The primary objective of this prospective, randomized, observer-blinded controlled trial is to compare the postoperative analgesic efficacy of continuous intermediate-approach sacral erector spinae plane (SESP) block versus continuous supra-inguinal fascia iliaca compartment block (FICB) in adult patients undergoing elective posterior-approach total hip arthroplasty under spinal anesthesia.

Sixty patients will be randomly allocated to receive either a continuous SESP block or a continuous supra-inguinal FICB. Both techniques involve an initial bolus of 20 mL 0.2% ropivacaine followed by intermittent boluses of 10 mL 0.2% ropivacaine every 6 hours for 72 hours via an indwelling catheter, in addition to standardized multimodal analgesia. The primary outcome is cumulative morphine milligram equivalents (MME) consumption over the first 72 postoperative hours. Secondary outcomes include pain scores at rest and during movement (VAS), quality of recovery (QoR-15) at 72 hours, motor function (modified Bromage scale), and adverse events.

We hypothesize that continuous SESP block will provide superior opioid-sparing analgesia compared with continuous supra-inguinal FICB while maintaining comparable pain control, quality of recovery, and motor function preservation.

Effective postoperative pain management after total hip arthroplasty (THA) via the posterior approach remains challenging because the surgical site receives innervation from both the lumbar and sacral plexuses. Multimodal analgesia incorporating regional anesthesia techniques is recommended to minimize opioid consumption and facilitate early rehabilitation. However, the optimal continuous regional technique for posterior THA has not been clearly established.

This prospective, randomized, observer-blinded trial compares two continuous ultrasound-guided regional analgesia techniques in patients undergoing elective posterior-approach THA under spinal anesthesia. Eligible adult patients (ASA I-III) are randomized in a 1:1 ratio to receive either continuous intermediate-approach sacral erector spinae plane (SESP) block or continuous supra-inguinal fascia iliaca compartment block (FICB).

In the SESP group, with the patient in the lateral decubitus position, a catheter is placed in the interfascial plane between the erector spinae muscle and the intermediate sacral crest under ultrasound guidance. In the FICB group, with the patient supine, a catheter is placed in the fascia iliaca compartment using the supra-inguinal approach with identification of the characteristic bow-tie sign. Both groups receive an initial bolus of 20 mL 0.2% ropivacaine followed by intermittent boluses of 10 mL 0.2% ropivacaine every 6 hours for a total of 72 hours via the indwelling catheter. All patients receive standardized multimodal analgesia consisting of intravenous paracetamol and diclofenac, with intravenous fentanyl PCA as rescue analgesia.

The primary outcome is cumulative opioid consumption expressed as morphine milligram equivalents (MME) during the first 72 postoperative hours. Key secondary outcomes include visual analog scale (VAS) pain scores at rest and during hip flexion at multiple time points up to 72 hours, the Quality of Recovery-15 (QoR-15) score at 72 hours, motor function assessed by the modified Bromage scale at 24 and 72 hours, and the incidence of adverse events. Outcome assessors, ward staff, and data analysts remain blinded to group allocation throughout the study period.

This trial aims to determine whether continuous SESP block provides clinically meaningful opioid-sparing benefits compared with continuous supra-inguinal FICB in patients undergoing posterior THA, without compromising recovery quality or safety.

Inclusion Criteria:

Adult patients aged 18 to 80 years. Scheduled for elective primary unilateral total hip arthroplasty due to osteoarthritis (not due to hip fracture or trauma).

American Society of Anesthesiologists (ASA) physical status I, II, or III. Ability to understand and cooperate with study procedures, including postoperative pain assessment using the Visual Analog Scale (VAS) and follow-up evaluations.

Provided written informed consent to participate in the study.

Exclusion Criteria:

Patient refusal to participate or inability to provide informed consent. Contraindications to regional anesthesia or peripheral nerve blocks (e.g., coagulopathy, therapeutic anticoagulation, infection at the injection site, or severe spinal deformity).

Known allergy or hypersensitivity to local anesthetics (ropivacaine or bupivacaine).

Previous surgery on the same hip. Severe cardiopulmonary disease (e.g., NYHA class III-IV, severe COPD) that may interfere with postoperative recovery assessment.

Neurological disorders affecting the lower extremities (e.g., stroke with residual motor deficit, peripheral neuropathy).

Cognitive impairment or inability to cooperate with catheter care and follow-up assessments.

Pregnancy or breastfeeding.

Public sharing of IPD is not planned due to patient confidentiality regulations in Vietnam. However, completely de-identified data underlying the results reported in this study may be made available to qualified researchers upon a reasonable and methodologically sound request, strictly subject to approval by the Principal Investigator and the Institutional Review Board.

Outcome assessors, ward nurses administering multimodal analgesia, and data analysts were blinded to group allocation. Patients and anesthesiologists performing the blocks could not be blinded due to the distinct anatomical locations of the two interventions.