Anatomy
The tendons travel from the muscles in the forearm across the palm and into the fingers.  They are surrounded by a layer of synovium (lining), allowing them to glide within this slippery canal.
Just before they enter the fingers, and along their course in the fingers, the tendons travel through tight tunnels called pulleys, which hold the tendons in their position close to the bones.

Symptoms
Normally the tendons glide easily along this course.  However, if there is swelling within the canals, and particularly as the tendon enters the first pulley at the distal (far) end of the palm, there may be pain as the fingers flex and extend, and tenderness in the palm close to the origin of the fingers.
When there is enough swelling that the tendon will not actually travel smoothly through the pulley system at the base of the finger, then, after full flexion (bending) of the finger, the tendon gets caught at the pulley.  With forceful extension (straightening) of the finger, the tendon ultimately glides into the pulley with a jerking motion, or a triggering of the finger.  At its most severe, in the case of what is termed a locked trigger finger, the tendon becomes entirely stuck within the pulley and the finger cannot move.

Diagnosis
A diagnosis of this condition is made according to three things:  tenderness in the palm, pain with movement, and a triggering or locking of the finger.
Triggering or synovitis is usually produced by cumulative-trauma problems, but it can also result from inflammatory disease such as rheumatoid arthritis, which affects the synovial lining of the tendon.  Triggering may also result from an acute injury or any process that produces swelling within the palm.

Treatment
The first line of treatment, as with all cumulative-trauma problems, is to attempt to reduce any activity that is causing ongoing injury.  This may be combined with oral antiinflammatory medications.
The most effective treatment for trigger finger is a cortisone injection into the region of the tendon at the edge of the pulley system.  This always reduces the inflammation and in most situations will resolve the problem.

Surgery
If there is no resolution with conservative means and there is ongoing disability and interference with daily activities, then a simple surgical procedure in which the pulley is incised to allow the tendon more room to glide will be effective.  This procedure can be done under local anesthetic, and only a very small incision is required.  Full healing usually occurs within two to three weeks.