Researchers aren't sure what is the cause of trigger thumb for most children. Does the child hold the thumb inside the palm because she can't straighten it? Or does it get stuck in a flexed position because she doesn't move it often enough?

It may be that the fingers are holding the thumb in a deeply flexed position. The position irritates the flexor tendon near the flexor tendon pulley mechanism. The tendon is covered with a lining called the tendon sheath. The beginning of the sheath is called the A-1 tendon pulley. The purpose of the pulley is to keep the tendons close to the bone. As the finger bends or flexes, the pulley keeps the tendons from sagging away from the bone.

Irritation leads to inflammation and closes off the space between the tendon and the tendon sheath. The tendon can no longer glide smoothly and gets stuck.

Treatment for this problem may include steroid injections, splinting, and passive and active motion to keep the tendon sliding and gliding. If these measures don't work, surgery to release the pulley may be needed.