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<v ->So let's begin our discussion

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concentrating first on Rheumatoid Arthritis.

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Classically we're taught that the imaging features

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relate early and to profound some cases synovitis.

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So we see thickening of the synovium shown in red.

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We see an enlarging joint effusion

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show in the darker green color.

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And over time, we begin to see bone abnormalities

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which take the form of marginal erosions of bone.

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I spoke about marginal erosions yesterday.

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They occur first because there's no protective cartilage

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in the areas at the edges of the joints.

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So these are characteristic of any synovial process,

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synovial inflammatory process.

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And these are the subchondral cysts in my diagram.

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They're filled with fluid but sometimes they're filled

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with synovial inflammation or as we call it panis

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or a mixture of fluid and panis.

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Now what I just described is known as the outside in

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hypothesis of Rheumatoid Arthritis.

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Meaning that the inflammation begins outside,

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in fact, in the synovial membrane

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and then extends through the cortex to reach inside,

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that is the bone.

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But there are at least a minority of people,

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experts in their field, who believe

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that's not exactly the case,

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that in some cases an important mechanism

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is an inside out mechanism.

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And by that they mean that the inflammation may begin

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within the bone or bone marrow,

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an osteitis and subsequent to that,

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it may extend through the cortex

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to reach the synovial membrane.

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And the reason that there is a belief

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in the inside out hypothesis, even by imagers,

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is the occasional example where the intra-osius component

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of Rheumatoid Arthritis is dominant, or even in some cases,

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the only abnormality that is visible.

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So here you look at an example where we're looking

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at an extensive bone erosion here,

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and there is of course a lot of synovial disease,

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but in some instances of Rheumatoid,

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only the bone erosion is apparent.

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So there is a school of thought that at least in some cases,

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think of this as an inside out hypothesis.