Low-Volume Pericapsular Nerve Group (PENG) Block With Dexamethasone vs Standard-Volume PENG Block After Total Hip Arthroplasty

The goal of this clinical trial is to compare two regimens of the pericapsular nerve group (PENG) block, an ultrasound-guided injection of local anaesthetic targeting the nerves supplying the front of the hip joint, for postoperative pain relief and quadriceps muscle function in patients undergoing primary unilateral total hip arthroplasty under spinal anaesthesia. The PENG block was originally proposed as a motor-sparing technique. However, the standard 20 mL volume can spread to the femoral nerve and cause quadriceps weakness. Reducing the injected volume may limit this spread and better preserve quadriceps strength, but smaller volumes risk shorter analgesic duration. Adding dexamethasone to a low-volume injection may compensate for the reduced volume. The main questions this trial aims to answer are: * Does a low-volume PENG block (10 mL bupivacaine 0.25%) with 4 mg dexamethasone differ from a standard-volume PENG block (20 mL bupivacaine 0.25%) in the time to first rescue analgesia? * Does it differ in the incidence and severity of quadriceps weakness on the operated side? Researchers will compare: * Group A (standard-volume PENG block): 20 mL bupivacaine 0.25%. * Group B (low-volume PENG block with dexamethasone): 5 mL bupivacaine 0.5% plus 4 mL normal saline plus 4 mg dexamethasone in 1 mL, giving 10 mL of bupivacaine 0.25%. Participants will: * Receive spinal anaesthesia with heavy bupivacaine 0.5% and fentanyl. * Be randomly allocated to Group A or Group B using computer-generated random numbers. * Receive the PENG block under ultrasound guidance, performed by an anaesthesiologist expert in regional anaesthesia. * Receive intravenous paracetamol 1 g every 8 hours, with intravenous nalbuphine 5 mg as rescue analgesia when the pain score (NRS) is 4 or more. * Be assessed by an anaesthesiologist blinded to group allocation for pain scores at rest and on movement at 2, 6, 12 and 24 hours; quadriceps strength (MRC scale and straight leg raise) at 6, 12 and 24 hours compared with the preoperative baseline; total opioid consumption over the first 24 hours; and time to first ambulation within the first 24 hours. The primary outcome is the time from completion of the PENG block to the first administration of rescue analgesia, recorded over the first 24 hours postoperatively. The study will be conducted at Ain Shams University Hospitals (El Demerdash Hospital) and will include 60 patients (30 per group), aged 50 to 70 years, ASA physical status I or II.