Inury Mechanisms: Internal Forces
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The second type of force that may occur
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during an injury is an internal force striking of one bone
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with the neighboring bone.
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Typically in the knee it's the femur and tibia colliding.
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But we could think perhaps of the patella striking the femur
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during a lateral, uh, dislocation of the patella.
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But let's think of the femur and tibia colliding here.
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The sphere of influence is often at the end of the bone,
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be it a chondral fracture, an osteochondral fracture,
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or a sub chondral fracture.
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So when we see involvement with footprints at the end
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of a long tubular bone, particularly about the knee,
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we think of an internal force.
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Now you come along and you look at the images
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and part of the path, your job is to figure out how severe
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that particular uh, injury is.
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That is the internal force may produce on one end a
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bone contusion.
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Okay? And we'll talk more about this
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in a subsequent lecture.
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On other end of the spectrum,
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you may see an osteochondral fracture.
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And the way you differentiate between a bone contusion
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and an osteochondral fracture is knowledge about the
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anatomy, the normal anatomy of the subcon bone plate.
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So if you're looking at the mid portion
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of the lateral femoral condyle, you recognize that
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although you may have a single notch there, you don't tend
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to get two areas in which the subcon bone
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plate is depressed.
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That is abnormal.
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The depression of the subc chondral bone plate,
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that is an osteochondral fracture.
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We recognize these particularly about the knee in the form
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of certain osteochondral fractures.
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The one that is classically described occurs in the lateral
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condyle patella sous, the mid portion
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of the lateral femoral condyle,
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where you may get exaggeration
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of a normal notch that is seen there.
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And if that notch is greater than two millimeters in depth,
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sometimes even between one
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and two millimeters, it's pretty good evidence
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that an ACL tear has occurred.
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There is also a long notch,
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although I don't have a measurement to tell you,
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but if in fact it is too long
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and looks too long, that too can be a sign of an ACL tear.
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And then particularly in the immature skeleton,
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you may get a double notch.
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And in all, almost all cases,
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the fracture is the posterior notch in these regions.
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So we'll call that a double notch.
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So these are three classic patterns of osteochondral
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injuries that we see involving fractures
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of the lateral femoral condyle.
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And to complete our story, as you know,
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you may see a fracture, an osteochondral fracture producing
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what I call a wrinkled tibia
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In the posterior aspect of the lateral tibial plateau.
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Now complicating our analysis of internal forces, those
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movements here, anterior translation
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and external rotation may be simultaneous or successive.
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So the path of abnormal movement
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of the tibia will vary depending upon the timing
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of these uh, forces.
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And therefore there will be a shift in the footprints left
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behind.
Donald Resnick, MD
Professor Emeritus, Department of Radiology
University of California, San Diego
Mini N. Pathria, MD, FRCP(C)
Division Chief, Musculoskeletal Imaging
University of California San Diego
Eric Y. Chang, MD
Adjunct Professor, Radiology
University of California, San Diego
Brady K. Huang, MD
Clinical Professor of Radiology
UC San Diego Medical Center
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