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To come to terms with the forearm, we need to cement certain relationships and assign priorities. In that sense we can say that at the elbow, the forearm is dominantly
the ulna. The ulna flexes and extends. At the wrist, the forearm is dominantly the radius. The radius is the structure which, at the wrist, supports the hand.
The radius is the forearm bone of the hand. The ulna is the forearm bone of the elbow. The two bones are not just associated at the elbow and wrist joints, but cross
attached by a flexible sheet - like the two posts of a canvas stretcher. The fibers are so organized so as to take the load when one pushes something heavy.
U=================o elbow => ulna
///////////////////////////// interosseus membrane
o================={Hand radius => hand
Note that the radius spins on the axis of the radial head-capitellum at the elbow but orbits around the head of the ulna at the wrist. The motion stops
when the radius bone abuts the ulna as the bones cross over.
Well, usually it stops when the bones press together. Sometimes something goes crack or pop.
Fractures of the forearm commonly follow a twist with a lot of momentum such as when the hand engages the ground as the body spins or topples
around. The point of contact is a fulcrum for what follows. Depending on the compression or distraction or angularity of the forces, fractures through the two bones may be at different levels.
The typical "angulation" seen in x-rays is an illusion of two dimensional projection. Typically the real angle is simply along the fold line
drawn between the two fractures and is a rotational deformation around that oblique and seldom square with the x-ray axis.
But chaos is chaos. Sometimes one bone - say the ulna - breaks and the other is levered out of
its lesser joint. In that case the radius relationship to the capitellum gets pulled apart. That is, one bone is broken and the other dislocated at one end.
Skin is quite elastic - to a point. The dense membranes which compartmentalize the forearm are
not. Swelling or bleeding into the forearm compartments - especially the deep one - can overcome venous flow. Pressure in the veins begins to rise. When that pressure equals arterial
pressure (blood pressure), flow stops. Pain is horrible. Any slight motion of the fingers is simply awful. That is an emergency. We don't worry about bones losing position - well, OK, a little.
We can always redo a setting of the bone or even undo bad healing and revise. But circulation damage is not forgiving. Squeezing and generating severe pain in the name of bone position is
like driving nails through your hat into your head to keep the wind from blowing your hat away. It had better be SOME hat!
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