ULTRASOUND GUIDED LUMBAR PLEXUS BLOCK - Usra.ca

ULTRASOUND GUIDED LUMBAR PLEXUS BLOCK
Dr. Manoj Kumar Karmakar
Associate Professor, Director of Paediatric Anaesthesia
Department of Anaesthesia & Intensive Care, The Chinese University of Hong Kong
Prince of Wales Hospital, Shatin, N.T, Hong Kong
e-mail: karmakar@cuhk.edu.hk
Introduction
Lumbar plexus block (LPB), also referred to as Psoas compartment block (PCB), is an advanced
regional anaesthetic technique that is used for anaesthesia or analgesia during hip or lower limb
surgery. During a LPB the local anaesthetic is injected into the fascial plane that is located within
the posterior aspect of the psoas major muscle. This produces complete blockade of the major
components of the ipsilateral lumbar plexus, namely the femoral nerve (FN), lateral femoral
cutaneous nerve (LFC) and the obturator nerve (OBN). The term PCB was originally coined by
Chayen and colleagues. They believed that the branches of the lumbar plexus and parts of the
sacral plexus lay close to each other in a “compartment”, between the psoas major and quadratus
lumborum muscle at the level of the L4 vertebra, and could be identified using “loss of
resistance”. However, recently it has been demonstrated that the lumbar plexus is located within
the substance of the psoas major muscle i.e. at the junction of the posterior third and anterior two
third of the muscle. PCB is also referred to as posterior lumbar plexus block and several
variations of this technique have been described in the literature.
Figure 1. Anatomy of the lumbar plexus with its three
major components - the lateral femoral cutaneous nerve,
obturator nerve and the femoral nerve.
Figure 2. Location of the lumbar nerve roots within the
substance of the psoas muscle and their relation to the
transverse process.
Anatomy
The lumbar plexus is formed by the union of the anterior primary rami of the L1, L2 and L3 and
the greater part of the L4 (Fig 1). The L1 nerve root may also receives a twig from the T12 nerve.
The plexus lies in a fascial plane within the posterior one third of the psoas major muscle (Fig 2-
4). This fascial plane is located between the fleshy anterior two third of the psoas major muscle,
which originates from the anterolateral surface of the vertebral body and the intervertebral
foramen, and the posterior one third of the muscle which originates from the anterior aspect of the
transverse process (Fig 2-4). The lumbar plexus is therefore very closely related to the lumbar
transverse processes. The plexus displays a triangular shape, narrow in its superior portion and
wider in its lower portion. The nerves that originate from the plexus also exhibit a fanned out

Figure 3. Cross sectional anatomical section through the L4 vertebral body and transverse process corresponding to the
level at which the PMOTS-TP (paramedian oblique transverse scan at the level of the transverse process) was
performed. ESM – erector spinae muscle, PM – psoas muscle, QLM – quadratus lumborum muscle, AP – articular
process, LF – ligamentum flavum, ES – epidural space and TP – transverse process.
Figure 4. Cross sectional anatomical section from just inferior to the L4 transverse process and through the lower part
of the L4 vertebral body corresponding to the level at which the PMOTS-ITS (paramedian oblique transverse scan
through the space between two adjacent transverse processes) was performed. Note the intervertebral foramen (IVF)
and the L4 spinal nerve root as it exits the IVF to enter the lumbar paravertebral space (LPVS). Also note the relation
of the L3 nerve root to the L4 nerve root close to the L4 IVF. This is because the L3 lumbar nerve root after it exits the
IVF takes a caudal course through the posterior part of the psoas muscle. PM – psoas muscle, QLM – quadratus
lumborum muscle, AP – articular process, ESM – erector spinae muscle and VB – vertebral body.
distribution, with the LFC being outermost, the OBN innermost and the FN in between. The
depth from the skin to the lumbar plexus also varies with gender and body mass index (BMI).
Ultrasound Guided Lumbar Plexus Block
LPB is traditionally performed using surface anatomical landmarks and there are various methods
of identifying the site for local anaesthetic injection (loss of resistance, paraesthesia or nerve
stimulation). Surface anatomical landmarks are useful but are only surrogate markers and can
vary among patients. This can result in a failure of the block (5-7%), failure to contact the
transverse process or elicit quadriceps muscle contraction resulting in inadvertent deep needle

insertion, renal or vascular injury. Recently there has been an increase in interest in the use of
ultrasound to guide upper and lower extremity blocks with improved outcomes. Ultrasound
guidance during peripheral nerve blocks offers several advantages. Ultrasound imaging is noninvasive,
simple to use and does not involve exposure to radiation. It allows the target nerves and
the surrounding structures to be directly visualized during block placement, which is particularly
advantageous in patients with difficult or variant anatomy and obesity. It also helps to decide on
the best possible site and maximum safe depth for needle insertion, allows real-time guidance of
the needle and needle tip to the target site, avoid inadvertent vascular or visceral injury and
visualize the spread of the injected local anaesthetic in real-time. All this in theory should lead to
fewer needle insertions, improved patient comfort during block placement, reduced
complications, improved quality of block and higher success rates and may be beneficial if used
for LPB.
Kirchmair et al. were the first group to describe the sonoanatomy relevant for LPB. Although
they were unable to visualize the lumbar plexus in the ultrasound scans of the lumbar
paravertebral region in the cadavers and volunteers they were able to accurately guide a needle
(in cadavers) using ultrasound to the posterior part of the psoas muscle, where the roots of the
lumbar plexus are located. Kirchmair and colleagues attribute their failure to visualize the lumbar
plexus to a loss of spatial resolution at depths due to the use of low frequency ultrasound
transducers. However in a recent report where we performed a paramedian sagittal scan of the
lumbar paravertebral region we were able to visualize parts of the lumbar plexus within the
substance of the psoas muscle in some of our patients. We attribute this to recent improvements
in ultrasound technology, improved image processing capabilities of ultrasound machine and the
use of tissue harmonic imaging (THI) and compound imaging. Despite these encouraging result
published clinical data on ultrasound guided LPB are limited. This may reflect the greater degree
of skill required to perform the ultrasound scan, interpret the paravertebral sonograms and
perform the intervention which is at a depth. Ultrasound guided LPB should therefore be
considered an advanced skill level block and only performed after one acquires the appropriate
level of training and skill. The following section describes our current understanding of
ultrasound guided LPB.
Sonoanatomy Relevant for Lumbar Plexus Block
Basic Consideration
The lumbar plexus is anatomically related to the psoas muscle and therefore is located at a depth.
This necessitates the use of low-frequency US (5-2 MHz) and curved array transducers to image
the lumbar paravertebral region. Low frequency US provides good penetration but lacks spatial
resolution at the depths (5-9 cm) at which the anatomy relevant for LPB is located. The latter
often compromises the ability to locate the lumbar plexus nerves within the psoas muscle.
However, recent improvements in US technology, image processing capabilities of US machines,
the availability of compound imaging and tissue harmonic imaging (THI), and the use of new
scan protocols have significantly improved our ability to image the lumbar paravertebral region.
Today, we are not only able to delineate the lumbar plexus nerve roots but also the adjoining
paravertebral anatomy.
Scanning Techniques
A scan for LPB can be performed in the transverse (transverse scan) or longitudinal (sagittal
scan) axis (Fig 5). The author prefers to position the patient in the lateral position with the side to
be blocked uppermost. The US scan can also be performed with the patient in the prone position
but the disadvantage is impaired visualization of the quadriceps muscle contraction. Since low
frequency US is used it is not always possible to accurately delineate the lumbar plexus nerve

oots. Therefore it is the authors opinion that nerve stimulation must always be used in
conjunction with ultrasound during ultrasound guided LPB to locate the lumbar plexus. Advanced
scanning modalities such as THI (tissue harmonic imaging) and compound imaging, which
Figure 5. Figure showing how the patient is positioned during the ultrasound scan (Fig 1A) and how the ultrasound
beam was projected during the sagittal and paramedian oblique transverse scans (PMOTS). The position of the US
transducer and the plane of the US beam have been superimposed on transverse anatomical slices, that were obtained
from the Visible Human Project ® male dataset to illustrate where the US scans were performed. A – midline, B –
intercristal line, C – sagittal scan line, X – a point 4 cm from the midline, SS – sagittal scan (Fig 1B), PMOTS-TP -
paramedian oblique transverse scan at the level of the transverse process (Fig 1C), PMOTS-ITS – paramedian oblique
transverse scan through the gap between two adjacent transverse processes (Fig 1D).
improve image quality, should be used when feasible. Liberal amount of ultrasound gel is applied
to the skin over the lumbar paravertebral region for acoustic coupling. The ultrasound transducer
is positioned approximately 3-4 cm lateral and parallel to the lumbar spine over the sagittal scan
line (Fig 5). The orientation marker of the transducer is directed cranially, for a longitudinal scan
and 4-5 cm lateral to the spinous process, with its orientation marker directed laterally, for a
transverse scan (Fig 5). This author also prefers to orient the transducer slightly medially so as to
produce a paramedian oblique transverse view of the lumbar paravertebral region (Fig 5C & 5D).
Also during a paramedian oblique transverse scan (PMOTS) of the lumbar paravertebral region
the ultrasound beam can be insonated either at the level of the transverse process (PMOTS-TP,
Fig 5C) or through the gap between the two adjacent transverse process (i.e. the inter-transverse
space, ITS) producing a PMOTS-ITS scan (Fig 5D). Since the acoustic shadow of the transverse
process obscures the posterior part of the psoas muscle during a PMOTS-TP one should perfrom
a PMOTS-ITS for optimal imaging of the lumbar plexus (authors unpublished data). The
ultrasound image is optimized by making the following adjustments on the ultrasound system: (a)
selecting an appropriate preset (abdominal preset), (b) setting an appropriate scanning depth (11-
13 cm), (c) selecting the “General” (mid range) frequency of the broadband transducer, (d)
adjusting the “focus” to the depth corresponding to the area of interest, (e) selecting the THI and
compound imaging option, (f) choosing an appropriate map or filter and (f) finally manually

adjusting the “gain”, “dynamic range” and “compression” settings to obtain the best possible
image.
Longitudinal Scan of the Lumbar Paravertebral Region
On a longitudinal sonogram, the lumbar transverse processes are identified by their hyperechoeic
reflection and an acoustic shadow distal (anterior) to them (Fig 6), which is typical of bone. The
transverse processes are located by initially identifying the sacrum, which appears as a flat
hyperechoeic band with a large acoustic shadow anteriorly, and then the “L5/S1 gap” the
intervertebral space between the L5 and S1 vertebra. Once the L5 transverse process is located
the other lumbar transverse processes are identified by counting them from below upwards. The
transducer is then positioned over the L2, L3 and L4 transverse processes. The acoustic shadow
of the transverse processes produce what we refer to as the “trident sign” (Fig 6-7) because of its
Figure 6. Sagittal sonogram of the lumbar paravertebral region showing the “Trident sign”. Note the lumbar plexus is
seen in the posterior part of the psoas muscle. PM – psoas muscle, ESM – erector spinae muscle.
Figure 7. Sagittal sonogram of the lumbar paravertebral region showing the hyperechoeic transverse processes and
their acoustic shadows, which produces the “Trident sign”. The psoas muscle is seen in the acoustic window.

similarity to the trident (Latin for tridens or tridentis) that is often associated with Poseidon (the
God of the sea in Greek mythology) and the Trishula of the Hindu God Shiva. The psoas muscle
is seen, through the acoustic window of the trident, as multiple longitudinal hyperechoeic
striations against a hypoechoeic background typical of muscle (Fig 6-7). In some patients the
roots of the lumbar plexus is seen as a longitudinal hyperechoeic structure in the posterior part of
the psoas muscle (Fig 6-7). However one must bear in mind that not all hyperechoeic shadows or
striations within the psoas muscle are nerve roots because the psoas muscle is known to contain
intramuscular tendons which also produce hyperechoeic shadows. Nevertheless as described
above the nerve roots are sonographically distinct from the muscle fibers. They are also thicker
than the muscle fibers and take an oblique course through the muscle and are better seen after the
local anaesthetic injection. A laterally positioned transducer produces a “sub-optimal” scan
without the ultrasound “trident” and the lower pole of the kidney, which lies anterior to the
quadratus lumborum muscle (QL) and can reach the L3-4 level in some patients, can be seen (Fig
8).
Figure 8. Longitudinal sonogram of the lumbar paravertebral region with the transducer positioned laterally showing a
suboptimal scan for LPB. Note the ultrasound trident is no longer visible and the lower pole of the kidney is now
visible.
Transverse Scan of the Lumbar Paravertebral Region
Kirchmair and colleagues were the first to describe the detailed transverse sonoanatomy of the
lumbar paravertebral region relevant for LPB. However they were unable to delineate the lumbar
plexus in the cadavers and volunteers that they had examined, which they attributed to a loss of
spatial resolution due to the use of low frequency ultrasound. We, at the Chinese University of
Hong Kong, have been using a modified transverse scan protocol to image the lumbar
paravertebral region and have been successful in identifying the lumbar plexus in majority of our
patients. In our transverse scan technique the ultrasound transducer is positioned 4-5 cm lateral to
the lumbar spinous process at the L3L4 level, with its orientation marker directed laterally. The
transducer is also tilted slightly medially so as to perform a paramedian oblique transverse scan of
the lumbar paravertebral region (PMOTS, Fig 5C & 5D).

Figure 9. Paramedian oblique transverse scan of the right lumbar paravertebral region. Note the IVC anterior to the
vertebral body. Picture in the inset shows the slight medial orientation of the transducer. VB – vertebral body, PM –
psoas muscle, QLM – quadratus lumborum muscle and IVC – inferior vena cava.
Figure 10. Paramedian oblique transverse scan of the right lumbar paravertebral region (zoomed view) at the level of
the transverse process (PMOTS-TP). Note how the acoustic shadow of the transverse process obscures the posterior
aspect of the psoas muscle and the intervertebral foramen. VB – vertebral body, PM – psoas muscle, QLM – quadratus
lumborum muscle, ESM – erector spinae muscle and IVC – inferior vena cava.

Figure 11. Paramedian oblique transverse scan of right lumbar paravertebral region through the space between two
adjacent transverse processes (PMOTS-ITS). Note the lumbar nerve root as it emerges from the intervertebral foramen
to enter the hypoechoeic lumbar paravertebral space (LPVS) and the posterior aspect of the psoas muscle. APFJ –
articular process of the facet joint, PM – psoas muscle, ESM – erector spinae muscle, QLM – quadratus lumborum
muscle, ESM – erector spinae muscle, IVC – inferior vena cava and VB – vertebral body.
Figure 12. PMOTS-ITS of the right lumbar paravertebral region (zoomed view). Note the lumbar nerve root as it exits
the intervertebral foramen (IVF) and the color Doppler signal of the dorsal branch of the lumbar artery in the posterior
part of the psoas muscle (PM). APFJ – articular process of the facet joint, LPVS – lumbar paravertebral space and VB
– vertebral body.

On a typical paramedian oblique transverse sonogram of the lumbar paravertebral region the
erector spinae muscle, the transverse process, the psoas major muscle, quadratus lumborum
muscle and the anterolateral surface of the vertebral body are clearly visualized (Fig 10-12). The
psoas muscle appears hypoechoeic but areas of hyperechogenicity are interspersed within the
central part of the muscle (Fig 10, 12). These dots and speckles represent the intramuscular
tendon fibers of the psoas muscle, described above, and are more pronounced below the level of
the iliac crest. The IVC (on the right side) and the aorta (on the left side) are also identified
anterior to the vertebral body (Fig 9-11) and are useful landmarks to look out for while
performing the scan. The lower pole of the kidney, which can extend to the L3 level, is closely
related to the anterior surfaces of the quadratus lumborum and psoas muscle and is seen as an
oval structure that moves with respiration in the retroperitoneal space.
During a PMOTS of the lumbar paravertebral region we have observed (unpublished data) that
when the US beam is insonated over the transverse process (Fig 10) the acoustic shadow of the
transverse process obscures the underlying psoas muscle and the angle between the transverse
process and the vertebral body i.e. the intervertebral foramen (Fig 10). Therefore the lumbar
plexus is rarely identified on the US image in this scan window. However if one performs a
paramedian oblique transverse scan with the US beam being insonated between two adjacent
transverse processes (PMOTS-ITS, Fig 11) then the acoustic shadow of the transverse process is
no longer a problem and one is able to clearly delineate the psoas muscle, quadratus lumborum
muscle, articular process (AP) and the anterolateral aspect of the vertebral body (Fig 11). The
angle between the APFJ and the vertebral body is where the intervertebral foramen (IVF) is
located (Fig 11). The lumbar nerve root, which is seen as a hyperechoeic structure, can often be
seen to exit the IVF (Fig 11-12) and enter the posterior aspect of the psoas muscle (Fig 11-12).
Occasionally a hypoechoeic area is seen adjacent to the IVF, which we believe is the true lumbar
paravertebral space and contains the lumbar nerve root and the lumbar blood vessels (Fig 12).
Since the lumbar plexus and the paravertebral anatomy are clearly delineated with this approach
we believe the PMOTS-ITS is the optimal window for ultrasound imaging during an ultrasound
guided LPB (unpublished data).
Techniques of Ultrasound Guided Lumbar Plexus Block
Currently there are limited data on ultrasound guided LPB. Two approaches have been described
in the literature.
1. Paramedian Transverse Scan with in-plane needle insertion (Technique 1). Originally
described by Kirchmair and colleagues in cadavers this technique involves performing a
transverse scan of the lumbar paravertebral region to delineate the psoas major muscle (as
described above) at the L3L4 or L4L5 level. It may be difficult to locate the psoas muscle at
the L4L5 level as the iliac crest interferes with transducer placement particularly curved array
transducers with a large foot print (60 mm). As described above the author prefers to perform
a PMOTS-ITS with the patient positioned in the lateral position. Once an optimal image is
obtained (Fig 11) the insulated block needle (nerve stimulating needle) is inserted medial to
the transducer and in the plane of the ultrasound beam (in-plane technique) (Fig 13). This
usually corresponds to a point 4 cm lateral to the midline and at the level of the iliac crest, the
same location where one would insert the block needle during a landmark based LPB. The
needle is slowly advanced under ultrasound guidance to the posterior part of the psoas muscle
and the correct position of the needle tip close to the lumbar plexus (Fig 14) is confirmed by
observing ipsilateral quadriceps muscle contraction. As described above, the lumbar plexus is
not sonographically visualized in all patients but when visualized are seen as a hyperechoeic
structure in the posterior part of the psoas muscle (Fig 11-12) or as it exits the

Figure 13. Ultrasound guided lumbar plexus block with a Paramedian oblique transverse scan (PMOTS-ITS) and inplane
needle insertion (Technique 1). Note the block needle is inserted 4 cm from the midline as one would do while
performing a traditional LPB.
Figure 14. Paramedian oblique transverse scan of the lumbar paravertebral region through the inter-transverse space
(PMOTS-ITS). Since the block needle is inserted in the plane of the ultrasound beam it can be visualized. ESM –
erector spinae muscle, PM – psoas muscle, QLM – quadratus lumborum muscle, APFJ – articular process of the facet
joint, VB – vertebral body and LPVS – lumbar paravertebral space.

Figure 15. Needle nerve contact visualized during an ultrasound guided LPB. (A) PMTOS – paramedian oblique
transverse scan and (B) Paramedian sagittal scan of the lumbar paravertebral region in the same patient. ESM –
erector spinae muscle, PM – psoas muscle, QLM – quadratus lumborum muscle, VB – vertebral body, IVC –
inferior vena cava, and TP – transverse process.
Figure 16. Paramedian oblique transverse scan of the right lumbar paravertebral region (PMOTS-ITS) after an
ultrasound guided LPB. Note the lumbar nerve at the level of the scan is surrounded by the local anaesthetic which is
shown using the white arrow heads. ESM – erector spinae muscle, PM – psoas muscle, QLM – quadratus lumborum
muscle and VB – vertebral body

IVF. Since the block needle is inserted in the plane of the ultrasound beam it can be
visualized (Fig 14). After negative aspiration an appropriate dose of local anaesthetic is
injected in aliquots over 2-3 minutes and the patient is closely monitored. Occasionally
needle nerve contact can be visualized on the ultrasound image during needle insertion or
after the local anaesthetic injection (Fig 15) It is also our observation that the lumbar plexus
is better visualized after the local anaesthetic injection as the hypoechoeic local anaesthetic
surrounds the nerve roots (Fig 16).
2. Paramedian Sagittal Scan with in-plane needle insertion (Technique 2). We have
described this technique in which an insulated block needle (nerve stimulating needle) is
inserted in the plane of the ultrasound beam from the caudal end (Fig 17) with the lumbar
ultrasound trident in view (described above). The aim is to guide the needle through the
acoustic window of the lumbar ultrasound trident, i.e. through the space between the
transverse process of L3 and L4 into the posterior part of the psoas major muscle. Correct
position of the needle tip close to the lumbar plexus is confirmed by observing ipsilateral
quadriceps muscle contraction. After negative aspiration an appropriate dose of local
anaesthetic is injected in aliquots over 2-3 minutes and the patient is closely monitored.
Spread of local anaesthetic in the posterior part of the psoas muscle can be visualized in
real-time and the nerves of the lumbar plexus are also better visualized after the local
anaesthetic injection (Fig 18).
Figure 17. Sagittal sonogram of the lumbar paravertebral region at the level of the transverse processes. Picture in the
inset shows the orientation of the transducer and the direction of needle insertion during ultrasound guided lumbar
plexus block. TP – transverse process

Figure 18. Paramedian sagittal (longitudinal) sonogram of the right lumbar paravertebral region (as in Fig 18) after an
ultrasound guided LPB. Note the spread of the local anaesthetic posterior to the lumbar nerve root in the posterior part
of the psoas muscle. TP – transverse process, ESM – erector spinae muscle and PM – psoas muscle.
Pearls and Pitfalls while performing Ultrasound Guided LPB
The lumbar paravertebral region is highly vascular and contains the ascending lumbar veins and
the lumbar arteries, which can be visualized using Color and Power Doppler ultrasound (Fig 19).
There is also a rich network of blood vessels (arteries and veins) within the substance of the psoas
major muscle (Fig 19C & 19D). The dorsal branch of the lumbar artery is also closely related to
the transverse process and the posterior part of the psoas muscle (Fig 19D) where the lumbar
plexus is located. Therefore this blood vessel may be at risk of needle related injury during LPB
because it is directly in the path of the advancing needle. Considering the rich vascularity of the
lumbar paravertebral region it is not surprising that inadvertent intravascular injection of local
anaesthetic or psoas haematoma have been described after LPB. It is for the same reason the
author believes that one must exercise caution when considering a LPB in patients with mild to
moderate coagulopathy because based on our current understanding it may be considered a
relative contraindication for LPB.
The echo-intensity (EI) of skeletal muscles is significantly increased in the elderly and there is a
strong correlation between EI of muscles and age (EI of the biceps increases 1.8% per year and EI
of the quadriceps increases 1.9% per year). The increase in EI of skeletal muscles with age is due
to age-related changes in the muscle. In the elderly there is a reduction in skeletal muscle mass
(sarcopenia), replacement of the contractile elements in the muscle by fat and connective tissue
and an increase in extracellular water content in the muscle. There is also increase in body fat.
Normally subcutaneous fat, water and skeletal muscle fibers are hypoechoeic but infiltration of
skeletal muscles by fat results in increased muscular EI. This may be due to a change in acoustic
impedance at the surface of the fat cells and an increase in scattering of the ultrasound energy by
the intramuscular fat. Therefore ultrasound images of the lumbar paravertebral region in the
elderly appear whiter and brighter and there is also loss of contrast between the muscle and the
adjoining structures (Fig 20) making it difficult to delineate the lumbar plexus when compared to
that in the young. Therefore ultrasound guided LPB in the elderly can be very challenging. The

same is also true in the obese when excessive fat can make ultrasound imaging of the lumbar
paravertebral anatomy and ultrasound guidance during LPB difficult.
Figure 19. Color Doppler ultrasound imaging of the lumbar paravertebral region. (A). Ascending lumbar vein (ALV).
(B). ALV and the lumbar artery (LA), (C). Dorsal branch of the LA in a paramedian transverse oblique scan (PMTOS)
and (D). Dorsal branch of the LA in a sagittal scan (SS). PMTOS – paramedian transverse oblique scan, SS – sagittal
scan, ESM – erector spinae muscle, PM – psoas muscle, QLM – quadratus lumborum muscle, VB – vertebral body,
IVC – inferior vena cava, and TP – transverse process, APFJ – articular process of facet joint and LPVS – lumbar
paravertebral space.
Figure 20. Paramedian transverse oblique scan of the lumbar paravertebral region in an elderly patient (65 years). Note
the increased echo-intensity of the paravertebral muscles and loss of contrast between the muscles and the surrounding
structures when compared to that in a young subject (Figure 9 & 12). QLM – quadratus lumborum muscle, PM – psoas
muscle, VB – vertebral body and APFJ – articular process of facet joint.

Gadsden and colleagues have recently demonstrated that injection of local anaesthetic under high
pressure (>20 psi) during lumbar plexus block results in unwanted bilateral sensory motor
blockade and a high incidence of neuraxial block. Therefore one must ensure that the injection
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Acknowledgement: All the figures have been reproduced with permission from
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