Ultrasound-guided interscalene block is commonly used for postoperative analgesia in patients undergoing shoulder arthroscopy. The location of local anesthetic injection may influence both block efficacy and the incidence of block-related complications. This randomized, double-blind clinical trial compares two different injection sites around the C5-C7 nerve roots using the same local anesthetic dose and volume. The study will assess block success, postoperative analgesia, opioid consumption, diaphragmatic function, and procedure-related complications.

Patients scheduled for elective shoulder arthroscopy under general anesthesia will be randomly assigned to one of two ultrasound-guided interscalene block techniques. In one group, the local anesthetic will be injected between the C5-C7 nerve roots and the anterior scalene muscle. In the other group, the injection will be performed between the C5-C7 nerve roots and the middle scalene muscle. The same local anesthetic solution, volume, and concentration will be used in both groups.

Block success, postoperative pain control, opioid consumption, diaphragmatic movement assessed by ultrasonography, and block-related complications including vascular puncture, Horner syndrome, hoarseness, dyspnea, nausea, vomiting, and prolonged motor block will be recorded and compared between the groups.

Inclusion Criteria:

Adults aged 18 years or older. ASA physical status I-III. Scheduled for shoulder arthroscopy under general anesthesia with a planned ultrasound-guided interscalene block for postoperative analgesia.

Able to provide written informed consent. Normal coagulation parameters and no antithrombotic therapy. No infection or anatomical abnormality at the injection site. No known allergy to local anesthetics. No contralateral diaphragmatic paralysis or pneumothorax. No dyspnea and adequate pulmonary function.

Exclusion Criteria:

Age younger than 18 years or ASA physical status IV. Antithrombotic therapy or abnormal coagulation parameters. Infection or anatomical abnormality at the injection site. Known allergy to local anesthetics. Contralateral diaphragmatic paralysis or history of pneumothorax. Dyspnea. Refusal to undergo interscalene block.

Individual participant data will not be shared because no data sharing plan has been established for this investigator-initiated study.

Participants were unaware of group assignment. Postoperative assessments were performed by an anesthesiologist who was not involved in block placement and was blinded to treatment allocation. The anesthesiologist performing the block was aware of group assignment.