The posterior cutaneous nerve of the leg and the Saphenous nerve both supplying the skin above the calf muscle passes closely to the operated site for you knee replacement. When they get irritated by the implant the project referred sensations on the back of your leg in your case the itch. It is one of the side effects of the total knee replacement surgery. The itch will be present for a short period and generally subsides when the portion of nerve affected heals. If itch persists for long time, i suggests you refer your orthopedician who might do an xray and other investigation to rule out possible infections.