The subaxial and cervicothoracic junction is a relatively difficult area for spine surgeons to navigate. Because of different transitional stressors at the junction of the smaller cervical vertebrae and the larger thoracic segments, proximity to neurovascular structures, and complex anatomy, extreme care and precision must be assumed during fixation in these regions. Lateral mass screws, pedicle screws, and translaminar screws are currently the standard of choice in the subaxial cervical and upper thoracic spine. This article addresses the relevant surgical anatomy, pitfalls, and pearls associated with each of these fixation techniques.
The subaxial and cervicothoracic junction is a relatively difficult area for spine surgeons to navigate. Because of different transitional stressors at the junction of the smaller cervical vertebrae and the larger thoracic segments, proximity to neurovascular structures, and complex anatomy, extreme care and precision must be assumed during fixation in these regions. Lateral mass screws, pedicle screws, and translaminar screws are currently the standard of choice in the subaxial cervical and upper thoracic spine. This article addresses the relevant surgical anatomy, pitfalls, and pearls associated with each of these fixation techniques.
Surgical anatomy
The subaxial cervical spine has a lordotic posture with each vertebra composed of a body, superior and inferior articular processes, pedicles, lamina, and one spinous process. Laminae at the cervical spine are thin. Pedicles are small and are medially oriented. The facet joint is formed from superior and inferior articulating processes of the lateral masses. Orientation of the facet joint at the cervical spine is coronal in nature and prevents the spine from overextension. With no articulations to the rib cage, the cervical spine allows for much more mobility than the thoracic spine.
In contrast, the upper thoracic spine has different properties due to the added elements of articulation with the thoracic rib cage. Physiologic kyphosis occurs here because of the greater height of the dorsal vertebral wall relative to the ventral vertebral surface. Moving inferiorly from C5 to T1, the height and width of the pedicles increase, with a concomitant decrease in the angle between the pedicle and vertebral body. Facet joint orientation in the thoracic spine is in the coronal plane and acts to limit motion. All these elements allow for less flexion and extension than the cervical spine.
Another factor to consider in the subaxial cervical region is the uniqueness of the C7 vertebra. Often described as the transitional vertebra and representing the cervicothoracic junction, its small/thin lateral mass size, increased biomechanical stressors, and close proximity to neurovascular structures make the C7 vertebra a challenge during instrumentation. In addition, the 5% incidence of vertebral artery passage through the transverse foramen makes avoidance of the foramen transversarium at C7 paramount during instrumentation of the pedicle and lateral mass at this level.
Several muscular groups warrant knowledge of posterior fixation techniques at the cervicothoracic area. The trapezius muscle inserts medially on the spinous processes of C7 to T12. The rhomboid muscles also surround the area. The serratus posterior inferior and superior muscles are present as well as several spinal muscles that extend from the spinous processes to the transverse processes and posterior angles of the ribs. These muscles form a 6-cm to 8-cm muscular band on each side of the midline, which inserts on the underlying bony elements. The neurovascular bundles rise from the intercostal vessels and the nerves run backward below the transverse process and reach the muscular layers.
Innervations of these muscles derive from the medial and lateral branches of the dorsal rami of the cervical nerves. Arterial supply to the posterior musculature inferiorly is provided by the deep cervical branch of the subclavian artery as it transverses the transverse process of C7 and the first rib. The occipital artery from the posterior external carotid supplies branches superiorly to the muscles and has branches to the vertebral and spinal arteries. The vertebral artery lies anterior to the anterior nerve roots as they exit the neural foramen. Inferiorly, the vertebral artery enters the transverse foramen at C6 and travels through each vertebral foramen through C2 where it courses posterior superior to the lateral aspect of C1. In the sagittal plane, the artery tends to move anterior in the transverse process as it runs from C6 cephalad to C2.
Lateral mass screws
Indications
Indications for lateral mass screw fixation include the following: acute and chronic instability resulting from tumors, infections, posterior element fractures, posterior ligamentous injuries, postlaminectomy instability, and following multilevel corpectomy and pseudarthrosis after anterior cervical fusion. Caution should be used in patients with abnormal bony anatomy as in those with erosive rheumatoid arthritis, osteoarthritis, or ecstatic coursing of the vertebral artery. These conditions can complicate screw placement. In cases of severe osteopenia/osteoporosis, lateral mass fixation may be supplemented with posterior wiring and/or pedicle screw fixation if the proper anatomy is present.
Technique
Imaging
Fine-cut (2 mm) computed tomography (CT) with two-dimensional reconstruction and T2-weighted sagittal magnetic resonance imaging (MRI) should be used to assess lateral mass quality in the lower cervical spine.
Positioning
Mayfield tongs may be used, rigidly fixing the head to the table in the prone position. Gardner-Wells tongs and a face pillow can also be used. Care should be taken to avoid pressure to the orbits. The neck is positioned in neutral alignment. If this might compromise spinal canal capacity to a detrimental degree, the neck is positioned flexed; an unscrubbed assistant can readjust the head holder to improve cervical lordosis after decompression once instrumentation begins. A hard collar and rotating table, such as the Jackson frame, may be used in a traumatic or severely stenotic spine to minimize cervical spine motion and increase stability during the turning process. Extreme flexion or extension of the head should be avoided to prevent fusion of the neck in a nonanatomic position. Horizontal gaze may be affected if cervical alignment is not appreciated while placing instrumentation. Lateral plain radiography should be used to visualize cervical alignment.
Exposures
A midline vertical skin incision can be made (as necessary) extending from the occipital protuberance past the spinous process of the seventh cervical vertebra (prominent vertebra). The nuchal ligament is divided in the midline and incised as far as the tips of the spinous processes. The deep muscle layer is stripped off the spinous processes close to the bone with the aid of electrocautery. Subperiosteral dissection is carried to the lateral boundary of the articular masses. Exposures carried too far ventrolaterally to the facet joints may result in increased bleeding and nerve root injury.
Procedure
Screw insertion is located 1 mm medial to the midpoint of the lateral mass. The direction of the screw is 15° cephalad and 30° lateral for C3 to C6. Drill trajectories in the sagittal plane that are too low may violate the facet joint. Trajectories that are too medial may violate the vertebral artery. Bicortical screw placement is recommended to ensure optimal screw anchorage. Holes are drilled with a 2.4-mm drill bit using the drill guide. Screw length should be selected 2 mm shorter than measured to avoid nerve root irritation when performing bicortical screw placement. Meticulous removal of soft tissue from the articular masses will allow for clear delineation of the anatomic landmarks. If the spinous process obstructs the application of the drill in the correct direction, it is trimmed with a rongeur. A small Penfield elevator can be placed in the joint space to keep the drill aligned in the sagittal plane parallel to the facet joint.
The Roy-Camille technique may also be used for the screw entry point. The starting point for screw insertion is located at the midpoint of the lateral articular mass perpendicular to the posterior cortex of the lateral mass in the sagittal plane of the spine. The screw is directed 10° lateral with no cranial-caudal inclination ( Fig. 1 ). This technique may, however, lead to cephalad articular joint violation.
Magerl modified the screw placement technique by moving the starting point cephalad and medially 1 mm, then aiming 25 to 30° laterally and 45° cephalad to parallel the surface of the facet joint ( Fig. 2 ). An’s technique uses a starting point 1 mm medial to the lateral mass center and angles the screw 30° lateral and 15° cephalad. Anderson describes an alteration of the Magerl technique with a 1-mm medial offset from the center of the lateral mass with angulation of the screw 20 to 30° cephalad and 10 to 20° lateral.
Auer and colleagues describe the following procedure for screw and rod placement. The surgeon estimates angulation of the screw. By placing a curette or Bovie tip in the facet for visual reference, angulation of the screw can be made parallel to the facet joints. Lateral angulation can be estimated with reference to the lateral mass or by resting the drill guide on the spinous process of the next caudad level. If the spine is unstable, a high-speed 2-mm burr can be used instead of a starting awl to penetrate the lateral mass cortex. Once the outer cortex is breached, a hand drill can be directed cephalad and lateral to the opposite side of the lateral mass. After the tapping the screw hole, the surrounding bone and the facet joint should be decorticated to maximize the fusion area. Placement at C7 can be difficult because of the angles of the posterior elements and thoracic transition. Elongation and thinning of the C7 lateral mass can also complicate screw fixation.
Minimally invasive technique
Wang and colleagues describe lateral mass fixation with the following minimally invasive technique. First, a 2.0-cm midline skin incision is made to introduce a set of tubular dilator retractors after a local anesthetic. The skin entry point is chosen so that the trajectory of the tube is parallel to the facet joint in the sagittal plane approximately 2 spinal segments below the level of interest. The tube trajectory is also directed laterally to dock on the posterolateral elements and to approximate orientation using the Magerl technique. The tubular retractor diameter is 20 mm. The surface of the lateral mass is then exposed with monopolar cautery and pituitary rongeurs to remove any overlying tissues. The facet joint synovium to be fused is removed with a curette and packed with autograft bone. A cancellous drill is then used to create a 14-mm-deep pilot hole in the center of the lateral mass. The trajectory is lateral and parallel to the facet joint. The pilot hole is then tapped, and a polyaxial screw (length 14 or 16 mm, diameter 3.5 mm) is then placed under direct visualization. This is done at neighboring levels. After screw placement on one side, a connecting rod is placed down the tubular retractor and advanced into the upper polyaxial screw head using slight elevation of the tubular retractor. The procedure is then repeated on the contralateral side. Fluoroscopic guidance is used during all steps.
Outcomes
Montesano and Lauch demonstrated that the Magerl technique provided greater pullout resistance and a higher load to failure than the Roy-Camille technique. Choueka and colleagues tested flexion failure strength and found the Magerl technique to be significantly stronger. Bicortical fixation has a potential risk of neurologic and vascular complications that range from 0.8% to 7.3%. Another study found long unicortical screws performed as well as bicortical screws. In addition, violation of the adjacent facet joint has been reported in bicortical insertion, raising the concern of extension of fusion to unintended levels. Lateral mass plating was also found to be safer with screws placed up to, but not through, the anterior lateral mass cortex while maintaining a mechanically stable construct.
Nerve root injury as well as vertebral artery injury (more rare) can occur if the screw trajectory is incorrect, if penetration is too deep (bicortical screw purchase), or if there is significant past pointing of the drill. If brisk, pulsatile arterial bleeding is encountered from the drill hole, hemostasis should be obtained using bone wax, thrombogenic agents, and, potentially, placement of screw in the hole. Postoperative angiography should be obtained to determine the status of the injured vertebral artery.
Auer and colleagues describe lateral mass fixation as having the advantages of earlier mobilization of the patient, decreased halo brace usage, and increased fusion rates at the cervicothoracic junction. Posterior instrumentation has allowed for increased fusion rates as a result of improved bony fixation and rigidity. These techniques also have diminished immobilization time and halo bracing while maintaining the structural alignment of the spine. Lateral mass screws have expanded our ability to maintain the alignment of the spine with deficient posterior elements and multiple-level injuries.
Pedicle screws
Indications
Several studies have suggested that cervical pedicle screws are superior to the more traditional lateral mass screws when there is a need for higher pullout strength, for decreased axial load at the disk space, or for stabilization of both ventral and dorsal aspects of the spine by traversing all 3 columns of the vertebrae. Lateral mass screws may be inadequate in cases with poor bone quality secondary to fracture, neoplasm, or revision surgery. In such cases, lateral mass screws can fail by loosening or avulsion, combined with the decreasing lateral mass size at the lower cervical spine.
Currently, the most accepted indications for cervical pedicle screws are: trauma-induced cervical spine fracture and dislocation; multilevel cervical instability; cervical instability after neoplasm resection; correction of cervical kyphosis; severe osteoporosis and/or absence of vertebral lamina or retrovertebral structures, making fixation difficult through other methods; and cervical instability caused by degeneration.
Pedicle screws at the upper thoracic spine have many indications for a myriad of conditions. These include better pullout strength, greater control in all planes due to increased stability by 3-column fixation, fewer vertebral motion segments arthrodesed, less need for postoperative bracing, and secure fixation after laminectomy or missing posterior elements. In patients with spinal deformity, thoracic pedicle screws have been shown to have greater 3-dimensional corrections with decreased rates of curve progression and higher fusion rates.
Cervical Technique
Successful placement of the screw depends on accurate entry point and trajectory. However, because the literature has not adequately established a standard approach for this, several approaches have been proposed based on healthy volunteers and cadaveric data.
Instrumentation
Generally, screw diameters of 3.5 to 4.5 mm are used depending on pedicle size as measured by preoperative imaging. Larger diameters are needed for larger pedicles to improve fixation. For C3 to C7, the insertion point is slightly lateral to the center of the lateral mass and close to the inferior articular facet of the cephalad segment ( Figs. 3 and 4 ). The sagittal plane trajectory is determined by fluoroscopy or navigational guidance. The transverse plane trajectory is 25 to 45° medial. The entry site is decorticated using a burr and a high-speed drill or a rongeur. A burr or awl is used to penetrate the dorsal cortex of the pedicle.
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