Study aim: Investigating the effect of perineural dexamethasone combination in brachial plexus interscalene block in shoulder surgery on postoperative pain Design: During this RCT, the number of 60 patients candidates for shoulder surgery are allocated randomly into 2 groups of 30 patients according to Including and excluding criteria. In the control group, lidocaine 2 cc (1.5%) , bupivacaine 20 mg (2cc, 0.5%) , and 2 cc normal saline are used for interscalene block. In the case group, lidocaine 2 cc (1.5%) , 2cc bupivacaine 0.5% (20mg), and 2cc dexamethasone (8mg) are used. VAS score is used to measure pain 12 hours and 24 hours after the operation Settings and conduct: During this RCT, the number of 60 patients candidates for shoulder surgery are allocated randomly into 2 groups of 30 patients according to Including and excluding criteria. In the control group, lidocaine 2 cc (1.5%) , bupivacaine 20 mg (2cc, 0.5%) , and 2 cc normal saline are used for interscalene block. In the case group, lidocaine 2 cc (1.5%) , 2cc bupivacaine 0.5% (20mg), and 2cc dexamethasone (8mg) are used. VAS score is used to measure pain 12 hours and 24 hours after the operation This research is double-blind and the physician injecting the drugs and the physician performing the post-operative evaluations are different and do not know about the content of the syringes and the cases. Participants/Inclusion and exclusion criteria: Patients with ASA class I and II candidates for shoulder surgery referred to Shohadaye Tajrish and Akhtar Hospitals affiliated to Shahid Beheshti University of Medical Sciences. Intervention groups: In the control group, 30 cc lidocaine1.5%, 2cc bupivacaine 0.5%(20mg), and 2cc of normal saline are injected between the C5-C6 roots of spine. In the case group, 30cc lidocaine 1.5%, 2 cc bupivacaine 0.5%(20 mg), 2cc of dexamethasone 8mg is injected between the same roots Main outcome variables: pain