Employee traveled to #6 Right of West Mains to check #6 pump. EE traveled through a mandoor and crouched down due to the low cavity height. The walkway had ruts from equipment EE stepped on the peak of a rut spraining EE's right knee. EE worked regular duties/shift and had surgery to repair meniscus on 8/18/2016. EE returned on EE's next scheduled shift on restricted duty.