Atlanto-dental Interval (ADI)

Medical Science Behind ADI

In terms of research, there are several excellent graded A and B level studies on the Atlantodental (dens) Interval (ADI) measurement of the craniocervical junction, which measure the atlanto-axial joint (C1-C2 vertebrae). An abnormal ADI can be affected by numerous diseases, hence multiple research papers look at the changes in the ADI within the patients affected. The last several years alone had several grade 1a papers and grade 2a papers published.

ADI is the distance between the atlas bone (C1) and the dens of the axis (C2) bone. The atlas bone acts as a base for the skull, and the axis is the second vertebrae and has an intimate relationship with the atlas, forming the atlantoaxial joint. The dens is a projection of the axis that attaches to the atlas like a finger through a large ring. Movements primarily include rotation of the head, and to some degree, flexion and extension allowing us to perform yes and no nodding.

The atlantoaxial junction is the most mobile joint of the body that is held together by ligaments that allow a great degree of freedom of rotation. The atlantoaxial joint is responsible for 50% of all neck rotation, 5° of lateral tilt (i.e. ear to shoulder), and 10° to 20° of flexion and extension (i.e. looking down or looking up).

The ADI measurement is the space between C1 and the front of the C2, and if the space is too large, it may mean there is Atlantoaxial Instability (AAI) or a problem like ligament damage or a fractured bone. The standard distance of ADI, although tiny, is usually a few millimeters, however it has a significant role in the atlantoaxial joint’s movement. It is measured in two parts, from the front (anterior ADI) and from the sides (lateral ADI). Normal measurements vary between adult males and females, with females having slightly smaller normal measurements.

ADI Radiology Reference Values

The ADI measurement is one measurement used to assess the Atlanto-axial Instability (AAI).

Normal Values: Less than 2 mm on CT scan and less than 3 mm on radiograph

Borderline Abnormal Values: 2 mm to 4 mm may indicate potential instability, particularly if accompanied by risk factors such as trauma, connective tissue disease or neurologic symptoms. Dynamic CT flexion-extension imaging is warranted.

Abnormal Values: Equal or greater than 5 mm is widely accepted as pathologic. Equal or greater than 7 mm suggests rupture or incompetence of the transverse ligament. These indicate significant instability and potential risk for spinal cord compression.

Radiology imaging such as CT scans and possibly MRIs (although less precise), are used to measure ADI. CT scans are the preferred modality as they provide more accurate measurements.

XRays / motion or movie dynamic digital Radiography by Konica Minolta are often utilized a the preliminary screening tool, although they are well known to be poor quality and therefore less than optimal for measurement accuracy.

Radiology mistakes are more common than most people think, and can lead to serious consequences such as delayed diagnosis, incorrect treatment, or long-term health problems. Some studies show that reported error rates in radiology can range from roughly 20% to 40%. This means that in some cases, “normal” imaging may not be accurate. 2nd radiology opinions whereby a radiologist actively measures and reports on ADI and other measurements of the craniocervical junction can be a crucial part of the diagnosis process.

What can disturb the normal atlantodental interval?

Any condition affecting the atlantoaxial joint can also affect the atlantodens interval.

Common causes include:

Trauma – One of the most frequent reasons for an irregular ADI is trauma, such as in falls or sports-related mishaps.
Congenital illnesses – Several congenital disorders, including Down syndrome, rheumatoid arthritis, and other connective tissue disorders, can weaken the ligaments holding the dens to the atlas.
Inflammatory conditions that directly affect the cervical spine.
Infections
Tumors that apply pressure on the bones and soft tissues of the cervical spine.
Iatrogenic causes refer to ADI irregularities caused by medical intervention, such as cervical spine surgeries.

What are some symptoms of an abnormal ADI?

Patients with an abnormality of the ADI, could be experiencing either general or specific symptoms. These are shown in the table below.

TABLE: General and specific symptoms of ADI

General symptoms	Specific symptoms
Neck pain	All general symptoms that start from childhood and continue into adulthood can be considered specific to a congenital deformity.
Limited range of motion	Feelings of neck instability or a sensation that the neck is not adequately supporting the head following a trauma.
Headache	Difficulty in swallowing and speaking is due to compression of the upper spinal cord segment, which controls the function of the pharynx and larynx.
Mild neurological symptoms such as numbness in the body	Severe neurological symptoms such as paralysis of limbs, imbalance, and loss of bladder control usually follow spinal cord compression.
Muscle spasms	Fever or chronic feeling of illness might be present in the case of osteomyelitis of the atlas or axis.

What treatment options are available?

Depending upon the severity and cause of the abnormality, the options to treat ADI abnormality are numerous. These include the following:

- - General measures include neck immobilization using rigid neck collars.
  - Physiotherapy such as training deep and superficial cervical flexors
  - In case of any inflammatory disease, non-steroidal anti-inflammation drugs, or in severe cases, steroids can be given to reduce inflammation and allow natural healing.
  - In case of infections, antibiotics are given.
  - Pain can be controlled by analgesics such as over-the-counter acetaminophen or narcotics. Additionally, muscle relaxant drugs can also be used to treat muscle spasms.
  - If the spinal cord is being compressed, surgical intervention is required.

Home

Having access to your medical information is a powerful thing and an important part of advocating for your healthcare. However, this website should never replace advice from your physician who understands your specific medical history and is trained to care for patients.

SBA Thanks Professor of Medical Education, A. Bohari M.D., for contributing to this page.

ADI Medical References

1. Chen Y, Zhuang Z, Qi W, Yang H, Chen Z, Wang X, Kong K. A three-dimensional study of the atlantodental interval in a normal Chinese population using reformatted computed tomography. Surgical and radiologic anatomy. 2011 Nov;33:801-6.
2. Yang SY, Boniello AJ, Poorman CE, Chang AL, Wang S, Passias PG. A review of the diagnosis and treatment of atlantoaxial dislocations. Global spine journal. 2014 Aug;4(3):197-210.
3. Wang C, Yan M, Zhou H, Wang S, Dang G. Atlantoaxial transarticular screw fixation with morselized autograft and without additional internal fixation: technical description and report of 57 cases. Spine. 2007 Mar 15;32(6):643-6.
4. Chen Q, Brahimaj BC, Khanna R, Kerolus MG, Tan LA, David BT, Fessler RG. Posterior atlantoaxial fusion: a comprehensive review of surgical techniques and relevant vascular anomalies. Journal of Spine Surgery. 2020 Mar;6(1):164.
5. Elbadrawi AM, Elkhateeb TM. Transoral approach for odontoidectomy efficacy and safety. HSS Journal®. 2017 Oct;13(3):276-81.
6. Munakomi S, Tamrakar K, Chaudhary PK, Bhattarai B. Anterior single odontoid screw placement for type II odontoid fractures: our modified surgical technique and initial results in a cohort study of 15 patients. F1000Research. 2016 Nov 21;5(1681):1681.
7. Michel C, Dijanic C, Abdelmalek G, Sudah S, Kerrigan D, Yalamanchili P. Upper cervical spine instability systematic review: a bibliometric analysis of the 100 most influential publications. Journal of Spine Surgery. 2022 Jun;8(2):266.
8. Joaquim AF, Appenzeller S. Cervical spine involvement in rheumatoid arthritis—a systematic review. Autoimmunity reviews. 2014 Dec 1;13(12):1195-202.
9. Shah A, Vutha R, Prasad A, Goel A. Central or axial atlantoaxial dislocation and craniovertebral junction alterations: a review of 393 patients treated over 12 years. Neurosurgical Focus. 2023 Mar 1;54(3):E13.
10. Manent L, Higuera JG, Lewis K, Angulo O, Solano ME. Radiological Improvements in Symmetry of the Lateral Atlantodental Interval and in C1 Tilt After the Application of the Atlasprofilax Method. A Case Series. Qeios. 2022 May 20.