Trigger finger is an inflammation of tissue inside your finger or thumb. It is also called tenosynovitis (ten-oh-sin-oh-VY-tis). Tendons (cordlike fibers that attach muscle to bone and allow you to bend the joints) become swollen. So does the synovium (a slick membrane that allows the tendons to move easily). This makes it difficult to straighten the finger or thumb.
Repeated use of a tool, such as a drill or wrench, can irritate and inflame the tendons and the synovium. So can arthritis or an injury to the palm of the hand. But often the cause of trigger finger is unknown.
Tendons connect muscles in your forearm to the bones in your fingers. The tendons in each finger are surrounded by a protective tendon sheath. This sheath is lined with synovium, which produces a fluid that allows the tendons to slide easily when you bend and straighten the finger. If a tendon is irritated, it becomes inflamed.
When a tendon is inflamed, it causes the lining of the tendon sheath to swell and thicken. Or the tendon itself may thicken. Then the sheath pinches the tendon, and the tendon can no longer slide easily inside the sheath. When you straighten your finger, the tendon sticks or “locks” as it tries to squeeze back through the sheath.
The first sign of trigger finger may be pain where the finger or thumb joins the palm. You may also notice some swelling. As the tendon becomes more inflamed, the finger may start to catch when you try to straighten it. When the locked tendon releases, the finger jumps, as if you were releasing the trigger of a gun. This further irritates the tendon, and may set up a cycle of catching and swelling.
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