Posterior Stabilization

The patient is placed in the prone position on a Jackson table or a radiolucent operating table to enable undisturbed anteroposterior (AP) x-rays or fluoroscopy. All pressure points and bony prominences should be thoroughly inspected and padded. The abdomen should hang free across appropriate cushioning to lower the intra-abdominal pressure and improve the venous return at the surgery site.

The standard midline posterior approach is performed. Any damage to fascial and muscular layers as well as the supraspinous ligament should be noted at the time of exposure. Careful exposure of the posterior bony and ligamentous structures follows. In cases where laminar or spinous process fractures have been documented, the dura might be bulging through the fracture defect. The ligamentum flavum should be carefully exposed for the same reason. The dissection is then carried out laterally to expose the facet joint capsule and the transverse processes completely. The facet joint capsule should be left intact in levels that are not to be fused. Exposure of the injured level typically involves a fair amount of bleeding from injured muscles, fractured bone surfaces, or damaged epidural vessels. Hemostatic agents such as thrombin-soaked Gelfoam and bone wax should be readily available. If spinal cord decompression is required, transpedicular resection can offer a corridor to the ventral spinal canal. The pedicle can be removed with a high-speed drill, and instruments such as Woodson elevators and down-going curettes can be used to push bony fragments anteriorly away from the canal (Fig. 178-5).

FIGURE 178-5 The patient underwent transpedicular decompression of L1. Canal diameter has been restored.

Throughout the years many instrumentation devices were developed for posterior spinal stabilization. Sublaminar wiring and sublaminar or transverse process hooks are valid instrumentation options and can be used both primarily and in cases where anatomic or pathologic parameters prevent the use of pedicle screws. Pedicle screws have superior biomechanical properties, including better pullout resistance and stabilization of all three spinal columns, as opposed to the posterior column alone with the other devices.^18–20 This has driven most spine surgeons toward the use of pedicle screws despite the higher skill level required, and this is now considered standard technique for posterior instrumentation.^21,22

Several techniques have been described for finding the entry point for pedicle screws. A freehand technique uses the anatomic landmarks.²³ In the lumbar spine, the entry point is found at the junction of the spinous process bisector and lateral border of the pars interarticularis (Fig 178-6). In the thoracic spine, the entry point varies with the level. As a general rule, in the mid-thoracic region, the entry points are relatively medial and cranial, at the junction of the proximal transverse process and just lateral to the middle of the superior facet location, though more lateral and caudal at the cranial and caudal levels²⁴ (Fig. 178-7). X-ray or fluoroscopy can be used to identify the bull’s-eye sign that is formed by the cortical borders of the pedicle.²⁵ This can be used to drive a guide wire or Steinman pin into the pedicle or to remove the posterior cortex. Imaging can be very challenging in the upper thoracic spine due to the thoracic kyphosis and rib angles, in obese patients, in patients with very poor bone quality, or in patients with spinal deformities. When an entry point cannot be verified, a laminotomy can be used to palpate the pedicle wall with a curved curette or a dental probe.

FIGURE 178-6 In the lumbar spine, the entry point is found at the junction of the spinous process bisector and lateral border of the pars interarticularis.

FIGURE 178-7 In the mid-thoracic region, the entry points are relatively medial and cranial, at the junction of the proximal transverse process and just lateral to the middle of the superior facet location, though more lateral and caudal at the cranial and caudal levels.

Once the entry point is identified, a sharp awl, Leksell rongeur, or high-speed drill is used to remove the posterior cortex. A blunt gearshift pedicle finder is then used to identify a trajectory through the pedicle’s cancellous bone and down to the posterior vertebral cortex. The posterior vertebral cortex is breached and the anterior vertebral cortex is palpated but not breached. A ball-tip probe is used to probe the trajectory walls and to verify intact bony wall at all four aspects of the canal and an intact anterior cortex. The probe or a depth gauge is used to determine screw length. Screw length varies from 25 mm for the upper thoracic and 60 mm for the lower lumbar.²⁶ Tapping of the pilot hole is recommended, usually 0.5 to 1 mm undertapping.²⁷ Probing of the borders can be repeated after tapping, because it is easier to feel gaps and breaches along the threaded bony walls.

On the sagittal plane, the straightforward trajectory that is parallel to the upper endplate of the vertebral body was found biomechanically better than the anatomic trajectory that is perpendicular to the dorsal cortex and points down to the antero-inferior corner of the vertebral body.²⁸ The axial angle might be more difficult to determine and ranges between 25 and 30 degrees of convergence in T1 to 0 to 4 degrees of divergence at T12. Screw diameter is also subject to great variation. Factors such as the patient’s age, sex, and body size influence overall screw diameter, and the spine level determines the specific screw size. Typically T6 and T7 have the smallest pedicles, and the lower lumbar levels have the largest.²⁹ Screw diameters of 4.5 mm to 6 mm should be available. Because most patients undergo preoperative CT of the spine, it is highly recommended to plan the trajectories and estimate screw lengths and diameters before surgery.²³

Once pedicle screw insertion is completed, x-rays are taken to verify screw trajectories at all levels. AP and lateral planes are used, although some surgeons perform lateral films only³⁰ (Fig. 178-8). Fracture-related deformity is corrected through positioning, and residual deformity is reduced by prebent rod fixation and careful compression and distraction maneuvers.³¹ The connecting rod is contoured to approximate the normal spinal contour and locked to the screw heads. Crosslink devices have been shown to increase the rigidity of the construct.³² Anterior bone fragments that bulge into the canal are often reduced when the vertebral height is restored.³³ Further reduction can be achieved via laminotomy when a curved curette or impactor is passed around the thecal sac and gentle tapping of the posterior vertebral cortex is possible.

FIGURE 178-8 Postoperative computed tomography shows good position of pedicle screws.

Fixation devices have been recommended to include two spinal levels above the fracture level and one or two levels caudal to the fracture level.^34,35 Such long segment spinal fixation fuses four or five spinal motion segments and therefore might result in significant loss of mobility and increased risk for adjacent level morbidity. In an attempt to lessen the latter, short segment instrumentation has been introduced, fusing only two motion segments. Mixed results with the use of short-segment instrumentation have been reported, with some good long- and short-term results³⁶ and some failures.^37,38 Better radiographic outcome does not fully correlate with clinical results.³⁹ Transpedicular bone grafting does not seem to improve short segment instrumentation outcome,^40,41 but insertion of pedicle screws in the fractured vertebra, when feasible, does.^42,43 Augmentation of the vertebral body with bone cement might present a valid adjunct to the posterior instrumentation.^44,45 We recommend short segment instrumentation be used in selected, mildly comminuted burst fractures only, and longer-segment instrumentation should be for all other fractures that are treated with the posterior approach only.^46,47

Decortication of the lateral gutters and the transverse processes is essential to ensure maximal bony fusion. A power drill, osteotome, or a curette is used to expose bleeding cancellous bone and remove cartilage from the facet joint at the levels to be fused. The use of autologous iliac crest bone graft is constantly declining owing to donor site morbidity.⁴⁸ Alternatives to iliac crest bone grafting include autologous bone from spinous processes and laminae, morselized allograft, or bone substitutes such as calcium phosphate or hydroxyapatite that can be mixed with a bone marrow aspirate or demineralized bone matrix.⁴⁹

Decompressive laminectomy was not found to improve neurologic outcome of spinal cord injuries accompanying thoracolumbar fractures.¹ It was, however, found to be related with higher rates of implant failure and is therefore not recommended.⁵⁰ New technologies have been introduced to improve the accuracy of pedicle screws and the reproducibility of the insertion technique. Intraoperative three-dimensional imaging⁵¹ and computer-assisted navigation systems⁵² have been found to improve the trajectory accuracy of pedicle screws and to reduce the radiation exposure of the operating room personnel (Fig. 178-9). Intraoperative electromyography (EMG) can be used to detect breached or misplaced pedicle screws by monitoring muscle activity in the rectus abdominis or lower limbs.⁵³ Intraoperative EMG can be time-consuming, and the actual benefits are yet to be proved.⁵⁴

FIGURE 178-9 Pedicle screws placed in T11.

Meticulous multilayer wound closure is imperative in posterior instrumentation for thoracolumbar fractures. The soft tissue damage can lead to accumulation of hematomas and seromas, wound infections, skin necrosis, and wound dehiscence. Most patients who do not suffer associated debilitating injuries are able to mobilize on the first postoperative day. We are not familiar with any study directly addressing the use of postoperative bracing, but a thoracolumbar sacral orthosis (TLSO) is usually recommended for a period of 4 weeks to 3 months after surgery depending on fracture type, instrumentation length, bone quality, and activity level (Fig. 178-10).

FIGURE 178-10 Postoperative x-rays with pedicle screws in place. The patient regained her ability to walk.

Anterolateral Approach

The anterolateral approach to the thoracolumbar spine is used for severely comminuted fractures of the anterior and middle spinal column that necessitate anterior stabilization, as well as spinal cord, conus medullaris or cauda equine injury, with obvious bony compression on the thecal sac, mandating direct release. Anterior decompression of the spine enables direct visualization of the dura and direct restoration of vertebral body height and anterior column stabilization.

Transpleural thoracotomy can be used for the treatment of fractures from T3 to L1; for the thoracolumbar region (T11–L1), the diaphragm should be divided and the retroperitoneal space entered. Purely lumbar fractures are approached through an anterior retroperitoneal approach.⁵⁵