FELLOW WORKER WAS KNOCKING OUT TOOTH KEEPER ON SHOVEL BICKET WHEN THE 16 LB SLEDGE HAMMER HE WAS USING CAME OUT OF HIS HAND. THE HAMMER TRAVELEDABOUT 18' TO 20' AND HIT EMPLOYEE ON HIS RIGHT K NEE. EMPLOYEE DID NOT SEE DR UNTIL 12-31-01 AND BEGAN MISSING WORK ON 1/2/02.