Atlanto-Occipital Dislocation (AOD)
What is the Occipitoatlantal Joint?
Atlanto-occipital joint is formed between facets of atlas and occipital condyles. It permits flexion and extension movement at craniocervical junction. Atlano-occipital joint: Bones, ligaments
What is Atlanto-occipital Dislocation?
Although once considered an invariably fatal injury, improvements in diagnosis and management have made atlanto-occipital dislocation (also called atlanto-occipital dissociation) (AOD) a survivable injury. The injury occurs mainly due to tear of ligaments at cervico-occipital junction due to hyper extension, hyperflexion, lateral flexion, or combination of these .
What is the cause of AOD?
Trauma is the main cause of Atlanto-occipital instability, however, connective tissue disorders can also lead to hypermobility and injury.
What are the types of AOD?
Occipitoatlantal subluxation is classified into three main types (Trayneli’s Classification), however, they can occur in combination as well. (Fig 6) [6,10]
- Anterior dislocation of occiput over atlas
- Traction/ Longitudinal Dislocation
- Posterior Dislocation
What are the symptoms of AOD?
The dislocation can be asymptomatic, or can present with symptoms of compression to brainstem or cervical cord. The symptoms can include motor or sensory neurological deficits, cranial nerve defects, or cervico-medullary syndrome .
How is AOD Diagnosed?
AOI is more accurately diagnosed on CT scan than X-Rays. Main diagnostic methods (Fig 7) used on both lateral X-Ray and sagittal CBCT / CT-Scan include following, however, their normal value differ on both modalities:
- Power’s Ratio: It’s defined as B*C/A*O, where B is basion, C is posterior arch of C1, A is anterior arch of C1 and O is opisthion. Value of less than 0.9 is normal. 0.9-less than 1 indicates gray zone, while value of more than 1 signifies atlanto-occipital dislocation.
- Basion-dens interval (BDI): Distance from basion to tip of dens. It’s less than 12 mm on X-Ray, and less than 8.5 mm on CT-Scan in adults. Although some neuroradiologist’s threshold values for BDI are greater than 10 mm in adults and greater than 12 mm in children.
Magnetic resonance imaging (MRI) flexion and extension is an important modality for evaluating the alar and apical ligaments and the tectorial membrane. Tell-tale signs of AOD are retroclival hematoma, and hematoma in the C0-C1 or C1-C2 joints.
Final diagnosis is usually established by a combination of CT and MRI. 5 10 11 12 A traction test under anesthesia using fluoroscopy may also be used; if the C0-C1 is distended more than 2 mm OCF is advised. Reference
CT and MRI cervical spine are mainly used pre-operatively to check the degree of instability, and cervical cord.
What are the conservative and surgical treatments for AOD?
Immediate management includes cervical spine immobilization using halo cervical collar. Although some articles state halo immobilization and traction are contraindicated in the management of AOD because of the risk of displacement of the injured occipito-cervical joint.
Most articles agree that cervical traction is never applied, but again, others state to apply cervical traction.
Surgical fusion is later done, when indicated . Postoperative hydrocephalus is frequent and should be suspected when neurologic decline occurs after fixation. Those who survive AOD will likely have residual neurologic deficits.
For more information, watch this video
- Fiester P, Rao D, Soule E, Orallo P, Rahmathulla G. Anatomic, functional, and radiographic review of the ligaments of the craniocervical junction. Journal of Craniovertebral Junction & Spine. 2021 Jan;12(1):4.
- Brockmeyer DL, Brockmeyer MM, Bragg T. Atlantal hemi-rings and craniocervical instability: identification, clinical characteristics, and management. Journal of Neurosurgery: Pediatrics. 2011 Oct 1;8(4):357-62.
- Atlantoaxial joint. www.kenhub.com. Updated August 2; 2022. Accessed: December 22, 2022. https://www.kenhub.com/en/library/anatomy/atlantoaxial-joint
- Cohen WI. Atlantoaxial instability: what’s next?. Archives of pediatrics & adolescent medicine. 1998 Feb 1;152(2):119-22.
- Goel A. Central or axial atlantoaxial instability: Expanding understanding of craniovertebral junction. Journal of Craniovertebral Junction & Spine. 2016 Jan;7(1):1.
- Greenberg MS. Handbook of Neurolsurgery. 9th edition. New York: Thieme; 2020.
- Atlantooccipital joint. radiopaedia.org. Updated November 25, 2022. Accessed: December 21, 2022. Atlanto-axial subluxation | Radiology Reference Article | Radiopaedia.org
- Neal KM, Mohamed AS. Atlantoaxial rotatory subluxation in children. JAAOS-Journal of the American Academy of Orthopaedic Surgeons. 2015 Jun 1;23(6):382-92.
- Craniocervical instability. me-pedia.org. Updated November 30, 2022. Accessed: December 21, 2022. https://me-pedia.org/wiki/Craniocervical_instability#Imaging
- Hall GC, Kinsman MJ, Nazar RG, Hruska RT, Mansfield KJ, Boakye M, Rahme R. Atlanto-occipital dislocation. World journal of orthopedics. 2015 Mar 18;6(2):236.