Ulnar Claw Pathology

Hi,

I am currently learning about ulnar nerve damage as part of my anatomy module. In my particular case is relating specifically to the ulnar clawing of the 4th and 5th fingers due to a complete severing of the ulnar nerve and flexor carpi ulnaris, slightly proximal to the wrist. I am trying to figure out why this injury means that the patient cannot zip up his/her jacket, based on both anatomical and biomechanical principles????

Any help would be GREATLY appreciated :)

Cheers,
Tom