Nerve blocks are an effective way to control pain after surgery. There are two major nerves that provide sensation to the knee: the femoral nerve provides sensation to the front of the knee and sciatic nerve provides sensation to the back of the knee. The femoral nerve has two branches. To locate the nerve, we use a machine called (Nerve stimulator) which is attached to the needle used for nerve block. Use of a nerve stimulator is standard practice for this procedure. Ultrasound is being increasingly used to locate nerves but is not used universally. We will use the nerve stimulator and ultrasound to locate the femoral nerve but patients will randomly be selected to enter a group looking at the stimulation of one or the other branches of the femoral nerve. The two branches when stimulated produce different muscle contractions in the thigh. We do not know from research which is the optimal contraction to position the needle to get the best block. After the knee replacement patients will still have the same analgesic medication available as patients would have received if they were not in the study.