Trigger finger, also called stenosing tenosynovitis, is a common condition affecting the fingers. The tendons that bend the fingers (the flexor tendons) glide through a tunnel made of pulleys. Triggering happens when the space in the flexor tunnel is narrowed and the flexor tendon catches on one of the pulleys.
A vicious cycle is then formed as the more friction there is on the tendon, the thicker the covering of the tendon becomes (synovium), until it forms a distinct swelling that gets caught in of the pulley.