Trigger finger occurs when irritation, inflammation, or swelling develops in a tendon or its surrounding synovial sheath. This condition prevents the tendon from gliding smoothly within the sheath, making it difficult to bend the affected finger or thumb. When an individual attempts to flex the digit, the inflamed tendon gets caught as it tries to pass through the narrowed sheath, leading to the characteristic catching, locking, and painful sensation associated with trigger finger.