That might happen when you have been posing for a while, or even using some furniture, I use a few trick that sometimes work
I press stop animating my avatar , i do this a few times in a row
I turn off and on my AO
I try pressing to edit something, no matter what, the point of this, is to make your hand try to point somewhere and sometimes makes the finger go normal again, I find this works most of the time in my avatar
or I use a cache cleaner, or one of those thing that you find sometimes in photo studios to fix deformation
Anyway, some of them work, most of the time, if nothing works, I log off and in again, and thats it