EMPLOYEE WAS WALKING FROM #3 SCALE TO THE PACKHOUSE PUMP ROOM WHEN HE STEPPED ON A PIECE OF PIPETHAT WAS PARTIALLY BURIED IN THE GROUND. WHEN HEDID SO, HE TWISTED HIS LEFT ANKLE AND KNEE. EMPLOYEE MISSED NO WORK UNTIL 12-19-01 WHEN HE WENT OFF FOR CORRECTIVE KNEE SURGERY.