What Causes Itching Sensation On Hands And Wrists Post Knee Replacement Surgery?

I had knee replacement surgery three weeks ago and developed an allergic reaction to the narcotic pain medication. I went to the emergency room and they gave me a steroid injection and then followed up with a weeks worth of prednisone pills. I finished taking the steroids yesterday. It has been eight days since my surgeon took me off the drugs. However, I still have itching on the palms of my hands and wrists. Most of the itching and redness is gone from my back, neck, and chest, but still remains on top of the hands, palms and wrists. I take Benydril at night which seems to help but the itching returns during the day. Why is this still happening? Are the extremities the last place to loose the itching sensation? Please help!