The effect of needle dimensions and infusion rates on injection pressures in regional anaesthesia needles: a bench-top study

Accuracy of injection pressure measurement at peripheral nerves using high-resolution 40 MHz ultrasound in an anesthetized porcine model

BACKGROUND

Fluid injection pressure measurement is promoted as a marker of needle tip position that discriminates between tissue layers. However, clinical ultrasound has insufficient resolution to identify the exact position of the needle tip. Our primary objective was to use 40 MHz ultrasound in anesthetized pigs in order to precisely locate the tip of the needle and measure opening injection pressure in muscle, at epineurium and in subepineurium.

METHODS

We surgically exposed the axillae of four anesthetized pigs. Two operators placed a 40 MHz ultrasound transducer over the pectoral muscle and imaged axillary, median and radial nerves. Injections (0.5 mL) were randomized to in-plane and out-of-plane needle trajectories and flow rates of 1, 6 and 12 mL/min.

RESULTS

We identified 541 fascicles in 23 nerves. The ratio of fascicle area to nerve area remained constant at ~0.30 for all nerves. Axillary nerves were smaller than median and radial nerves, difference in diameter (95% CI) 1.61 (0.87 to 2.36) mm, p<0.001 and 1.59 (0.82 to 2.36) mm, p=0.001, respectively. Axillary nerves had less fascicles per nerve than median nerves, difference 7.63 (2.43 to 12.83) and radial nerves, difference 9.02 (3.64 to 14.40). We visualized the circumneurium and injection within the subcircumneural compartment. Intraneural injection increased nerve area (SD) from 5.7 (2.2) mm2 to 13.7 (5.5) mm2, difference 8.0 (5.4-10.6) mm2, p<0.001. Mean injection pressure was greater in subepineurium compared with muscle, geometric ratio 2.29 (1.30 to 4.10), p<0.001; and greater on epineurium compared with muscle, geometric ratio 1.73 (1.03 to 3.00), p=0.01. Twenty-two out of 23 injections in muscle, 14 out of 23 injections at epineurium and 11 out of 22 injections in subepineurium were <138 kPa (20 psi).

CONCLUSION

Needle tip position was not discernible using pressure monitoring. The circumneurium and subcircumneural injection compartment were observed but not intrafascicular injection.

"Knowing It Before Blocking It," the ABCD of the Peripheral Nerves: Part C (Prevention of Nerve Injuries)

"A clever person solves the problem. A wise person avoids it" (Albert Einstein). There is no convincing evidence that any modality 100% effectively prevents nerve injury. The risk of nerve injury remains the same even with the ultrasound due to limitations in the resolution of images and inter-operator and inter-patient differences. In a nutshell, caution is required when dealing with precious nerves in the perioperative period, either during peripheral nerve blocks (PNBs), patient positioning, or surgery. Identifying pre-existing nerve injury, either due to trauma or an existing neuropathy, and preventing further nerve injury should be an important goal in providing safe regional anesthesia (RA). Multimodal monitoring is key to avoiding multifactorial nerve injuries. The use of triple guidance (ultrasound + peripheral nerve stimulator + injection pressure monitor) during PNBs further improves the safety of RA. The ultrasound helps in real-time visualization of the nerve, needle, and drug spread; the peripheral nerve stimulator helps confirm the target nerves; and the injection pressure monitor helps avoid nerve injury. Such multimodalities can also give the confidence to administer PNB without risk of nerve injury. This article is part of the comprehensive overview of the essential understanding of peripheral nerves before blocking them. It describes various preventive measures to avoid peripheral nerve injuries while administering PNBs. It will help readers understand the importance of prevention in each step to avoid perioperative PNIs.

Flexible Needle Steering with Tethered and Untethered Actuation: Current States, Targeting Errors, Challenges and Opportunities

Accurate needle targeting is critical for many clinical procedures, such as transcutaneous biopsy or radiofrequency ablation of tumors. However, targeting errors may arise, limiting the widespread adoption of these procedures. Advances in flexible needle (FN) steering are emerging to mitigate these errors. This review summarizes the state-of-the-art developments of FNs and addresses possible targeting errors that can be overcome with steering actuation techniques. FN steering techniques can be classified as passive and active. Passive steering directly results from the needle-tissue interaction forces, whereas active steering requires additional forces to be applied at the needle tip, which enhances needle steerability. Therefore, the corresponding targeting errors of most passive FNs and active FNs are between 1 and 2 mm, and less than 1 mm, respectively. However, the diameters of active FNs range from 1.42 to 12 mm, which is larger than the passive steering needle varying from 0.5 to 1.4 mm. Therefore, the development of active FNs is an area of active research. These active FNs can be steered using tethered internal direct actuation or untethered external actuation. Examples of tethered internal direct actuation include tendon-driven, longitudinal segment transmission and concentric tube transmission. Tendon-driven FNs have various structures, and longitudinal segment transmission needles could be adapted to reduce tissue damage. Additionally, concentric tube needles have immediate advantages and clinical applications in natural orifice surgery. Magnetic actuation enables active FN steering with untethered external actuation and facilitates miniaturization. The challenges faced in the fabrication, sensing, and actuation methods of FN are analyzed. Finally, bio-inspired FNs may offer solutions to address the challenges faced in FN active steering mechanisms.

How Much Does Filler Apparatus Influence Ease of Injection (and Hence, Potential Safety)?

PURPOSE

To document the relative contributions of intrinsic filler fluid dynamics versus delivery systems for ease of injection-specifically, to measure extrusion force variability across different syringes and needles (with the characterization of intrinsic rheological fluid properties vs. delivery apparatus contributions to ease of injection).

METHODS

Six different fillers were tested: Belotero balance (Bel), Juvederm Voluma XC (Vol), Revanesse Versa (Rev), Restylane Lyft (Res), Radiesse (Rad), and Teosyal RHA3 (RHA). Extrusion force was measured in Newtons (N) for each by testing using the provided injection apparatus (needle + syringe), and also by standardizing all fillers to the same syringe and then varying needle sizes (30-ga, 27-ga, 25-ga/1.5-inch, 25-ga/2-inch, and 22-ga). Five trials were conducted for each scenario, with comparison via t -test (2-tailed, unpaired, assuming unequal variance).

RESULTS

The following results were noted: (1) in order of least to highest extrusion force in box-provided syringe + needle at 0.2 ml volume, the following were noted: Vol < RHA = Bel (27-ga) < Bel (30-ga) < Rev < Res = Rad; (2) for each filler (except for Vol which was similar), the box-provided syringe involved greater extrusion force than the standardized syringe used in this study (each 1-cc, p < 0.05); (3) for 27-ga and 30-ga needles, after standardization of delivery syringe at 0.2 ml volume, a significant difference was noted (proportional to increasing resistance): Bel = Vol = RHA3 < Res < Rev < Rad (for needles of 30-ga [ p < 0.05] and 27-ga [ p < 0.01]); (4) for testing cannulas after standardization of syringes no reproducible order was noted with increasing resistance when using 25-ga/1.5-inch long, 25-ga/2-inch, and 22g/2-inch cannulae; and (5) confirming expectation (validating study technique), the extrusion force was significantly higher for smaller needles and longer needles.

CONCLUSIONS

The delivery apparatus appeared to be the most influential contributor to filler injection extrusion force, with significant changes in ease of injection correlated to the filler's intrinsic rheological properties, such as viscosity (when standardized to the same syringe for needles tested). Knowledge of such data could influence the injector's ability to maximize patients' safety and clinical results.

Is Jedi Grip efficient and effective in ultrasound-guided peripheral nerve blocking? A prospective, randomized, observer-blinded study

Background

In this prospective, randomized, controlled observer-blinded study, we aimed to compare the efficacy of a single-operator technique called the Jedi Grip and a conventional technique requiring a double operator in ultrasound–guided axillary brachial plexus blocking.

Methods

Ninety-two patients (ASA I–II; aged 18–65 years old) who underwent elective hand, wrist and forearm surgery were randomly assigned to Group Conventional (C) or Group Jedi (J). In both groups, axillary plexus blockade was performed by applying 5 cc of a mixture of 10 cc of 0.5% bupivacaine and 10 cc of 2% prilocaine to the ulnar, radial, median, and musculocutaneous nerves. Parameters such as the performance time and number of needle passes were recorded during the procedure. Subsequently, a blinded observer evaluated and recorded parameters related to the blockade success. The main outcome variables were the performance time and success rate (surgical anesthesia).

Results

The block performance time of the Jedi technique was slightly longer than that of the conventional technique (220 (50), 202 (78) s, respectively) (median (IQR); p = 0.05). No significant difference was found between groups in terms of blocking success; 9 (20%) from the conventional group and 3 (6.4%) from the Jedi group were unsuccessful (p = 0.053). No differences were found in terms of arterial puncture, and no other complications occurred in either group. The motor-sensory block onset and termination times and initial analgesia requirements were similar.

Conclusion

The Jedi technique may be applied safely with similar block success and performance results as the conventional technique.

Effect of needle type on plane block spread in a cadaveric porcine model

INTRODUCTION

Plane blocks are an increasingly common type of regional anaesthesia technique in the perioperative period. Increased spread of local anaesthesia during plane blocks is thought to be related to an increased area of pain coverage. This study sought to assess differences in injectate spread comparing Tuohy needles with standard insulated stimulating block needles.

METHODS

10 Yorkshire-Cross porcine cadavers were used in this study. Immediately following euthanasia, the cadavers underwent bilateral ultrasound-guided transversus abdominis plane (TAP) block injection with radiopaque contrast dye, with one side placed with a 20 g Tuohy needle, and the other side with a 20 g insulated stimulating block needle. Injectate spread was assessed using plain film X-ray and area of spread was measured to compare differences.

RESULTS

All 10 animals underwent successful ultrasound-guided TAP block placement. In all 10 animals, the area of contrast spread was greater with the Tuohy than stimulating needle. Wilcoxon signed-rank test was used to analyse the difference between the groups. The average difference between the two sides was 33.02% (p=0.002).

CONCLUSIONS

This is the first study to demonstrate differences in injectate spread with different needle types. This suggests enhanced spread with Tuohy needle compared with standard block needle, and may encourage its use during plane blocks.

Combination of real-time needle-tip pressure sensing and minimal intensity stimulation limits unintentional intraneural injection during an ultrasound-guided peripheral nerve block procedure: A randomized, parallel group, controlled trial

STUDY OBJECTIVE

Ultrasound guidance does not eliminate the risk of intraneural injection, which must be avoided during PNB. Combining ultrasound guidance (USG), nerve stimulation (NS), and injection pressure monitoring is advocated to prevent nerve injury during PNB. We hypothesized that combining patient-tailored dynamic NS and real-time pressure sensing (RTPS) could reduce the incidence of intraneural injection and nerve puncture during USG PNB compared with a traditional fixed thresholds (Control) procedure.

DESIGN

Randomized, prospective study.

SETTING

Operating room.

PATIENTS

One hundred ASA physical status I to III patients undergoing orthopedic surgery.

INTERVENTIONS

Patient anesthetized using axillary, sciatic or femoral USG PNB were randomized to the PresStim group (Dynamic RTPS and NS set at 1.5 mA then decreased; n = 50) or Control group (fixed thresholds for in-line pressure mechanical manometer and NS at 0.2 mA; n = 50).

MEASUREMENTS

Procedural ultrasound images and videos were recorded, stored and reviewed in random order by two experts in ultrasound-guided PNB blinded to the group. They noted the needle-to-nerve relationship and intraneural injection for all blocked nerves.

MAIN RESULTS

One hundred and twenty-three USG PNBs were performed (56 axillary brachial plexus blocks, 40 femoral nerve blocks and 27 sciatic popliteal nerve blocks); 235 blocked nerves and videos were recorded and analyzed (PresStim, 118; Control, 117). Less paresthesia was noted in the PresStim group (12.7%) compared with the Control group (18.8%). The risk of intraneural injection was significantly higher in the Control group (odds ratio [OR], 17.1; 95% confidence interval [CI], 2.2-135, P = 0.007). The risk of nerve puncture (OR, 22.7; 95% CI, 2.9-175, p = 0.003) and needle-nerve contact (OR, 4.7; 95% CI, 2.4-9.5, p < 0.001) was significantly higher in the Control group than the PresStim group.

CONCLUSIONS

Under the conditions of the study, dynamic triple monitoring combining RTPS, NS and USG decreases intraneural injection and unintentional needle-nerve contact and puncture during a PNB procedure.

Real-time visualisation of peripheral nerve trauma during subepineural injection in pig brachial plexus using micro-ultrasound

BACKGROUND

Nerve damage is consistently demonstrated after subepineural injection in animal studies, but not after purposeful injection in patients participating in clinical studies. There is a need to better visualise nerves in order to understand the structural changes that occur during subepineural injection.

METHODS

We scanned the brachial plexuses of three anaesthetised pigs using micro-ultrasound imaging (55-22 MHz probe), inserted 21 gauge block needles into the radial, median, and axillary nerves, and injected two 0.5 ml boluses of saline into nerves at a rate of 12 ml min^-1. Our objectives were to measure the area and diameter of nerves and fascicles, and to describe changes in nerve anatomy, comparing our findings with histology.

RESULTS

Images were acquired at 42 sites across 18 nerves in three pigs and compared dimensions (geometric ratio; 95% confidence interval; P value). As expected, the nerve cross-sectional area was greater in the proximal brachial plexus compared with the mid-plexus (2.10; 1.07-4.11; P<0.001) and the distal plexus (2.64; 1.42-4.87; P<0.001). Nerve area expanded after 0.5 ml injection (2.13; 1.48-3.08; P<0.001). Using microultrasound, subepineural injection was characterised by nerve and fascicle rotation, uniform, or localised swelling and epineural rupture. Micro-ultrasound revealed a unique pattern suggestive of subperineural injection after a median nerve injection, and good face validity with histology. Histology showed epineural trauma and inflammation to the perineurium.

CONCLUSION

We accurately identified fascicles and real-time structural changes to peripheral nerves using micro-ultrasound. This is the first study to visualise in vivo and in real-time the motion of nerves and fascicles in response to anaesthetic needle insertion and fluid injection.

Validation of the soft-embalmed Thiel cadaver as a high-fidelity simulator of pressure during targeted nerve injection

INTRODUCTION

Although administration of regional anesthesia nerve blocks has increased during the COVID-19 pandemic, training opportunities in regional anesthesia have reduced. Simulation training may enhance skills, but simulators must be accurate enough for trainees to engage in a realistic way-for example, detection of excessive injection pressure. The soft-embalmed Thiel cadaver is a life-like, durable simulator that is used for dedicated practice and mastery learning training in regional anesthesia. We hypothesized that injection opening pressure in perineural tissue, at epineurium and in subepineurium were similar to opening pressures measured in experimental animals, fresh frozen cadavers, glycol soft-fix cadavers and patients.

METHODS

We systematically reviewed historical data, then conducted three validation studies delivering a 0.5 mL hydrolocation bolus of embalming fluid and recording injection pressure. First, we delivered the bolus at 12 mL/min at epimysium, perineural tissue, epineurium and in subepineurium at 48 peripheral nerve sites on three cadavers. Second, we delivered the bolus at using three infusion rates: 1 mL/min, 6 mL/min and 12 mL/min on epineurium at 70 peripheral nerve sites on five cadavers. Third, we repeated three injections (12 mL/min) at 24 epineural sites over the median and sciatic nerves of three cadavers.

RESULTS

Mean (95%) injection pressure was greater at epineurium compared with subepineurium (geometric ratio 1.2 (95% CI: 0.9 to 1.6)), p=0.04, and perineural tissue (geometric ratio 5.1 (95% CI: 3.7 to 7.0)), p<0.0001. Mean (95%) injection pressure was greater at 12 mL/min compared with 1 mL/min (geometric ratio 1.6 (95% CI: 1.2 to 2.1), p=0.005). Pressure measurements were similar in study 3 (p>0.05 for all comparisons).

DISCUSSION

We conclude that the soft-embalmed Thiel cadaver is a realistic simulator of injection opening pressure.

Real-Time Injection Pressure Sensing and Minimal Intensity Stimulation Combination During Ultrasound-Guided Peripheral Nerve Blocks: An Exploratory Observational Trial

BACKGROUND

Nerve damage can occur after peripheral nerve block (PNB). Ultrasound guidance does not eliminate the risk of intraneural injection or nerve injury. Combining nerve stimulation and injection pressure (IP) monitoring with ultrasound guidance has been suggested to optimize needle tip location in PNB. In this prospective observational study, we hypothesized that measured pairs of IP and minimum intensity of stimulation (MIS) might differentiate successive needle tip locations established by high-resolution ultrasound during PNB.

METHODS

For this exploratory study, 240 observations for 40 ultrasound-guided PNBs were studied in 28 patients scheduled for orthopedic surgery. During the progression of the needle to the nerve observed by ultrasonography, the IP was measured continuously using a computerized pressure-sensing device with a low flow rate of solution. Stimulation thresholds and electrical impedance were obtained by an impedance analyzer coupled to the nerve stimulator at 6 successive needle positions. The median (quartile) or mean (95% confidence interval [CI]) was reported. A mixed model analysis was used, and the sample was also explored using a classification and regression tree (CART) algorithm.

RESULTS

Specific combinations of IP and MIS were measured for subcutaneous, epimysium contact, intramuscular, nerve contact (231 mm Hg [203-259 mm Hg] and 1.70 mA [1.38-2.02 mA]), intraneural location (188 mm Hg [152-224 mm Hg] and 0.58 mA [0.46-0.70 mA]), and subparaneural location (47 mm Hg [41-53 mm Hg] and 1.35 mA [1.09-1.61 mA]). The CART algorithm shows that the optimal subparaneural needle tip position might be defined by the lowest pressure (<81.3 mm Hg) and MIS (<1.5 mA) cutoffs.

CONCLUSIONS

Our exploratory study evaluated concepts to generate hypotheses. The combinations of IP and MIS might help the physician during a PNB procedure. A low IP and low MIS might confirm a subparaneural location, and a high IP and a low MIS might be an alert for the intraneural location of the needle tip.

Early detection of intraneural and intravascular injections with real-time injection pressure monitoring in cadavers

BACKGROUND AND OBJECTIVES

Injection pressure monitoring can help detecting the needle tip position and avoid intraneural injection. However, it shall be measured at the needle tip in order to be accurate and reproducible with any injection system and non operator-dependent. With an innovative system monitoring the injection pressure right at the needle tip we show that it is possible to early detect an intraneural and also an intravascular injection.

METHODS

We performed supraclavicular block-like procedures under real-time ultrasound guidance on two fresh cadaver torsos using a sensing needle with an optical fiber pressure sensor within the shaft continuously measuring injection pressure at the needle tip. A total of 45 ultrasound-guided injections were performed (15 perineural, 15 intraneural and 15 intravenous injections).

RESULTS

Mean (SD) injection pressure after only 1 mL injected volume was already significantly higher for the intraneural compared to the perineural injections: 70.46 kPa (11.72) vs 8.34 (4.68) kPa; P < .001. Mean (SD) injection pressure at 1 mL injected volume was significantly lower for the intravascular compared to the perineural injections: 1.51 (0.48) vs 8.34 (4.68) kPa; P < .001.

CONCLUSIONS

Our results show that injection pressure monitoring at the needle tip has the potential to help identifying an accidental intraneural or intravascular injection at a very early stage.

[Distal hand block]

OBJECTIVE

Peripheral nerve blocks in hand surgery.

INDICATIONS

Short operations on the hand.

CONTRAINDICATIONS

Long operations, polyneuropathy, local anesthesia allergy.

TECHNIQUE

Identification of landmarks, skin disinfection, local anesthesia injection, surgery.

POSTOPERATIVE MANAGEMENT

Postoperative monitoring.

RESULTS

We performed 75 hand surgeries using distal nerve blocks over 3 years (between 2015 and 2018) in our department. In all, 60% (n = 45) of cases belong to the innervation region of the median nerve, 28% of cases (n = 21) belong to the innervation area of the radial nerve and about 5% (n = 4) to the innervation area of the ulnar nerve. In 7% (n = 5) of cases, anesthesia of two neighboring innervation areas was necessary. The distal nerve block provides a reliable form of anesthesia in hand surgery with low complication rates.

Intracoronary pressure increase due to contrast injection for optical coherence tomography imaging

BACKGROUND

Optical coherence tomography (OCT) requires intracoronary injection of contrast media to remove blood from the field of view during image acquisition. Contrast injection may cause a temporal increase in intracoronary pressure. The aim of this study was to compare the intracoronary pressure during contrast injection between OCT and coronary angiography.

METHODS

We measured intracoronary pressure by using a pressure guidewire during contrast injection for OCT and angiography in 30 coronary arteries (mean fractional flow reserve = 0.90 ± 0.03). Contrast media was injected into coronary artery through the guiding catheter by using a mechanical injector pump.

RESULTS

Intracoronary pressure before contrast injection was similar between OCT and angiography (systolic pressure: 123 ± 18 mmHg vs. 122 ± 19 mmHg, p = 0.863). Intracoronary pressure was increased due to contrast injection in both OCT (systolic pressure: 123 ± 18 mmHg to 132 ± 18 mmHg, p < 0.001) and angiography (systolic pressure: 122 ± 19 mmHg to 128 ± 19 mmHg, p < 0.001). The increase in intracoronary pressure was slightly greater in OCT compared with angiography (absolute increase of systolic pressure: 9 ± 2 mmHg vs. 6 ± 1 mmHg, p < 0.001; and relative increase of systolic pressure: 8 ± 2% vs. 5 ± 1%, p < 0.001). Intracoronary pressure during contrast injection was not significantly different between OCT and angiography (systolic pressure: 132 ± 18 mmHg vs. 128 ± 19 mmHg, p = 0.831).

CONCLUSIONS

Contrast injection for OCT induced significant but small increase in intracoronary pressure compared with that for angiography.

Evaluation of the B-Smart manometer and the CompuFlo computerized injection pump technology for accurate needle-tip injection pressure measurement during peripheral nerve blockade

Background and Objectives

The exact mechanism of peripheral nerve blocks causing/leading to nerve injury remains controversial. Evidence from animal experiments suggests that intrafascicular injection resulting in high injection pressure has the potential to rupture nerve fascicles and may consequently cause permanent nerve injury and neurological deficits. The B-Smart (BS) in-line manometer and the CompuFlo (CF) computerized injection pump technology are two modalities used for monitoring pressure during regional anesthesia. This study sought to explore the accuracy of these two technologies in measuring needle-tip pressures in a simulated environment.

Methods

In seven simulated needle–syringe combinations, the BS and the CF devices were connected in series through a closed system and attached to a digital manometer at the tip of various needles. The pressures were evaluated in three trials per needle-syringe combination. Sensitivity, specificity, positive predictive value, negative predictive value, and accuracy (F1 Score) were determined for each needle type and overall.

Results

For pressures ≥15 psi and ≥20 psi, respectively, the CF device demonstrated a sensitivity of 100%, 100%; specificity of 96%, 98%; positive predictive value 93%, 93%; and negative predictive value of 100%, 100%. The BS device demonstrated a sensitivity of 60%, 100%; specificity of 99%, 95%; positive predictive value of 96%, 85%; and negative predictive value of 85%, 100%. Accuracy, as measured by the F1 Score, for detecting a pressure of ≥15 psi was 0.96 for the CF and 0.74 for the BS.

Conclusions

Future research is needed to explore in-vivo performance and evaluate whether either of these devices can impact on clinical outcomes.

Landmark Technique for a Wrist Block

Effective anesthesia of the hand and wrist has many uses inside and outside the operating room. In the emergency department or fracture clinic, a wrist block may be used for closed reductions of dislocations and fractures or for effective inspection and treatment of wounds. In the operating room, surgery may be carried out under a wrist block alone or a wrist block may be used as an adjunct to general anesthesia as the block is an opiate-sparing option to facilitate outpatient surgery and to provide many hours of postoperative analgesia, particularly if administered prior to the commencement of surgery. The landmark technique for distal peripheral nerve blocks at the wrist is a well-recognized method and is described for the median nerve, ulnar nerve, superficial branch of the radial nerve, and dorsal branch of the ulnar nerve at the wrist. To make this technique more effective for carpal surgery, blocks of the posterior interosseous and anterior interosseous nerves are added.Step 1: The patient is counseled about the procedure and the expected outcomes.Step 2: Drug allergies are checked.Step 3: The maximum safe dose of the chosen local anesthetic agent is calculated using the weight of the patient.Step 4: The drug ampules are checked for the name and concentration of the drug as well as the expiration date.Step 5: The drug is drawn up into a 10-mL syringe, and a needle is fitted for injection.Step 6: For each of the 6 nerves to be blocked, the anatomical landmarks are identified along with surrounding structures at risk.Step 7: The skin is prepared with an antiseptic agent.Step 8: The nerve block injections are administered using the techniques in the accompanying video, while checking that there are no signs of intraneural or intravascular injection. The landmark technique for wrist block is an effective method in the situations described above. However, there are occasional failures to provide sufficient analgesia as with all peripheral nerve block techniques. There is a possibility of intraneural injection, which must be avoided. There is also a risk of direct nerve fascicular injury with the needle, which therefore requires a thoughtful technique. There is little if any motor block, which makes the technique particularly suitable when intraoperative active motion is required or when physiotherapy is started directly postoperatively. The technique is very rapid to administer, and it does not require any equipment other than a syringe and needle, making it very inexpensive and suitable for austere environments.