**16. Conclusion**

296 Rheumatoid Arthritis – Etiology, Consequences and Co-Morbidities

biomechanical studies have shown the more laterally divergent screws of the Magerl technique to have greater pull-out strength compared with the 10 degree Roy-Camille screw (Chin et al 2006), and we believe that our own variation on this with increased screw length at least partly accounts for our success in achieving fusion despite significant comorbidities

Inclusion of the C7 lateral mass may be required in a minority of cases. In such instances the surgeon's task is eased somewhat by the absence of the vertebral artery from the vertebral foramen, but it must be borne in mind that the C7 lateral mass is the smallest of all in the cervical spine. For this reason placement of a C7 pedicle screw with a 30 degree medial and perpendicular rostrocaudal trajectory is our usual practice. Should the occiput be included in such constructs however we advocate inclusion of at least T1 and T2 in the construct due to the dangers (screw pull-out and kyphosis) of stopping such a large moment arm at a transition junction. Careful review of 3D CT cervical spine pre-operatively will allow the surgeon to gauge pedicle size, and also recognise any aberrant vertebral artery anatomy.

Misplacement and breach of pedicle cortex occurs in approximately 20% of attempted screw placements. Given the much softer consistency of rheumatoid bones, and the importance of avoidance of creating any false tracts down narrow pedicles, it is of utmost importance to ensure that each "screw placement counts". A screw which breaches the pedicle will not be able to contribute its required resistance to flexion, extension or torque forces, and so will significantly weaken the entire construct, perhaps even placing the patient at risk of

(Heller et al 1991).

**15. Use of PediGuard™** 

Fig. 9. PediGuard™ Picture

Rheumatoid arthritis affecting the craniovertebral junction and subaxial cervical spine remains a challenging surgical entity despite recent technological advances. Such cases need a pre-operative assessment by a multi-disciplinary team to ensure adequate medical optimisation prior to such invasive procedures, thereby limiting the risk of post-procedure medical deterioration. Symptomatic instability may require instrumentation, and success in such cases depends on the specialist knowledge of the unique altered bone morphology and the plethora of traversing neural and vascular structures. An appreciation of the biomechanical forces which instrumented constructs in this area experience is mandatory if a safe solid pain-free end-result is to be achieved.
