2 The procedure
2.1.1 Atlantoaxial instability (excessive movement between the first and second cervical vertebrae) can be caused by trauma, malignancy and inflammatory or congenital defects. It can present with local pain, but if the spinal cord is compressed it can cause clumsiness, lack of coordination, difficulty walking and, rarely, paralysis or death. Treatment is by stabilisation of the C1 (atlas) on to the C2 (axial) vertebra.
2.1.2 Traditional methods of atlantoaxial fusion involve using wires and bone grafts, but these require external support in the postoperative period, including halo devices. Methods of rigid fixation with transarticular screws between C1 and C2 have been described. These do not require external fixation, but are not appropriate for every patient.
2.2.1 Under general anaesthesia, the patient is placed in the prone position and standard posterior exposure of the cervical spine is performed. Screws are inserted into the lateral masses of C1 and fixed by rods corresponding to screws in the lateral masses or pedicles of C2. The posterior arch of bone compressing the spinal cord may be removed. An onlay graft of bone permits a permanent fusion between C1 and C2.
2.3.1 The primary endpoint in the literature was successful fusion. In two case series of 37 and 160 patients and one case report, immobilisation of C1 on C2 was achieved in 196 patients (in the remaining two patients the intent was not to procure fusion, see Sources of evidence for more information).
2.3.2 In a case series of 157 patients, clinical and neurological recovery was documented in 100% but assessment measures were not described.
2.3.3 A case series investigating a lateral mass and plate system reported that 6% (9/157) of procedures could not be completed because of inadequate exposure of the atlantoaxial region. For more details refer to the Sources of evidence.
2.3.4 The Specialist Advisors considered this to be a variation on existing fusion techniques.
2.4.1 In two case series of 157 and 37 patients, there were no reports of implant failure. No vertebral artery injuries occurred in the three case series (n = 212) or in the one case report where safety outcomes were reported.
2.4.2 In a case series, sensory loss in the distribution of the C2 nerve root was reported by 11% (18/157) of patients undergoing screw and plate fixation. In the same series, six screws were found to be penetrating more than 4 mm from the anterior cortex of C1 but with no clinical complications. One screw was found to be broken at 18 months' follow-up. In another case series, deep wound infection was reported in 3% (1/37) of patients. For more details refer to the Sources of evidence.
2.4.3 The Specialist Advisors stated that potential adverse events include haemorrhage from the vertebral venous plexus and screw failure or loosening. Less common but more serious complications may include injury to the vertebral artery, and spinal cord injury caused by screw misplacement.
2.5.1 It was recognised that the evidence on this procedure was limited, but it was considered sufficient in the context of the conditions requiring treatment.
2.5.2 These recommendations are based on evidence from studies using two different techniques – the plate system and the more recent polyaxial screw and rod systems.