The movements of the fingers are the result of a complex process: ligaments move the bones according to the muscular functions. The flexor tendon runs through a tunnel, called tendon sheath. This sheath is covered with synovium, which helps the tendon to glide easily.
Trigger finger occurs if the tendon thickens or a node forms on its surface or the pulley inflames and swell. The tendon cannot glide through the thickened pulley, which causes the popping sensation and noise.
If the glide of the tendon becomes difficult or it stuck, that intensifies the inflammation and the swell around the pulley, which results in an infinite loop without treatment. Sometimes the finger could lock in a bent or stretched position.
The causes of trigger finger are unclear, an excessive load of the hand, repeating movements, the injury of the palm, scarring, certain diseases (diabetes, rheumatoid arthritis) could also predispose to its formation.