15 Instrumentation in Craniovertebral Junction Surgery

10.1055/b-0034-81392

15 Instrumentation in Craniovertebral Junction Surgery

Fessler, Richard G., Ogden, Alfred T.

Typical pathologies for occipitocervical fusion (OCF) include congenital and developmental malformations, rheumatoid arthritis, primary bone tumors, metastatic disease, and trauma. Although rates of OCF have decreased with the advent of effective medical therapy for inflammatory arthritides, OCF will remain an essential tool for spine surgeons in select cases. Posterior instrumentation specifically designed for OCF is available from several vendors, and assorted fixation strategies have been advocated and tested biomechanically. Some authors have reported novel anterior constructs, although these have not gained widespread popularity. This chapter touches briefly on the history of OCF and relates preferred methods.

Background

Initial experience with occipitocervical fusions reflected the need to address decompression and stabilization in patients with rheumatoid arthritis with compressive myelopathy secondary to basilar invagination, ventral pannus, or instability at the atlantoaxial joint. Some early reports also relate experience treating congenital and developmental anomalies. Given the progressive nature of rheumatoid arthritis and the deleterious effects of corticosteroids on bone, many surgeons advocated fusion at the first sign of clinical symptoms (myelopathy and neck pain) and/or demonstration of instability. The procedure was often applied to patients with pure C1–C2 instability because of the high rate of pseudoarthrosis with C1–C2 fusions. Surgeons argued that the added immobility from incorporation of the occipitoatlantal joint in a C1–C2 construct was clinically inconsequential. Lacking advanced instrumentation, early fusion strategies ranged from non-structural onlay grafts of morcellized iliac crest1,2 to wired cortical strips of the ilium or tibia,3,4 followed by lengthy immobilization and traction.

Early results with patients with rheumatoid arthritis—which arose as the index population for this procedure—were disappointing, with high rates of pseudoarthrosis/reoperation and high morbidity and mortality.5 As the development of fixation techniques continued, various surgeons reported their experience with wiring techniques and supplementation with acrylic. In the 1980s, rigid plate6 and loop systems using wires7 or cables8 for fixation were introduced. The incorporation of rigid internal devices led to earlier mobilization and decreased the need for external immobilization. Ultimately, success with screw fixation was reported. For the cervical end of the fixation, Grob et al. reported the use of C1–C2 transarticular screws, and Goel and Laheri reported the use of a C2 pars screw in isolation or in combination with C1 lateral mass screws.9–11 Both groups fixed the occipital end of the implant with screws. Today successful outcomes with OCF are related to improvements in techniques and instrumentation, as well as a changing patient population. One recent surgical series reported an 87% rate of improvement in myelopathy and a 97% fusion rate in a group of 69 patients undergoing OCF for a variety of indications.12 Goel and Kulkarni reported atlantoaxial lateral mass plate and screw fixation even in cases where there was assimilation of the atlas.13

The current trend toward a reduction in the implementation of occipitocervical fusion has been heavily influenced by the advent of effective medical therapy for rheumatoid arthritis, the employment of highly effective C1–C2 fusion strategies, and the recognition of significant morbidity from immobilization of the craniovertebral junction. The first two developments have significantly reduced the number of patients for whom OCF is indicated, and the third has pushed surgeons to opt for OCF only as a last resort. Still, craniovertebral instability is unlikely to disappear completely, and, as such, OCF is likely to remain an essential tool for a subset of patients with relatively severe and complex pathology.

Indications

The indications for OCF are globally defined as pathological conditions resulting in instability of the articulation of the atlas and the skull base and the treatment of an iatrogenic instability resulting from a decompressive procedure. Typical disease processes that produce such situations are inflammatory arthritis (in particular, rheumatoid arthritis), trauma (occipital condyle–C1 [O–C1] dislocation and complex high cervical fractures), malignancy (primary bone tumors or metastases), congenital malformations (dysgenesis of the C1 lateral masses and occipital condyles), and developmental disorders (Grisel and Down syndromes). The anatomical abnormality can range from a reducible instability that requires reduction and fixation, either in the vertical plane, such as basilar invagination with rheumatoid arthritis, or in the sagittal plane, such traumatic O–C1 dislocation, to a compressive pathology whose surgical decompression results in instability requiring fusion (rheumatoid pannus and malignancy). Often the decision involves the relative merits of extension of a high cervical construct to the occiput. For example, in cases of complex C1–C2 fractures, where C1–C2 instability is combined with destruction of the C1 lateral masses, OCF may be considered as a primary procedure or as a backup strategy.

Instrumentation

Current OCF strategies usually entail fixation of a rigid contoured rod to multiple points in the occipital bone and C2 at a minimum, with the addition of other fixation points as needed depending on pathology, technique, and surgeon preference. Fixation may be achieved using a variety of techniques.

Rod/Wiring Technique

An older but still effective technique is a pure rod and cable construct. This technique is attributed to Ransford et al.7 and was discussed by Menezes.14 It can be accomplished with minimal equipment and at low cost. Exposure of the craniovertebral junction and the cervical spine down to C3 is followed by a small suboccipital craniectomy of ~2 × 2 cm, saving bone for subsequent arthrodesis. Burr holes are drilled around the edge of the craniectomy to create fixation points for cables in the occipital bone. Menezes prefers four burr holes of 6 to 8 mm placed in the occipital bone, two rostral and two lateral to the craniectomy.14 The rostral burr holes are 1 cm of the midline and 1 cm above the craniectomy, and the lateral burr holes are 1 cm above the foramen magnum and 1 cm lateral to the craniectomy. The dura is freed between the craniectomy and the burr holes and under the lamina of the vertebra to be fused. Sublaminar cables are passed under C1, C2, and C3 (if necessary). A loop titanium rod, which is precon-toured or contoured by the surgeon, is placed into the field and cabled in place to a pressure of 15 lb (6.8 kg) for C1 and 30 lb (13.6 kg) for the occipital and other cervical cables. Following decortication, bone grafting is performed with occipital bone and the surgeon’s choice of other material. Goel and Achawal reversed the foramen magnum bone flap following a small craniectomy such that the inner surface of the occipital squama faced outside, and the inner curve of the occipital bone faced away from the neural structures, a procedure they labeled foramen magnotomy.11

Screw Rod/Plate Constructs

Although cabling remains an important tool, most surgeons prefer screw fixation points because of increased rigidity and durability.15 Several current instrumentation systems have been specifically designed for OCF, and these offer the convenience of contoured rods and malleable plates. All of these constructs offer rigid occipital bone screw fixation with integration into cervical screw rod/plate constructs. The use of screws in the cervical spine, however, remains “off-label” according to the U.S. Food and Drug Administration. Designs vary mostly in terms of whether a single-piece contoured loop is used versus two curved rods whose biomechanics are integrated with cross-linkers. The significant technical advance in dedicated OCF systems lies in the ability to create rigid, integrated constructs with multiple screw fixation points in the cervical spine and the occipital bone.

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