EMPLOYEE WAS ATTEMPTING TO TAKE OFF THE LUG NUTS ON THE WHEEL OF THE ANFO HAULING TRAILER. A LUG THAT WAS TIGHT BROKE LOOSE. WHEN THE LUG BROKE LOOSE, THE EMPLOYEE WENT WITH IT AND CAUSED HIM TO HIT HIS HEAD (JUST ABOVE HIS LEFT EYE) ON THE TRAILER. EMPLOYEE'S GLASSES WERE BENT AND HEAD WAS CUT.