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 high-energy trauma translation or distraction injuries that destabilize the occipitocervical junction 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 anterior and posterior median atlanto-odontoid joints paired atlanto-occipital joints paired atlantoaxial joints Ligaments intrinsic ligaments are located within the spinal canal and provide most of the ligamentous stability. They include: transverse ligament primary stabilizer of atlantoaxial junction connects the posterior odontoid to the anterior atlas arch, inserting laterally on bony tubercles paired alar ligaments connect the odontoid to the occipital condyles relatively strong and contribute to occipitocervical stability apical ligament relatively weak midline structure runs vertically between the odontoid and foramen magnum tectorial membrane connects the posterior body of the axis to the anterior foramen magnum and is the cephalad continuation of the PLL 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 used to diagnose occipitocervical dislocation C-D: distance from basion to posterior arch A-B: distance from anterior arch to opisthion significance ratio ~1 is normal >1.0 raises concern for anterior dislocation <1.0 raises concern for posterior atlanto-occipital dislocation odontoid fractures ring of atlas fractures Harris rule of 12 basion-dens interval or basion-posterior axial interval >12 mm suggests occipitocervical dissociation Condyle-C1 interval (CCI) distance between the occipital condyles and the superior articular surfaces of the atlas (C1) most sensitive and specific test to diagnose occipitocervical dislocation <2.5 mm is normal 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 traumatic instability with distraction of the atlanto-occipital joint techniques halo vest tongs prolonged cervical orthosis is not recommended due to poor stabilization of the atlanto-occipital joint outcomes use of traction should be avoided in most cases traction may be considered in stage 2 injuries when MRI demonstrates soft-tissue injury with preserved alignment Operative occipitocervical fusion indications most traumatic cases require stabilization acquired cases when evidence of myelopathy or significant symptomatic neck pain invagination and atlantoaxial impaction secondary to inflammatory arthropathy (e.g. rheumatoid arthritis) tumor Technique Occipitocervical fusion approach posterior midline incision with patient in prone position Mayfield retractor used to obtain proper craniocervical alignment establish preoperative O-C2 angle with lateral fluoroscopy prior to draping 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 posterior segmental instrumented fusion is usually performed from the occiput to C3 occiput occipital plates usually allow for 3 or 4 total screws with adjustable rod holders occipital screws usually unicortical to avoid injury to venous sinus major dural venous sinuses are located just below the external occipital protuberance and are at risk of penetrative injury some institutions prefer bicortical screws but they come at an increased risk occipital screw safe zone the safe zone for occipital screws is located within an area measuring 2 cm lateral and 1 cm inferior to the external occipital protuberance along the superior nuchal line C1 lateral mass screws often skipped due to angle at base of skull making it more difficult to place a rod may choose a unilateral screw to provide some rotational stability for C1 ring C2 fixation pars, pedicle, transarticular, or translaminar screws C3 fixation standard lateral mass screws aimed cephalad and lateral to avoid vertebral artery arthrodesis may require bone grafting or removal of bony fragments compressing neurovascular structures Complications Nonunion Bleeding Internal carotid artery Vertebral artery