Extensor Tendon Injuries:
Extensor tendons are just under the skin. They lie next to the bone on the back of the hands and fingers and straighten the wrist, fingers and thumb. Extensor tendon injuries can happen from a minor cut or jamming a finger, which may cause the thin tendons to rip from their attachment to bone. Common injuries include mallet finger, boutonniere deformity, and cuts on the back of the hand can injure the extensor tendon. This can make it difficult to straighten your fingers. (ASSH American Society for Surgery of the Hand)
Flexor Tendon Injuries:
The Muscles that bend(flex) the fingers are called flexor muscles. The flexor muscles move the fingers through cord like extensions called tendons, which connect the muscles to bone. The flexor muscles start at the elbow and forearm area and turn into tendons just past the middle of the forearm and attach the bones of the fingers. In the finger, the tendons pass through tunnels that keep them close to the bones, which helps them work better. Deep cuts can injure the tendons and nearby nerves and blood vessels. These injuries require you to wear a splint and exercises can be tricky and should be done with your Occupational Therapist.