Contraindications

There are no specific contraindications to the performance of lateral mass plate and screw fixation if the lateral masses of the atlas and axis are normal. We have observed that such a fixation is possible even in cases where there is lateral mass destruction, erosion, or significant osteoporosis. The anomalous course of the vertebral artery in the facet of axis and in the vicinity of the posterior arch of the atlas may rarely preclude the use of our technique.

Relevant Surgical Anatomy

Covered with a large plexus of veins, the vertebral artery adopts a serpentine course in relationship to the craniovertebral region. The venous plexuses are the largest in the region lateral to the C1-C2 joint. After a relatively linear ascent of the vertebral artery in the foramen transversarium of C6 to C3, the artery makes a loop medially toward an anteriorly placed superior articular facet of the C2 vertebra, making a deep groove on its inferior surface. The extent of medial extension of the loop varies. The distance of the artery from the midline of the vertebral body of C2 as would be observed during a transoral surgical procedure is 12 mm on average.4,5 The vertebral artery loops away from the mid-line underneath the superior articular facet of the C2. The course of the vertebral artery in relationship to the inferior aspect of the superior articular facet of the C2 makes its susceptible to injury during transarticular and interarticular screw implantation techniques.

Operative Technique

Cervical traction is set up prior to induction of anesthesia, and weights are progressively increased to ~5 to 8 kg or a sixth of total body weight. The patient is placed prone with the head end of the table elevated to ~35 degrees. Cervical traction stabilizes the head in an optimally reduced extension position and prevents rotation. The traction also ensures that the weight of the head is directed superiorly toward the direction of the traction and the pressure of the headrest on the face or eyeball is avoided. Although placed on the headrest, the head is essentially floating as the traction pulls it away from the headrest. Elevation of the head end of the table, which acts as countertraction, reduces venous engorgement in the operative field. The suboccipital region and the upper cervical spine are exposed through an ~8-cm, longitudinal, midline skin incision centered on the spinous process of the axis. The process is identified, and the paraspinal muscles attached to it are sharply sectioned. The C2 ganglion is placed transversely over the atlantoaxial joints. The large ganglion is widely exposed and then sectioned and resected. The ganglion is closely related to the vertebral artery on its lateral aspect, and all dissection in the region must be done under direct vision. On some occasions, the ganglion can even be mobilized superiorly or inferiorly, and sectioning can be avoided. However, the author has found that sectioning the ganglion provides a wide and panoramic exposure of the lateral masses of the atlas and axis and the atlantoaxial joint region.2,3 Such an exposure is essential when manipulation and distraction of the facets are contemplated. Numbness related to the ganglion sectioning is marginal and easily tolerated by the patient ( Fig. 42.3 ). Bleeding from the large venous sinuses in the region and in the extradural space can be troublesome. Packing the region with Surgicel (Ethicon, Somerville, NJ) and Gelfoam (DuPuy, Raynham, MA) can assist in the control of venous bleeding. The joint capsule is cut sharply, and the articular surfaces of the joint are exposed. The adjacent synovial articular surfaces of the atlantoaxial joint are decorticated widely with a microdrill, and pieces of bone harvested from the iliac crest are stuffed into the joint space. The lateral aspect of the lamina and a part of the pars of the axis are drilled to make the posterior surface of the lateral mass of the axis relatively flat so that the metal plate can be placed snugly and parallel to the bone. Drilling helps reduce the length of the plate and places the screw superiorly and almost directly into the lateral mass of the axis. Actual vertebral artery exposure is unnecessary lateral to the pars of the axis or superior to the arch of the atlas.

Screws are implanted in the previously created guide holes in the lateral mass of the atlas and axis through a two-holed (~2 cm in length) stainless steel or titanium plate ( Fig. 42.1A ). First, a screw is placed into the atlas and directed at an angle of ~15 degrees medial to the sagittal plane and 15 degrees superior to the axial plane. The preferred site of screw insertion is at the center of the posterior surface of the facet, 1 to 2 mm above the articular surface. Whenever necessary, careful drilling of the inferior surface of the lateral aspect of the posterior arch of the atlas in relation to its lateral mass can provide additional space for the placement of the plate and screw implantation. The screw may be implanted by choosing an insertion point on the posterior surface of the posterior arch of the atlas, just superior to the facet or even through the articular surface of the lateral mass of the atlas. Such sites are useful more frequently in children than in adults.

Screw implantation in the axis is relatively unsafe because of the intimacy of vertebral artery relationships. The preferred site of screw implantation in the lateral mass of the axis is in the medial and superior third of the pars. The direction of screw implantation must be sharply medial and superior and should be toward the superior aspect of the body of the axis vertebra toward the midline. The medial surface of the pars/pedicle of the axis is identified before the implantation of the screw. The screw is directed at an angle ~25 degrees medial to the sagittal plane and 15 degrees superior to the axial plane. The angle of screw insertion varies, depending on the local anatomy and size of the bones. The quality of cancellous bone in the lateral masses of the atlas and axis in the proposed trajectory of screw implantation is generally good, providing an excellent purchase of the screw and avoiding the vertebral artery.

The screws are 2.9 mm in diameter in adult patients and 2.7 mm in diameter in pediatric patients. The length of the required screw is calculated on the basis of the size of the lateral masses observed on the preoperative radiological studies. The approximate lengths of the atlas screws are 26 mm in adults and 22 mm in children. The screws in the atlas and axis are similar in length. One of the criteria of good screw placement includes engagement of anterior and posterior cortices of the lateral masses. Some authors have recently reported dangers to the carotid artery in the prevertebral region. However, we have never encountered this complication. It appears that, even if the screw is extra long, it will displace the soft tissues and carotid artery rather than cause injury. Intraoperative fluoroscopic control and neuronavigation was found to be helpful but not essential in determining the state of the screws. Large pieces of corticocancellous bone graft from the iliac crest bone are then placed over the adequately prepared posterior elements of the atlas and axis. After the wound is closed, cervical traction is discontinued. Patients are mobilized as soon as possible and advised to wear a hard cervical collar for 3 months.

Vertebral Artery Management

The most dreaded complication of the procedure is injury to the vertebral artery. Appropriate anatomical information of the region in general and of the case in question is required to avoid this injury. The vertebral artery can be injured during the process of lateral dissection of the C2 ganglion. A second potential site of injury is during insertion of the screw in the axis. To control the bleeding in the second situation, one has to pack the bleeding bone hole with bone wax. One can then insert the screw through the same hole, prepare for an alternative site of screw insertion, or use an alternative method of atlantoaxial fixation. Suturing the artery should be attempted whenever the injury is during its course outside the confines of the bones. Respect and care of all neural and vascular tissues and employment of precise technique are critical to success.6

This technique of lateral mass fixation and opening the joint provides an opportunity for manipulating the atlas and axis independently by obtaining fixation points in their strongest elements and, hence, has very versatile applications ( Table 42.1 ).