The setup for C1-C2 fusion using a screw and rod construct technique is the same as for placement of transarticular screws, except that it is not mandatory to reduce the spine before exposure and the head and neck may be placed in physiologic alignment. Refer to the previous section for further details. Once the patient is prepared and draped, an incision is made over the midline of C1 and C2. Dis- section is carried down to the C2 spinous process. Again, care should be taken
to preserve the attachments of the semispinalis cervicis muscle to the C2 spinous process if possible. The superior aspect of the C2 lamina should be identified and subperiosteally dissected to identify entry points for the C2 pars screws. Dissec- tion of the posterior arch of the atlas is carried out laterally to identify the lateral masses of C1. The vertebral artery should be avoided as described previously.
Bleeding often occurs during dissection around the epidural venous plexus near the C1-2 joint. This can be controlled with thrombin-soaked absorbable gelatin sponges and judicious use of bipolar electrocautery (Tips from the Masters 12-5).
The C2 pars interarticularis or pedicle screw entry points are identified by locat- ing the medial border of the pars. The entry point is marked with a 2-mm bur.
Generally the direction of the bit is approximately 25 degrees medial and cepha- lad. The entry point should be in the cranial and medial quadrant of the isthmus surface of C2.3
Tips from the Masters 12-5 • When the almost inevitable bleeding is encountered during placement of a C1 lateral mass screw, use a hemostatic agent, cottonoid patty, and patience rather than blind or excessive bipolar cautery.
Attention is then turned to the C1 lateral mass screws. The dorsal root ganglion of C2 is retracted caudally to expose the entry point for the C1 lateral mass screw.
The analog of the C1 pedicle can be identified with a Penfield 4 dissector to assist in finding the appropriate starting point. The entry point is in the middle of the poste- rior aspect of the lateral mass of C1 just caudal to the point where the analog of the pedicle becomes confluent with the lateral mass.3,34 This entry point is marked with a 2-mm bur. A hand drill is then used to drill the screw path. The drill bit should be directed medially 5 to 10 degrees and parallel to the plane of the pedicle analog of C1. Fluoroscopy or image guidance can be used to guide the drill bit; the target point is the same as that described for a transarticular screw (see previous section).
The hole is probed, tapped, and probed, and a 3.5-mm polyaxial screw of appro- priate length is inserted. Use of a smooth-shanked screw can be useful in reduc- ing potential nerve root irritation from the threads of the screw. These steps are repeated on the contralateral side at C1. An alternative entrance point for the screw is on the inferior one third of the posterior arch to enter the lateral mass through the pedicle analog.35 Use of this entrance point avoids the troublesome bleeding that is often encountered when dissecting in the region of the lateral mass. Unfortunately, the bone caudal to the groove of the vertebral artery in the arch is often insufficient to allow safe placement of a screw in this location.34,36-38
The focus is then shifted back to C2, where the entrance holes are already in place. The hand drill is used to make a pilot hole that is approximately 25 degrees medial and cephalad. This hole is probed, tapped, and probed, and a fully threaded 3.5- to 4.0-mm screw is inserted, depending on preoperative planning. The same steps are repeated on the contralateral side at C2. The location of the vertebral artery in C2 usually necessitates placing the screw cranial and medial in the pars interarticularis, hugging the medial border of the pars. If the bone quality is good, it may be possible to carefully place a shorter screw into the lower pars to avoid the vertebral artery. A prominent or high-riding vertebral artery may preclude placement of a pars screw, in which case a C2 laminar screw may be chosen.29,30,37
With four polyaxial screws in place, any further reduction of C1 on C2 can be completed using fluoroscopic guidance. The screws can be used for leverage to assist with the reduction. Once optimal reduction has been achieved, the rods are tightened to maintain this reduction. Decortication of C1 and C2 is followed by placement of bone graft. The recommendation is to use cables to secure a block of iliac crest autograft to the posterior elements of C1 and C2 to provide three-point fixation and optimal biomechanical rigidity, especially when the bone quality is mar- ginal. Others prefer to onlay graft over the decorticated posterior elements without wires or cables.3 Finally, the wound is irrigated and closed in a layered fashion over a drain. A sterile dressing and collar are applied. The patient’s neurologic status is checked postoperatively.
The patient remains in a cervical collar for 3 months. Weight-lifting restrictions are 10 lb for most patients. The patient is seen at 3 months and flexion and exten- sion cervical spine radiographs are obtained to verify solid fusion of the atlantoaxial complex. After 3 months, the patient is weaned off the collar using isometric neck exercises for strengthening and reconditioning, and the patient is allowed to ease into noncontact activities.21
Careful consideration was given to the appropriate type of fixation for this 81-year-old patient in whom closed-reduction internal fixation of a type II odontoid fracture had failed and who was developing subluxation of C1 on C2. Because of the patient’s age, extensive medical comorbidities, and rib fractures, it was determined that he would likely do poorly with halo immobilization. Therefore rigid fixation, which would not require halo immobilization, was elected. A screw and rod con- struct was chosen in this case for several reasons. The patient’s thoracic hyperkypho- sis and compensatory cervical lordosis would have made the acute angle needed to insert transarticular screws nearly impossible to obtain. In addition, the lateral mass of C1 was posteriorly displaced relative to C2 on the left, and a transarticular screw would not have engaged C1 unless the spine could be reduced before drilling the pilot hole (Figure 12-7). On the right side, a high-riding vertebral artery precluded use of a transarticular screw (Figure 12-8). Since the patient’s bone quality was excellent, relatively short screws placed into the lower portion of the pars interar- ticularis were chosen to avoid injury to vascular structures (Figures 12-9 and 12-10).
Had these screws not achieved excellent fixation, an intralaminar C2 screw would have been placed. Using the screw and rod technique, the screws were placed into the ideal locations in C1 and C2 and then the spine was reduced and the con- struct tightened (Figure 12-11). Anatomic reduction was achieved, and this brought the odontoid screw back into position, which made it unnecessary to remove it (Figures 12-12 and 12-13). In this case, a corticocancellous block of iliac crest bone graft was secured to the posterior elements with a titanium cable to enhance the stability and increase the fusion rate (Figure 12-14).
FIGURE 12-7 Sagittal CT image revealing posterior subluxation of the left C1 lateral mass on C2.
FIGURE 12-8 Sagittal CT image showing a high-riding vertebral artery on the right side.
This case illustrates that preoperative planning is paramount to avoid compli- cations in C1-C2 fusions. For the patient in the Case Presentation, the vertebral artery would have been in jeopardy had the surgeon chosen to use transarticular screws instead of a screw and rod construct. Both of these more modern techniques provide greater stability than posterior wiring techniques alone, which eliminates the need for postoperative immobilization with a halo vest. In younger, healthier patients, a cable and graft construct without screws would be a reasonable alterna- tive if augmented with halo immobilization. When it is essential to avoid the use of a halo, the risk and cost of screw augmentation techniques are justified.
DISCUSSION OF BEST EVIDENCE
The current evidence concerning surgical techniques for C1-C2 arthrodesis consists of Level III, IV, and V studies. There is no Level I or Level II evidence from clinical studies to direct treatment. Levels of evidence are generally not assigned to basic science studies, but since the biomechanical and anatomic investigations cited in the
FIGURE 12-9 Sagittal CT image showing a right vertebral artery path necessitating use of a short screw in the right C2 pars.
FIGURE 12-10 Sagittal CT image showing a left vertebral artery path necessitating use of a short screw in the left C2 pars.
FIGURE 12-11 Sagittal CT image illustrat- ing rod reduction of C1 on C2.
FIGURE 12-12 Preoperative CT lateral view of the odontoid showing malreduction.
FIGURE 12-13 Postoperative CT lateral view of the odontoid showing re-reduction.
following sections are prospective, well designed, and adequately powered, they will be considered Level I evidence.