A FROZEN CHUNK OF MATERIAL BECAME CAUGHT IN THE UNLOADING DOORS OF A RAILCAR. THE EMPLOYEE ENTERED THE CAR TO BREAK UP THE MASS OF MATERIAL USING BARS AND A CHIPPING HAMMER. AS HE WAS WORKING ON THE FROZEN MATERIAL THE CHUNK BROKE INTO TWO PIECES AND SHIFTED. ONE PIECE STRUCK HIS LEG, THE FORCE OF THE BLOW HYPEREXTENDING HIS KNEE. A DOCTOR DIAGNOSED KNEE STRAIN & GAVE MEDICATION.