I am 12 days post op open achilles repair. i am non-weight bearing in a plaster cast. i fell on my scooter last night and accidentally stepped on this foot. should i be worried about tearing the incision or sutures? or does the cast usually protect?

Of course you should be worried about ripping open the wound and tearing the incision. They whole point of putting you in a cast is to hold everything in place. While you may have got away with it you do need to check the would site to make sure that you have not ripped the surgery site and have things healing in the wrong position.

Since it is only 12 days past surgery, I recommend going back to the surgeon and let him evaluate situation. He will decide whether or not the cast needs to be removed to examine the surgical wound