Employee was repairing access ladder on #4 pit pump. The pump sled was covered w/uneven dirt/buildup, the dirt shifted and he felt a slight pain in his L knee. Reported to clinic on 5/1 w/discomfort in his L knee & received first aid treatment. On 8/20 he returned to clinic c/o continued discomfort. Referred to Dr. and on 9/12 was given restrictions and referred to ortho surg.