Major surgery involving the knee is often associated with severe postoperative pain. Postoperative pain control remains a significant contributor to delayed recovery and length of hospital stay. Approaches to minimize postoperative pain after surgery are a matter of major concern due to the need for early mobilization, a crucial factor in good postoperative rehabilitation. Femoral nerve blocks by either continuous infusion or single injection of anesthetics provide an effective method for analgesia while minimizing the need for systemic opioid therapy, reducing the opioid induced side effects, and facilitating early ambulation. Administration of fentanyl has shown to be a highly effective method to control pain after Total Knee Replacement (TKR). The investigators hypothesize that fentanyl infusions will result in greater post-operative strength in the operative knee.