Grafting Methods: Posterior Occipitocervical Junction and Atlantoaxial Segment

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Grafting Methods: Posterior Occipitocervical Junction and Atlantoaxial Segment

Robert K. Eastlack, Bradford L. Currier, and Alexander R. Vaccaro

Description

To facilitate arthrodesis of the occipitocervical or atlantoaxial segments.

Key Principles

Prepare the donor sites or bony bed adequately.
Autograft bone is the gold standard choice in graft material.
Adequate internal fixation plays an important role in maximizing fusion success.

Expectations

Thorough preparation of the grafting site and use of autograft bone with adequate stabilization should result in a healthy fusion mass.

Indications

Occipitocervical instability
Atlantoaxial instability

Contraindications

Grossly infected sites should be debrided and treated with appropriate surgical/medical management before application of metal instrumentation and bone grafting in some cases. The timing of the graft placement and instrumentation must be addressed on an individual basis. Shortened life expectancy (less than 3 to 6 months) may obviate the usefulness of bone grafting, but it is prudent to err on the side of overtreatment.

Special Considerations

Occipitocervical fusions require careful intraoperative positioning to leave patients in an optimized functional alignment.
Preserve the subaxial muscular and ligamentous attachments to the spinous process of C2 (semispinalis cervicis muscles, interspinalis muscles, interspinous ligament) when not planning for subaxial fusion.

Special Instructions, Position, and Anesthesia

Be sure that anesthesiology is aware of the need for neuromonitoring, as the choice of anesthetic may change when motor evoked potentials are employed.
Position the patient prone with the head held with a pinion or halo secured to the operating table with a Mayfield attachment.
Position the occipitocervical junction intraoperatively using fluoroscopy. The intersection of lines drawn parallel with the opening of the foramen magnum and the superior end plate of C3 should approximate 45 degrees. Ensure that neutral rotation has been obtained before fusing the atlantoaxial junction as well.

Tips, Pearls, and Lessons Learned

Autograft bone may be harvested from the posterior-superior iliac spine or ribs. Bicortical specimens should be obtained when the graft is providing structural support in the area of fusion. If there is inadequate structural or morselized autograft bone specimen available, allograft bone may be used or autograft bone can be supplemented with demineralized bone matrix adjuncts. Do not place bone graft directly on neural elements, including the C2 exiting nerve root or ganglion.

Remove the cortex on the occipital bone below the external occipital protuberance to expose bleeding cancellous bone. This can be done with a high-speed burr following placement of occipital hardware, which may include a plate and screws. Analyze the preoperative computed tomography (CT) images for bone dimensions and the location of neurovascular structures. There is considerable anatomic variability. The occiput is thickest in the region of the external occipital protuberance (EOP), but the adjacent venous sinuses must be avoided. A thick midline keel of bone runs from the EOP to the foramen magnum and may be used for rigid fixation. Do not stray greater than 2 cm laterally from midline, as the bone thickness diminishes considerably.

Prepare the atlas and axis similarly prior to graft placement. It is very important to adequately expose and decorticate the C1 arch and laminae of C2. Using a Kerrison rongeur to decorticate the caudal portion of C1 and cranial portion of C2 will avoid a dural tear from the burr.

Difficulties Encountered

Absence of the C1 posterior arch, or the need for laminectomy at C1 or C2, may necessitate placement of bone graft into the atlantoaxial joints. Carefully elevate the C2 ganglion to expose the atlantoaxial joint on each side. Remove cartilage joints with a curette or burr and pack morselized cancellous graft into the joints. Corticocancellous strut grafts can be applied laterally and held in place with wires or cables secured to the bone or the instrumentation. This bone graft can be carried up to a prepared occipital bony surface when occipitocervical fusion is desired. The occipital bone should be prepared sufficiently laterally from midline to accommodate the graft placement over the C1–2 articulation.

Key Procedural Steps

Occipitocervical Grafting

This step can be accomplished in a variety of ways, depending on the extent of fusion required below the axis, as well as the accompanying instrumentation. Corticocancellous strips placed parasagittally in a longitudinal fashion can be employed (Fig. 7.1). Alternatively, a large unicortical piece of iliac crest autograft can be placed over the occiput-C2 region, using a midline caudal notch to help with its positioning on the cephalad aspect of the C2 spinous process (Fig. 7.2). Prepare the dorsal aspect of the posterior elements and occiput by decorticating them, and ensure that the cancellous surface of the graft abuts those areas well. The graft can be secured to the occiput by a midline screw or wire, or held in place by the overlying soft tissues. With newer rod/screw fixation techniques, morselized cancellous autograft bone appears to be sufficient if applied liberally to well-decorticated surfaces. Bilateral instrumentation in combination with secure occipital fixation obviates the apparent need for structural bone graft material. Because of the small surface area comprising the posterior arch of C1, we typically use a supplemental interlaminar bicortical bone graft (via one of the methods described below) at the atlantoaxial site.