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<v ->Now a relative of pigmented villonodular synovitis,

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a more localized process that we have been calling

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localized nodular synovitis.

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This is not giant cell tumor of a tendon sheath.

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This is a lesion, generally single,

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but occasionally more than one lesion,

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occurring in an intra-articular location.

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And like most of what I'm gonna be talking about

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the most common location being that of the knee.

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It has a characteristic distribution,

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as I'm gonna show you in a moment,

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favoring, Hoffa's fat pad.

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And what we look for is a lesion,

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well-defined in most cases, of low signal,

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sometimes also containing hemosiderin.

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It looks a bit like the cyclops lesion

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that we see usually following ACL reconstructive surgery.

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So here are two examples.

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And the example on the left with the arrow on it

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is the classic example.

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The deep part of Hoffa's fat pad,

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a somewhat well marginated mass of low signal,

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probably, or seems to be containing hemosiderin.

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The second favored location shown by the other image

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is behind the posterior cruciate ligament

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but it is quite variable in it's distribution.

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Here's another case, beautifully shown,

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with T1 and T2 fat suppressed imaging,

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a more elongated lesion

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occupying the deep part of Hoffa's fat pad

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and nearby joint lumen.

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One further a case demonstrating again

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that location behind the posterior cruciate ligament.

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As you look at this, you can see low signal

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and then some focal areas of very low signal

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representing areas containing hemosiderin deposition.

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So this is the classic appearance

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of localized nodular synovitis.

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Now you run into cases like this where you can't decide,

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is this localized or is this more diffused?

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It's more than one area,

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both these areas being shown by arrows,

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where we have focal masses, low signal,

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with areas that look like hemosiderin deposition.

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In these cases, sometimes as you follow them,

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a more diffused pattern will occur over time.

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So this is probably the transition stage

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between localized nodular

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and diffuse pigmented villonodular sinusitis.