Cervical Pedicle Screw Fixation

Posterior segmental fixation of the cervical spine has been widely performed for a number of years. Posterior segmental fixation reduces the need for external orthosis, which may allow for earlier patient mobilization. Additionally, it provides rigid internal fixation of the cervical spine, which immobilizes the desired segments while arthrodesis develops.

Older techniques have included sublaminar wires, cables, and hooks; however, these approaches depend on the presence an intact lamina or spinous process. Also, they may result in low fusion rates resulting from decreased biomechanical stability, particularly in lateral rotation and extension. More recent spinal instrumentation advances have led to the development of posterior cervical screw systems, including lateral mass and pedicle screws, and these have been recently cleared by the US Food and Drug Administration.

In general, cervical segmental pedicular fixation requires more precision than does lateral mass fixation. The anatomical margin for error with cervical pedicle screw fixation is slim as a result of the small pedicle sizes, primarily in the subaxial cervical spine, and proximity of the screw to critical structures such as the vertebral arteries, nerve roots, and spinal cord. Pedicle screw fixation at the C3–6 levels carries greater risk to vascular and neural structures than does lateral mass fixation at those levels, and as such, pedicle screw fixation at these levels is less commonly performed.

20.2 Patient Selection

Cervical pedicle screws can be used to provide rigid internal fixation in situations that require postoperative immobilization before maturation of an arthrodesis. Indications for cervical arthrodesis vary widely to encompass cases of instability from or after the treatment of degenerative disease, deformity, stenosis, trauma, and tumors.

When considering whether to use pedicle screws or lateral mass screws for posterior cervical segmental fixation, it is important to consider that there is a stratification of anatomical risk for neurovascular injury by cervical level. At C2, the a shorter pars screw, which takes a more lateral trajectory, has been used. A recent systematic review of the literature identified similar rates of vertebral artery injury with placement of pedicle versus pars screws (1.1 versus 1.5%, respectively), with a slightly increased rate of pseudarthrosis with placement of pars screws. ¹ Regardless of technique, segmental fixation at this level is relatively safe and efficacious.

The anatomical margin of error for placement of pedicle screws at the C3–6 levels is smaller; however, pedicle screws have greater resistance to pullout than do lateral mass screws. At The C7 level, anatomy may favor the use of a pedicle screw over a lateral mass screw as a result of the transitional and thin nature of the C7 lateral mass. Extension of cervical segmental fixation constructs to T1 or T2 is also commonly performed to allow for enhanced fixation and stabilization of the cervicothoracic junction.

Situations in which there is a fractured or unacceptably atretic lateral mass may also call for cervical pedicle screw fixation. Although skipping segmental fixation of the fractured or atretic lateral mass is an option, segmental fixation of each vertebral level can theoretically shorten the needed length of construct. Shorter constructs create a shorter moment arm, which may put less biomechanical stress on adjacent levels.

Pedicle screw placement is generally contraindicated with destruction of the pedicle from tumors, traumatic injuries, infections, absent or atypically small pedicles, vertebral artery ectasias, or an abnormally oblique angle of the pedicle with respect to the sagittal plane. ²

20.3 Preoperative Preparation

If cervical pedicle screw fixation is considered necessary, preoperative imaging consisting of computed tomography (CT) and/or magnetic resonance imaging (MRI) should be obtained. CT imaging is important for measuring pedicle sizes, as well as for examining the vertebral foramina at each level. Sagittal reconstructions should also be obtained to assess for vertebral artery ectasias, which are of paramount importance for determining the suitability of placing C2 pedicle screws, as well as those in the subaxial cervical spine ( ▶ Fig. 20.1). CT–myelography or MRI is frequently used as an adjunct to assess the anatomy of the spinal cord and nerve roots in relation to compressive or unstable pathology.

Fig. 20.1 Preoperative computed tomographic scans obtained in a patient in whom aberrant left C2 pedicle anatomy prevented use of the approach we have described. Sagittal view of the left C2 pedicle (a), sagittal view of the right C2 pedicle (b), and axial (c), and coronal (d) views of C2 are shown. (Reproduced, with permission, from Fig. 4 in Sciubba et al. ⁶)

20.4 Operative Procedure

20.4.1 Operative Setup

After intubation, a Foley catheter is placed, and appropriate intravenous or arterial access is obtained. Electrodes should be placed for neurophysiologic monitoring if desired. A Mayfield head-holder (Integra LifeSciences Corporation, Cincinnati, Ohio) is applied to 27 kg of torque. The patient is rolled into the prone position on chest rolls, and the Mayfield head-holder is secured to the bed with the patient in the desired anatomical position. Fluoroscopy or a lateral radiograph can be used to confirm the appropriate alignment and lordosis of the spine. Both arms are secured at the patient’s sides with a sheet, with care being taken to protect the ulnar nerve with foam padding. For male patients, a check of the genitalia is conducted to ensure that they are not compressed; for female patients, the breasts should be examined to ensure that the nipples are not compressed on the chest rolls. Heavy cloth adhesive tape is then placed circumferentially around the patient to secure the positioning of the upper extremities and to help secure the patient to the bed. The adhesive tape can also be used to retract both shoulders inferiorly to minimize soft tissue redundancy and enhance radiographic visualization of the cervical spine; however, care must be taken when retracting the shoulders as excessive traction may lead to a brachial plexus injury.

The operating room bed is then adjusted so that the patient’s knees are flexed, with the feet elevated on pillows. The bed is lowered to minimum height, and the patient is placed in a reverse Trendelenburg position. With this positioning, the working area of the cervical spine is elevated to the surgeon’s level, with the cervical spine roughly parallel to the floor. Additionally, this positioning maintains the torso is at lower height, which can assist with venous drainage to the heart and decrease blood loss.

A standard midline cervical incision is planned based on palpable anatomical landmarks, such as the external occipital protuberance, C2 spinous process, and C7 spinous process. The midline cervical incision is made, and a standard exposure of the posterior cervical structures is undertaken for the area of interest. Care is taken to remain in the midline ligamentum nuchae to minimize blood loss and create a substantial fascial layer for closure.

The entire lateral masses of the levels of interest should be exposed, as the lateral mass is used as one of landmarks for the pedicle screw entry point. Uninstrumented levels should not be exposed, however, as there may be an increased risk of degeneration at these levels with disruption of the joint. Additionally, in children, there may be a risk of unintended fusion with exposure of uninstrumented levels. Furthermore, particular care should be taken when exposing C1 laterally because of the course of the vertebral artery, which runs along the superomedial vertebral sulcus on the posterior ring of C1. Therefore, safe dissection zones are usually within 8 mm lateral to midline on the superior surface of C1 and 1.5 cm lateral to midline along the posterior surface of C1. Subsequent discussion of pedicle screw entry points and placement will be divided based on vertebral level.

20.4.2 Insertion of C1 Pedicle Screws

The possibility of placing screws in the pedicle analog of the posterior arch of C1 has been suggested. ³ After exposure of C1 and C2 posteriorly, the vertebral artery is identified along the vertebral sulcus and retracted superiorly following a subperiosteal dissection. An entry point is created in the cortical bone using a high-speed bur in the C1 posterior arch in the same plane as the midpoint of the C2 lateral mass. The lateral mass is palpated medially to verify an adequate position of the entry point. A power drill or hand-held drill is then positioned medially 10 degrees and in line with the C1 ring in the vertical plane and used to create a pilot hole. The pilot hole is palpated with a blunt ball-tipped probe, followed by tapping and screw placement.

The advantages of this technique over more standard C1 lateral mass screws is that the C1–2 joint does not need to be exposed, which decreases blood loss from dissection of the vascular plexus overlying the joint. Additionally, as the joint is not exposed, there is a decreased risk of injury to the C2 nerve roots and a decreased risk of irritation from the screws, with no need to sacrifice the C2 nerve roots. Similarly sized screws in diameter (typically 3.5 mm) and length are used as in placing lateral mass screws; however, the screws are threaded along their entire length as the screw is placed entirely within C1, whereas in C1 lateral mass screws, the posterior region of the screw is nonthreaded as it is adjacent to the nerve root and serves as an extension to connect to the rod. C1 pedicle screws are unicortical but have a biomechanical advantage over unicortical C1 lateral mass screws owing to their entire length within bone, although an advantage over bicortical C1 lateral mass screws is debatable. ⁴

20.4.3 Insertion of C2 Pedicle Screws

Although some advocate the use of intraoperative fluoroscopy for placement of C2 pedicle screws, it often provides poor visualization of the course of the vertebral artery and foramen and may not prevent injury. We use the freehand approach using anatomical landmarks, which can be done safely with a slightly more extensive dissection. ⁵^, ⁶ Use of intraoperative image-guided navigation is increasingly popular and should be used if there is inexperience with the freehand technique.

As discussed already, examining a sagittal CT reconstruction preoperatively is critical for determining the suitability of the C2 pedicle for screw placement. A pedicle screw can generally be safely placed if the entire pedicle can be visualized on a single 3-mm slice without seeing the transverse foramen. The anatomy of C2 can be confusing, and inconsistent nomenclature is used in the literature. The C2 pars interarticularis is found between the adjacent joint spaces; the pedicle connects the posterior elements to the vertebral body. A Penfield no. 1 dissector can be used to dissect the C2 pars from the lateral mass to the C1–2 joint space to allow for direct visualization of the screw trajectory.

We use an entry point located at the lateral aspect of the C2 lateral mass, just caudal to the transition of the lateral mass into the C2 pars, at least 1.75 mm caudal to the lateral mass-pars transition zone, which allows for placement of a 3.5 mm diameter screw ( ▶ Fig. 20.2). ⁶ Although this entry point is slightly more superior and lateral than others reported in the literature, it reduces the risk of vertebral artery injury. The trajectory of the screw is parallel to the exposed pars or isthmus in the rostral–caudal dimension and along the medial–lateral trajectory from the entry point along the pars into the vertebral body. One should keep in mind that the transverse foramen is located on the inferolateral surface of the pedicle. After perforating the cortical bone with a small bur, at the screw entrance site, a 3.5-mm fluted tap is used to make a pilot hole, as the tap is able to perforate the cancellous bone of the pedicle without penetrating the cortical bone, again reducing the risk of cortical breach.

Fig. 20.2 Three-dimensional reconstructions of C–1 and C-2 vertebral segments from CT-acquired data highlighting the entry point of a C-2 pedicle screw path and lateral-medial trajectory (a), the inferosuperior trajectory (b), a “bulls-eye” view down the screw (c), and an anterior view showing the relationship of the transverse foramen to the C-2 pedicle (d). Dashed arrows indicate the screw trajectory. (Reproduced with permission from Sciubba et al. Radiographic and clinical evaluation of free-hand placement of C-2 pedicle screws. J. Neurosurg. Spine 2009; 11:15–22.)

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