Tendons connect bones to muscles in the body. Flexor tendons of the thumb and fingers pull the fingers into a fist. The tendons are enclosed in a synovial sheath. When this sheath becomes inflammed it is called trigger finger.
Usually, tendons slide easily through the sheath as the finger moves. In the case of trigger finger, the synovial sheath is swollen. The tendon cannot move easily. This causes the finger to remain in a flexed (bent) position. In mild cases, the finger may be straightened with a pop. In severe cases, the finger becomes stuck in the bent position. Usually this condition can easily be treated. Contact your doctor if you think you may have trigger finger.