At first, your affected finger may seem stiff and may click when you move it. You may notice a bump (nodule) or tenderness at the base of the affected finger in your palm. This is the spot where the tendon is likely catching.

As trigger finger worsens, your finger may catch at times in a bent position and then suddenly pop straight. Eventually, the finger may not fully straighten. Trigger finger is not the same as Dupuytren's contracture — a condition that causes thickening and shortening of the connective tissue in the palm of the hand — though it may occur in conjunction with this disorder.

Trigger finger most often affects your thumb or your middle or ring finger. More than one finger may be affected at a time, and both hands might be involved. Triggering is usually more pronounced in the morning, while firmly grasping an object or when straightening your finger.

The cause of trigger finger is a narrowing of the sheath that surrounds the tendon in the affected finger. Tendons are fibrous cords that attach muscle to bone. Each tendon is surrounded by a protective sheath — which in turn is lined with a substance called tenosynovium. The tenosynovium releases lubricating fluid that allows the tendon to glide smoothly within its protective sheath as you bend and straighten your finger — like a cord through a lubricated pipe.

But if the tenosynovium becomes inflamed from repetitive strain injury or overuse or due to inflammatory conditions, such as rheumatoid arthritis, the space within the tendon sheath can become narrow and constricting. The tendon can't glide through the sheath easily, at times catching the finger in a bent position before popping straight. With each catch, the tendon itself becomes irritated and inflamed, worsening the problem. With prolonged inflammation, scarring and thickening (fibrosis) can occur and bumps (nodules) can form.