Indications for Occipitocervical Fixation1

The occipitoaxial joint is among the strongest joints of the body. The strength of the ligaments and the orientation of the joint surface provide remarkable stability and mobility to the region. We have identified that, in most cases of craniovertebral junction region–related instability, it is the atlantoaxial joint that is unstable whereas occipitoaxial instability is infrequent or rare. In cases with trauma and in congenital instability of the region, atlantoaxial instability is the principle issue in question. In cases of craniovertebral instability related to rheumatoid arthritis, the issue of occipitoaxial instability is debated, whereas the presence of atlanto-axial instability is clearly evident in most cases. Such an instability results in both basilar invagination and in atlantoaxial dislocation. On the basis of our experience, it is clear to us that occipitocervical fixation in the presence of atlantoaxial dislocation or instability is a suboptimal operation and can be avoided. The technique of manipulation and distraction and direct fixation of the atlantoaxial joint can be the key to a variety of craniovertebral junction instability–related problems like reducible and irreducible atlantoaxial dislocation, basilar invagination, rheumatoid arthritis, and similar such problems. Craniovertebral realignment can be possible after such a joint manipulation and distraction. Although technically relatively difficult, it is possible to perform direct screw implantation in the facet of atlas, even in the presence of assimilation of the atlas. Accordingly, it is crucial for the surgeon to clearly identify the presence or otherwise of occipitoaxial instability prior to resorting to occipitocervical fixation. The improvements in understanding the anatomy of the craniovertebral junction and advancements in the techniques of atlantoaxial fixation have limited the indications of occipitocervical fixation.

Direct Atlantoaxial Fixation in the Presence of an Occipitalized or Assimilated Atlas

An occipitalized or assimilated atlas is relatively common.2 An occipitalized atlas is usually associated with fusion anomalies of adjoining bones in the region and with basilar invagination. We had earlier analyzed 190 cases of basilar invagination treated between 1987 and 1997 and isolated 153 (77%) having occipitalization of the atlas.3 Occipitalization is frequently associated with maldevelopment of the occipital bone, reduced length of the clivus and platybasia, occipital condylar and adjoining bone hypoplasia, and complete or incomplete fusion of the occipitoaxial joint. Fusion of the C2–C3 spinal elements and a range of Klippel-Feil spinal abnormalities are frequently associated. Occipitalization of the atlas is usually associated with basilar invagination and compression of the cervicomedullary cord by the odontoid process. Transoral surgery followed by posterior fixation is the generally adopted protocol for such an anomaly. Several methods have been discussed. These include use of the occipital squama for wire or screw fixation of the occipital end of the implant (plate, rods, or metal loops). Jain et al.4 described the technique of drilling the occipital bone close to the foramen magnum and formation of an artificial arch of the atlas, which is subsequently used for atlantoaxial fixation. Although occipitocervical fixations have been performed for more than 50 years using various types of instrumentation, and each type has its own advocates, it is fair to say that an ideal, universally applicable, and acceptable technique has not evolved. Crockard et al.,5 after reviewing the literature, remarked that occipitocervical fixation is always difficult.

Although all authors do not uniformly agree, the craniovertebral region in the presence of an occipitalized atlas has been observed by us to be potentially unstable.2 Complete or incomplete occipitalization of the atlas is frequently seen in cases of basilar invagination. Mobile and reducible atlantoaxial dislocation in the presence of an occipitalized atlas is not common and has been reported by us earlier.2,6 In a more recent publication,7 we discussed that fusions in the craniovertebral junction are more frequently above (assimilation of the atlas) and below (C2–C3 fusion) the site of neural compression by the odontoid process. The bone fusions may not be due to an embryonic dysgenesis but are a result of chronically reduced neck size. Although direct screw implantation into the lateral masses of the atlas in the presence of occipitalization of the atlas is a relatively difficult surgical procedure, we found that the screw purchase and stability achieved were much stronger than screw implantation or wire fixation in the occipital squama, which is a relatively thin shell of bone. Direct application of screws to the atlas and axis, thus using the firm purchase in their thick and large corticocancellous lateral mass, provided a biomechanically firm fixation of the region. The drilling of the articular cartilage of the atlantoaxial joint and placement of the bone graft provided an opportunity for joint distraction, reduced the dislocation and basilar invagination, and helped in the ultimate bone fusion of the joint8–10 ( Figs. 22.1, 22.2, and 22.3 ).

Screw Implantation in the Spinous Process and Spinolaminar Region of the Axis

The feasibility of direct screw implantation in the spinous process or in the spinolaminar region for occipitocervical fixation was first described by Goel and Kulkarni in 2004.11 The relatively strong and stubby spinous process of the axis facilitated screw implantation. Screws that were implanted in the base of the spinous process or spinolaminar junction and those that extended into the substance of the lamina were the strongest in their purchase ( Figs. 22.4, 22.5, and 22.6 ).

Preoperative computed tomography (CT) scan and intraoperative navigation assisted in the identification of the thickness of the spinous process and in determining the best site and direction of screw implantation. The selection of the site for screw implantation varied and was primarily based on the shape and thickness of the components of the spinous process. The spinous process was first denuded of all its ligamentous attachments and periosteum. The strongest part of the spinous process was at its base at the spinolaminar junction. The safety of the trajectory of the screw implantation was verified with intraoperative navigation and direct physical observation. The screw diameter ranged from 2.5 to 2.7 mm, and the screw length ranged from 8 to 12 mm. In cases where the screw will subsequently traverse into the lamina of the axis, the longer screw length ranging from 20 to 26 mm can be used. In some cases, the spinous process can be so thick that two or more screws can be implanted into its substance.