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 ED50 of levobupivacaine with the ED50 of ropivacaine for preventive pain relief after total knee replacement. Further, calculation of the equipotent ED50's of each local anaesthetic allied to objective measurement of quadriceps motor block using an electromyogram (EMG) will allow us to determine the sensory - motor split of each local anaesthetic.