I'm not a doctor (just a mom ;-) But my son fractured his wrist this past week, and they called it a "bubble fracture" or a "wrinkle fracture." My son fell off of his bicycle and braced his fall with his hand. The force of the fall on his wrist caused his wrist bone to "flex" (non-scientific term). Since they are kids, their bones still bend a bit, rather than break. (That's a good thing!) But if they bend too much, there is an area of the bone that weakens. I suppose in the old days they would just heal on their own, but now doctors seem more inclined to put a cast or splint on them, so there is no further damage.