Mechanism of Injury
The legs and buttocks generate torque which rotates the trunk. The hand is like the end of the whip, and the trunk rotates forcefully leaving the hand behind. The shoulder and elbow act as the links between the trunk and hand. As the shoulder rotates back and then suddenly forward (external rotation and then internal rotation) the momentum generated by the trunk rotation is multiplied.
Think of how much faster the tip of a whip moves than the handle. This is how much faster the hand is moving than the trunk. The momentum change when the shoulder changes from cocking to acceleration (i.e. when the shoulder changes from external rotation to internal rotation, or from rotating backward to forward) puts the elbow under tremendous stress.
The ligament on the inner side of the elbow (the Ulnar Collateral Ligament “UCL” also known as the Medial Collateral Ligament) is subjected to forces large enough to tear it on almost every hard throw. It doesn’t tear because the nearby muscles help protect it.
But, if the athlete throws too hard too often, or if his/her mechanics are poor the stress on the ligament can be too great causing a tear. When this happens the thrower often feels a pop on the inner side of the elbow and may try one or two more throws but has to stop because of the pain. Sometimes it feels as if the elbow “isn’t there” or is unstable.
When the ligament is torn the elbow is not able to tolerate the forces of throwing and hurts primarily during cocking up through ball release. This is a serious injury, and often requires surgery to fix (Ulnar Collateral Ligament Reconstruction or “Tommy John” surgery). Unfortunately, if this ligament is torn it very rarely heals and the thrower must choose between having surgery and giving up throwing.