Extensor tendons located on the back of your hand and fingers allow you to straighten your fingers and thumb. They lie just under the skin and thus can be injured pretty easily by a cut. Also, jamming a finger can tear your extensor tendon off of the fingertip bone.
Mallet finger, or inability to extend the joint at the very end of the finger, results from the tendon being torn from the bone. Sometimes the tendon tears off a piece of bone; this is called a “mallet fracture.” These are treated best by splinting the joint in hyperextension, allowing the tendon to heal. This splint usually is kept in place constantly for six to eight weeks.
Tendon lacerations on the back of the hand cause you to have difficulty straightening your finger at the large joint where the fingers join the hand; these need to be repaired surgically.