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Posted by email@example.com, 10/19/04 at 6:31:40 AM.
When deciding to write one of these stories, the last thing I wanted
to do was to write a long story. I've read a lot of long stories, but
very little of long stories seem to stick around. I figured by choosing
a zebra, that would allow a quick chat, rather than a story, as not
much literature would be around from which to draw, say, on POEMS
syndrome. Sigh... Instead, I'll try to write something pithy about a
common entity. I'll try to keep this simple since we'd all prefer it
that way. Moreover I figure what we're all interested in is the
radiologic stuff, so that's what I'll mainly talk about.
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 invovled 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. Here's a CT example that
gives you an idea of the fine nature of the erosions:
Erosions are much less than seen with other spondyloarthropathies. Further down the line you might see something like this:
The synovial portion of the SI joint, i.e. the anteroinferior
1/2 to 2/3 of the joint, ankyloses first, follwed by the ligamentous
portion. Ankylosis of the posterosuperior ligamentous portion is
considered to look like a "star." The first CT above gave you an idea
of the initial involvement of the anteroinferior SI joint portion,
while the next CT shows the more diffuse involvement at a later stage.
Obviously the end result is ankylosis, which we see starting here on CT:
and here on a plain radiograph:
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 invovlement 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. We see
those pearly whites nicely here:
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. Here we see a nice AP and
lateral example of 'dem syndesmophytes:
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. Now I just made my pet panda hungry.
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, which we saw above, in case you're paying attention:
as well as here:
Although this last one seems more like a Monorail. 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. Although the odontoid
is still present and accounted for, here's a nice example of an AS
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. Here's a neat example of pseudoarthrosis:
with a CT 3D recon for grins 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. Here's a
fine example of the uniform joint space loss and axial migration:
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. Here's a neat example of
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.
Da capo... No, just kidding, unless you perseverate. I really meant to say these are the take home points:
- Normal mineralization pre-ankylosis; subsent osteopenia post-ankylosis.
- Subchondral bone pre-ankylosis.
- Erosions are not significant part of the picture, but if seen are small and focal.
- No subluxation.
- No subchondral cysts.
- Ankylosis, as the name suggests.
- Bilateral, symmetrical.
- First axial: SI joints, then spine, starting lumbar and
progressing to cervical. Later appendicular, in decreasing
prevalence: hips, shoulders, knees, hands, and feet.
For greater detail, I urge you to read Resnick (Resnick D,
Niwayama G: Ankylosing spondylitis. In Resnick D (ed): Diagnosis of
Bone and Joint Disorders. 3rd ed. Philadelphia, W.B. Saunders, 1995, p.
1003.), if you're inspired, or Baby Resnick, if you want to start with
baby steps, but it still gives a great encompassing capsule.
Brower's summary in her classic book: Arthritis in Black and White
(2nd ed. Philadelphia, W.B. Saunders, 1997.) is also a great start for
getting the skinny on AS.
Additional classic AS reads:
- Berens DL: Roentgen features of ankylosing spondylitis. Clin Orthop 74:20, 1971.
- Cawley MI, Chalmers TM, Kellgren JH, Ball J: Destructive lesions of vertebral bodies in ankylosing spondylitis. Ann Rheum Dis 31:345, 1972.
- Cruickshank B: Pathology of ankylosing spondylitis. Bull Rheum Dis 10:211, 1960.
- Dwosh IL, Resnick D, Becker MA: Hip involvement in ankylosing spondylitis. Athritis Rheum 19:683-692, 1976.
- McEwen C, DiTata D, Longg C, et al: Ankylosing spondylitis and
the spondylitis accompanying ulcerative colitis, regional enteritis,
psoriasis, and Reiter's disease. A comparative study. Arthritis Rheum 14:291, 1971.
- Pascual E, Castellano JA, Lopez E: Costovertebral joint changes in ankylosing spondylitis with thoracic pain. Br. J Rheumatol 31:413, 1992.
- Resnick D: Patterns of peripheral joint disease in ankylosing spondylitis. Radiology 110:523, 1974.
- Trent G, Armstrong GWD, O'Neil J: Thoracolumbar fractures in ankylosing spondylitis. high-risk injuries. Clin Orthop 227:61, 1988.
- Wilkinson M, Bywaters EGL: Clinical features and course of
ankylosing spondylitis as seen in a followup of 222 hospital referred
cases. Ann Rheum Dis 17:209, 1958.