Norman B. Chutkan

Description

Review of fixation options for C1-C2 posterior fusion including posterior wiring, transarticular screws, lateral mass screws, and translaminar screws. The relevant anatomy and structures at risk are reviewed.

Key Principles

Although posterior wiring is an accepted method of atlantoaxial fixation, newer techniques have been developed that provide rigid fixation. These include transarticular screw fixation, C1-C2 lateral mass screw fixation, and C2 translaminar screws. Although these techniques do allow for more rigid fixation, they are technically more demanding and are associated with an increased risk of complications.

Expectations

In cases of C1-C2 instability, there are several options for secure fixation of the atlantoaxial complex. The suitability of each technique depends on the specific anatomy and the presence or absence of intact posterior elements.

Indications

• C1, C2 instability
  
• Nonunion or delayed union of odontoid fractures
  
• Pseudarthrosis of previous atlantoaxial fusion

Contraindications

Related to specific techniques, as follows.

Posterior Wiring

• Absent or fractured C1 or C2 lamina; marked C1 canal encroachment by bone, pannus, or other tissue; congenital occipital-C1 fusion

Transarticular Screws

• Aberrant vertebral artery anatomy
  
• Inability to reduce the atlantoaxial complex
  
• Relative contraindication: previous injury or known unilateral vertebral artery occlusion

C1-C2 Lateral Mass Screws

• Aberrant vertebral artery passage
  
• Destruction or fracture of C1 lateral mass or C2 pedicle

C2 Translaminar Screws

• Fractured or absent C2 lamina

Special Considerations

Detailed preoperative evaluation of the bone morphology and neurovascular anatomy is critical to maximize surgical success and minimize the risk of complications. Computed tomography (CT) and magnetic resonance imaging (MRI) facilitate determining the location of the vertebral arteries, assessing the reduction of the atlantoaxial complex, assessing the integrity of the posterior elements, and evaluating the pars of the axis. CT angiography provides better visualization of the vertebral arteries than does standard CT. Stereotactic image guidance systems may be beneficial in demonstrating the proposed screw trajectory. In cases where posterior wiring is being considered, care should be taken if there is significant anterior canal encroachment by hypertrophied bone (odontoid), pannus, infected tissue, or large tissue mass, as this may increase the risk of neurologic injury.

• With gross instability, consider awake intubation and a rotational table such as an OSI Jackson (Orthopedic Systems, Inc., Union City, CA).
  
• Prone position with rolls or spine frame
  
• Head fixed with Mayfield pin holder or graphite halo ring
  
• Neurophysiologic monitoring may be helpful.
  
• Lateral intraoperative fluoroscopy is mandatory. Anteroposterior (AP) fluoroscopy is optional, as is stereotactic guidance.

Tips, Pearls, and Lessons Learned

For cases of posterior wiring, flexing the head on C1 can facilitate wire passage. Anatomic reduction should be achieved prior to passing wires. Large curved blunt needles can be used to pass sutures under the lamina. Wires or cables can then be pulled through. Unicortical grafts with a thick cancellous component should fit snugly against the decorticated posterior and inferior ring of C1 and the lamina of C2. In cases where screw fixation is being considered, it is important to remember that there is significant variation in bony morphology and vascular anatomy among individuals. Careful preoperative assessment facilitates determining the best options for screw fixation. The use of intraoperative tactile and visual landmarks as well as fluoroscopy is critical. Although not mandatory, stereotactic imaging may be of benefit. Direct palpation of the medial border of the C2 isthmus is most important, as it serves as a guide for correct medial-lateral screw placement.

Difficulties Encountered

Increased thoracic kyphosis may present a challenge for percutaneous placement of transarticular screws. The venous plexus overlying the C1-C2 facet joint can lead to significant bleeding. This can be minimized with careful dissection and packing with hemostatic agents. Appropriate positioning prior to beginning the case is critical. The neck should be positioned with lower cervical flexion and midcervical protraction while maintaining reduction of the atlantoaxial complex. With posterior wiring, if wires or cables cannot be passed safely or easily under the lamina of C2, passing it through or around the spinous process should be considered.

Key Procedural Steps