I've spent the past few days Googling for strings like "pitcher tommy john surgery" and then looking at the resulting images for patterns. One thing I'm picking up on, increasingly, is a specific movement of the thumbs of many pitchers who need Tommy John Surgery.
This movement pattern also overlaps with some of the other patterns I've noticed...
Tommy John Thumb
Tommy John Thumb is a specific movement of the thumb, into or at foot plant, that predisposes pitchers to needing Tommy John Surgery. Technically what I'm referring to is pronation of the forearm of the pitching arm. I call this Premature Pronation because it occurs well before advocates of pronation, like Dr. Mike Marshall, want pronation to happen.
At a minimum, Tommy John Thumb causes problems by creating a severe Timing problem that I call Flat Arm Syndrome.
The first pitcher I noticed Tommy John Thumb in was Ivan Nova, and it let me see his elbow problem coming in advance.
Ivan Nova and Mariano Rivera
Contrast what Ivan Nova does with what you see in Mariano Rivera at the same moment in time; Ivan Nova is thumb-down while Mariano Rivera's thumb would be pointing at the catcher, if he were to extend it.
Hunter Harvey is a more recent example of Tommy John Thumb.
Contrast Hunter Harvey's thumb-down position at foot plant with Roy Halladay's thumb-up position into foot plant.
In the picture above notice how, at foot plant, Hunter Harvey's thumb is turned down.