EE WAS WORKING ON 9483 WITH COWORKERS, ATTEMPTING TO REMOVE PIN FROM SURE-STRIKE ATTACHMENT WHICH WAS OFF THE UNIT. WHILE EE WAS WALKING AWAY TO GET A LARGER HAMMER, A COWORKER CONTINUED TO HIT THE PIN WITH A HAMMER. A SMALL STONE CAME OFF THE PIN AND STRUCK EE IN THE KNEE. REQUIRED STITCHES.