Thoracic Pedicle Screw Placement

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Lawrence G. Lenke

Orthopaedic Surgery, Pediatric and Adult Spinal, Scoliosis and Reconstructive Surgery, Washington University Medical Center, Shriners Hospital in St. Louis, St. Louis, Mo., USA

The use of thoracic pedicle screws in the treatment of various spinal disorders has shown a dramatic increase in the last decade [1-4]. The bio-mechanical stability afforded by transpedicular fixation, the ability to correct various sagittal and coronal plane spinal malalignments and deformities, and the available room for bone grafting around the implants provide an optimal environment for spinal fusion success [5-10]. This chapter will discuss the technique of free-hand thoracic pedicle screw placement using anatomic landmarks, and a special thoracic pedicle probe combined with appropriate surgical 3-dimensional orientation and 'feel' to access the vertebral body via the pedicle channel [11]. This free-hand technique can be adopted by any surgeon comfortable placing lumbar pedicle screws and experienced with thoracic spinal anatomy and surgical techniques, and does not require the use of continuous or intermittent intraoperative fluoroscopy, radiography or navigational systems.

Preoperative Assessment

One of the first assessments that needs to be made by the surgeon contemplating the use of thoracic pedicle screws for the treatment of various surgical spinal disorders is whether the thoracic pedicle is large enough to probe down with a pedicle probe and subsequently accept a screw of appropriate and available diameter and length. There is a wide spectrum of individual pedicle dimensions, with the limiting dimension always being the medial to lateral diameter at the isthmus in millimeters [12-15]. This can range from only 2-3 mm in the mid thoracic region up to >10 mm at the lower thoracic levels. Interestingly, the smallest pedicle dimensions are usually found at the T5-T7 levels. Pedicle isthmus dimensions increase both above and below these mid thoracic levels, being consistently larger in the lower thoracic region (T10-T12), and somewhat smaller in the proximal thoracic region (T1-T3). Because of the varied anatomy and pedicle sizes, it is extremely important to have a variety of choices for pedicle screw diameters available ranging from a 4.0-mm all the way through a 7.5-mm diameter in 0.5-mm increments. The goal should be to maximize fit into the pedicle cortex with the largest screw that will safely fit. The lengths of the screws will vary but range from 40-50 mm in length in the lower thoracic regions to 25-35 mm in length in the proximal thoracic levels. The lengths will depend on patient size, and orientation of the screw, and will be shorter if the trajectory is somewhat lateral. It is also helpful to have both fixed head and multiaxial screws available. I tend to use fixed head screws in the thoracic spine, for they are lower profile and allow more room for bone grafting. However, multiaxial screws can easily be used as well.

In most instances, standard AP and lateral radiographs will adequately demonstrate the pedicle dimensions required to confirm the size of the thoracic pedicles and the ability of the surgeon to navigate down them into the body. One must remember that in a true frontal plane radiograph of the thoracic spine, the pedicles that are perfectly perpendicular to the x-ray beam will have the most accurate representation of their overall dimensions. With increasing thoracic kyphosis, the ability to adequately assess the pedicles above and below the apex will be diminished and occasionally additional views may be helpful in assessing pedicle dimensions in these specific cases. As an alternative, a preoperative CT scan can be obtained with a single slice through each mid pedicle level to check on overall isthmus dimensions. In my experience, these CT dimensions somewhat underestimate the diameter of the screw that can be placed since there does appear that cortical expansion occurs with aggressive tapping and final screw placement. This will be discussed further in the techniques section of this report. I would caution against using a preoperative MRI to assess pedicle dimensions as it appears to underestimate the true cortical size due to artifacts that can occur during the acquisition process.

It is much more difficult to assess rotated pedicles, such as those located at the concave apex of scoliosis deformities. For scoliosis cases, we rely on the assessment of pedicle dimensions noted at proximal and distal levels away from the apex where the vertebrae become more neutral in orientation. Routinely, if the pedicle dimensions are adequate at these more neutral levels, the pedicles will be accessible at the concave apical levels. Also, analyzing the convex-sided pedicle dimensions can provide information on the corresponding concave pedicles. The major difference is that the concave pedicles will tend to be more cortical than the more cancellous centered convex pedicles, although they appear to be relatively the same size at least in adolescent idiopathic scoliosis patients [16].

Free-Hand Pedicle Screw Technique

The technique of 'free-hand' thoracic pedicle screw placement is identical to that utilized in the lumbar spine and includes eight specific steps: (1) exposure,

(2) locating the appropriate starting point with burring of the dorsal cortex,

(3) use of the thoracic pedicle probe to navigate down the pedicle into the vertebral body, (4) intraosseous palpation, (5) undertapping by 0.5 mm of the intended screw diameter, (6) repalpating to confirm intraosseous borders, (7) screw placement, and (8) confirmation of intraosseous screw placement (table 1, fig. 1-8).

It is extremely important to have a well-exposed posterior spine with minimal oozing of blood into the field. The free-hand technique requires the identification of specific posterior element landmarks to help confirm the appropriate starting point. In addition, the vertebral body can often bleed a fair amount through the pedicle and thus blood loss can be an issue if multilevel screws are placed throughout the thoracic spine. So the initial spinal dissection must be meticulous and the pedicle tract should be liberally plugged with bone wax to minimize oozing from the body through the pedicle. It is also recommended to perform a partial inferior facetectomy which serves several purposes including to remove the cartilage off the facet joint, to improve the fusion rate, and also to gain access to the base of the facet joint which will be utilized as the starting point for the mid thoracic pedicle levels (T7, T8, T9).

Next a specific starting point on the posterior elements must be located for the creation of a cortical defect which should be at the superficial base of the pedicle. We have discovered that there are specific starting points for each thoracic pedicle between T1 and T12 that are fairly consistent for patients with or without a coronal and/or sagittal spinal deformity present [11]. Beginning distal at T12, the starting point is the down slope of the medial portion of the bisected transverse process where it meets the lamina which should be just at or lateral to the lateral portion of the pars. And as one proceeds to the T11 and T10 spinal levels, the starting point moves slightly more proximal and more medial such that by T10 the starting point is the proximal edge of the transverse process where it meets the lamina and facet. The starting points for T7, T8 and T9 are fairly consistent being the proximal portion of the transverse process where it meets the facet joint, just lateral to the mid portion of the superior facet. Then as one proceeds proximal from T7, the starting point becomes more lateral and distal, based off the anatomic landmarks of the thoracic transverse process-lamina junction. Thus at T6, similar to T10, the starting point is the proximal edge of the transverse process base at the junction of the lamina. Then by T3, the starting point is the bisected transverse process where the transverse process meets the lamina. This also holds true for T1 and T2, which are slightly larger pedicles, but have a greater medial inclination in the transverse plane [13].

Table 1. 8 steps of the 'free-hand' technique of thoracic pedicle screw placement

Step

Description

Exposure (see fig. 1)

Starting point and cortical burr (see fig. 2)

Pedicle gearshift-lateral (see fig. 3)

Pedicle palpation (see fig. 4)

Pedicle tapping (see fig. 5)

Thorough and meticulous exposure to the tips of all transverse processes to be included in the instrumentation/fusion.

Visualize the starting point based upon as much anatomical information as possible. For noncontiguous levels, this may be limited to a review of the pre- and intraoperative radiographs and orientation of the posterior elements. However, with successive levels, much information is provided by making fine adjustments to the trajectory of the previous level's screw or contralateral screw. Use a 5.0-mm acorn-tipped burr (or smaller burr in smaller patients) to thin the posterior cortex and enter the pedicle. The pedicle 'blush' should be visualized suggesting entrance into the cancellous bone of the pedicle. In cases with smaller pedicles, especially in the apical concavity of scoliosis patients, there will be very limited intrapedicular cancellous bone and therefore a pedicle blush may not be observed.

Use a 2-mm blunt-tipped, slightly curved pedicle finder (thoracic gearshift) facing laterally to enter the pedicle. Often the endosteal diameter of the pedicle is quite small, so allowing the finder to 'fall' into the pedicle. The thoracic gearshift should be perpendicular to the plane of the superior facet and/or lamina. Aiming slightly laterally initially will help to avoid medial wall perforations. Rotate the pedicle finder to a medially faced orientation after a depth of approximately 15-20 mm is attained (the average length of a typical pedicle). Carefully place the tip to the base of the prior hole before advancing the pedicle finder. Advance the finder to approximately the length of the desired screw and then rotate the finder 180° to make room for the screw. Make sure you feel bone the entire length of the pedicle. Any sudden advancement of the gearshift suggests penetration into soft tissue and thus a pedicle wall violation or vertebral body violation. These should be investigated immediately in order to possibly salvage the pedicle and avoid complications.

Use a flexible ball-tipped sounding device to palpate all 5 walls of the pedicle (cephalad, caudad, medial, lateral, and floor). Pay special attention to the junction of the middle and upper portions of the tract as this is the region of the pedicle where the spinal canal is located. If any wall besides the medial wall has been breached, the pedicle may be salvageable. In this circumstance, place bone wax in the pedicle hole to limit bleeding and reangle the pedicle finder with a more appropriate trajectory.

Tap the pedicle with a tap that is 0.5 mm smaller than the proposed screw. If there is difficulty passing the tap, use the next smaller tap and then retap the pedicle. If there is no anterior wall violation, one can use a K wire to assist with guiding the path of the cannulated taps. If there is any question of whether the anterior wall is intact, do not use a K wire as cardiac tamponade due to K-wire-induced trauma to a coronary artery has been reported.

Table 1 (continued)

Step

Description

Repeat pedicle palpation (see fig. 6)

Screw placement (see fig. 7)

Confirmation of intraosseous screw placement (see fig. 8)

Use the palpating device a second time to assess the bony pedicle walls and remeasure the tract length with a hemostat. Compare this measurement directly adjacent to the screw to be placed to ensure appropriate screw length.

Place the screw slowly to confirm it is threaded properly and allow for viscoelastic expansion of the pedicle. Make sure the angle of screw insertion matches the tract previously palpated and tapped.

After all the screws have been placed, intraoperative radiographs are repeated prior to placing the rods in order to confirm accurate placement of the screws. Some surgeons prefer to use fluoroscopy in order to obtain a true AP of each level. The coronal plane should show harmonious screw positions from proximal to distal. The sagittal plane should demonstrate parallel orientation to the superior end-plate so that no screw tip is anterior to the anterior vertebral body line. Perform EMG assessment of the screws based upon rectus abdominis data. Frequently we use active EMGs via the same approach through the pedicle finder or cannulated tap to assess real-time monitoring of the thoracic nerve root. The screws with the lowest impedance are removed and rechecked with a ball-tipped sounder prior to replacement of the screw if intraosseous borders are confirmed.

ffii

Thoracic Pedicle Screw Starting Points

Level

Cephalad-Caudad Starting Point

Medial-Lateral Starting Point

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