The goal of this clinical trial is to learn whether an ultrasound-guided serratus posterior superior intercostal plane block (SPSIP block) can lower the need for opioid pain medicine after breast reduction surgery.

The main question it aims to answer is: Does adding an SPSIP block to standard anesthesia and pain treatment lower opioid use during the first 24 hours after surgery? Researchers will compare participants who receive standard anesthesia and pain treatment with participants who receive standard anesthesia and pain treatment plus an SPSIP block.

Participants will:

• Have planned bilateral breast reduction surgery under general anesthesia
• Be randomly assigned to the SPSIP block group or the control group
• Receive standard local anesthetic infiltration during surgery and standard pain treatment after surgery
• Have their pain score and pain medicine use recorded during the first 24 hours after surgery
• Be followed for safety and complications for 30 days after surgery

Reduction mammaplasty, also known as breast reduction surgery, may be associated with early postoperative pain because it is usually performed bilaterally and involves wide surgical dissection and tissue resection. Multimodal pain treatment is commonly used after this procedure to provide adequate pain control and to limit opioid exposure.

The serratus posterior superior intercostal plane block (SPSIP block) is an ultrasound-guided fascial plane block used for thoracic wall analgesia. Recent studies have evaluated this block in breast surgery, but data specific to reduction mammaplasty remain limited. In routine practice, surgical local anesthetic infiltration is often used during reduction mammaplasty. Therefore, this trial will evaluate whether adding bilateral SPSIP block to standard anesthesia, surgical local infiltration, and standard postoperative pain treatment provides additional opioid-sparing benefit.

This study is designed as a prospective, randomized, parallel-group, assessor-blinded, single-center clinical trial. Participants scheduled for elective bilateral reduction mammaplasty will be assigned in a 1:1 ratio to either the SPSIP block group or the control group. In the SPSIP block group, bilateral SPSIP block will be performed under ultrasound guidance after induction of general anesthesia and before surgical local infiltration and skin incision. The control group will not receive SPSIP block.

Both groups will receive the same standardized general anesthesia, surgical local anesthetic infiltration, postoperative multimodal analgesia, and rescue analgesia protocol. The local anesthetic agent, concentration, volume, and maximum safe dose limits for the block and surgical infiltration will be standardized before study enrollment.

The trial is planned to reflect routine plastic surgery practice. Surgical technique, pedicle selection, resection amount, drain use, and the use of dermal graft or internal support when clinically needed will be decided by the surgical team according to participant anatomy and routine clinical judgment. These variables will be recorded to describe surgical variability and may be considered in additional analyses if needed.

Postoperative pain assessments will be performed by ward nurses who are blinded to group allocation. Opioid use will be obtained from medication administration records. Block-related events and surgical complications will be recorded during routine postoperative care and up to 30 days after surgery.

Inclusion Criteria:

• Female participants aged 18 to 65 years
• Scheduled for elective bilateral reduction mammaplasty
• Planned surgery under general anesthesia
• American Society of Anesthesiologists physical status class I to III
• Planned postoperative clinical follow-up for the first 24 hours after surgery
• Able to understand and report pain using the 0 to 10 Numeric Rating Scale
• Able to provide written informed consent for study participation

Exclusion Criteria:

• Known allergy or hypersensitivity to local anesthetic drugs
• Contraindication to regional anesthesia or fascial plane block
• Coagulopathy or clinically significant bleeding disorder
• Use of anticoagulant or antiplatelet medication that prevents safe block application
• Infection at the planned block injection site
• Chronic opioid use, chronic pain syndrome, or regular analgesic use that may affect postoperative pain assessment
• Revision breast surgery
• Planned major additional surgical procedure other than reduction mammaplasty
• Pregnancy or lactation
• Unable to reliably report pain scores

Individual participant data will not be shared because the study data contain clinical and perioperative information from surgical participants and sharing individual-level data was not included in the ethics approval or informed consent process.

Participants will receive the intervention under general anesthesia and will not be informed of their group allocation during postoperative assessment. Postoperative pain scores and early clinical outcomes will be recorded by ward nurses who are blinded to group allocation. The anesthesia team performing the block will not be masked.