Posterior Thoracic Instrumentation with Pedicle Screws, Hooks, Wires, and Rods

Patient Selection

Spinal instrumentation is used to augment the treatment of traumatic, neoplastic, congenital, and degenerative disorders of the thoracic spine in circumstances where there is structural instability. Instrumentation provides immediate stabilization, allowing for earlier mobilization, and has been shown to improve the rates of fusion in traumatic and degenerative conditions. ¹ The addition of instrumentation to a fusion construct stabilizes the vertebrae to allow arthrodesis to occur. Universal spinal instrumentation systems, such as laminar hooks and pedicle screws, are used in isolation or in combination for single or multiple column fixations.

The indications for thoracic instrumentation are as follows. In patients with thoracic instability after trauma, instrumentation is often recommended for immediate protection of the neural elements. Other common uses include management of iatrogenic instability after spine surgery, for deformity correction, and for stabilization following treatment of infections or tumors.

Each form of instrumentation has its own advantages and drawbacks. Hooks and wires require intact posterior elements, and implants are placed within the spinal canal. These constructs would be contraindicated for use in spinal canal stenosis; however, hooks are ideally suited for use as terminal components in three-point bending constructs and serve as excellent adjuncts to avoid screw pullout. Pedicle screws avoid the spinal canal and provide rigid fixation to all three columns of the spine and therefore are increasingly used to replace hook and wire constructs. The thoracic spine, however, has small and variable pedicles, making screw placement more difficult and prone to complications. Therefore, multiple instrumentation techniques should be learned and applied as dictated by the patient’s anatomy and pathology. The relevant contraindications, advantages, and disadvantages to the use of wires, hooks, and pedicle screws are detailed in ▶ Table 37.1.

**Table 37.1** Relevant contraindications, advantages, and disadvantages to the use of wires, hooks, and pedicle screws
Relative	Pedicle screws	Hooks	Wires
Contraindications	Severe osteoporosis	Disrupted posterior elements	Disrupted posterior elements
		Canal stenosis	Canal stenosis
Advantages	Three-column fixation	Simpler application	Simpler application
	No implant placed into the spinal canal	Lower cost	Lower cost
Disadvantages	Technically demanding	Posterior column fixation only	Posterior column fixation only
	Neurologic injury with screw misplacement	Multiple segments included for stability	Multiple segments included for stability
	Visceral/vascular injury with screw misplacement	Hooks placed into the spinal canal with risk of neurologic injury	Wires placed into the spinal canal with risk of neurologic injury
	Higher cost	Hook dislodgement	Specific indications
	Increasing imaging artifact		No resistance to axial loading

37.2 Preoperative Preparation

Before entering the operating theater, patients should have completed a full history and physical examination by the surgeon as well as the primary care physician for perioperative optimization of concurrent comorbidity. Preoperative laboratory studies and cardiopulmonary tests should be performed as indicated. Radiographic studies should include plain radiographs or computed tomographic (CT) scans or both to assess for any anatomical constraints to placing hardware (narrow pedicles for pedicle screws). Preoperative plain films are also useful as comparisons for intraoperative localization. Magnetic resonance imaging (MRI) should be performed to assess for spinal canal compromise or stenosis. Intraoperative C-arm fluoroscopy should be reserved, and when available, neuronavigation can be used for both preoperative planning and intraoperative guidance.

37.3 Operative Procedure

The procedure should be performed with the patient prone after undergoing general anesthesia with endotracheal intubation. It is important to reduce intra-abdominal pressure by allowing the abdomen to hang freely, which will help reduce central venous pressure and blood loss. This can be done by using a standard bed with bolsters under the patient’s chest and hips or by using a table that suspends the patients over a frame with pads suspending the upper chest, hips, and legs (e.g., Jackson table [OSI, Union City, California]). Care should be taken to position patients in the desired alignment. The arms should be placed to the side when operating on the high thoracic levels to facilitate surgeon comfort and imaging localization. All pressure points are appropriately padded to avoid compressive neuropathies, a Foley catheter is placed for fluid status monitoring, and mechanical thromboembolism devices are used to prevent deep venous thrombosis. We do not perform arterial line monitoring under normal circumstances unless blood loss or cardiac comorbidities are a concern, or we routinely use electrophysiological monitoring unless spinal cord injury or gross spinal instability is a likely.

Before the incision is made, radiographs are used to localize the appropriate surgical levels. In taller patients, a standard radiograph may be insufficient. Imaging options include long-cassette scoliosis films or fluoroscopy to localize midthoracic lesions. When using fluoroscopy to account for parallax effect, care must be taken, which often requires counting spine levels using continuous fluoroscopy. High thoracic localization (T1–4) is difficult. Imaging options include plain X-rays or fluoroscopy with a saline bag overlying the patient’s lateral neck. This seeks to simulate the tissue density of the shoulders so that the X-ray beams are attenuated, similarly reducing the variation in density between the shoulders and the neck. Another technique is to use an oblique fluoroscopic view along the axis of the patient’s laminae. This so-called foraminal view allows the pedicles to be counted from C2 into the upper thoracic spine and looks around the shoulders. Finally, if available, intraoperative cone-beam CT imaging (e.g., Medtronic O-arm) is an option to image the upper thoracic spine. In these cases, it is important to carefully evaluate the preoperative imaging to rule out anatomical variants that might contribute to errors in localization (e.g., lumbarized sacral vertebrae, cervical rib). When in doubt, consult radiology preoperatively or intraoperatively to assist with localization. Finally, perioperative antibiotics are initiated before marking, preparing, and draping the skin in the usual sterile fashion.

37.3.1 Relevant Anatomy

The thoracic spine consists of 12 vertebrae oriented in a kyphotic alignment with an average curve of 42 degrees in normal adults ² ( ▶ Fig. 37.1). The vertebral bodies increase in size as they progress from the first to the last thoracic level. The upper thoracic spine resembles the cervical vertebrae, whereas the lower thoracic spine resembles the lumbar vertebrae with more rounded anterior columns. This is in contrast to the midthoracic spine, where the vertebral bodies are triangulated anteriorly with a heart-shaped appearance. The rib heads articulate with the transverse process at the level of the rib, the pedicle, and across the disk space to the level below.

Fig. 37.1 Thoracic vertebra gross anatomy. (Reproduced with permission from the Mayfield Clinic).

The facets in the thoracic spine are coronally oriented, with the superior facet facing posteriorly and cranially and the inferior facet facing anteriorly and caudally. These facets resemble shingles on a roof and allow the thoracic spine to perform lateral bending, flexion, and axial rotation. At the thoracolumbar junction, the facets convert to a more sagittal orientation, thereby transitioning to the lumbar spine.

The thoracic pedicles also vary widely in terms of their width and height throughout the thoracic spine ( ▶ Fig. 37.2). The upper and lower thoracic pedicles have greater pedicle width, whereas the T3 to T9 pedicles have more narrow pedicles. Fortunately, in the midthoracic spine, the thoracic pedicle–rib complex provides ample cortical purchase (through an in-out-in technique) for placing thoracic pedicle screws at all levels.

Fig. 37.2 Pedicle anatomy. (a) Mean pedicle and pedicle–rib unit widths from T1 to T12 for previous studies. (b) Mean pedicle height from T1 to T12 for previous investigations. (c) Mean pedicle transverse angles from T1 to T12 for previous studies. (Reproduced with permission from the Mayfield Clinic.)

The spinous processes of the upper and middle thoracic spine vary from their cervical and lumbar counterparts in that they are long and significantly overlap the vertebral level below. One should recognize this variation so as not to misjudge the level of interest. The spinous processes begin to resemble those of the lumbar spine as the level descends toward the thoracolumbar junction. Transverse processes of the thoracic spine arise at the junction of the superior facet and pedicle and serve as a good landmark for pedicle screw placement but become smaller at the thoracolumbar junction. The interlaminar space is small between adjacent thoracic laminae.

37.3.2 Wire–Rod Techniques

Wire–rod techniques were originally used to supplement the Harrington rod system and consist of straight rods attached to the lamina, spinous, or transverse processes via wires. ³ This fixation technique required exposing one level above and below the area of instrumentation to reveal the laminae or transverse processes bilaterally. The most common wire–rod technique is sublaminar wiring ( ▶ Fig. 37.3), but spinous process wiring can also be used to provide rotational stability. Spinous process wiring does not prevent flexion or extension movement as well as sublaminar wiring, but the latter is contraindicated at levels with significant canal stenosis and requires the posterior elements to be intact.

Fig. 37.3 Wire–rod techniques. The most commonly used wiring techniques use the placement of sublaminar wires, although spinous process wiring can be used. (a) Sublaminar wires are passed under the lamina in a caudal to rostral direction. (b) Once pulled through, the wire loop is cut to allow the use of each half of the wire on either side of the midline. The wires are pinched over the lamina to prevent wire migration ventrally into the spinal canal. (c) The wires are then twisted around rods. The wires on one side are tightened at the caudal end while the contralateral rod is secured with the wires at the rostral end. The rods are then gradually secured by sequential tightening of the wires to gradually restore alignment. (d) Spinous process wires are passed through a hole at the base of the spinous process but avoiding entering the spinal canal. The hole should be just above the internal cortex. Wires are passed through the hole, and buttons may be applied to prevent the wire from fracturing the bone. (Reproduced with permission from the Mayfield Clinic.)

To pass a sublaminar wire, the designated superior and inferior laminae are identified, along with the adjacent ligamentum flavum underneath. The caudal ligamentum flavum is incised to expose the underlying epidural fat, and a small laminotomy is created at the rostral and caudal levels of each lamina to enlarge the interlaminar space and facilitate the safe passage of wires. The mesial border of the superior facet may also need to be shaved to widen the epidural space. A 16-gauge (1.2-mm) braided cable or looped wire is passed underneath the lamina in a caudal–cranial direction with the tip angled toward the undersurface to prevent injuring the dura underneath. Smaller wires tend to sheer through the laminae, whereas thicker wires risk exacerbating spinal stenosis. Once the wire pierces through the interlaminar space above, a needle-holder is used to grab and tension the wire away from the dura during further advancement. Next the wire loop is cut and pinched over the lamina. Rods are contoured and cut to the appropriate length and placed on each side. The wires are looped and tightened around either end of the rod and sequentially tightened. L-shaped rods can be used to prevent rotational forces, prevent caudal or rostral migration of the rod, and increase resistance to lateral bending

37.3.3 Hook–Rod Techniques

Hook–rod constructs are ideally suited for use as terminal components in three-point bending constructs and serve as excellent adjuncts to avoid screw pullout. Hooks can be rostrally or caudally facing along the laminae, rostrally facing along the pedicle, or caudally facing along the transverse process ( ▶ Fig. 37.4).

Fig. 37.4 Hook–rod techniques. An array of hooks are available for use in hook–rod constructs, including laminar, pedicle, and transverse process hooks. (a) Laminar hook placement. Laminar hooks can be inserted rostrally or caudally (supralaminar or infralaminar), but care must be taken to prevent neurologic compromise. The ligamentum flavum and lamina may be partially resected to facilitate hook insertion but may reduce the extent of bone for proper seating of the laminar hook. Once adequate space is created underneath the laminar edge, a feeler instrument is used to assess the fit and determine the correct hook size. The general principle is to select the largest possible hook to optimize bony purchase without encroachment on the spinal canal. Once the hook is selected, it is attached to a hook holder and carefully inserted, taking care to closely oppose the undersurface of the lamina to avoid neurologic injury. (b) Pedicle hook placement. The ideal location of a pedicle hook is underneath the inferior articulating process just adjacent to the base of the pedicle. For this reason, pedicle hooks should not be placed at the distal end of a construct to avoid destabilizing the facets. To facilitate placement, the caudal one-third of the inferior facet is typically removed parallel to the axis of the vertebral body to allow optimal hook purchase; however, if too much bone is removed, the hook may cut into the pedicle. If too little bone is removed, there may be inadequate purchase, resulting in hook migration. As previously mentioned, a “feeler” can be passed underneath the inferior facet to engage the pedicle and facilitate proper sizing. The hook, which is attached to its holder, is then inserted and directed cephalad to engage the pedicle base. (c) Transverse process hook placement. Placement requires full exposure of the transverse process and resection of the costotransverse ligament. Once adequate exposure has been obtained, a “feeler” instrument can be inserted underneath the rostral aspect of the transverse process and directed caudally. (Reproduced with permission from the Mayfield Clinic.)

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