Medial uni-compartmental knee replacement (UKA) has been used for over 40 years to treat osteoarthritis (OA) of the knee. It is becoming more widely accepted as a treatment for early stage OA of the knee in order to preserve bony and ligamentous structures. The majority of the subjects who have received this knee are older (over 65 yrs) however, it is being used increasingly more frequently in younger individuals who have uni-compartmental knee disease so that bone may be spared in case the need arises in the future for a total knee arthroplasty. The study will take place at two sites within the United States and be managed by the Clinical Affairs Department at DJO Surgical in Austin, TX. The study will include only subjects who meet the indications for use criteria for the EPIK™ Uni-compartmental Knee System and who are candidates for UKA. Investigators will enroll up to 60 subjects at each of 2 sites within the United States, for a total of 100 subjects maximum.