped pedicle-sounding device are critical components of the pedicle screw instrumentation set.
For pediatric and adult deformity cases, it is important to have a variety of screw sizes available, as dictated by preoperative planning. In the immature spine, the pedicle widths are smaller and increase with age. A significant decrease is seen in the width of the concave pedicles compared with the convex side in patients with thoracic scoliosis. The instrumentation sets should contain pedicle screws ranging in diameter from 4 to 7 mm, in 0.5-mm increments. Lengths should range from 25 mm for small-diameter screws to 55 mm for large-diameter screws. Monoaxial screw heads have a lower profile, which is conducive to the superficial thoracic spine and allows better manipulation of the spine during derotation maneuvers. Additionally, these screws are less expensive than their multiaxial counterparts. Polyaxial screws may be helpful for rod placement after curve correction because their ability to vary the angle of the screw head allows better seating of the rod to the screw. More recently, uniaxial screws have become available; these screws combine the benefits of the monoaxial and multiaxial screw heads. They are locked in the axial plane to allow for vertebral column derotation but permit variance in the cephalocaudal direction, resulting in easier seating of the rod.
SURGICAL TECHNIQUE
Incision and Exposure
The extent of the skin incision is marked, from the highest planned instrumented vertebra to the lowest. The incision should be a straight, vertical line connecting these two points to ensure a straight incision after the scoliosis is corrected. For short-segment procedures, levels should be confirmed using surface anatomy landmarks with fluoroscopic confirmation. In deformity or scoliosis procedures, the neutral vertebrae are selected carefully at both ends of the curve. Exposure typically involves an incision from the spinous process above the most cranial vertebra to the spinous process of the most caudal vertebra in the segment to be instrumented. For an expeditious approach, the anesthesia team should paralyze the patient pharmacologically to facilitate exposure of the soft tissues. Muscle paralysis allows for better retraction of the paraspinal muscles during dissection and reduces muscle contraction during unipolar cautery, thereby reducing blood loss. Additionally, pharmacologically induced hypotension further aids in reducing blood loss. The anesthesiologist should be reminded to reverse the paralysis before instrumentation, to avoid its interfering with electrophysiologic monitoring.
 FIGURE 3 Intraoperative photograph shows the posterior elements of the spine exposed to the edge of the transverse processes bilaterally. The inferior facets are removed with a 0.5-in straight osteotome (down to T10) or rongeur (below T10). Articular cartilage is removed from the dorsal side of the superior facet of the inferior vertebra using a small curet. (Reproduced with permission from Kim YJ, Lenke LG, Bridwell KH, Cho YS, Riew KD: Free hand pedicle screw placement in the thoracic spine: Is it safe? Spine [Phila Pa 1976] 2004;29[3]:333-342.) |
The surgeon carries the dissection along the midline and carefully dissects subperiosteally laterally to the tips of the transverse processes (Figure 3). Meticulous exposure of the posterior elements is required to place thoracic pedicle screws successfully. In an immature spine, the dorsal spinous process epiphysis can be bluntly scraped away to provide an obvious periosteal edge. The posterior elements are exposed by staying strictly subperiosteal to reduce bleeding. Bovie electrocautery is used as the periosteum is scraped laterally to the tips of the transverse processes, bilaterally. Next, a wide facetectomy is performed. The facetectomy facilitates access for the starting point, allows for facet fusion, provides local bone graft via the resected facet to facilitate fusion, and allows the spine to become more flexible to facilitate better correction. Using an osteotome, the inferior 3 to 5 mm of the inferior facet is removed, and the exposed cartilage is scraped from the top of the superior facet surface to enhance the fusion bed for intra-articular arthrodesis.
Starting Point and Trajectory
A good starting level is the most neutrally rotated and most distal instrumented vertebra. By working in successive levels from caudal to cephalad in the thoracic spine, with an appreciation for the general trends of the starting points, fine adjustments may be made to the trajectory of each screw based on the screw position of the previous level or contralateral pedicle. The starting points can be grouped according to their location (Figure 4). The T1 through T3 and T12 starting points are at the midpoint of
the transverse process in the vertical direction and 2 mm lateral to the midpoint of the facet. This is known as the superior facet rule (Figure 5). The starting points for T4, T5, and T11 lie at the proximal third of the transverse process and 2 mm lateral to the midpoint of the facet. The T6 and T10 starting points lie along the ridge, or confluence, of the cephalad aspect of the lamina, whereas the vertical starting points of T7 through T9 lie at the junction of the facet and lamina.
the transverse process in the vertical direction and 2 mm lateral to the midpoint of the facet. This is known as the superior facet rule (Figure 5). The starting points for T4, T5, and T11 lie at the proximal third of the transverse process and 2 mm lateral to the midpoint of the facet. The T6 and T10 starting points lie along the ridge, or confluence, of the cephalad aspect of the lamina, whereas the vertical starting points of T7 through T9 lie at the junction of the facet and lamina.
 FIGURE 4 Illustration shows pedicle screw starting points using a 3.5-mm acorn-tipped burr. The posterior elements are burred to create a posterior cortical breach approximately 5 mm in depth. (Adapted with permission from Kim YJ, Lenke LG, Bridwell KH, Cho YS, Riew KD: Free hand pedicle screw placement in the thoracic spine: Is it safe? Spine [Phila Pa 1976] 2004;29[3]:333-342.) |
In our own surgical experience, we have noted a reliable and unique anatomic structure known as the ventral lamina. The ventral lamina is formed by the roof of the spinal canal that becomes confluent laterally with the medial aspect of the pedicle wall (Figure 6). An increased understanding of its three-dimensional structure and relationships has improved our ability to place screws safely in pedicles previously thought impossible, or too difficult, to instrument. A consistent anatomic relationship exists between the ventral lamina and the superior articular facet (SAF) at all thoracic levels, except at T12 because of its transitional nature. The ventral lamina can be used as a reliable guide for the starting point in pedicle screw placement. Taking advantage of the fact that the medial cortical wall is thicker than the lateral wall and that the ventral lamina consistently is medial to the midpoint of the SAF, the medial cortical wall can be used as a guide for the path of the pedicle screw while avoiding the feared medial cortical breach. We recommend that the starting point for the placement of pedicle screws be 2 to 3 mm lateral to the midpoint of the SAF.
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