Trigger finger is a common problem in the hand that causes locking and pain in the involved finger or thumb. The technical name used to describe trigger finger is stenosing tenosynovitis. Stenosing means a narrowing of a tunnel or tube-like structure (the sheath of the tendon). Tenosynovitis means inflammation of the tendon.
Your fingers flex (make a fist) and extend (straighten out). Two sets of tendons make this possible - flexor tendons (on the palm side) and extensor tendons (on the back side of the hand). Trigger finger involves the flexor tendons of the hand.
Flexor tendons are normally smooth, white bands of tissue that start as muscles in the forearm and connect to the bones in your fingers. As they go from the forearm into the wrist and fingers, they pass through tight tunnels (sheaths) that keep them close to the bones of your hand.
The sheath is lined with a lubricating tissue called synovium. This tissue sits between the tunnel wall and the tendon. Trigger finger is caused by inflammation of the synovium, enlargement of the tendon, and thickening or narrowing of the sheath itself, usually in the area of the palm closest to the fingers.
The name of the tendon sheath in this area is the "A1 pulley".
These problems with the tendon's lining, the tendon, and the sheath, all combine to make it hard for the tendon to glide smoothly through the tunnel as your finger bends and straightens.
As the patient tries to flex (bend into a fist) the affected finger, the enlarged tendon has trouble passing underneath the sheath's tight opening. When the patient grips tight enough, the swollen tendon is suddenly pulled through the sheath with a painful snap, which may lock the finger in a bent position (it won't straighten except with help from the other hand). The action of straightening the finger from its locked position creates another often painful snapping sensation as the enlarged tendon passes back through the tunnel.