Total hip replacement is a common orthopaedic procedure that improves pain and mobility in a variety of pathologies like osteoarthritis, rheumatoid arthritis and avascular necrosis. Post-operative complications, for instance, venous thromboembolism and chest infection have long been documented in literature. These complications can have a bearing on long term survival, and may be prevented by early mobilisation. Therefore, pain control plays an important role in enhancing post-operative recovery, which may also shorten length of stay and reduce overall cost.

Multimodal analgesia is applied to these patient, with combination of opioid, oral adjuvant and regional anaesthesia. Each of the components has its own limitation; for opioid, post-operative nausea and vomiting (PONV) and respiratory depression limits its use, and adjuvants like non-steroidal anti-inflammatory drugs (NSAID) are contraindicated in certain patient populations (renal impairment, ischaemic heart disease, coagulopathy). Various regional techniques like femoral nerve block, fascia iliaca block, lumbar plexus block, paravertebral block and epidural anaesthesia are proposed but may be limited by incomplete coverage (due to the innervation by femoral and obturator nerve for the anterior aspect of the joint and sciatic nerve for the posterior aspect, with contribution of lateral cutaneous nerve of thigh for the wound), the invasive nature of the regional technique (psoas haematoma for lumbar plexus block, epidural haematoma for epidural anaesthesia (EA)) or cardiovascular effects like hypotension from EA.

Erector spinae block, first introduced by in 2016 as a chronic pain intervention, was also used in hip surgery from a case report in 2018. However, currently the evidence for lumbar ESP block is limited mainly to case reports, while randomised control trial is scarce. More concrete data are required to determine the efficacy of this novel technique.

It is postulated that single shot lumbar ESP injected at L1 level can 1) reduced post-operative pain score 2) reduced post-operative 24 hour opioid (fentanyl) use. This study is conducted in Tuen Mun hospital (TMH) and Pok Oi hospital (POH) in Hong Kong. Patient are recruited for the study during pre-anaesthetic assessment, and they are counselled for risk of general anaesthesia and erector spinae plane block (i.e. local infection/bleeding, injury to neighbouring structure, local anaesthetic toxicity).

After a pilot study regarding opioid use for patient after THR, the sample size for either treatment or control arm is calculated to be 35 patients per group to achieve adequate power to detect such a difference.

Sealed opaque envelopes with allocation inside is prepared forehand and is subsequently drawn on the day of surgery. The attending anaesthetist receives the envelope right before induction, and opens the envelope after the patient is put under general anaesthesia (GA). If the operation is cancelled or the patient cannot use PCA post-operatively (e.g. post-operative mechanical ventilation) or post-operative delirium), the envelope is put back into the pool.

After the patient is put under GA and turned lateral, the envelope is opened. For the treatment arm, lumbar ESP block is performed while for control arm no regional anaesthesia is performed nor saline is injected into the ESP. Before proceeding to ESP block, the back is cleaned with aseptic technique and draped. 40 mL of 0.25% levobupivacaine (or maximum of 2mg/kg body weight made up to same volume) is injected into the ESP.

After the injection, the surgeon is told not to infiltrate any local anaesthetics into the incision site or the wound. Intra-operatively, intravenous fentanyl is given as the only analgesic by the attending anaesthetist. Post-operatively, the patient is given fentanyl PCA of standard setting anaesthetist). The patient is also given all the pre-operative regular oral analgesic agents post-operatively, including paracetamol, NSAID, gabapentinoid and/or weak opioid (DF118 or tramadol).

The patient is assessed by an independent assessor from the acute pain service team 24 hours after the operation, who is blinded from the allocation result. Primary outcomes, that are post-operative 12 and 24 hour PCA fentanyl use and pain score in numeric rating scale (NRS) from 0 to 10 at rest and upon mobilisation are assessed and documented in the electronic system and also the patient chart. Secondary outcomes like presence/absence of PONV and knee flexion power in MRC scale are also noted.

The data are collected by the investigator and input to Excel 2003. 2-sided student's t test is used to compare the primary outcomes and also knee flexion power between treatment group and control group, while the rate of PONV between two groups is compared by Chi-square test. Multi-variate regression is done to control for the effect of oral analgesics on study outcomes.

Inclusion Criteria:

• Adult patients (age >=18)
• American Society of Anesthesiologists (ASA) class 1-3
• primary elective unilateral THR
• understand and accept the risk for general anaesthesia and ESP block
• counselled of post-operative patient controlled analgesia (PCA) and deemed fit for its use.

Exclusion Criteria:

• emergency THR
• bilateral THR
• revision THR
• THR done under neuraxial technique (e.g. spinal anaesthesia, combined spinal epidural anaesthesia)
• Patient with contraindication for ESP block (i.e. patient refusal, injection site infection or spine pathology/surgery, coagulopathy with international normalised ratio (INR) > 1.4 and thrombocytopenia < 75 x 10^9/L)
• patient who are mentally incompetent
• severe obesity (Body Mass Index >= 35)
• severe obstructive sleep apnea syndrome (Apnoea hypopnoea index >= 30, no matter on treatment or not)
• patient on regular strong opioid (e.g. morphine, oxycodone, methadone, buprenorphine, fentanyl)
• patient who have undergone hip neurolysis

data including patient number, age, gender, ASA class, anaesthetic technique, assignment, 24 hour fentanyl use, pain score, PONV and knee flexion power are available upon request by email to principal investigator

data available up to 1 year after completion of study

for validation of study results or for compilation of systemic review/meta-analysis

24 hour post-operatively

After the patient is put under GA and turned lateral, the sealed opaque envelope is opened. Unilateral lumbar ESP block is performed in participants under treatment arm, while no regional block is performed for control arm.

The patient is assessed by an independent assessor from the acute pain service team 24 hours after the operation, who is blinded from the allocation result.