Yeah they way his wrists are bent in that scene also bothered me--his claws aren't going to bend on the way out.
AFTER his claws have come out, then he can bend his fist at the wrists and the claws will be fine. But if he bends his wrist down or up before they come out, the claws will come out of his wrist or out of his palm, respectively.
As to the suggestion that the claws passed in the spaces between the ribs, this of course occured to me. But the problem is that even if it just so happens that all 3 claws get thru a space instead of jamming against a rib upon both entry and exit (which is unlikely), the only way they can get thru is sideways, and the way they come out of his back is not like that. Also, even if they made it thru the ribs, how would they make it past his adamanitum coated shoulder blades?