Atlantoaxial (C1-C2) Fusion

C1-C2 fusion is offered when the atlantoaxial segment is unstable, painful beyond salvage, or destroyed — and when the pathology is confined to C1-C2 (extension to the occiput or subaxial spine changes the operation).

• Confirm reduction on lateral fluoroscopy before drilling — reduce with positioning, Mayfield adjustment, or a temporary clamp; if irreducible, abandon Magerl.
• Entry 2-3 mm superior and 2-3 mm lateral to the medial edge of the C2-C3 facet on the C2 inferior articular process; trajectory strictly sagittal (0-10 degrees medial), steeply cephalad along the C2 isthmus, across the joint, into the C1 lateral mass aiming at the anterior arch tubercle.
• The steep trajectory often requires percutaneous stab incisions near the cervicothoracic junction; thoracic kyphosis or a barrel chest can make the trajectory unachievable — simulate on preoperative sagittal CT.
• Always supplement with a posterior interlaminar graft with wiring or clamp (a Gallie-type graft) — screws alone leave the graft uncompressed and fusion rates fall.
• Unilateral screw plus posterior wiring is an accepted compromise when only one side has a safe VA course.

• AO Spine upper cervical classification and knowledge forum publications frame odontoid fracture and atlantoaxial injury management; operative thresholds for type II fractures (displacement greater than 5 mm, angulation greater than 11 degrees, age greater than 50, comminution) are widely adopted internationally.
• Congress of Neurological Surgeons guidelines on odontoid fractures acknowledge equipoise between rigid immobilisation and surgery in selected type II fractures but note high nonunion rates with non-operative care in the elderly.
• Global practice variation: modern segmental screw-rod constructs dominate in well-resourced centres; posterior wiring with halo immobilisation remains a legitimate, effective option where fluoroscopy, navigation, or implant availability is limited — examiners expect candidates to justify technique choice by resources as well as anatomy. In parts of Asia, Goel joint-spacer techniques for basilar invagination and congenital craniovertebral anomalies are a major practice stream reflecting regional case mix.
• Epidemiology: odontoid fractures are the commonest cervical fracture in the elderly worldwide, driven by low-energy falls in ageing populations; rheumatoid atlantoaxial instability is declining in incidence in regions with early access to disease-modifying therapy, but remains prevalent where such access is limited.
• No arthroplasty registry applies; fusion outcome data derive from institutional series and multicentre AO Spine cohorts, consistently reporting fusion above 95 percent for segmental constructs.

• C2 nerve root sacrifice versus preservation: sacrifice improves joint exposure and haemostasis with mostly well-tolerated occipital numbness; preservation avoids sensory loss but limits joint preparation. Comparative series show broadly similar patient satisfaction — both positions are defensible in a viva.
• Asymptomatic stable os odontoideum: prophylactic fusion versus surveillance remains unresolved; most fuse once instability, cord signal change, or symptoms appear.
• Type II odontoid fracture in the very elderly: operative fixation versus accepting a fibrous nonunion in a collar — surgery improves union and possibly survival in fit patients, but perioperative risk is substantial in the frail; shared decision-making is the examinable answer.
• Bicortical versus unicortical C1 screws: bicortical purchase is stronger but risks the ICA; most surgeons accept unicortical fixation within a rigid segmental construct.
• Joint fusion versus posterior onlay only: direct C1-C2 joint decortication and grafting (Goel philosophy) probably maximises fusion, at the cost of more plexus bleeding and root handling.
• Navigation and robotics reduce breach rates in anomalous anatomy in cohort data, but fluoroscopy-guided freehand technique by experienced surgeons remains standard in much of the world.