LOADER OPERATOR WAS STARTING TO CLIMB HIS LADDER ON HIS LOADER WHEN HE REACHED UP, NOT PAYING ATTENTION TO WHAT HE WAS DOING AND HAMMED HIS RIGHT THUMB INTO THE UNDERSIDE OF THE TOP STEEL STEP . HE THOUGHT IT WOULD GET BETTER, SO HE DIDN'T REPORT IT TO HIS SUPERVISOR UNTIL 11-19-01. HE WENT TO MD ON 11-20-01 & MISSED ONE DAY OF WORK.