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<v ->Now the question always arises,

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when you diagnose osteonecrosis of the femoral head,

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how do you establish the prognosis?

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That is,

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how likely is it that that femoral head will collapse?

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Now, there are a number of articles that suggest

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that you can decide this based upon measurement.

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What you do is you determine the size

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of the area of osteonecrosis

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in the coronal plane and in the sagittal plane,

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using the mid coronal and mid sagittal images,

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showing the largest diameter of the area of necrosis.

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You figure out the angle from the center

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of the femoral head,

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and you add those 2 angles,

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one in the coronal, one in the sagittal plane.

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And, of course, the larger the number that you arrive at,

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particularly if they,

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you get more than 240 degrees,

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the higher likelihood that collapse

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of the subchondral bone plate will occur.

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Now, this can be a reliable technique in some instances,

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but it's been pointed out it can be misleading.

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Look at this example.

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Here, we have two possibilities:

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A large area of osteonecrosis

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located fairly superficial

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in the femoral head;

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and here is one deeper in the femoral head,

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far smaller.

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And yet, if you go ahead and figure out the angle

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of that necrosis in the coronal plane,

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you can see it's the same.

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So you do run into problems,

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and that's why

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in some articles people suggest volumetric measurements.

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They're more meaningful.

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And you have to go ahead and look at the amount

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of involvement in multiple slices,

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and calculate that on each image.

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In my practice, to be totally honest,

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we don't really do that calculation.

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We do, in fact, decide how large the area,

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and if we think it is large,

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based upon, really, just visual picture of it,

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we will indicate a higher likelihood of collapse.