11.3.5\ Occipitocervical Fusion

11.3.5.1\ Discussion

Indications for occipitocervical fusion include anterior and posterior bifid C1 arches with instability, absent occipital condyles, severe reducible basilar invagination, unstable dystopic os odontoideum, unilateral atlas assimilation, traumatic occipitocervical dislocation, complex craniovertebral junction fractures of C1 and C2, transoral craniovertebral junction decompression, cranial settling in Down’s syndrome, tumors, and inflammatory disease such as Grisel’s syndrome.

A variety of internal fixation methods have been developed for posterior craniocervical junction fusion including sublaminar wiring (Fig. 11.33) and occipital rods and plates (Fig. 11.34). Bone grafts are often added alongside the posterior elements in order to promote bony fusion. Interestingly, the degree of stabil-

a

ity does not seem to correlate with the presence or absence of radiographically evident bone graft fusion. Sublaminar wires have the potential to unravel, resulting in recurrent malalignment and instability (Fig. 11.35) and generally provide less stability than screw constructs. In addition, wire fracture can lacerate the spinal cord. The occipital screws can sometimes penetrate the inner table of the occipital bone (Fig. 11.36), which may not necessarily result in cerebellar injury, especially if it is only by a small extent. Transarticular screw fixation of the cervical spine can encroach upon the transverse foramina and potentially injure the vertebral arteries or even impinge upon the internal carotid arteries (Fig. 11.37). The incidence of nonunion or loosening is about 7% for occipitocervical fusion and atlantoaxial fusion, which appears as lucency around the hardware on CT (Fig. 11.38).

b

Fig. 11.33  Atlantoaxial fusion with sublaminar wiring. The patient has a history of an unstable dens fracture. Lateral radiograph (a) and axial CT image (b) demon-

strate posterior atlantoaxial fixation with bilateral cables that pass into the spinal canal and around iliac crest bone grafts applied posterior to the C1 and C2 arches

Fig. 11.34  Occipitocervical fusion with rods and screws. Sagittal CT image (a) shows the curvilinear posterior rod (arrow) attached to the occipital bone via plate (arrow)

and screws and to the upper cervical spine via lateral mass screws. Photograph of an occipital plate (b)

Fig. 11.35  Unraveled sublaminar wire. The patient has a history of unstable os odontoideum. Preoperative sagittal (a) CT image shows a dystopic os odontoideum angled anteriorly and a widened C1–C2 interspinous space (bracket). Initial postoperative sagittal CT image (b) shows interval fixation of the posterior elements of C1 and

C2 with sublaminar wires and application of bone graft. There is resulting decreased C1–C2 interspinous distance and angulation of the os odontoideum. Subsequent sagittal CT image (c) shows interval widening of the C1–C2 interspinous distance and angulation of the os odontoideum similar to the configuration before surgery

Fig. 11.36  Occipital screw intracranial penetration. Sagittal CT image shows penetration of the inner table with intracranial extension of a lateral occipital plate screw (arrow)

Fig. 11.37  Vascular compromise by screw malposition. Axial CT image (a) shows a right lateral mass screw within the right foramen transversarium (arrow). Coronal CTA image in another patient (b) shows impingement of the left internal carotid artery (encircled)

Fig. 11.38  Hardware loosening. Sagittal CT image (a) shows lucency surrounded the hardware in the occipital bone (arrow). Sagittal CT image (b) shows lucencies

surrounding­ multiple lateral mass screws (arrows), which have begun to pull out