**12. C1 Lateral mass screw placement**

292 Rheumatoid Arthritis – Etiology, Consequences and Co-Morbidities

not subaxial fixation were included as part of the construct. Criticisms such as the femaleonly nature of the cadaveric specimens and the use of non-contiguous subaxial fixation points have been robustly refuted by the authors as being of limited clinical relevance. Nowhere in spine surgery is the concept of ultimate mechanical fatigue and subsequent failure of greater importance than at the occipitocervical junction. The large moment arm generated with an adult head and the steep angles required in excessively lordotic porotic rheumatoid cervical spine account for these high failure figures. We use a combination of the autologous bone-chippings on the decorticated bone surfaces, and osteoconductive and osteoinductive bone void-fillers superimposed on our construct to maximise the chance of securing bony fusion. Osseous fusion is necessary for ultimate success of this procedure, as in its absence metal fatigue and subsequent catastrophic construct failure is virtually guaranteed. Rigid internal fixation at time of surgery obviates the need for use of postoperative HALO-bracing, though a custom-fitted Miami-J collar beneath the patient's chin will prevent premature excessive neck flexion and screw pull-out. Careful adherence to a professionally devised and supervised nutrition programme is also part of any follow-up

Fig. 8. Occipitocervical fusion using a transarticular screw fixation of the C1C2 complex, and lateral mass fixation of C3-C7. The construct was extended into the upper thoracic levels.

Note that single screws were only possible at the C3 and C4 levels

Though we prefer to use C1/C2 transarticular screws, anatomical or surgical circumstances pertaining to this challenging patient cohort occasionally mandate the use of C1 lateral mass screws. C1 lateral mass screws are technically demanding, but we do use them regularly in cases of rheumatoid C1-2 fusions. These may be inserted in cases in which transarticular screws are contraindicated because of anatomic constraints. Such cases include patients with anomalous vertebral arteries, though in such cases it is essential to show on 3D CT that placement of a C2 pars screw is possible. Seventeen (18%) of 94 patients had a high-riding transverse foramen on at least one side of the axis that would prohibit the placement of conventional C1/C2 transarticular screws (Mummaneni & Haid 2005, Nagaria et al 2009). C1 lateral mass screw-rod constructs are preferred over conventional atlantoaxial transarticular screws by certain authors due to a variety of factors (Paramore et al 1996, Currian & Yaszemski 2004). The C1 lateral mass screws can be inserted before reduction of the atlantoaxial joints, thereby enabling the surgeon to use the screws as method of achieving a reduction. The screws do not violate the C1-2 joints, and therefore they can be used for temporary immobilization in trauma patients; however this is not a consideration in the rheumatoid patient. C1 lateral mass screws can also be used when the C1 arch is deficient. The presence of an arcuate foramen (ponticulus posticus) in the atlas, seen in up to 18% of cases, through which the vertebral artery and first cervical nerve traverse, precludes the use of this technique (Gunnarsson et al 2007).

Beginning the passage of C1 lateral mass screw can be quite a challenge due to the almost constant presence of a venous plexus at the insertion site caudal to the posterior lateral arch of the atlas. We use a combination of Surgicel and thrombin glue to achieve haemostasis, and a slightly more rostral entry point, on the posterior lateral arch itself using a pneumatic drill to drill away the undersurface of the posterior lateral arch. Using such an entry point, in conjunction with neuronavigation, allows one to avoid the vertebral artery and spinal cord, whilst also avoiding the worst of the bleeding from the venous plexus. Such an approach is possible in over 85% of cases (Huang & Glaser 2003). The internal carotid artery and hypoglossal nerve lie over the anterior aspect of the lateral mass of the atlas and are at risk from bicortical C1 lateral mass screws. Some authors have advocated use of unicortical C1 lateral mass screws in order to avoid such potential complications. Such opinions are supported by biomechanical data showing greater pull-out strength of both unicortical and bicortical C1 lateral mass screws compared with subaxial lateral mass screws. Our practice however is to aim for bicortical purchase, given the absence of adequate comparative data for rheumatoid patients, and the greater risk of screw pullout due to the tendency of the underlying rheumatoid disease to cause osteoporosis of the vertebrae (Wordsworth et al 1984, Lee et al 2006).
