Tendons are cord-like extensions that connect muscles to bones. Extensor tendons are thin tendons located on the back of the hand, just under the skin. These particular tendons allow you to straighten your fingers and thumb and can be injured by a simple cut or jammed finger. These injuries can cause the tendons to rip from their attachment to the bone, making it hard to straighten your fingers or thumb as you usually do.
Here are two common conditions that result from an injured extensor tendon: