The IE tripped on a forklift pocket of a stand. When EE fell EE landed on left knee. Other than a little pain and some initial swelling the employee was feeling fine and able to walk and move the knee as normal. About 3 weeks later the knee had some fluid build up in a bursas sack and the employee had it drained. It became reportable when EE had the knee drained on 6/26/17.