Trigger finger, also known as stenosing tenosynovitis, affects the flexor tendons of the fingers or thumb. It most commonly affects the ring finger or thumb.

The tendons in the hand help control finger movement and pass through small tunnels called tendon sheaths. When a flexor tendon and its surrounding sheath become inflamed, a small lump or nodule can form near the base of the finger. As the finger bends, the lump moves through the tendon sheath. When the finger straightens, the lump can become caught, causing the finger to click, catch or lock in a bent position.