Trigger finger is a form of tendonitis, or irritation, involving the tendons that bend the finger.
These tendons connect the muscles in the forearm to the bones in the hand. The tendons are supported by fibers called pulleys which create several tunnels along the length of the finger.
When the muscles in the forearm contract (as when making a fist), the tendons slide under the pulleys. If there is inflammation present, the tendons do not pass freely under these pulleys because of the swelling and scar tissue.
It is like trying to pull a knotted thread through the eye of a needle. The ring and middle fingers are the most commonly affected, but trigger finger can also affect the thumb.
The exact cause of trigger finger is unknown, but many other hand and upper extremity problems and repetitive motion disorders have been associated with trigger finger.