Cervical Pedicle Screws

Chapter Preview

Chapter Synopsis	Cervical pedicle screws offer a biomechanical advantage over other types of posterior cervical instrumentation, but at the expense of increased risk of iatrogenic neurovascular injury. Careful preoperative planning with evaluation of three-dimensional imaging and meticulous surgical technique, however, leads to a low incidence of complications, particularly in experienced hands in high-volume centers. The purpose of this chapter is to review the preoperative considerations, surgical technique, complications, and results for cervical pedicle screw fixation.
Important Points	This procedure is indicated for most cervical disorders requiring stabilization. It is contraindicated in patients with narrow or absent pedicles. It is contraindicated where the pedicle is destroyed by trauma or tumor. Preoperative computed tomography is essential to assess pedicle morphology. Three-dimensional imaging of vertebral arteries is indicated if any suspicion exists of involvement in a pathologic process or an aberrant anatomic course.
Clinical and Surgical Pearls	Portals are used percutaneously to probe, sound, and tap the pedicle. The entry point is determined by local bony anatomy. The trajectory of insertion is guided by direct visualization of the medial cortical pedicle wall, the outer cortex of the ipsilateral lamina, and the direction of the contralateral lamina. The entry point and trajectory in the sagittal and transverse planes are confirmed by oblique orthogonal intraoperative fluoroscopy.
Clinical and Surgical Pitfalls	Injury to the vertebral artery is best avoided by careful selection of patients. Cases of aberrant arterial anatomy or disadvantageous pedicle morphology can usually be determined preoperatively. The surgeon should avoid injury to the spinal cord by decompressing developmentally narrow spinal canals before kyphosis correction. The surgeon should avoid iatrogenic foraminal stenosis secondary to excessive reduction of translational deformity by prior foraminotomy.

Posterior cervical instrumentation is frequently indicated for the treatment of conditions of traumatic, degenerative, cancerous, or inflammatory origin. This instrumentation has evolved from wires to facet and lateral mass screws and laterally based pedicle screws. The evolution in cervical instrumentation has coincided with the increased availability and higher resolution of three-dimensional medical imaging. Cervical pedicle screws, first described in the 1960s as C2 pedicle screw insertion for osteosynthesis of a hangman fracture, and popularized by Abumi and colleagues for reconstructive surgery of the subaxial cervical spine in 1994, offer a biomechanical advantage over other techniques that makes them an attractive option to surgeons, but at the expense of increased risk of iatrogenic damage to the adjacent neurovascular structures. Cervical pedicle screws offer a potential benefit in patients with deficient or dysplastic lateral masses or laminae.

Studies performed since the 1990s in an attempt to reduce the risks of pedicle screw insertion have addressed pedicle morphology, optimal entry point, and trajectory, and preoperative and intraoperative imaging. Nonetheless, the technique is inherently risky. Preoperative radiologic evaluation of the pedicles and adjacent neurovascular structures is mandatory, as is meticulous operative technique. The purpose of this chapter is to review the preoperative considerations, surgical technique, complications, and results for cervical pedicle screw fixation.

Preoperative Considerations

History

When considering the use of pedicle screws to stabilize the cervical spine, the surgeon should take a focused history directed toward the potential for congenital vertebral anomalies and abnormalities of the vertebral arteries. Cervical malignant disease or spondylodiskitis can involve the arteries in the pathologic process. Furthermore, vertebral artery injury is associated with 0.5% of all cases of blunt trauma, and this rate approaches 30% to 40% in patients with cervical fractures because of the tortuous semiosseous course of the artery.

Signs and Symptoms

Damage to the dominant vertebral artery from trauma or involvement of the artery in a pathologic process can lead to symptoms and signs of posterior circulation stroke or transient ischemic attack. The presence of any of these features should direct the clinician to the potential for involvement of the vertebral artery and necessitates imaging of the arteries.

Physical Examination

The focus and the extent of the physical examination should be directed by the nature of the disorder for which the operative intervention is being considered. The patient is observed for the syndromic features of conditions with known cervical spine involvement and for cutaneous manifestations of systemic diseases such as neurofibromatosis. In cases of deformity correction, the cervical alignment is assessed, and consideration is given to the overall spinal balance. Assessment of horizontal gaze is paramount in correction of cervical kyphosis, to help calculate the amount of correction required intraoperatively. A detailed neurologic examination then follows to exclude ischemic posterior circulation stroke secondary to vertebral artery occlusion, spinal cord compression or injury, and radicular pattern nerve root dysfunction.

Imaging

Plain anteroposterior, lateral, and oblique radiographs may provide an indication of when pedicle cannulation would be difficult, for example, by the gross absence of a pedicle as a result of infiltration by tumor or involvement of a pedicle in a fracture. However, plain radiography alone provides insufficient detail of pedicle morphology. Fine-cut (1.0 to 1.5 mm) computed tomography (CT) with bone windows is recommended to aid surgical planning. Morphometric studies have shown that the outer diameter of most cervical pedicles is greater than 5 mm. Investigators have recommended that pedicle screw placement should not be attempted if the outer pedicle diameter is less than 4 mm, when screw placement may be impossible. The lateral pedicle cortex is typically thinner than the medial, thus increasing the risk of violation of the foramen transversarium. Furthermore, the relative expansion of the dominant vertebral artery may be associated with a narrower pedicle than on the contralateral side of the same vertebra ( Fig. 42-1 ). Evaluation of the axial sections of the CT scan through the pedicles allows for detection of sclerotic pedicles, pedicles infiltrated with tumor ( Fig. 42-2 ), or pedicles involved in fractures ( Fig. 42-3 ). Pedicle screw insertion is not recommended in any of these conditions.

Magnetic resonance imaging (MRI) is performed for diagnosis and in preoperative planning for most cervical disorders. The addition of magnetic resonance angiography (MRA) sequences allows identification of the dominant vertebral artery and provides increased detail of the precise course of the arteries. The vertebral artery occasionally loops into the vertebral body ( Fig. 42-4 ), and ipsilateral pedicle screw insertion is not advised in such patients. MRA should be performed whenever CT or MRI results suggest anomalies in the course of the vertebral arteries or when the arteries may be involved in the disease process.

Furthermore, preoperative CT and MRI can be helpful in patients with preexisting foraminal stenosis when cervical instrumentation is being used to correct deformity. The presence of foraminal stenosis in these cases is a relative indication for prophylactic foraminotomy as a result of the high incidence of iatrogenic neural injury in kyphosis correction.

Indications and Contraindications

Cervical pedicle screws are indicated for potentially all conditions of the cervical spine in which stabilization is required, including subaxial deformity correction ( Fig. 42-5 ), occipitocervical reconstruction, trauma, metastatic or primary malignant disease, rheumatoid or seronegative destructive spondyloarthropathy, and accompanying posterior cervical decompression by laminectomy to address myelopathy secondary to cervical spondylosis, ossification of the yellow ligament, or ossification of the posterior longitudinal ligament.