This randomized controlled trial aims to compare the effectiveness of interscalene (ISB) and serratus posterior superior intercostal plane block (SPSIP) for postoperative analgesia in patients undergoing elective arthroscopic shoulder surgery. The primary outcome is total 24-hour opioid consumption. Secondary outcomes include pain scores, hemidiaphragmatic paresis incidence and severity, duration of analgesia, and changes in lung function. Participants are randomized into ISB or SPSIP groups; blocks are performed under ultrasound guidance. Postoperative pain is managed with patient controlled analgesia(PCA).