Summary Occipitocervical instability can be traumatic or acquired through a degenerative process such as rheumatoid arthritis or Down syndrome Diagnosis is usually confirmed with a combination of CT scan, MRI, and lateral flexion-extension radiographs Traumatic instability is treated with occipitocervical fusion. Acquired instability is treated with observation or occipitocervical fusion depending on the presence of neurologic deficits Epidemiology traumatic occipitocervical instability incidence ~15-30% of cervical spine injuries occur at the occipitocervical junction prevalence identified in 19% of fatal cervical injuries acquired occipitocervical instability most frequently seen in the Down syndrome population usually asymptomatic and identified in screening for surgery or special Olympic participation Etiology Terminology also called atlanto-occipital dissociation (AOD) occipitocervical dislocation Pathophysiology traumatic mechanism of injury login to view 2 more bullets head most often displaces anteriorly acquired due to bony dysplasia or ligament/soft-tissue laxity Associated conditions atlantoaxial instability also seen in Down syndrome patients neurologic deficits vertebral or carotid artery injuries Down syndrome Anatomy Osteology morphology occipital condyles are paired prominences of the occipital bone oval or bean shaped structures forming lateral aspects of the foramen magnum joint articulations intrinsic relationship between occiput, atlas, and axis to form the occipitoatlantoaxial complex or CCJ 6 main synovial articulations login to view 3 more bullets Ligaments intrinsic ligaments are located within the spinal canal and provide most of the ligamentous stability. They include: transverse ligament login to view 2 more bullets paired alar ligaments login to view 2 more bullets apical ligament login to view 2 more bullets tectorial membrane login to view 1 more bullet Vascular system occipital condyles in proximity to vertebral arteries Nervous system occipital condyles are in close proximity to: medulla oblongata spinal cord lower cranial nerves (CN IX-XII) Classification Traynelis Classification (direction of displacement) Type I Anterior occiput dislocation Type II Longitudinal dislocation Type III Posterior occiput dislocation Harborview Classification (degree of instability) Stage I Minimal or nondisplaced, unilateral injury to craniocervical ligaments Stable Stage II Minimally displaced, but MRI demonstrates significant soft-tissue injuries Stability may be based on traction test Stable or unstable Stage III Gross craniocervical malalignment (BAI or BDI >2 mm beyond normal limits) Unstable Imaging Radiographs recommended views AP, lateral, and odontoid views findings low sensitivity in detecting injury (57%) measurements Powers ratio = C-D/A-B login to view 10 more bullets Harris rule of 12 login to view 2 more bullets Condyle-C1 interval (CCI) login to view 3 more bullets CT indications considered gold standard for osseous injuries of the spine midsagittal CT reconstruction CT angiogram indications evaluate for injury to vertebral artery identify anatomy of vertebral artery prior to occipitocervical fusion MRI indications suspected ligamentous injury with preserved alignment or occult injury neurologic deficits Treatment Nonoperative provisional stabilization while avoiding traction indications login to view 1 more bullet techniques login to view 3 more bullets outcomes login to view 2 more bullets Operative occipitocervical fusion indications login to view 4 more bullets Technique Occipitocervical fusion approach posterior midline incision with patient in prone position Mayfield retractor used to obtain proper craniocervical alignment login to view 1 more bullet deep dissection if performing C1 lateral mass screw fixation, work within safe zone and do not dissect more than 1 cm lateral to midline above the posterior arch of C1 to avoid injury to vertebral artery instrumentation length login to view 1 more bullet occiput login to view 7 more bullets C1 lateral mass screws login to view 2 more bullets C2 fixation login to view 1 more bullet C3 fixation login to view 1 more bullet arthrodesis may require bone grafting or removal of bony fragments compressing neurovascular structures Complications Nonunion Bleeding Internal carotid artery Vertebral artery