Med student here. Just learned about this the other day. Its called "triphalyngeal thumb." You know how there are three joints in your fingers, but only two in your thumb? An error in embryological development causes the thumb to have three joints.
There are different classifications of triphalyngeal thumb. This case appears to not be opposable. Normally there is loss of function because the thumb has the same functions as a finger, so no grasping. Only flexion, extension, adduction and abduction, no "crossing" or "grasping" like a normal thumb. Here is a better picture.
Did I say suffering? I meant rocking out:
I saw in the comments "give this woman a piano" I play guitar, piano and bass (studied music in college) and yes it is pretty useful!