PERSON WAS OPERATING SCOOP AND STOPPED TO PICK UP SOME TRASH TO PUT IN SCOOP BUCKET. WHEN PERSON STEPPED OUT OF OPERATOR'S DECK ON SCOOP HE TWISTED HIS LEFT KNEE WHEN HIS FOOT SLIPPED IN SMALL HOLE WHICH HAD WATER OVER IT. PERSON HAD INJECTED IN LEFT KNEE ON 1/31/05.