I've spent the past few days Googling query 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 a pitcher's thumb that reflects a combination of the Tommy John Twist and a particularly serious type of Timing problem that I call Flat Arm Syndrome, that I believe significantly increases the risk for Tommy John Surgery.
Technically what I'm referring to is, into or at foot plant, the Premature Pronation of the forearm of the pitching arm combined with little to no external rotation or even internal rotation.
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 and his pitching arm is FLAT while Mariano Rivera's thumb would be pointing at the catcher, if he were to extend it and his pitching arm is UP.
Hunter Harvey is a more recent example of Tommy John Thumb.
Contrast Hunter Harvey's thumb-down, flat arm position at foot plant with Roy Halladay's thumb-up, arm-up position into foot plant.
In the picture above notice how, at foot plant, Hunter Harvey's thumb is turned down.