The Posteromedial Corner: Semimembranosus Expansions
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Knee anatomy on MRI, posteromedial corner,
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focusing on the semimembranosus and its
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expansions. Semimembranosus is a large muscle.
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The semitendinosus rides along its posterior margin
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or back up more approximately.
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And as it comes down, it has
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a very deep component known as the direct arm.
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And this is the one you frequently
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see attaching to the tuberculum tendinous along
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the posteromedial aspect of the tibia.
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It'll bifurcate into two structures
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that one sees one anteriorly, not labeled here.
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It's underneath the POL and the deep
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portion of the superficial MCL, or superficial
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portion of the deep MCL. Also known
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as the tibial collateral ligament.
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And here is the anterior reflected arm underneath, called the pars reflexa.
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So you'll probably be able to see that.
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You'll also be able to see the
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distal continuation of the direct arm or
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deep arm, more distally along the posteromedial aspect of the tibia.
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More variable and often not seen on MRI
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is the inferior (popliteal arm), which goes in between these two.
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And then more posteriorly is the distal attachment of the inferior arm.
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This goes a long ways down, and because of that,
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it's sometimes more easily identifiable because of its distal extension.
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And then, supporting the posteromedial capsule of the knee,
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a little more in the front though,
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is the capsular arm.
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And then all the way in the back, becoming the oblique poploteal ligament,
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is the OPL extension of the semimembranosus.
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So the semimembranosus provides support
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to the entire posterior capsule running from posteromedial
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to posterolateral, along the back of the knee.
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It's a little hard to appreciate on
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this oblique projection, but you'll see what
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we mean on other vignettes.
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So those are the major expansion heads
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of the semimembranosus.
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As you can see, we've added an
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important division here, the pars reflexa, which runs
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anteriorly underneath the POL and the direct
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arm, which runs more oblique inferior
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and inserts on the tuberculum tenderness
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a little protrusion or bony protrusion along the
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posteromedial edge of the tibia.
Stephen J Pomeranz, MD
Chief Medical Officer, ProScan Imaging. Founder, MRI Online
ProScan Imaging
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