Comparison of Levobupivacaine and Ropivacaine for Femoral Neural Block

The anaesthetic management of patients undergoing total knee replacement is still not standardised. Epidural analgesia is common, but is associated with bilateral lower limb motor block and limited mobilisation. Spinal anaesthesia with intrathecal morphine provides good pain relief, but only for 12 to 16 hours, and is often associated with nausea and vomiting. Combined single injection femoral / sciatic blocks give good pain relief, but for a variable length of time (between 12 and 24 hours). In contrast, continuous femoral perineural infusion of local anaesthetic provides very good pain relief for several days. Pain relief is maintained by a constant infusion of local anaesthetic using an elastomeric ball. The investigators overriding aim is to develop a local anaesthetic regimen which offers complete pain relief for at least 48 hours, yet allows full mobilisation within 24 hours Given that ropivacaine may offer a more advantageous pharmacological profile (less lipid solubility) compared to levobupivacaine, the investigators feel it is pertinent to investigate the capacity of ropivacaine to prevent pain relief after surgery. Thus, the investigators aim in this study is to compare the median effective dose of levobupivacaine with the of ropivacaine for preventive pain relief after total knee replacement. Further, calculation of the equipotent median effective dose's of each local anaesthetic allied to objective measurement of quadriceps motor block using an electromyogram will allow us to determine the sensory - motor split of each local anaesthetic.