On-Track/Off-Track: Dislocation Mechanics
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There are actually two methods that you can use to
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look at the glenoid track on the humerus to see what
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kind of risk you have for recurrent dislocation.
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So let's take a humerus.
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This would be, this would be lateral,
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greater tuberosity, and this would be medial.
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Doesn't really look like a humerus.
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And then let's draw our, our track.
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So our track will use a different color.
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And our track might be something like
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this, from supralateral to inframedial.
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Now, what does the track really consist of?
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Because you can, you can measure
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this track directly off the axial.
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We've shown you how to do that in other vignettes.
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Uh, it consists of a ridge right here,
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where you have your defect, a bony ridge.
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And what's this side of the track?
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That side of the track, which is a little
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counterintuitive, we'll make it purple,
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is the medial margin of the rotator cuff.
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So from the medial edge of the rotator cuff to
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this spot right here is the width of the track.
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So if you take the patient from a position of abduction,
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external rotation, and then start moving the arm
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forward, you could actually retrace that track.
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Or, if you're doing a dynamic examination, you could
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show in the abduction external rotation position,
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and then in the less extreme position, the engagement
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of this locus on the anterior aspect of the glenoid.
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Now why is that happening here?
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You've seen other vignettes where there was
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a defect, but it did not engage this point.
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And the reason is twofold.
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This Hill-Sachs lesion is bigger and broader, number one.
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And number two, this Hill-Sachs lesion is more medial.
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When it's more medial, and you have the arm
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in certain positions, it is more likely to
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engage the anterior portion of the glenoid.
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So, and if you lost this right here, if
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you were missing the glenoid cup, wow.
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Now, now you're engaging something that is completely flat.
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And it just slides right off.
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So, there may be a little pause
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right here if you've got a point.
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In other words, if you have a glenoid point,
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there'll be a pause where you're locked, and then
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all of a sudden you unlock, and then you dislocate.
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Whereas if you don't have a glenoid,
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it'll just slide right off the front.
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So, Hill-Sachs lesions that are big and broad, not so
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much depth, but big and broad, and more medially
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located, are more prone to dislocation as one moves
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from extremes of abduction external rotation forward.
Stephen J Pomeranz, MD
Chief Medical Officer, ProScan Imaging. Founder, MRI Online
ProScan Imaging
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