Placement of Thoracic Pedicle Screws



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




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.




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.






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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Feb 2, 2020 | Posted by in ORTHOPEDIC | Comments Off on Placement of Thoracic Pedicle Screws
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