Hi guys im trying to rig a hand, and to be honest have no experience in the area, i have the bones created, and have smooth bind them to the hand model, and have painted some weight maps for the joints for he index finger to start out, my problem is the when i rotate the finger in (as in clenching into a fist) the finger squishes flat (images attached) . any hints and tips to stop that? I know i can set up handles and set driven keys but i didnt think they would help with this problem.

thanks in advance if ya can help