On-Track/Off-Track: Mid-Range Summary
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And we're talking on track and off track morphology.
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This is a companion to the A bear position,
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on track, off track summary discussion.
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Except this time now, we're not all the way
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in the abduction external rotation position.
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We're starting to move our arm forward.
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Maybe we're throwing a javelin.
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Maybe we are throwing a baseball.
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Or some other sport.
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And as we start to come out of this, This extreme position,
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where the capsule is very taut, now the capsule loosens up.
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And you might say, well, wow, we've got this big
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defect anteriorly, why aren't we dislocating?
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We're not dislocating because, even though we're a little
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loose here, we still have articular surface contact.
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And even here, we start to come forward a little
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bit, but we don't completely dislocate because The
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inferior glenohumeral ligament and capsule is helping
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to keep us in place, at least partially in place.
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There is some translation, no doubt about it.
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The humerus is translated anteriorly.
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Now, what if I had a big hatchet
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chop V shaped defect right here?
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It would get stuck on this little point.
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That point would go in here, and then it
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would allow with further degrees of Anterior
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rotation and less abduction external rotation.
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It would allow this to, to basically
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fulcrum or jump out of this area.
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And you would have an unstable situation.
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So this is known as engagement.
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Now you might think, well, this is kind of strange.
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People that are, you know, throwing
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with their arm all the way back.
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They don't, they're not, they don't dislocate.
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But they lie in bed and their arm comes out.
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Well, here's why.
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They're in this neutral position, this abduction
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external rotation position, if you've got
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a decent capsule or a repair, that's tight.
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But when you're in the mid range position, it's loose, so
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it allows this translation phenomenon, and if this thing is
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big enough, or this thing is deep enough, that's lax, and
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now the shoulder, just lying in bed, flops out of the joint.
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And it has to do, with the capsular laxity from
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the position you're in when you're lying in bed.
Stephen J Pomeranz, MD
Chief Medical Officer, ProScan Imaging. Founder, MRI Online
ProScan Imaging
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