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Diagnostic Radiology/Musculoskeletal Imaging/Joint Disorders/Ankylosing spondylitis

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Ankylosing spondylitis (AS) represents a chronic inflammatory disease, primarily affecting the axial skeleton and secondarily affecting the appendicular skeleton. It typically presents in men, 15–35 years old. Remember it as the least erosive and most ossifying arthropathy. Joint ankylosis is the hallmark, as the name less than subtlely suggests. Some details on different pieces of anatomy: SI joints are radiographically involved first, bilaterally and symmetrically. Joint edges have a serrated "postage stamp" appearance due to tiny erosions, which start on the iliac side, due to the thinner cartilage, then progress to the sacral side.

Radiologic Findings

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Erosions are much less than seen with other spondyloarthropathies.

The synovial portion of the SI joint, i.e. the anteroinferior 1/2 to 2/3 of the joint, ankyloses first, followed by the ligamentous portion. Ankylosis of the posterosuperior ligamentous portion is considered to look like a "star."

Other findings you might see in the pelvis, if you're not overwhelmed by those SI joints and a glimpse of the spine, are ossification of ligamentous attachments in the iliac crests and ischial tuberosities, classically giving a purported "whiskered" look. That sounds cute doesn't it. Looks just kind of fuzzy to me. The symphysis pubis can show tiny "serrated" erosions like the SI joints, before it ankyloses. Purportedly, ~25% of ankylosing sponylitis eventually has symphysis pubis involvement. Probably since SI joints are the first radiographic evidence of AS and the pelvis is being imaged for that, lumbosacral AS involvement is typically seen first, as we see on the above pelvis radiograph, although apparently the thoracolumbar junction can be the first site of involvement in the spine for AS. Involvement progresses cranially to involve the entire spine. At first, there is slight erosion of the vertebral body corners with secondary sclerosis, giving a classic squared vertebral body with "ivory" corners.

The ivory corners disappear, not unlike true elephant ivory has, leaving simply square vertebral bodies. There is an explanation for these ivory corners, even if I can't explain where all the elephants' ivory went. The outer portion of the annulus fibrosus, i.e. Sharpey's fibers, ossify first. Apparently this may not always be seen radiographically, but decreased ROM will suggest this to an astute radiologist. The ossification progresses deeper to involve the longitudinal ligaments, resulting in the classic syndesmophyte seen with AS, linking adjacent vertebral bodies.

Disc spaces tend to remain normal at first, with no loss of height, but they may eventually calcify. Apophyseal joints in the spine can be involved, if they choose to, with resultant erosions followed by ankylosis. All spinal ligaments can eventually ossify giving the classic "bamboo" spine.

Once you get a bamboo spine, a classic sign is that of the "tram track," namely the syndesmophytes and ossified ligaments between spinout processes look like "tram tracks." We get a hint of that here in the pelvis.

For further details on tram tracks, talk to MSK Fellow Durkee, as he does a thorough job at explaining trams. One point to remember, as forgetting it could give grave results, is that odontoid erosions can be seen with AS. Atlantoaxial subluxation can be seen as well. So, don't jump to conclusions and label someone with "arthritis" and such cervical findings with RA, when in fact they may have AS. The bamboo spine, though, would probably give away the diagnosis.

A couple of things to be aware with AS. As with DISH, intubating someone with an ankylosed C-spine can result in horrendous fracture with paralyzing results. Similarly, fracture in the thoracolumbar region can result in pseudoarthrosis. This can also result from an area that failed to ossify. At this area one can see DDD, erosion, and bony sclerosis. These findings can resemble severe DDD, discitis/osteomyelitis, or "neuropathic" spinal disease. Now that you are aware of the possibility of pseudoarthrosis, you can consider that as a possible diagnosis too.

Moving beyond the axial skeleton, the hip is the most common appendicular joint involved. Two kinds of patterns can be seen when the hip is involved, i.e. nondestructive and destructive. Kind of reminds me of relationships. The former is fortunately more common, as hard as that is to believe sometimes. In AS the hips are involved bilaterally and symmetrically, with, surprise surprise, ankylosis being the characteristic feature. There can be no joint space loss, or uniform joint space loss with axial migration of the femoral head.

Tiny erosions can be seen, but overall the femoral head contour is preserved. Pre-ankylosis there will be a osteophyte collar with normal mineralization, while post-ankylosis the phytes go byebye and the bones become osteopenic. The destructive hip is unilateral and insidious, resulting in marked destruction of the femoral head prior to ultimate ankylosis. The shoulder is the next most frequent appendicular joint involved. Again we see destructive and nondestructive patterns. With the latter there is a normal humeral head ankylosed to the glenoid with much ossification of the coracoclavicular ligament. With the destructive type there is a "hatchet" erosion of the humeral head. Not that I know what a hatchet blow looks like, but I think sometimes I could benefit from having such a weapon.

With the hatchet shoulder, ankylosis will eventually occur after much destruction has been wreaked. The knee is involved in ~30% of longstanding cases, not because they're standing, but, well you know what I mean. The AC, sternoclavicular, and sternomanubrial joints can all be involved. Purportedly only ~10% of longterm AS shows elbow, hand, and feet involvement. In all of these less involved joints, erosions, if present at all, are slight, with minimal bone sclerosis, but as you guessed, intra-articular ankylosis is the characteristic. Progression of an ankylosing joint tends to be rapid.

Take Home Points

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  1. Normal mineralization pre-ankylosis; subsent osteopenia post-ankylosis.
  2. Subchondral bone pre-ankylosis.
  3. Erosions are not significant part of the picture, but if seen are small and focal.
  4. No subluxation.
  5. No subchondral cysts.
  6. Ankylosis, as the name suggests.
  7. Bilateral, symmetrical.
  8. First axial: SI joints, then spine, starting lumbar and progressing to cervical. Later appendicular, in decreasing prevalence: hips, shoulders, knees, hands, and feet.

References

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