Occiput, C1, and C2 Instrumentation

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CHAPTER 298 Occiput, C1, and C2 Instrumentation

The occipitocervical junction (OCJ) represents a complex interface between the cranium and the rostral cervical spine. Composed of the occiput, atlas, and axis, it provides nearly all of the rotation and most of the flexion-extension of the head and neck. This region, with its osseous articulations and ligamentous support structures, must resist forces in eight axes of rotation, including flexion, extension, bilateral lateral bending, bilateral rotation, distraction, and axial loading.¹ The occipitoatlantal joint allows 13 degrees of flexion and extension, 8 degrees of lateral bending, and no rotation, whereas the atlantoaxial complex allows 10 degrees of flexion and extension, no lateral bending, and 94 degrees of rotation.² Therefore, it is imperative that these unique anatomic and kinematic features be taken into consideration when proceeding with occipitocervical fusion. These unique movement capabilities also provide challenges to stabilization compared with the subaxial spine.

Until the past few decades, the mainstay of treatment for cervical instability was external immobilization. However, the nonunion rate and morbidity associated with its use in certain populations precluded its use in some instances.³ In 1910, Mixter and Osgood⁴ described the first surgical treatment of atlantoaxial instability in the literature. They reported securing the posterior arch of the atlas to the spinous process of the axis with a heavy silk thread. Since then, the evolution of the techniques has spanned the gamut from the Gallie type in 1939⁵ to more recent techniques such as the C2 laminar technique.⁶^–⁸

Indications for Fusion—Occipitocervical

Occipitocervical fusion is indicated primarily for instability. Causes of instability include, but are not limited to trauma (including occipitoatlantal dislocation), rheumatoid settling, basilar invagination (primary or secondary), neoplastic (primary or metastatic), congenital abnormalities (e.g., certain cases of Chiari malformation), or iatrogenic instability after surgical decompression. In the setting of trauma, occipitoatlantal dislocation is a devastating injury usually resulting in death or significant neurological morbidity.⁹ If the patient survives the initial injury, occipitocervical fusion is often required. Acquired causes of instability at the craniocervical junction, such as rheumatoid settling or basilar impression, result from pathologic changes of the occipitocervical junction instigated by inflammation of the synovium resulting in bony, cartilaginous, and ligamentous destruction leading to deformation and instability.¹⁰ Such destruction can often cause brainstem compression and/or spinal cord compression, and patients may experience severe neck pain or myelopathy.¹¹ Additional indications for occipitocervical fusion include persistent atlantoaxial instability secondary to failure of previous C1-2 fixation or complicated C1-2 fractures where wiring or screw techniques are considered unsafe or contraindicated in the patient. Also, certain C1 fractures with wide displacement of the lateral mass cannot only destabilize the C1-2 articulation, but also the condyle-C1 articulation, requiring occipital-C2 stabilization.

Indications for Fusion—C1-2

Trauma

Traumatic fractures of C1 or C2 can lead to significant instability of the atlantoaxial complex. Fracture of the dens is the most common traumatic injury encountered requiring stabilization, but fortunately multiple options exist for the management of this fracture.¹² Anderson and D’Alonzo¹³ divided the dens fracture into three groups. Type 1 fractures involve fractures through the upper dens, and although they are generally considered stable, one study suggested that the stability is not absolute.¹⁴ Type 2 fractures occur at the junction of the dens and the body of C2, and type 3 fractures extend into the body of C2. Odontoid screw fixation can address certain C2 fractures; however, it is contraindicated in the following: disruption of the transverse atlantal ligament, associated comminuted fractures of one or both atlantoaxial joints, unstable type 3 fractures, atypical type 2 fracture with oblique comminuted fracture lines, irreducible fractures, associated thoracic kyphosis, and pathologic fractures.¹⁵

Dorsal fusion techniques offer an alternative approach for stabilization of odontoid fractures in complex situations. Such examples include associated fractures to one or both of the atlantoaxial joints or associated Jefferson fractures of the atlas, which places the C1-2 joint at risk for subluxation.¹⁶ Additionally, the patient with extensive, multisystem trauma who requires constant access to the chest and neck, when halo vest immobilization is not recommended, and those patients who have suffered a type 3 dens fracture when surgical fusion could facilitate earlier mobilization can benefit from dorsal fusion.¹⁷

Other indications for dorsal fusion include type 2A fractures. These fractures not only have significant comminution at the base of the dens and behave differently from other type 2 fractures, they exhibit a high rate of nonunion with external immobilization.¹⁸ Posterior displacement of the dens leads to nonunion rates of 70% to 89%,^17,¹⁹ and anterior displacement of more than 6 mm leads to nonunion in 67% of cases compared with 9% in those displaced less than 6 mm.²⁰ Significant displacement in either direction prevents adequate placement of ventral screws through the body and into the dens if the fracture cannot be reduced, therefore requiring dorsal stabilization.

Dorsal fusion also proves to be a valuable alternative in patients with marked thoracic kyphosis and type 2 or shallow type 3 fractures, which often cannot be approached ventrally because the rib cage obscures the correct screw trajectory. Finally, in pathologic fractures from neoplastic disease of the dens, dorsal fusion techniques prove to be a safer option because they prevent instrumentation of the diseased dens.¹²

Ligamentous Injury

Ligaments of the atlantoaxial complex limit flexion of the upper cervical spine. These include the tectorial membrane, cruciform ligament, transverse ligament, and alar ligaments.^21,²² If the atlantodental interval (normal value, 2 to 4 mm) has a value greater than or equal to 5 mm, ligamentous laxity should be suspected. If the interval is more than 10 to 12 mm, complete incompetence of the ligamentous complex is likely.²³ The alar ligaments limit axial rotation of the upper cervical spine and damage to these ligaments increases rotation in the contralateral side by 30%.²⁴ If failure of any component of the atlantoaxial ligament complex is suspected, dorsal surgical fusion is indicated.

Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a systemic disease that can affect the both the atlantoaxial junction and the subaxial spine. RA progression in the cervical spine can cause pain in as many as 49% of patients and symptoms of cord compression or myelopathy in 20% of patients.²⁵ Dorsal fusion has been more successful than ventral approaches for stabilizing the atlantoaxial complex in these patients.²⁶

Congenital

Os odontoideum is the failure of the dens to fuse with the body of the axis. Although it is often confused with type 2 dens fractures, it is considered a stable process. However, both os odontoideum and odontoid agenesis may lead to incompetence of the cruciate ligament, resulting in subsequent atlantoaxial instability.²⁷ The evidence-based indications and treatment options for os odontoideum have previously been reported.²⁸

Preoperative Preparation and Exposure

The patient with occipitocervical instability typically arrives in the operating room with some form of external immobilization, such as a halo orthosis or hard cervical collar. These patients should remain in a flat or semirecumbent position while undergoing awake fiberoptic endotracheal intubation to minimize the risk of neurological injury. This technique allows constant surveillance of the patient’s neurological examination, which in turn allows the anesthesiologist to cease the induction if the examination changes during this critical period. Preparation for intraoperative somatosensory and brainstem auditory evoked potentials for continuous monitoring throughout the operation is done before positioning. Evoked potentials are measured following induction of general anesthesia and compared with those measured after the patient is placed prone. In the supine position, the Mayfield head holder (Integra Life Science, Plainsboro, NJ) is placed on the cranium for fixation or, if a halo brace has been placed preoperatively, a Mayfield head holder adapter for the halo is used. The patient is carefully placed in the prone position and craniocervical alignment is assessed with fluoroscopic guidance before removal of any cervical orthosis. Recent studies suggest that positioning patients with cervical instability may be safer on a rotating bed rather than with conventional logrolling.²⁹

A linear posterior midline incision is made from the inion to the spinous process of the desired caudal level. The skin and subcutaneous tissue are incised down to the dorsal fascia, followed by development of an avascular midline plane between the paraspinal muscles. Soft tissue dissection is continued until the occipital squamosa, foramen magnum, and posterior elements of the first three cervical vertebrae are clearly exposed. The exposure should extend to the lateral margin of the facet joints bilaterally, whereas the occipital surface should be wide enough to support the chosen construct. All soft tissue should be meticulously removed from the bony surfaces of the segments selected, followed by use of a high-speed bur drill to decorticate these same levels to promote fusion before placement of instrumentation.

Operative Techniques—Wiring

Wiring Techniques—Occipitocervical

In 1927, Foerester first described reconstruction of the occipitocervical junction with the use of fibular strut grafts.³⁰ Since that time, multiple techniques of fusion have been described, have evolved, and been improved. Simple, stand-alone onlay bone grafting precluded the morbidity from instrumentation, but required prolonged external halo immobilization. Later, however, the addition of wiring to stabilize the segments posteriorly, and securing the bone grafts, enhanced stability in some planes of motion.³¹

Wiring techniques for the occipitocervical junction require the passage of wires between bur holes in the occiput as the rostral fixation point, with spinous process, sublaminar, or facet wires as the caudal fixation points.³²^–³⁹ Although wiring techniques have largely been supplanted by rigid screw fixation, they remain a useful tool in the surgeon’s armamentarium in cases where screw fixation is precluded or undesired.

The occipital bone located near the foramen magnum or the midline nuchal line provides the thickest site of bone for secure fixation. The occipital bone is prepared by enlarging the posterior rim of the foramen magnum and then drilling bur holes into the bone approximately 0.5 cm above the rim of the foramen magnum. The dura is subsequently elevated from the inner table before passing the wire through the bur holes. The wires are then passed from one drill hole to the other. Following preparation of the occipital bone, bone struts or contoured metal rods are fixed to the occiput by twisting the wire tightly and then fixed to the posterior elements of the cervical spine with wire by the following techniques.

Spinous process wiring of C2 and the subaxial spine is achieved by drilling a hole in the base of the spinous processes. The hole is enlarged with a towel clip and then a standard braided 22-gauge stainless steel wire is passed through the hole and around the base of the adjacent spinous process. Fixation between segments is achieved by wiring bone graft struts to the spinous processes.

Sublaminar wiring around C1 and below requires visualization of the underlying dura, which is achieved by making laminotomies and removing the ligamentum flavum. The lamina is also notched where the wire is passed to optimize visualization of the dura. The wire is passed under the lamina by feeding and pulling simultaneously to prevent neurological injury. Then bone struts or rod constructs are wired to the lamina bilaterally.

Facet wiring can be performed if decompression with a laminectomy is indicated and requires the facet joints to be opened and the articular cartilage to be removed. A drill hole is made at a 90-degree angle to the inferior facet and the wire is passed through the hole. Facet wiring alleviates the risk of neurological injury associated with sublaminar wires.

Steinmann Pin Fusion

Sonntag and Dickman ^43,⁴⁴ have described a rod and wire technique using a contoured -inch diameter threaded Steinmann pin. This pin is contoured into a U shape and also bent to accommodate the lordotic curvature of the occipitocervical region. After obtaining the desired curvature and length, the pin is wired to the occiput and cervical lamina or facets. If a suboccipital craniotomy or cervical laminectomy is performed, a plate of cortical iliac crest bone is wired to the central portion of the pin to protect the site of decompression. Fusion rates of 89% were reported in their series using this technique.

C1-2

First reported by Mixter and Osgood,⁴ initial techniques for dorsal atlantoaxial fusion involved variations of wiring together the posterior elements of the axis and atlas. Although these techniques are technically simple and require no special intraoperative equipment, such as fluoroscopy, all of them require rigid postoperative immobilization to obtain successful fusion.³

Because these techniques have largely been supplanted by more recent rigid screw fixation methods, they are only discussed briefly here. Several good review articles describe these techniques in fuller detail.^3,¹²

Gallie Fusion

In 1930, Gallie ⁵ first described the stabilization of a subluxed atlantoaxial complex by using a sublaminar wire placed around the posterior arch of C1 and looped around the spinous process of C2, holding in place a median bone graft notched over the spinous process of C2. Although the procedure is technically simple, it remains the poorest biomechanical construct, therefore requiring supplementation with other techniques. Gallie type of fusion requires an intact posterior arch of C1; therefore, it cannot be used if there is an associated Jefferson fracture or rheumatic involvement.

Brooks-Jenkins Fusion

The Brooks-Jenkins fusion uses doubled 20-gauge wires, which are passed under the laminae of the atlas and axis bilaterally. Two posterolateral autologous iliac crest bone grafts are beveled in to both interlaminar spaces and held in place with the overlying wire.⁴⁰ The Brooks-Jenkins fusion also requires an intact posterior arch of C1.

Sonntag’s Modified Gallie Fusion

In this modification technique,³² the sublaminar wires are eliminated under C2, using the spinous process of C2 as a fixation point. A single bicortical bone graft is fit into the interlaminar space by wiring the C1-2 interlaminar space and notched to accommodate the spinous process of C2. This technique provides increased stability without using two levels of sublaminar wires seen in the Brooks-Jenkins technique. To obtain optimal anatomic realignment, patients are kept in a halo vest preoperatively and intraoperatively. The C1-2 interlaminar space is widened with a high-speed bur drill and the spinous process and lamina of C2 are decorticated. The inferior aspect of the spinous process is also notched to seat the wire. Iliac crest graft (4 cm long) is shaped to fit the interlaminar space, placing the concave cortical surface toward the dura. The inferior aspect of the graft is notched to lie over the spinous process of C1 and two strands of no. 24 wire are passed around the posterior arch of C1, over the bone graft, and around the notched spinous process of C2. Finally, wires are tightened to three turns per centimeter. Patients are recommended to stay in a halo for 3 months postoperatively, and then changed to a hard collar for 4 to 6 weeks thereafter. The fusion rate for this technique has been described as 97%.

Rigid Screw Fixation

Previous wiring techniques introduce the risk of neurological injury by passage of the wire adjacent to unprotected dura, and in some patients with incompetent or absent cervical lamina, may prove to be a suboptimal method of stabilization. Therefore, over the past decade, rigid fixation with plate-screw or screw fixation with rod constructs have been studied and popularized. Biomechanical studies ⁴¹^–⁴³ also suggest superiority of screw-based fixation over wiring-based stabilization methods. Increased rigidity of screw-based constructs provide resistance to fatigue and vertical settling, while allowing fusion over fewer segments and decreasing the length of external immobilization after surgery.

Screw Fixation—Occiput

Screw placement for a rigid occipitocervical construct requires specific selection of both cranial and caudal anchors. Occipital screw placement requires both careful measurement of the thickness of the occipital bone and identification of the proximity of the dural sinuses preoperatively. Screws must also be carefully selected to provide optimal purchase, while avoiding violation of the dura, which may cause cerebrospinal fluid (CSF) leaks or cerebellar injury. Anatomic studies ⁴⁴^–⁴⁷ evaluating occipital bone morphology have shown that the occipital protuberance is the area of greatest bone thickness and was consistently located midline on the superior nuchal line. The bone thickness also decreases radially in both lateral and inferior directions from the occipital protuberance.⁴⁷ Ebraheim and others⁴⁴ reported that the maximum thickness of occipital bone ranged from 11.5 to 15.1 mm in males and 9.7 to 12.0 mm in females at the level of the external occipital protuberance (EOP) and that the occipital bone was thicker than 8 mm in an area extending lateral to the external occipital protuberance for up to 23 mm. Therefore, screws up to 8 mm long may be inserted in the region of the superior nuchal line up to 2 cm laterally from the center of the external occipital protuberance, 1 cm from the midline at a level 1 cm inferior to the EOP, and 0.5 cm from the midline at a level 2 cm from the EOP. Haher and associates⁴⁵ examined the pullout strength of unicortical and bicortical screws relative to occipital bone morphology and reported that on average, the pullout strength of bicortical fixation was 50% greater that unicortical screw fixation. However, unicortical screw pullout strength at the occipital protuberance was not significantly different than that of bicortical screws at other anatomic locations. Zipnick and associates⁴⁷ showed that the outer cortex contributed 45% of the total thickness of the occipital bone but the inner cortex only contributed 10%, so unicortical fixation at the occipital protuberance may offer acceptable pullout strength to secure the occiput to prevent the complications of bicortical screw placement.

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