The purpose of the study is to determine the minimum effective concentration of bupivacaine (EC50) in femoral block for analgesia by ultrasound after knee surgery.

45 patients classified as ASA P1 or P2 aged between 18 and 65 candidates for elective knee operation via arthroscopy were included in the study. All patients received femoral nerve block guided by ultrasound with bupivacaine, 22 ml. The first patient at a concentration of 0.25%. The other patients with higher or lower concentrations if the first patient has pain> 3 or <3 on visual analogic scale respectively after awakening of surgical anesthesia. For surgical anesthesia each patient will undergo general anesthesia. At the end of the surgical procedure and anesthetic awakening patients will be evaluated for analgesia by verbal numeric scale. The modified Dixon method was used to find the EC50 and EC95 of bupivacaine for analgesia of femoral nerve block guided by ultrasound. Concentration still correlated with motor block and side effects.

Inclusion Criteria:

• patients to undergo knee surgery

Exclusion Criteria:

• patients with coagulopathy
• pregnant
• infection at the puncture site
• chronic pain