The muscle that bends the tip of the thumb is called flexor pollicis longus (FLEX-er PAHL-i-sis LONG-us). This muscle starts in your child’s forearm. Near the wrist, the muscle turns into tendon. The tendon runs along the palm side of your child’s thumb and connects to the bone in the tip of the thumb.
When your child moves their thumb, this tendon should glide smoothly inside a wrapping called a tendon sheath. Near the base of the thumb, a tough band (ligament) crosses the tendon and tendon sheath, acting like a pulley. Doctors call this the A1 pulley.
Trigger thumb occurs when the tendon swells, forming a bump (nodule) near the A1 pulley. The nodule gets stuck at the pulley, so the tendon cannot glide inside the sheath. You may be able to feel this bump on your child’s palm at the base of their thumb.