The ADI is measured from the inferior border of the anterior tubercle of the C1 arch to the Odontoid process (Dens) of C2. The normal width is 1–5 mm for children & 1–3 mm (maximum) in adults.
This space may increase in old age as a response to degeneration of the atlantodental joint complex. This is best appreciated in flexion views. It may also increase with trauma, which may rupture the transverse atlantal ligament.
Laxity of this ligament is noted in Down's Syndrome.
The ligament may also rupture with inflammatory arthropathies like rheumatoid or psoriatic arthritis,
Reiter's Syndrome, and ankylosing spondylitis.
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