EE crossed the belt to remove a flag pin. He stepped off belt onto belt rail. His foot slipped off the rail & his leg went between the belt rubber & rail twisting his right knee. He finished his shift with a swollen knee & worked lt duty till 11/13/08, when he notified safety dept he had a torn meniscus & would have surgery 11/24/08. Off a minimum of 2 wks after surgery.