In patients undergoing ultrasound-guided fascia iliaca compartment blockade before knee replacement surgery, does the addition of hyaluronidase to the local anaesthetic solution improve the success rate and hasten the onset of the nerve blockade?

INTERVENTION: 30mL 0.5% ropivacaine with/without hyaluronidase powder 1500IU (i.e. 50IU/mL hyaluronidase) will be used for fascia iliaca compartment block. A single block fascia iliaca compartment procedure is performed approximately 30minutes before commencement of surgery. The solution is delivered to the fascia iliaca compartment (deep to the fascia iliaca and superficial to the iliacus muscle in the groin). The needle tip is directed into the correct position with ultrasound guidance. CONDITION: Improving regional anaesthesia efficacy PRIMARY OUTCOME: Success of facia iliaca compartment block (success is defined as anaesthesia of 3 nerves ‐ the lateral cutaneous nerve of the thigh (LCNT), the femoral nerve (FN) and the obturator nerve(ON)). Anaesthesia of the LCNT is defined as reduced/absent of cold sensation to ice on the skin of the lateral aspect of the mid‐thigh. Anaesthesia of the FN is defined as reduced/absent sensation to cold (using an ice cube) on the anterior aspect of the thigh AND weakness of knee extension with the hip passively flexed. ON anaesthesia is defined as > 20% decrease in 'strength' of thigh adduction (baseline is determined by force measured (mmHg) when teh patients thighs squeeze a sphygmonamometer cuff which has been pre‐inflated to a pressure of 40mmHg). SECONDARY OUTCOME: Plasma ropivacaine levels. Blood samples will be collected at 10, 20 and 30 minutes after block administration. Plasma ropivacaine levels will be determined using a Gas Chromatograph Mass Spectrometer. Speed of onset of fascia iliaca compartment blockade. The proportion of successful LCNT and FN and ON will be measured/compared between groups at 5 minutely time points. Methods to assess success/failure are defined in the explanation of primary outcome INCLUSION CRITERIA: Patients undergoing single‐sided, first‐time total knee replacement Male or female; 18‐80 years of age at the time of surgery Must be able to understand the possible anaesthetic complications and study protocol