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<v ->When we talk about ankylosing spondylitis, in particular,

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one of the SpAs,

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the most common joints in the periphery

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are really not in the periphery.

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We call them root joints.

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And the most common root joints that are involved

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in ankylosing spondylitis occur at the root

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of the lower extremity, which is the hip,

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and the root of the upper extremity, which is the shoulder,

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and they're characteristic findings

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of ankylosing spondylitis that occur in those joints.

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Now in this case, we have sacroiliac joint involvement,

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I'll talk about that later today,

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we have involvement of the symphysis pubis,

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but this is the characteristic hatchet deformity

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that you may see in ankylosing spondylitis

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with extensive erosion on the lateral aspect

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of the humeral head.

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This patient also had knee involvement with synovitis.

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Well, we're gonna finish up

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in the last five to seven minutes

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just beginning our discussion of the sacroiliac joint.

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So, let's look a little bit at its anatomy,

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and this is a nice picture transverse section

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through a portion of the sacroiliac joint.

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So, let's first define the difference

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between the sacroiliac space and the sacroiliac joint.

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The sacroiliac joint is part of what occupies

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the interosseous space between the sacrum and the ilium.

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It is a synovial-lined joint, all right?

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Now, it may eventually have fibrosis and no joint lumen,

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but at least early on it has a joint lumen,

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it has cartilage on the sacrum and ilium,

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something unique about it, I'll show you in a moment,

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and it has nearby capsular ligaments.

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So, it occupies a portion of the space

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between sacrum and ilium.

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The other portion of the space is occupied by

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a number of ligaments,

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some among the strongest in the human body,

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such as the posterior sacroiliac ligament.

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It's a joint that connects surfaces

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that are auricular in shape, all right?

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They have elevations and depressions,

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they're not smooth looking,

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and that creates diagnostic difficulty for us.

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So, if we go ahead and look at transverse sections

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through the sacroiliac interosseous space,

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at the very top I'm showing you the synovium line portion.

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This is the sacroiliac joint.

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It's located anteriorly.

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The blue area represents the interosseus ligaments

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connecting the sacrum and ilium,

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and you can see they're located mainly posteriorly.

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As you go to the mid-portion,

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the synovial joint increases in its extent.

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Again, the ligamentus portion mainly posteriorly.

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You get to the bottom,

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mainly you're dealing with synovial, the joint,

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the sacroiliac joint.

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So, if you're ever asked to aspirate the sacroiliac joint,

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you better not go at the top unless you're good at golf

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because it looks like one of the golf holes.

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It would be very difficult to get a needle in there.

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You wanna go toward the bottom

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where you have a better chance of getting a needle

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into the sacroiliac joint.

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Now, there are some variants that cause diagnostic problems.

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One of these is an accessory sacroiliac joint

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that occurs posteriorly.

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Here's the other part of the joint

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we normally think of posteriorly.

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This is said to occur in 10 to 15% of cases.

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It can become arthritic and cause symptoms and signs.

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The more troublesome, although less frequent, anomaly

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has the name of an accessory iliac projection.

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It looks like an iliac horn,

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not the classic iliac horns that we think of

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associated with Fong's disease and HOOD Syndrome.

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These are iliac horns that occur in this location

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and they produce mechanical changes in the sacrum

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and these can become symptomatic as well,

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and not only that, on MR they produce edema

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and it looks very much like the sacroiliitis

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that we're looking for in the spondyloarthropathies.

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Now one other point I wanna make,

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here is a coronal section through that sacroiliac joint.

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Here is the joint, synovium line joint,

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above it, the interosseus base,

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which is filled with a ligament.

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When you look at the cartilage of the sacroiliac joint,

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the cartilage on the sacral side, far more healthy.

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This is highland cartilage.

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The cartilage on the ilial side, thinner,

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less healthy looking fibrocartilage.

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So here's the rule.

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Virtually every disease that involves the sacroiliac joint

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begins on and dominates within the ilial side

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of the sacroiliac joint,

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including the spondyloarthropathies.