Improving pain relief following Total Knee Replacement

INTERVENTION: the treatment arm of the study will have a femoral nerve infusion of Bupivicaine 0.125% running at 10ml/hr for 48 hours after total knee replacement while the placebo will have saline running for 48 hours. all patients will have an intraoperative femoral nerve injection of 15ml Bupivicaine 0.25% along with subarachnoid blockade using 10‐20mcg 0.5% bupivicaine + 3mcg/kg Morphine. Sedation using Midazolam/Propofol titrated to effect or General Anaesthetic (GA) CONDITION: Analgesia following total knee replacement PRIMARY OUTCOME: pain at rest (visual analogue scale) SECONDARY OUTCOME: cumulative dose of narcotics calculated using infusion pump information nerve infusion rate using information from infusion pump and bedside chart INCLUSION CRITERIA: 18‐85 years Body Mass Index (BMI) less than 40 undergoing primary total knee replacement ability to give informed consent