Which findings on radiography are diagnostic of ankylosing spondylitis (AS)?

Updated: Sep 03, 2019
  • Author: Lawrence H Brent, MD; Chief Editor: Herbert S Diamond, MD  more...
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Involvement of the SI joint is a requirement for the diagnosis of AS. Sacroiliitis is a bilateral inflammatory condition leading to bony erosions and sclerosis of the joints (see the image below).

Anteroposterior radiograph of sacroiliac joint of Anteroposterior radiograph of sacroiliac joint of patient with ankylosing spondylitis. Bilateral sacroiliitis with sclerosis can be observed.

The sacroiliitis seen in AS is usually bilateral, symmetric, and gradually progressive over years. The lesions progress from blurring of the subchondral bone plate to irregular erosions of the margins of the SI joints (pseudowidening) to sclerosis, narrowing, and finally fusion. Erosions of the subchondral bone of the SI joint are generally seen earlier in the lower portion of the joint (because this portion is lined by synovium) and on the iliac side (because of the thinner cartilage covering this side of the joint).

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