Why might a 12 year old boy with a swollen ankle and pain on the distal fibula be treated with a cast despite normal xrays?
To consider the questions about ankle sprains and Achilles injuries, let’s first go over the anatomy, AP view to the left, lateral on the right:
In this next view, I have added the calcaneo-fibular ligament –one of the major supporting structures that is stretched with an inversion sprain (twisting your ankle medial side up, like when you partially land on another player’s shoe).
And now I have added the fibular growth plate
Ok, so you are 12 year old and twist your ankle medial side up, by partially landing on another player’s shoe
The force is shown in blue. This is TENSILE on the lateral side and compressive medially:
Something’s got to give:
If you have no growth plate, left panel, you sprain your CF ligament; but in a twelve year old, the growth plate is weaker, so it gets injured.
Now, it there are “normal xrays” then by definition this a non-displaced fracture.
This is an urgent medical situation, however, as we want to keep it non-displaced! It needs a cast, typically.
TAKE HOME MESSAGE: 12 year olds don’t get sprains. (OK, Maybe they do, but you have to assume that a sprain is a non-displaced growth plate fracture until proven otherwise.
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