Chances of a complete recovery after surgery are estimated today at 85 to 90 percent. At the time of Tommy John's operation, Jobe put his odds at 1 in 100. After his surgery in 1974, John went on to pitch in the major leagues until age 46. Today, the procedure takes about an hour.
It is not uncommon today for pitchers to throw harder after the surgery than they did before the injury that caused the surgery to be necessary.
A torn elbow ligament--the most common cause of what was simply called "dead arm injury" during most of the 20th century--can be caused by a number of things, but the injury is most common in pitchers and the most frequent cause is throwing too hard or overwork.