The Aim of This Study is to Determine the Minimum Effective Volume of Local Anesthetic (Ropivacaine 0.5%) Required to Produce an Effective Ultrasound-guided Supra-anterior Superior Iliac Spine Fascia Iliaca Compartment Block（SASIS-FICB） for Surgical Anesthesia in 90% of Patients Scheduled for Elective Surgery of One Lower Limbs.

The study is based on biased coin design, and the volume of Local Anesthetic (Ropivacaine 0.5%) for the next patient is determined by the result of the last one. In the case of block failure, the volume will be increased by 2.5ml. Conversely, block success will result in either a reduction in volume by 2.5ml (probability 11%) or no change in volume (probability 89%). A blinded assistant will assess sensory and motor blockade in each nerve territory ( the femoral, obturator and lateral femoral cutaneous nerves) at 5-min intervals up to 30 min after completion of the block. Finally, MEV90 is calculated by isotonic regression.

Inclusion Criteria:

• patients undergoing hip/knee surgery

Exclusion Criteria:

• age <18 years
• body mass index >35
• inability to consent to the study
• allergy to local anesthetics
• a history of liver or renal insufficiency, coagulopathy
• clinical evidence of peripheral neuropathies, abnormalities of sensory or motor function of the FN, ON or LFCN.