Atlantoaxial Instability (AAI)
What is the Atlantoaxial Joint?
The upper cervical vertebra (C1) is known as atlas and second cervical vertebra (C2) is called axis. Atlantoaxial Joint is formed by 3 joints: one median and two lateral joints. The movement of dens of axis over osteoligamentous ring of atlas allows rotation  when moving your head i.e. side to side.
What is Atlantoaxial Instability?
Atlantoaxial instability (AAI) is characterized by excessive movement at the junction between the atlas (C1) and axis (C2) as a result of either a bony or a ligamentous abnormality/injury. . The normal range of motion for rotation at the joint is 40 degrees. The excess mobility at the articulation can lead to AAI.
What Causes Atlantoaxial Instability?
Atlantoaxial instability can originate from congenital conditions, but in adults, it is primarily seen in the setting of acute trauma or degenerative changes due to the inflammatory response of rheumatoid arthritis (RA).
Infection has been found to be an additional cause of instability, with the rich arterial supply and venous plexus in this region of the body providing a route for infectious sequelae…” Reference
AAI can also happen in connective tissue disorders .
What are some common symptoms of AAI?
It can be symptomatic or asymptomatic . AAI places a lot of mechanical stress on the central nervous system. Neurologic symptoms along with fatigue and brain fog can occur when the spinal cord, vertebral artery or adjacent nerve roots are compressed.
Patients presenting with atlantoaxial instability can suffer from a spectrum of clinical signs and symptoms, although most are asymptomatic. These include:
Vertigo and/or Tennitus
Face, neck and shoulder pain
Severe headaches and migraines
Ears full or plugged
Restricted neck movements
Pyramidal signs and myelopathy
Lower cranial nerve palsies
Vertebral artery dissection
How is AAI Diagnosed?
Radiographic Diagnostic Imaging with measurements to determine if there is an abnormal or excessive movement of the atlantoaxial joint. ADI is a typical measurement used to determine AAI.
- Atlantodental Interval (ADI) – The normal distance between dens and medial cortex of lateral masses of atlas (Atlantodens interval- ADI measurement) is less than 3 mm in adults (Fig 4). Atlantoaxial instability occurs when ADI is > 3mm (some but not many articles indicate >5 mm), or when there is intentation of C2’s dens into cervicomedullary cord or the space available for the cord (SAC) of less than 13 mm.
- Rule of Spence, otherwise known as “lateral mass overhang” has been used since 1970s and is being retired by some physicians while others use it regularly for screening purposes. “Rule of Spence” classically determines the stability of C1 fractures by measuring the lateral overhang of the lateral masses of C1 on C2 when viewing an AP radiograph. If the sum of both lateral masses of C1 on C2 is greater than 7mm, the fracture is considered unstable. Reference
- Degree of Rotation – Although there are opposing opinions on the use of Rotational CT Scan for investigating rotational atlantoaxial instability, some neurosurgeons still use it in investigating rotational AAI. Reference
Are there different types of Atlantoaxial Instability?
Atlantoaxial subluxation can be anteroposterior, rotatory, horizontal or vertical [5-8].
i) Anteroposterior Atlantoaxial Instability can be due to:
- Rupture of transverse atlantal ligament (connects the medial surfaces of both lateral masses of atlas, behind the dens), which may lead to anterior displacement of atlas over axis. This will lead to increase in the atlantodens interval.
- Incompetence of odointoid process: In case of dens fracture or congenital hypoplasia. ADI is normal.
ii) Rotatory Atlantoaxial Instability can be due to ligament or facet injury.
- It can range from from muscle spasm to a fixed mechanical block to atlantoaxial complex reduction.
- Some cases may resolve spontaneously if left untreated; however, other cases may result in the development of secondary changes in the bony anatomy of the atlantoaxial joint, resulting in persistent deformity. Fielding and Hawkins classified it into four types, given below.
Fielding and Hawkins Classification
Fielding and Hawkins suggested a four-part classification scheme for evaluating rotatory displacement,  as follows:
- Type I – Simple rotatory displacement with an intact transverse ligament
- Type II – Anterior displacement of C1 on C2 of 3-5 mm with one lateral mass serving as a pivot point and a deficiency of the transverse ligament
- Type III – Anterior displacement exceeding 5 mm
- Type IV – Posterior displacement of C1 on C2
**Both type III and type IV are highly unstable.
iii) Vertical Atlantoaxial Instability is when the dens is often above the McGregor line by over 8 mm in men and 9.7 mm in women.
iv) Horizontal Atlantoaxial Instability is when there is increased lateral movement of atlas over axis.
A TOP NEUROSURGEON & PROFESSOR, DR. ATUL GOEL, INDIA 2019 ARTICLE: A Review of a New Clinical Entity of ‘Central Atlantoaxial Instability’: Expanding Horizons of Craniovertebral Junction Surgery. Reference
For several decades’ abnormal alteration of atlantodental interval was considered to be the sole parameter to diagnose atlantoaxial instability. Apart from clinical evidences, neural indentation and deformation and alteration in cord signals in the region of the tip of odontoid process have been other parameters that establish the presence of abnormal craniovertebral dynamics and indicate the need for surgical intervention. In the year 2014, we proposed a novel concept that atlantoaxial instability can be present even when there is no bone deformity or malalignment on dynamic imaging and the conventional and validated parameters that determine the presence of atlantoaxial instability are within the range of normal . Such instability was identified as ‘central’ or ‘axial’ atlantoaxial instability (CAAD) [2,3]. Our further experience in the subject identifies that understanding of presence of CAAD can have wide implications in the treatment of a number of commonly encountered clinical entities involving the craniovertebral junction and rest of the spine. We review our published reports on the subject in this article. Other authors are yet to validate these observations.
Atlantoaxial joint is the most mobile joint of the body. Its round and flat articular surfaces that facilitate circumferential movements at this joint also predispose it to development of instability. Our analysis of the subject, over several years, concludes that amongst all spinal segments, atlantoaxial instability is the most common. The concept is that more is the mobility; more is the possibility of instability. It seems that atlantoaxial instability is an ill-understood and undertreated clinical entity.
CLASSIFICATION OF CAAD
Atlantoaxial instability was diagnosed on the basis of evaluation of alignment of facets on lateral profile imaging with the head in neutral position and by manual manipulation and visual inspection of the atlantoaxial facetal articulation during surgery . Type 1 atlantoaxial facetal instability indicates when the facet of atlas was dislocated anterior to the facet of axis (Fig. 1). Type 2 atlantoaxial facetal instability indicates when the facet of atlas was dislocated posterior to the facet of axis (Fig. 2). In type 3 atlantoaxial facetal instability, the facets were in alignment and instability was diagnosed on the basis of telltale clinical and radiological evidences and confirmed by manual manipulation of bones during the surgical procedure (Fig. 3). In types 2 and 3 atlantoaxial facetal instability, the atlantodental interval may not be abnormally altered and there may not be any neural or dural compression by the odontoid process. Due to these reasons, types 2 and 3 atlantoaxial facetal instability is labeled as CAAD. It is obvious that high degree of clinical suspicion and understanding of the subject is crucial to make such a diagnosis. Considering that there can be a wide spectrum of therapeutic implications of such an understanding, a comprehensive discussion on the subject is mandatory.
DR. GOEL ALSO WROTE ARTICLE: Atlantoaxial instability associated with single or multi-level cervical spondylotic myelopathy. Reference. and also, Goel’s classification of atlantoaxial “facetal” dislocation in 2014 Reference
DIAGNOSTIC IMAGING TO RULE OUT ATLANTOAXIAL INSTABILITY (AAI)
MRI would be preferred in all cases with suspected cervical cord injury, that is, in case of neurological deficit. A few articles demonstrate the diagnosis and show that radiographs alone have a low diagnostic rate and that functional (dynamic) MRI may be able to better quantify instability.
CBCT / CT Scan 3D provide better models and more accurate measurements as well as motion-capture systems (DMX or DDR) have the potential to increase our understanding of atlantoaxial instability.
i) Anteroposterior Atlantoaxial Instability – Dynamic XRay / CBCT / CT Scan Cervical Spine in Lateral C-Spine Neutral, Flexion and Extension positions and open-mouth odontoid, anterior-posterior, and lateral side bending.
ii) Horizontal Atlantoaxial Instability – Dynamic Upright MRI with neutral, flexion and extension positions.
iii) Rotational Atlantoaxial Instability – Dynamic CBCT/ CT scan with rotation
iv) Vertical Atlantoaxial Instability – Invasive cervical traction (ICT) with fluoroscopy
- Rheumatoid Arthritis
WHAT ARE THE CONSERVATIVE AND SURGICAL TREATMENTS FOR ATLANTOAXIAL INSTABILITY?
Conservative treatments generally include:
- pain and inflammation control
- immobilization with bracing during car rides of activities that worsen symptoms
- activity modification to avoid motions which may worsen instability
- Immobilization can be done via a halo vest but not so often that muscles weaken. The halo vest was originally designed to treat patients with severe scoliosis or neck muscle paralysis due to poliomyelitis, however since then it has been widely used to immobilize the neck in pre- or postoperative settings for cervical spine injuries or deformities and upper cervical disorders. Braces are typically used for a minimum of three months until reassessment of the neurological condition with repeat imaging
- Formal physical therapy can also aid in treatment
- Minimally invasive treatment options are not well described, with no medical articles describing the use of steroid injections, pulsed radiofrequency, peripheral nerve stimulation, or transcutaneous electrical nerve stimulation in the management of these patients. Reference
- Orthobiologic Injections is a newly emerging area with few studies proving results however orthopaedic physicians are optimistic about these kinds of treatments for conservative, minimally invasive treatment.
Atlantoaxial fusion Surgery: Occurs when spinal cord compression occurs. Several anterior and posterior surgical techniques are suitable for atlantoaxial fusion. Each technique has its own indications, contraindications, risks, and technical difficulty. Future prospective clinical trials are necessary to more adequately determine which surgical technique is optimal based for respective clinical presentations. Reference
- 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.