2 thoughts on “SI joints 38M referred for evaluation of Ankylosing Spondylitis. MRI Please Help.”

  1. hilary.umans
    hilary.umans

    Thanks to Winston Rennie and Gideon Flusser, I’ve been set straight…T1W
    is more helpful and clearly shows the erosions.
    I was not confused by the subchondral lesions or erosions….rather, I was
    confused by the posteroinferior distribution—I understood this should be
    anteroinferior.
    I gather this degree of variability is acceptable and is clearly
    sacroiliitis.

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  2. hilary.umans

    After I summarized the responses yesterday, I heard from a dissenter from
    Australia who said We see these changes very frequently. They are typically
    due to mechanical overload and not related to a seronegative arthritis.
    That left me very confused, so I reached out to Rob Lambert, who answered
    in detail:

    1. AS affects any part of the subchondral bone in the cartilage compartment.
    2. There are a lot of myths that originate from data based on x-ray
    interpretation. Because the most inferior part of the joint is the part
    best seen in profile on x-ray, x-ray shows erosion most often in that
    location. But CT and MRI confirm that the lesions occur anywhere.
    3. Biomechanical changes predominate at the perimeter of the joint, more
    anterosuperior and posteroinferior but truthfully anywhere around the
    perimeter where stress is greater. The center of the SIJ is protected from
    focal stress.

    Tips:
    Always review T1 and STIR/T2FS simultaneously.
    BME with no T1 lesions is uncommon in adults.
    Erosion is more specific.
    BME, fat metaplasia and sclerosis are relatively non-specific however,
    The relationship of the lesions to each other, multiplicity of lesions, and
    random distribution of lesions all favour SpA.

    Your case is 100% AS – because of the patterns on T1.

    Winston Rennie added:
    The axial T1 is the clincher in this case. As I said in one of our many
    email exchanges the only differential is mechanical change- this does not
    look like it on the axial especially the sacral lesion.
    Do note that mechanical changes can occur along with or secondary to SpA
    involvement of the joint. See paper below.
    pubmed.ncbi.nlm.nih.gov/30601197/

    I have found this all very helpful—thanks so much for clarifying this for
    us!

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