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

Updated: Feb 02, 2021
  • Author: Lawrence H Brent, MD; Chief Editor: Herbert S Diamond, MD  more...
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Answer

Involvement of the SI joint is a requirement for the diagnosis of AS. Sacroiliitis is an inflammatory condition leading to bony erosions and sclerosis of the joints (see the image below).

The sacroiliitis seen in AS is usually bilateral, symmetric, and gradually progresses over years. The lesions progress from blurring of the subchondral bone plate to irregular erosions of the margins of the SI joints (pseudo-widening) 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).

Bilateral symmetric sacroiliitis in a patient with AS showing blurring of the margins of the joints and pseudo-widening.

Bilateral symmetric sacroiliitis in a patient with Bilateral symmetric sacroiliitis in a patient with AS showing blurring of the margins of the joints and pseudo-widening.

Bilateral symmetric sacroiliitis in a patient with AS with narrowing and sclerosis of the joints.

Bilateral symmetric sacroiliitis in a patient with Bilateral symmetric sacroiliitis in a patient with AS with narrowing and sclerosis of the joints.

Bilateral sacroiliac joint fusion in a patient with AS.

Bilateral sacroiliac joint fusion in a patient wit Bilateral sacroiliac joint fusion in a patient with AS.

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