12: Cervical Spine: Lateral Mass Screw Fixation

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Procedure 12 Cervical Spine

Lateral Mass Screw Fixation

Kern Singh, Jonathan A. Hoskins, Vamshi Yelavarthi, Alexander R. Vaccaro

Indications

image Acute and chronic instability

• Posterior element fractures
• Posterior ligamentous injuries
• Postlaminectomy instability

image Destruction of bony anatomy secondary to neoplasm

image Stabilization after multisegment anterior decompression and fusion (long anterior fusions for tumor, infection, ankylosing spondylitis, diffuse cervical spondylosis)

image Stabilization after failed cervical arthroplasty

image Pseudarthrosis following anterior cervical fusion

Indications Pearls

• Beware of patients with aberrant bony anatomy making screw placement difficult, such as those with erosive rheumatoid arthritis or osteoarthritis, or ectatic coursing of the vertebral artery.

Indications Controversies

• In severe osteopenia/osteoporosis, lateral mass fixation may be inadequate. It may be supplemented with posterior wiring if posterior elements are present and/or with pedicle screw fixation if the anatomy allows.

Examination/Imaging

image Fine-cut (2-mm) computed tomography with two-dimensional reconstruction allows assessment of the lateral mass quality in the lower cervical spine.

image T2-weighted sagittal magnetic resonance imaging may allow identification of neural compression and determination of the vascular anatomy (Figure 12-1).

image

FIGURE 12-1

Treatment Options

• Posterior wiring

• Posterior hook fixation

• Posterior cervical pedicle screw fixation

Surgical Anatomy (Figure 12-2)

image Nerve root injury may occur if the screw trajectory is incorrect, if the screw penetration is too deep (bicortical screw purchase), or if there is significant past pointing of the drill.

image Vertebral artery injury is an exceedingly rare complication that may occur if the trajectory is medial and the screw penetration is too deep.

image If brisk, pulsatile arterial bleeding is encountered from the drill hole, hemostasis should be obtained using bone wax, thrombogenic agents, and, potentially, placement of a screw in the hole. Postoperative angiography should be obtained to determine the status of the injured vertebral artery.

image

FIGURE 12-2

Positioning

image Mayfield tongs are applied, rigidly fixing the head to the table in the prone position (Figures 12-3 and 12-4).

image The neck is slightly extended. If this compromises spinal canal patency to a detrimental degree, lordosis may be obtained following decompression by having an unscrubbed assistant readjust the head holder to improve cervical lordosis.

image The arms and elbows are placed adjacent to the torso and are well padded to prevent pressure ulcers.

image The shoulders are gently pulled caudad by adhesive tape.

image The knees are flexed to prevent distal migration of patient.

image

FIGURE 12-3

image

FIGURE 12-4

Positioning Pearls

• A slightly reverse Trendelenburg position allows venous drainage and less bleeding during surgery.

Positioning Pitfalls

• Extreme flexion or extension of the head should be avoided to prevent fusion of the neck in a nonanatomic position.

Positioning Equipment

• Mayfield tongs

• Lateral plain radiography to visualize cervical alignment

Positioning Controversies

• Placing instrumentation without appreciating cervical alignment may lead to loss of horizontal gaze, or the need for compensatory mechanisms to maintain horizontal gaze, in the postoperative period.

Portals/Exposures

image A midline vertical skin incision can be made (as necessary) extending from the occipital protuberance past the spinous process of the seventh cervical vertebra (typically the most superficially prominent vertebra).

image The nuchal ligament is divided in the midline and incised as far as the tips of the spinous processes.

image The deep muscle layer is stripped off the spinous processes close to the bone with the aid of electrocautery (Figure 12-5, A and B).

image Subperiosteal dissection is carried to the lateral boundary of the articular masses.

image image

FIGURE 12-5, A-B

Portals/Exposures Pearls

• The bifid nature of the spinous processes may result in dissection into the paraspinal musculature, affecting the superficial plexus of veins.

• Care should be taken not to violate the facet joint capsules until the target vertebrae have been radiographically confirmed.

Portals/Exposures Pitfalls

• Exposures performed too far ventrolaterally to the facet joints may result in increased bleeding and nerve root injury.

Portals/Exposures Equipment

• Bovie electrocautery

• Cobb elevator

Portals/Exposures Equipment

• The use of minimal incision portals may decrease postoperative neck discomfort and accelerate rehabilitation.

Procedure

Step 1: Determining the Entry Point

image The entry point for screw insertion is located 1 mm medial to the midpoint of the lateral mass. The direction of the screw is 15 degrees cephalad and 30 degrees lateral for C3-6 (Figure 12-6, A).

image image

FIGURE 12-6, A-B

Step 1 Pearls

• A lateral radiographic or fluoroscopic image is mandatory for confirming the correct levels of instrumentation.

• Meticulous removal of soft tissue from the articular masses will allow clear delineation of the anatomic landmarks.

Step 1 Pitfalls

• Lack of preinstrumentation localizing radiography may lead to wrong-level surgery.

Step 1 Instrumentation/Implantation

• 2-mm round-tip burr, drill, or Kirschner wire

Step 1 Controversies

• The Roy-Camille technique may be used for screw entry point (see Figure 12-6, B). The starting point for the screw insertion is located at the midpoint of the lateral mass. The screw is directed 10 degrees lateral with no cranial-caudal inclination. This technique may lead to cephalad articular joint violation.

Step 2: Drilling the Screw Hole

image Holes are drilled with a 2.4-mm drill bit using the drill guide.

image The drill depth can be increased in 2-mm increments.

image A depth gauge is used to confirm the appropriate screw length.

Step 2 Pearls

• If the spinous process obstructs application of the drill in the correct direction, it may be trimmed with a burr or 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.

Step 2 Pitfalls

• Past pointing of the drill may result in nerve root irritation.

• Improper drill trajectory may result in injury to the spinal cord or vertebral artery.

• Drill trajectories in the sagittal plane that are too low may violate the facet joint.

• Drill trajectories that are too medial may encroach upon the vertebral artery.

Step 2 Instrumentation/Implantation

• Power or hand drill

Step 3: Tapping and Screw Insertion

image The tap size may equal the outer screw diameter or be slightly undersized. A self-tapping screw may avoid the need for additional tapping.

image An appropriate-size screw is then placed in the same trajectory as the tap (Figure 12-7).

image

FIGURE 12-7

Step 3 Pearls

• To ensure optimal screw anchorage in the lateral masses, bicortical screw placement is recommended.

• To avoid nerve root irritation when performing bicortical screw placement, screw length should be selected 2 mm shorter than measured.

Step 3 Instrumentation/Implantation

• Tap

• Polyaxial screws

Step 4: Rod Insertion

image The determined length of rod is cut using the rod cutter (Figure 12-8, A) and bent utilizing the rod bender (Figure 12-8, B).

image Contouring of the rod is performed in gentle, limited steps until the desired shape is achieved (Figure 12-8, C).

image image image

FIGURE 12-8, A-C

Step 4 Pearls

• In extended and complex instrumented fusions, additional stability can be achieved by linking cross connectors to the longitudinal rods.

• In patients with poor bone quality unsuitable for placement of lateral mass screws, sublaminar wires can be secured to open cable connectors attached to the rods for additional fixation.

Step 4 Pitfalls

• Titanium rods should not be contoured multiple times, because fatigue may occur to the rod.

• A cross connector should be carefully placed in a dorsal location following a laminectomy to avoid posterior dural impingement.

Step 4 Instrumentation/Implantation

• Polyaxial screw and rod system, reduction tools

Step 5: Placement of Screw Caps

image Screw tightening (once the inner nut is placed in the polyaxial screw head) should be performed in a gentle and systematic fashion to facilitate rod seating, allow controlled manipulation of the spinal segments, and to avoid screw torque, which may result in screw loosening (Figure 12-9).

image Final nut tightening is performed utilizing the torque driver and antitorque device to guarantee optimal tightening of the nuts and to avoid late rod loosening.

image

FIGURE 12-9

Postoperative Care and Expected Outcomes

image Depending on the length and extent of the entire surgical procedure, the pathology, and bone quality, the required postoperative care may differ significantly (Figure 12-10).

image Patients are routinely monitored in the hospital overnight.

image A soft collar orthosis may be used in the setting of short segment reconstructions in degenerative disease. A hard collar is may be applied in the postoperative period for approximately 6 weeks, depending upon the pathology and length of the instrumented construct. Rigid external fixation with a halo-vest orthosis is rarely indicated, except in severe osteoporosis, questionable compliance, neoplasms, and certain traumatic lesions.

image Deep or superficial wound infections should be addressed at an early stage. Extensive infections may require hardware removal and rigid external immobilization.

image Postoperative neurologic deterioration resulting from hardware malpositioning requires hardware revision or removal.

image Failure of bony fusion may result in instrumentation failure, necessitating hardware revision.

image

FIGURE 12-10

Evidence

Bayley E, Zia Z, Kerslake R, Klezl Z, Boszczyk BM. Lamina-guided lateral mass screw placement in the sub-axial cervical spine. Eur Spine J. 2010;19:660-664.

Results of this study indicate that using the subaxial cervical lamina as a reference plane for the insertion of lateral mass screws in the cervical spine decreases the likelihood of vertebral injury.

Deen GH, Birch BD, Wharen RE, Reimer R. Lateral mass screw-rod fixation of the cervical spine: a prospective clinical series with 1-year follow-up. Spine J. 2003;3:489-495.

These data indicate that posterior cervical stabilization with polyaxial screw–rod fixation is a safe, straightforward technique that appears to offer some advantages over existing methods of fixation.

Merrola AA, Castro BA, Alongi PR. Anatomic consideration for standard and modified techniques of cervical lateral mass screw placement. Spine J. 2002;2:430-435.

This study indicates that there are significant differences in potential neurovascular injury, which are dependent on the technique used for screw entry, the level instrumented, and the angle of screw trajectory in the parasagittal plane.

Wang MY, Levi AD. Minimally invasive lateral mass screw fixation in the cervical spine; initial clinical experience with long-term follow-up. Neurosurgery. 2006;58:907-912.

The results of this study indicate that the minimally invasive approach preserves the integrity of the components that provide stability to the cervical spine.

Xu R, Haman SP, Ebraheim NA, Yeasting RA. The anatomic relation of lateral mass screws to the spinal nerves: a comparison of the Magerl, Anderson, and An techniques. Spine. 1999;24:2057-2061.

The results of this study indicate that the potential risk of nerve root violation is higher with the Magerl and Anderson techniques than with the An technique.

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17: Anterior Thoracolumbar Spinal Fusion via Open Approach for Idiopathic Scoliosis 13: Cervical Pedicle Screw Fixation 9: Occipital-Cervical Fusion 39: Lumbar Internal Laminectomy 25: Sacropelvic Fixation 2: Halo Placement in the Pediatric and Adult Patient