Trigger finger and trigger thumb are conditions affecting the movement of the tendons as they bend the fingers or thumb toward the palm of the hand. This movement is called flexion. Trigger thumb is much more common than trigger finger among babies and young children.
In children, trigger thumb or finger is an acquired (not congenital or present at birth) condition. In other words, like your daughter, the child isn’t born this way but instead, develops the condition early on. A common anatomic cause of trigger thumb is a mismatch in the size of the flexor tendon and the pulley.
The tendons that move the fingers are held in place on the bones by a series of ligaments called pulleys. These ligaments form an arch on the surface of the bone that creates a sort of tunnel for the tendon to run in along the bone.
To keep the tendons moving smoothly under the ligaments, the tendons are wrapped in a slippery coating called tenosynovium. The tenosynovium reduces the friction and allows the flexor tendons to glide through the tunnel formed by the pulleys as the hand is used to grasp objects.
Triggering is usually the result of a thickening in the tendon that forms a nodule, or knob. The pulley ligament may thicken as well. The constant irritation from the tendon repeatedly sliding through the pulley causes the tendon to swell in this area and create the nodule.
Trigger thumb or finger in children is not from overuse, trauma, or injury (those are more common causes in adults). And in children, the thumb is more likely to be fixed or stuck in what is referred to as a flexion contracture rather than a true triggering mechanism. Flexion contracture means the child cannot actively (or sometimes cannot even passively) straighten the thumb.
No one really knows the reason why some children develop these triggering digits. There are plenty of theories but no actual scientific evidence to explain it. There may be some linked to habit (constantly holding the thumb in a bent or flexed position). A static position can cause build up of synovial tissue and a break down of the natural tissue. The end result is a thickening of the tendon sheath as the body tries to heal itself.