The tommy John surgery is the Ulnar collateral ligament reconstruction (UCL). What it does it helps athletics get there elbow back to 100%. It will replace a tendon from a different part from the human body or from an organ donor. From my personal experience nothing was taken from me. I had an organ donor tendon to replace my bad ligament. Doctors will cut open your arm by the elbow (not a huge cut, but big enough for them to get into). Then they will drill holes into the ulna and humerus bones to help for the next step. With the doctors drilling through your bones, it doesn't cause your arm to be broken they need the holes there so that the doctors can replace the ligament in a figure eight sort of way to help get the range of motion back faster. The cause of getting the injury that leads to the surgery is when an Athlete puts a lot of stress into there elbow. For example I over used my elbow a lot to throw baseballs and bowling balls. On top of that I would be lifting heavy weights which cause my ligament to tear. Athletes need to be careful when training and playing their sport!
For society a lot of major league players have many surgeries. Probably because they are always playing their sport. The tommy John surgery mostly happens in baseball because it easy for a pitcher to tear their ligament faster by throwing an average of 100 fast balls per outing on the mound. Since 1999 235 MLB players underwent the tommy John procedure. About 7% of pitchers like to throw the fastball more instead of their breaking ball. Pitchers have a tendency to stick with the fastball because it is there most fast and dominate pitch against a batter. Also on top of that they probably feel more confident with that pitch slowly making it easier to injure their elbows.
For myself I underwent the tommy John procedure the past summer. I was out for 9 months and was suppose to be out for 11 months but I recovered super fast because I wanted to get back out on the diamond. I had a great comeback this season hitting 0.629 batting average, 0.638 slugging average, OBP 1.2 something, and OPS 1.297.