Before induction of general anesthesia, all patients will receive interscalene block using 15ml of ropivacaine which concentration will be based on the response of the previous patient.~Based on biased-coin design up-down sequential method: if patient has pain free recovery, a successful block, the next patient will be randomized to receive either the same ropivacaine concentration or a concentration 0.01% w/v less. However, if he has an unsuccessful block, the next patient will receive a concentration 0.01% w/v higher.~Patients with pain free recovery will be considered to have positive response.~Patients' responses will be analyzed to calculate the MEAC90.
This study aim to calculate the minimal effective ropivacaine concentration required achieve successful analgesic interscalene block in 90% of patients (MEAC90)