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<v ->Let's move on now to osteonecrosis.

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And we'll talk about its general features,

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differential diagnosis and something

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about its classification.

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Here again, terminology becomes important.

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Ischemic necrosis or osteonecrosis

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is a term applied to involvement of an epiphysis

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particularly in the subchondral bone.

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So we would call this ischemic necrosis

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or osteonecrosis of the femoral head.

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The term bone infarct applies to an area of osteonecrosis

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in a tubular bone that involves a

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metaphysis or diaphysis.

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So the proper term for this area

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of necrosis would be a bone infarct.

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So there's a difference between the two.

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Now, when there is ischemia involving bone and bone marrow

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there is a progression of abnormalities that occur.

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The first cells to die are those

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of the hematopoietic elements.

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So, right and that

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occurs generally within six or 12 hours.

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Subsequent that the bone cells themselves may die.

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And that occurs between 12 and 48 hours.

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And finally the marrow fat cells may die.

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And that occurs between two and five days.

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So when we look at examples of osteonecrosis

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there are two characteristic findings that I look for.

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We see in a regular serpintine, like reactive border

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more about that in a moment.

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And we see within the areas of osteonecrosis intra fat.

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So some of the fat cells are still intact.

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Those are the characteristics of bone necrosis

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be it in the epiphysis or

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involving a diaphysis or metaphysis.

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Here's another example,

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conventional radiography.

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We can see a lucency, not really specific T1.

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We can see fat and we see the serpintine-like area

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and with T2 fat suppress

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we can see the ademinis response to this area

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of necrosis.

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And by the way, for those of you

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looking at the edges of the film,

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this looks like cystic degeneration

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of the anterior cruciate ligament.