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<v ->Now these stress fractures can occur in the cortex.

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They can occur in the spongiosa

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and sometimes they can occur specifically

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in the subchondral bone.

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The type of fracture that is fatigue versus insufficiency

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will vary according to which of these three

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particular locations we are considering.

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Let's start by talking about the cortex.

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The majority of stress fractures

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that we see involving cortical bone are fatigue fractures.

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And their distribution and their direction

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are influenced by the anatomy of the cortical bone.

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I tried to give you a kind of a three dimensional drawing

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of what the cortical bone looks like.

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And I would draw your attention to the aversion systems

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which consists of osteon, as well as a aversion canal

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through vessels and nerves may be located.

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Well these osteons are surrounded by cement lines

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and the distribution of the stress fractures,

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again, mainly fatigue and tight are influenced

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with the distribution of the osteons and cement lines.

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Here, I'm trying to give you an idea

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that these fractures will appear

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often between the aversion systems.

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We see them radiographically in the cortex

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because we're dealing with dense compact bone

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and we're looking for a radiolucent line within them.

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So it's the ideal situation.

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One of the most common locations that we see them

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is in the anterior aspect of the tibia

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more often as in this particular case

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the tibial cortex is thicken.

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I see these most frequently in this location in runners

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and sometimes in fact, in this particular location

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these fractures, which are fatigue and tight are multiple

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and you will see multiple radiolucent stripes.

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These can progress to complete fractures.

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So generally a period of rest is advised in those athletes,

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those runners who develop

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this sort of cortical stress fracture.

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Sometimes the fractures will extend not just

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horizontally but vertically

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and they become longitudinal stress fractures.

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We see these most frequently

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in the bones of the lower extremity, the femur and tibia.

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I show you one here,

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an obliquely near vertical fracture line

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extending up along the medial cortex.

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These can extend for considerable distance.

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And since they're associated with alterations in signal

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both in the marrow and soft tissues

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the findings can be concerning on MRI

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and simulate the appearance of infection or tumor.

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Again, their course is often slightly oblique

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but maybe almost vertical explaining why

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they can extend over a large distance.

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Cortical fractures, again usually fatigue,

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but we may see insufficiency fractures

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isolated to the cortex.

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These often occur on the tensile aspect of the bone.

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And the disease that comes to mind immediately

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would be Paget's disease.

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Intertrochanteric and subtrochanteric fracture lines

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involving the lateral cortex of the femur.

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As shown here are not uncommon in Paget's disease.

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I'm not gonna go through all of the radiographic features

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of Paget's disease.

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The diagnosis should be obvious in this particular case

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but it's important that you look for these fractures

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because they can go onto complete fractures

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and even displaced fractures.

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And a similar phenomenon is associated

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with bisphosphonate therapy.

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Very characteristic fracture along the outer cortex

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of the proximal femoral shaft

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may be seen unilateral or bilateral.

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In the case that I'm illustrating here it's bilateral,

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it's rather symmetrical.

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And it leads to what we would call

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cortical buttressing kind of focal nodular

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thickening of the cortex in that particular region.

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Sometimes these patients will present with hip pain

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and a pelvic radiograph does not include

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lower down on the femur.

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So you may not be able to see them.

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So if your patient does have hip pain

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and is on a bisphosphonate

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you might wanna consider a slightly larger field of view.

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These may go on to complete fracture

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with various angulation at the fracture site

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and what has been designated as a medial beak of bone.

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We turn our attention now to the spongiosa

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or medullary bone

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where we see both fatigue and insufficiency fractures

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with about similar frequency.

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Cancellous bone is relatively

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resistant to compressive forces

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but it is weaker against tensile forces.

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So orientation of these fractures are often perpendicular

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to the primary compressive trabeculae.

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This is a classic appearance

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and also a classic orientation of a near complete

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fatigue fracture involving the femoral neck.

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But as I say, insufficiency fractures

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and I'll show you some can also occur in this location.

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We do see fatigue and insufficiency fractures

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involving the calcaneus.

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If you look at the trabecular arrangement in the calcaneus

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we see the primary compressive trabeculae curved in yellow

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and the primary tensile trabeculae curved in blue.

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And this is a typical appearance, the morphology

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the orientation of these fatigue

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and insufficiency fractures in the calcaneus.

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Most of us learn to look fairly low down

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and particularly in this location, but I would remind you

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these fractures can extend higher up

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and even reach the location of the posterior subtalar joint.

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Here, I show you a nice example, classic location,

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classic orientation of stress fracture.

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This one, I believe was an insufficiency fracture

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occurring in an older person in a,

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the posterior aspect of the calcaneus.

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Here, we can appreciate two more and you can see

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the one on the left is a little bit higher up

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approaching the area of the posterior facet

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and posterior subtalar joint.

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And in this case, again

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still perpendicular to the compressive trabeculae.

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We see a fracture that is quite long, quite wide

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beneath both the posterior

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and middle facets of the calcaneus.

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So these can occur fairly high up in the calcaneus.

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We now move on to the third category,

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the one that I wanna emphasize in this talk

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and that is the subchondral stress fracture.

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And these, the vast majority

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are insufficiency and not fatigue fracture.

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There are a lot of predisposing factors,

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but in general they relate to the loss of the cushion

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that normally protects the cartilage

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and the subchondral bone.

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So whether the cushion is articular cartilage

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and that is lost.

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Whether or not it's the bone trabecular themselves

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that are weakened,

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whether or not it is a meniscus that has been removed

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or is torn, or is displaced.

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All of that owing to the loss of the cushion

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can produce a subchondral stress fracture,

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usually again, of the insufficiency type.

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So let's go ahead and look at how these develop.

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This is my drawing, showing you the particular cartilage

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the subchondral bone plate, the cortical bone

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and the trabeculae chambers that I talked about

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with both vertical and horizontal trabeculae

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separated by marrow spaces

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typically filled with fatty marrow.

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Repetitive stress applied to the articulate surface

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of normal or weakened bone leads to cyclical loading.

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And what occurs is loading of the trabeculae

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both the vertical and the horizontal trabeculae

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as well as pressurizing of the marrow spaces

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between those trabecular walls.

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What occurs is the phenomenon known as bending,

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bending of a column, as you can see here

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and subsequent to the bending of these trabeculae,

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what occurs is a response to that stress

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with a wandering team of osteoblast and osteoclast

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the latter removing damaged tissue

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and weakening the structure and integrity

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of the subchondral bone.

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Here, you can see

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particularly the osteoclast

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at work leading to trabeculae resorption.

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Then with continued stress applied

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to the surface of articular cartilage

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insufficiency fractures will appear

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and fatigue fractures as well

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classically in a transverse direction as shown here.

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Eventually, the fractures associated with callus formation.

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The histologic picture taken again

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from Yamamoto shows a beautiful example

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of a trabecular stress fracture

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not too far from the subchondral bone plate.

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Here a classic example of the appearance and location

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of a subchondral stress fracture involving the femoral head

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with extensive marrow edema.

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The subchondral bone plate in this particular case

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probably intact there isn't collapse yet.

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There is some reaction in the adjacent acetabulum as well.

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There's another one, a subchondral stress fracture.

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This could be fatigue or insufficiency.

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Please observe the meniscus is blunted, it's abnormal

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and more important than that

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it is peripherally displaced,

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leading to bowing of the medial collateral ligament.

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Now what's interesting and not as well known

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that although we think of these subchondral fractures,

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stress fractures as linear

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there is a second pattern that is seen

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that I'm gonna say is nodular in appearance.

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What we get is nodular callus and granulation tissue

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in those areas of trabeculae injury.

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The specimens on the right from Yamamoto

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show very, very nicely the dots

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that we can see a nodular callus that's forming

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about some of these injured trabeculae.

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One of the classic sites

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that this particular pattern is seen

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is involving the acetabulum.

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This can be a fatigue fracture

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but more likely it is an insufficiency fracture.

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And as you look at this,

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all of these are small nodules areas of radio density.

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Seeing a few nodules in this location can be normal

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but if you see an increase number of nodules

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some of which becoming more linear in shape

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here's what it looks like on the MRI.

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You better consider that you're dealing

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with a stress fracture likely of the insufficiency type.

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Here's another example of a limited insufficiency fracture

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involving the calcaneus

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producing as a focal nodular region

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just below the posterior facet of the calcaneus.

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Now there have been articles that have

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studied the distribution of insufficiency fractures

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of this element.

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This dominate in the spine, in the ASIS pelvis

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and in the lower extremities.

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They do occasionally occur in the upper extremities

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in certain selective sports where

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a lot of overhead throwing and movement is required.

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When they are found in the lower extremity

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a subchondral location is a frequent place to look for them.

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You can see by the frequency in the table on your right.

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The sacrum is the first place

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to look for when there is pelvic pain.

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And you're considering an insufficiency fracture,

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the symphysis pubis, the acetabulum,

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a femoral neck and head.

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And then you can read down the list to less likely places.

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Any process that weakens the bone

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can lead to an insufficiency fracture.

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Osteopenia related to osteoporosis,

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osteomalacia, hyper power thyroidism disuse

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or diseases such as rheumatoid arthritis

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all can produce insufficiency fractures,

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irradiation, a total joint replacement

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that changes the stress about the pelvis

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and then a variety of medications.

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Some of which we've already talked about.