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

Background: Hip surgery is associated with moderate to severe postoperative pain that, if poorly controlled, delays mobilization, prolongs hospital stay, and increases complications in a frequently elderly population. Opioid-based analgesia is limited by dose-dependent adverse effects such as sedation, respiratory depression, nausea, vomiting, and constipation, particularly harmful in older patients. Peripheral nerve blocks provide effective analgesia, reduce opioid requirements, and are therefore central to enhanced recovery pathways after hip surgery.

The anterior hip capsule, the most densely innervated region of the joint, receives sensory articular branches from the femoral, obturator, and accessory obturator nerves. These arise cephalad to the inguinal ligament and are therefore not reliably captured by traditional infra-inguinal techniques such as the femoral nerve block or fascia iliaca compartment block.

The pericapsular nerve group (PENG) block was described in 2018 as an ultrasound-guided injection of local anaesthetic into the musculofascial plane between the psoas tendon and the iliopubic eminence, targeting these articular branches while theoretically sparing motor components. Meta-analyses of randomized trials confirm that the PENG block reduces postoperative opioid consumption and pain scores after hip surgery and facilitates positioning for neuraxial anaesthesia.

Despite being promoted as motor-sparing, the PENG block performed with the conventional 20 mL volume has been repeatedly associated with quadriceps weakness, reported in approximately 25% or more of patients, attributed to spread of local anaesthetic toward the femoral nerve proper within the fascia iliaca compartment. Cadaveric and imaging studies support a volume-dependent relationship: larger volumes increase femoral nerve staining, whereas smaller volumes reduce this risk. A cadaveric study estimated the maximum effective volume sparing the femoral nerve in 90% of specimens to be approximately 13.2 mL.

However, lowering the volume may shorten the duration of analgesia. Dexamethasone is among the most studied adjuvants for peripheral nerve blocks and can prolong analgesia of long-acting local anaesthetics while reducing postoperative nausea and vomiting; the analgesic benefit is largely achieved at a dose of approximately 4 mg.

A preliminary randomized trial reported that adding dexamethasone to ropivacaine for the PENG block prolonged postoperative analgesia compared with ropivacaine alone. The PENG block has also been shown to improve early functional recovery and reduce time to ambulation after hip arthroplasty. More recent work suggests that combining dexamethasone with a reduced local anaesthetic volume in PENG block can maintain analgesia while preserving quadriceps strength. However, the available evidence largely employs ropivacaine and time-to-first rescue as the principal endpoint; data on a bupivacaine-based low-volume regimen with dexamethasone, evaluated as a complete analgesic strategy, remain limited, which forms the rationale for the present study.

Aim: To compare the analgesic efficacy and quadriceps motor function of two PENG block regimens, a low-volume block (10 mL bupivacaine 0.25%) with 4 mg dexamethasone versus a standard-volume block (20 mL bupivacaine 0.25%), in patients undergoing primary unilateral total hip arthroplasty.

Study design and setting: Prospective, randomized, controlled clinical trial conducted at Ain Shams University Hospitals (El Demerdash Hospital). Study period: 1 year.

Randomization and blinding: Eligible patients who fulfil the inclusion criteria and consent to participate will be randomly allocated into two equal groups using computer-generated random numbers, such that patients with even numbers will be enrolled in Group A (standard-volume PENG block) and patients with odd numbers will be enrolled in Group B (low-volume PENG block with dexamethasone). The participants, the outcome assessors and the data analyst will be blinded to group allocation.

Study procedure:

• History, clinical examination, and routine investigations (complete blood count, prothrombin time, partial thromboplastin time, random blood glucose) will be performed for all patients.
• All patients will be fasting for 2 hours for clear fluids and 8 hours for heavy meals.
• Standard monitoring (electrocardiography, non-invasive blood pressure, peripheral oxygen saturation) will be applied in the operating room and baseline readings recorded.
• Under strict aseptic conditions, standard intrathecal anaesthesia will be administered using a 25-gauge spinal needle with 3-4 mL heavy bupivacaine 0.5% and 10-12.5 mcg fentanyl as additive.
• In the event of hypotension, ephedrine 5-15 mg will be administered by slow titration.
• Adequacy of spinal anaesthesia will be confirmed after 5 minutes by a cold sensation test and the modified Bromage scale; an adequate block will be considered if the sensory level is at or above the tenth thoracic dermatome (T10).
• Study syringes will be prepared as follows: Group A (standard-volume PENG): 20 mL of bupivacaine 0.25% (50 mg). Group B (low-volume PENG with dexamethasone): 5 mL of bupivacaine 0.5% (25 mg) plus 4 mL of normal saline plus 4 mg dexamethasone in 1 mL, giving a total injectate volume of 10 mL of bupivacaine 0.25%.
• An 80-100 mm, 22-gauge echogenic needle is advanced in-plane from lateral to medial into the plane between the psoas tendon and the iliopubic eminence, and the local anaesthetic is injected in increments after negative aspiration.
• Correct injectate spread deep to the psoas tendon will be confirmed sonographically; failure to achieve appropriate spread will be recorded as a block failure.
• Both blocks will be performed by an anaesthesiologist who is an expert in regional anaesthesia, under strict aseptic conditions and ultrasound guidance using a low-frequency curvilinear transducer (2-5 MHz; C60xp; SonoSite X-Porte; SonoSite Inc., Bothell, WA, USA).
• The total bupivacaine dose (intrathecal plus block) will be kept within the recommended maximum of 2 mg/kg; the dose will be verified against body weight for each patient before injection.
• All patients will receive the same multimodal analgesic regimen: intravenous paracetamol 1 g every 8 hours. Rescue analgesia will be intravenous nalbuphine 5 mg, administered slowly when the Numerical Rating Scale (NRS) pain score is 4 or more.
• Intralipid emulsion 20% and full resuscitation facilities will be immediately available for management of local anaesthetic systemic toxicity.
• All postoperative pain and motor assessments will be performed by a single anaesthesiologist who is blinded to group allocation and who is not involved in randomization, block performance, or intraoperative care.

Primary outcome: Time to first rescue analgesia, defined as the interval (in hours) from completion of the PENG block to the first administration of rescue analgesia (NRS 4 or more), recorded over the first 24 hours postoperatively.

Secondary outcomes:

• Incidence and severity of quadriceps weakness on the operated side, assessed using the Medical Research Council (MRC) scale (0-5) together with the ability to perform a straight leg raise, at 6, 12 and 24 hours postoperatively compared with the preoperative baseline; clinically significant weakness defined as a reduction of one or more MRC grades or inability to perform a straight leg raise.
• Postoperative pain assessed using the NRS (0-10) at rest and on movement at 2, 6, 12 and 24 hours postoperatively.
• Total opioid consumption over the first 24 hours postoperatively.
• Time to first ambulation within the first 24 hours.

Sample size: Using the Power Analysis and Sample Size (PASS) 11 program, results from a previous randomized trial showed that the PENG plus dexamethasone group demonstrated a significantly longer time to first rescue opioid analgesia (15.0 +/- 1.5 hours vs 9.1 +/- 1.7 hours). Based on these findings, a sample size of 30 patients per group achieves 100% power to reject the null hypothesis of equal means when the population mean difference is 5.9 hours with standard deviations of 1.5 for group 1 and 1.7 for group 2, and with a significance level (alpha) of 0.05 using a two-sided two-sample unequal-variance t-test.

Statistical analysis: Data will be coded and analyzed using the Statistical Package for the Social Sciences (SPSS) version 23. The primary outcome (time to first rescue analgesia) will be analyzed using Kaplan-Meier survival analysis with the log-rank test, and a Cox proportional-hazards model will be used to estimate the hazard ratio with 95% confidence interval. Quantitative parametric data will be presented as mean +/- standard deviation (SD) and compared using the unpaired Student t-test; non-parametric data as median (interquartile range, IQR) using the Mann-Whitney test; and qualitative data as number and percentage using the chi-square or Fisher exact test. Repeated measurements over time (NRS, MRC) will be analyzed using a linear mixed-effects model. Analyses will follow both the intention-to-treat and per-protocol principles, with block failure and conversion to general anaesthesia handled as protocol deviations and reported. A p-value < 0.05 will be considered statistically significant.

Ethical considerations: The study will be conducted after approval from the Research Ethics Committee, Faculty of Medicine, Ain Shams University, and will be prospectively registered before enrolment of the first participant. Informed written consent will be obtained from all patients after a detailed explanation of the study procedure, including the off-label use of dexamethasone, the potential risks of the block (nerve injury, infection, local anaesthetic systemic toxicity), and the right to withdraw at any time without affecting their care. The privacy of participants and confidentiality of data will be guaranteed throughout all phases of the study.