Trigger finger is a painful condition that causes the fingers or thumb to catch or lock when bent. In the thumb its called trigger thumb.Trigger finger happens when tendons in the finger or thumb become inflamed. Tendons are tough bands of tissue that connect muscles and bones. Together, the tendons and muscles in the hands and arms bend and straighten the fingers and thumbs.A tendon usually glides easily through the tissue that covers it (called a sheath) because of a lubricating membrane surrounding the joint called the synovium. Sometimes a tendon may become inflamed and swollen. Prolonged irritation of the tendon sheath can produce scarring and thickening that impede the tendon's motion. When this happens, bending the finger or thumb can pull the inflamed tendon through a narrowed tendon sheath, making it snap or pop.