The aim of this clinical trial is to compare the analgesic effect of pericapsular nerve block (PENG) with intrathecal morphine in patients scheduled for total hip replacement surgery. The main question to be answered is whether the PENG block is equivalent to intrathecal morphine in reducing postoperative pain. Participants will be randomised into two groups. Patients assigned to the PENG group will receive spinal anaesthesia with local anaesthetic (isobaric bupivacaine) alone and a PENG block. Patients assigned to the intrathecal morphine (ITM) group will receive spinal anaesthesia with a mixture of local anaesthetic (isobaric bupivacaine) and morphine (100 mcg) and a sham PENG block to ensure patient blinding.