Shinya Aoki broke Mizuto Hirota's humerus with a hammerlock on Dynamite 2009. It took him out of MMA for almost 2 years.
There's a good picture of Mir holding the lock and the break is right where it pivots on Mirs arm. Watching the replays though I would have sworn his shoulder popped out.
Mit -JasonGV - Pretty similar to the Renzo arm break right?
I believe the Renzo one was a dislocation not an actual break, but it's been a while.
Yeah. I thought the Renzo one he dislocated his elbow. 
It takes less force to snap the bone than to rip the joint. The bone is under severe pressure in the lock and usually gives out first.
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