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Occipitocervical fixation (OCF) is a maximally invasive surgical technique that results in significant loss of flexion, extension, and rotation. Therefore, surgical indications for OCF are either conditions that result in cervicomedullary compression, the treatment of which would cause instability, or entities that themselves result in overt instability at the occipitocervical junction. These surgical indications include traumatic injuries, rheumatoid arthritis (RA), congenital malformations, and primary and metastatic neoplastic lesions of the craniocervical junction. The earliest description of occipitocervical fusion was by Foerster in 1927. In subsequent decades, additional reports described similar bone onlay techniques. Occipitocervical techniques have evolved extensively since their initial description, and competent spine surgeons should possess a mastery of these techniques in their surgical armamentarium.
Preoperative Considerations
Traumatic Injuries of the Craniovertebral Junction
Traumatic injuries of the craniovertebral junction include occipitoatlantal dislocation (OAD), occipital condyle fractures, atlas fractures, and axis fractures and dislocations.
Occipitoatlantal Dislocation
Considerable force is required to cause OAD, and patients often present with significant head, spinal cord, or multisystemic traumatic injuries. Mechanical ventilation, which can be needed as a result of brainstem compromise, often makes neurologic assessment difficult. Cranial nerve deficits or vertebral artery injury can be present. Despite the significant nature of the injury, some patients may have no neurologic deficits.
Once OAD is suspected based on examination or mechanism of injury, strict cervical spine precautions are mandatory to prevent further complications. Sandbags should be used for initial head immobilization because rigid cervical collars can further distract the occipitoatlantal joint. The authors agree with other investigators who recommend early halo fixation once the diagnosis of OAD is confirmed. Even if surgical fixation is planned, a halo vest minimizes motion of the cervical spine during intubation and positioning.
A wide range of sensitivities has been reported for the techniques used to diagnose OAD, and none of these criteria is fail proof. Available methods include the Power ratio, the X-line method, the condylar gap method, the basion-dens interval (BDI), and the basion-axial interval (BAI). A universal theme underlying the difficulties of diagnosing OAD using plain lateral cervical radiographs is the ability to visualize the anatomic landmarks required for application of these methods. Dedicated studies using computed tomography (CT) to diagnose OAD have supported the use of the BDI (with 10 mm as the cutoff) and the occipital condyle–C1 interval (CCI) (>4 mm is abnormal) as the diagnostic tests of choice.
The increased use of magnetic resonance imaging (MRI) in trauma patients raises the question of how to interpret equivocal findings in the occipitoatlantal region. The primary dilemma is how to treat patients with equivocal occipitoatlantal joint disruptions noted on MRI whose measurements on CT are normal. Further research may uncover a less severe but still unstable occipitoatlantal joint injury that threatens the neural structures enough to warrant internal fixation of the occiput to the cervical spine.
Once the diagnosis of OAD has been established, OCF is the appropriate treatment. Contraindications to treatment include medical instability in patients.
Occipital Condyle Fractures
The initial neurologic evaluation of patients presenting with occipital condyle fractures is often confounded by a concomitant head injury. Other patients can become symptomatic with neurologic injury or just neck pain. CT of the cervical spine is critical in diagnosing these fractures, which are often missed on plain radiographic imaging. Most isolated occipital condyle fractures can be treated with either a hard collar or halo immobilization. Surgical intervention is indicated in cases of concurrent ligamentous injury and instability on dynamic imaging.
Atlas and Axis Fractures
Patients with atlas fractures often present with neck pain, although symptoms can include difficulty swallowing related to retropharyngeal edema or neurologic deficit related to vertebral artery injury or lower cranial nerve injury. Although plain radiographs can detect an atlas fracture, fine-cut CT with sagittal and coronal reformatted scans can rule out pseudospread of the atlas, and MRI can be used to evaluate the integrity of the transverse ligament. Surgical fixation is rarely indicated for isolated C1 fractures. C1 fractures associated with C2 fractures demonstrating dynamic instability, an atlantodens interval (ADI) greater than 5 mm, more than 11 degrees of C2-C3 angulation, or an incompetent transverse ligament may require OCF. Isolated axis fractures rarely require OCF.
Rheumatoid Arthritis
RA is the most common inflammatory disease of the spine. One percent of the world’s population is affected by RA, and 50% of these patients have cervical spine involvement. Two of the most common spinal findings in patients with RA are basilar invagination and atlantoaxial instability. The degree of involvement of the cervical vertebral junction is related to the length and severity of the disease. Patients with RA who have involvement of the craniovertebral junction can present with neck pain, cervical deformity, or progressive neurologic decline. However, in more recent years, disease-modifying antirheumatic drugs have made a major impact on the natural history of RA in the cervical spine. Dynamic imaging, MRI, and CT are all critical in assessing instability, the degree of cervicomedullary compression, and abnormal bony anatomy.
Preoperative considerations in patients with RA are critical because of the potential multisystemic involvement of the disease. A significant cohort of patients can have cardiovascular involvement, including pericarditis, valvular dysfunction, and conduction abnormalities. Furthermore, patients with RA should be evaluated for pulmonary involvement before surgical intervention, especially patients with pulmonary fibrosis, who tend to have worse outcomes than do other patients with RA.
Pain, myelopathy, spinal cord compression, and symptomatic vertebral artery compression are all indications for OCF in patients with RA and atlantoaxial instability. Patients with basilar invagination undergo surgical procedures to ameliorate neurologic symptoms to prevent progressive neurologic decline. Contraindications to surgical treatment include significant medical comorbidities.
Congenital Malformations
Congenital malformations of the craniovertebral junction include basilar invagination, atlas assimilation, C1 congenital anomalies, atlantoaxial fusion, and odontoid process anomalies. These anomalies occur in isolation or in known syndromes such as Klippel-Feil syndrome, Down syndrome, and Chiari malformations. Indications for OCF include instability, spinal cord compression, and progressive neurologic decline.
Neoplasms of the Craniocervical Junction
Neoplasms of the craniocervical junction range from primary tumors, including osteoid osteomas, osteoblastomas, osteochondromas, hemangiomas, aneurysmal bone cysts, plasmacytomas, osteosarcomas, chondrosarcomas, giant cell tumors, Ewing tumors, hemangiopericytomas, and chordomas to metastatic tumors. Patients with tumors in this region often have a late presentation. These patients typically seek treatment for neck pain that can be exacerbated by motion. They also have prominent nocturnal pain and persistent, progressive pain. These patients rarely exhibit neurologic symptoms because of the generous subarachnoid space at the craniocervical junction. Diagnosis is commonly made with MRI. Dynamic imaging can reveal craniocervical instability related to bony destruction associated with the lesion. Indications for OCF in patients with lesions at the craniocervical junction include atlantoaxial instability, pain, and neurologic dysfunction.
Surgical Technique
Positioning
Before patients are positioned for the surgical procedure, appropriate leads are placed if monitoring of somatosensory- or motor-evoked potentials is planned. Depending on the degree of instability, the patient should be transferred to the prone position on the surgical bed in a hard cervical collar or halo brace. The head is positioned in a neutral to slightly flexed position. Excessive flexion could result in discomfort and swallowing difficulty for the patient, and excessive extension results in poor visualization of the ground. The head is rigidly fixed to the surgical bed with a Mayfield skull clamp or the halo ring ( Fig. 38-1 ). If intraoperative navigation is to be used, the stereotactic reference frame should be attached to the Mayfield skull clamp. The occiput, neck, upper thoracic, and infrascapular region should be prepared in sterile fashion for a midline incision and harvest of a rib graft.
Exposure
A routine midline skin incision is made to access the occipital bone and the vertebral levels to be included in the fusion ( Fig. 38-2 ). Dissection proceeds to expose a 5- to 6-cm width of the occipital bone and foramen magnum. The posterior ring of C1 is exposed laterally in a subperiosteal fashion by using the sulcus arteriosus as a landmark to identify and protect the horizontal portion of the vertebral artery on the superior aspect of C1. Dissection is also extended laterally over the remaining cervical levels until the lateral edges of the facet joints are visualized. If necessary, the posterior aspect of the C1 lateral masses can be exposed by using bipolar cautery and sharp dissection ventrally along the medial inferior aspect of the lateral ring of C1. The C1 lateral mass can usually be exposed by identifying and retracting the C2 nerve root, but if necessary it can be sacrificed with minimal consequence.
Occipital Instrumentation
Many devices, such as midline plates, clamp constructs, or plate and rod constructs, have been developed for occipital fixation ( Fig. 38-3 ). The authors’ current practice uses an occipital plate that allows fixation by midline occipital keel screws, typically, a 10-mm screw. The thickness of the keel should be measured preoperatively on the CT scan. Intraoperative navigation can be helpful to identify regions of greatest thickness along the keel.
Occipital keel screws are strongest when placed bicortically, but care must be taken to avoid a penetrating screw injury that can result in cerebrospinal fluid leak, cerebellar injury, or venous sinus injury. Because of the thickness of the occipital keel, drilling with a manual drill requires patience. Care must be taken to avoid applying excessive force with the drill and plunging through the bone. Use of a drill guide set to the appropriate depth and a manual drill allows the surgeon to receive tactile feedback and to confirm bicortical placement. The drill hole is then tapped, and the appropriately sized blunt occipital screw is placed to secure the plate to the occiput.
The size of the midline occipital plate is chosen so that the rod attachment points are aligned with the cervical screws. Modern occipital plates often have rod attachment points that rotate and slide to allow easy connection of the cervical construct to the occipital plate. The occipital plate should be placed 5 to 10 mm superior to the posterior edge of the foramen magnum for placement of the rib or other graft material.
Atlantoaxial Instrumentation
If the patient’s anatomy is appropriate, instrumentation should include the C1 level to provide an additional point of fixation to the fusion construct. When a fracture or abnormal anatomy increases the risk of placing a C1 lateral mass screw, that screw should be excluded from the construct. Multiple techniques for upper cervical spine instrumentation exist, including C1-C2 transarticular screws, C1 lateral mass screws, C2 interarticularis pars or pedicle screws, C2 laminar screws, and laminar wiring techniques.
The authors prefer the C1 lateral mass and C2 pedicle or pars screw and rod construct to the transarticular technique, although both offer similar biomechanical stability. The risk for vertebral artery injury is higher with the transarticular technique, and the patient’s vascular anatomy often precludes its use. The choice of technique should be based on anatomic and safety considerations.
When a C1 lateral mass screw is used, the entry point is placed at the midline of the posterior aspect of the lateral mass at its junction with the inferior surface of the posterior C1 ring ( Fig. 38-4 ). A small area of the undersurface of the posterior ring can be drilled to improve clearance for the drill and screw. A manual drill and drill guide are used to drill the hole while fluoroscopic guidance can be used to verify the depth and trajectory. The trajectory of the drill should be 10 to 15 degrees medial and aimed directly at the anterior tubercle of C1. The anatomy of the anterior ring on preoperative imaging helps guide the depth of the drill. If the curvature of the anterior arch is minimal or “flat,” the tip of the screw on lateral fluoroscopy should be aligned with the anterior tubercle. If the curvature of the anterior arch is significant or “steep,” a screw aligned with the anterior tubercle on lateral fluoroscopy will breach the anterior surface of C1 and place the carotid artery at risk. A partially threaded screw is inserted so that its threaded portion is buried in the lateral mass and its smooth portion extends beyond the C2 nerve root to a height that is level with the remaining cervical screws.
