Welcome to another talk in André Boezaart’s series on the
static sonoanatomy for regional anesthesia and acute pain
medicine. With this talk, we will discuss the sonoanatomy of
the sciatic nerve.
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The authors are André Boezaart, a professor of anesthesiology and
orthopaedic surgery at the University of Florida, Division of Acute and
Perioperative Pain Medicine.
And
Barys V. Ihnatsenka, an assistant professor of anesthesiology at the
University of Florida, Division of Acute and Perioperative Pain
Medicine.
The reader is strongly encouraged to systematically duplicate every
one of these images on a model while studying the sonoanatomy and
also to view the movie that covers the dynamic sonoanatomy of this
area.
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The ultrasound probe that was used was a 2-‐ to 5-‐MHz curvilinear
ultrasound transducer probe with a 60-‐mm footprint (C-‐60, SonoSite
Fujifilm, Bothell, WA, USA).
To create the images in this first secMon for the parasacral
approach to the sciaMc nerve, we posiMoned the model
(“paMent”) in the lateral or Sim’s posiMon, with the buQock area
exposed. We placed the ultrasound transducer probe axially on
the ala or wing of the ilium, which brings the gluteus maximus,
medius, and minimus muscles into view as well as the hard line
of the wing of the ilium.
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Sonoanatomy of the parasacral approach to the scia2c nerve.
Lat = lateral; Med = medial; G Max M = gluteus maximus
muscle; G Min M = gluteus minimus muscle; G Med M = gluteus
medius muscle.
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We now slide the ultrasound transducer probe caudally and the sciaMc
notch comes into view, with the body of the ischium bone inferolaterally
and the sacrum medially. If we slide the probe more caudad, the body of
the ilium bone becomes the ischial spine, posteromedially, and
eventually the ischial tuberosity. In addiMon to the gluteus muscles, the
piriformis muscle can also be seen here. The sciaMc nerve starts here at
the confluence of the 4th and 5th lumbar roots and first three sacral
roots. At Mmes, we see spinal roots and other Mmes, the sciaMc nerve,
which renders a high sciaMc nerve block in this area a paravertebral block
with all the aQributes (good and bad) that accompany a paravertebral
block. Chief of these is that the spinal roots may, similar to all spinal
roots, be surrounded by dura, which essenMally creates a sacral
paravertebral and extradural block.
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Sonoanatomy of the scia2c nerve or sacral plexus in the scia2c
notch or sacral paravertebral space.
Inf-‐lat = inferolateral; Post-‐med = posteromedially; G Max M =
gluteus maximus muscle; P-‐f M = piriformis muscle; SP/Sciat
nerve = sacral plexus or scia@c nerve. 6.8 cm is the distance from
the skin in this par@cular model.
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We can now slide the ultrasound transducer probe further distally
and follow the sciaMc nerve down to the area of the body of the
ischium where the “Labat block” or transgluteal sciaMc block is
performed, and even more distally to the subgluteal area, where the
nerve is no longer in a confined space. In the mid-‐buQock or Labat
area, the nerve is sMll in a relaMvely Mght space between the
piriformis muscle and the ischial bone and there are a large number
of blood vessels in the same area as the nerve, making this an
unfavorable area to rouMnely place conMnuous or single-‐injecMon
blocks of the sciaMc nerve. In this posiMon, however, the posterior
cutaneous nerve of the thigh is sMll in close proximity to the sciaMc
nerve. Some important nerves to the hip joint are also located here.
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Sonoanatomy of the scia2c nerve in the subgluteal area.
Lat = lateral; Med = medial; G Max M = gluteus maximus
muscle (inferior edge); Origin of hamstring m = origin of
hamstring muscle. 6.8 cm is the distance from the skin.
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As we move the ultrasound probe further distally, we reach the
subgluteal area where the sciaMc nerve is no longer under the
piriformis muscle, the blood vessels are fewer, and the
posterior cutaneous nerve of the thigh has coursed away from
the nerve. This makes this an ideal and safe posiMon to place
single-‐injecMon and conMnuous blocks of the sciaMc nerve if
posterior cutaneous nerve or hip capsule blocks are not also
sought.
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The posterior cutaneous nerve of the thigh gives off inferior
cluneal nerves and perineal branches that one may or may not
want to avoid blocking. The depth at which the nerve is situated
is again very variable and depends mainly on the amount of
subcutaneous adipose Mssue in this area – very minimal in this
model. The deeper (or more anterior) the nerve is situated, the
more difficult it becomes to visualize with ultrasound. As a rule
of thumb, the nerve will consistently be approximately 45% to
50% of the distance from the anterior inguinal crease to the
posterior subgluteal crease from the skin as shown by Crabtree
et al.
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The posiMon of the nerve has oaen been described as in a
“hammock” of fascia spanning the femur and the ischial
tuberosity. We do not need much imaginaMon to see this in this
image. The nerve courses distally on the quadratus femoris
muscle and further down on the adductor minimus, part of the
adductor magnus, and further down on the adductor magnus
itself. If we follow the nerve down from underneath the
piriformis muscle, it lies from superior to inferior, first on the
superior gemellus muscle, and then on the obturator internus,
and finally on the inferior gemellus muscle before it lies on the
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We can now follow the sciaMc nerve further down to the mid-‐femoral area. This is an especially valuable exercise if the clinical situaMon, paMent habitus, or posiMoning dictates a more distal
approach. Furthermore, if it is essenMal to preserve hamstring funcMon in a paMent for some reason (mobilizaMon, for example), a
more distal approach may be indicated because the nerves to the hamstring muscles branch off of the sciaMc nerve just distal to the subgluteal area – usually just distal to its posiMon on the quadratus
femoris muscle. (When using nerve sMmulaMon to idenMfy the sciaMc nerve, it may be of value to know that the muscular branches of the sciaMc nerve branch off medially. Thus, if one encounters a
hamstring motor response, the sMmulaMng needle needs to be reposiMoned slightly more laterally).
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Sonoanatomy of the scia2c nerve in the mid-‐femoral area.
Med = medial; Post = posterior; Lat = lateral; BFM – long head =
long head of the biceps femoris muscle; VLM = vastus lateralis
muscle; AMM = adductor magnus muscle; SN = scia@c nerve.
(To prepare this image, the model was posiMoned in the Sim’s
posiMon, and the HFL-‐38 ultrasound probe was used.)
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We now follow the nerve further down with the paMent sMll in the
Sim’s posiMon. This view of the sciaMc nerve in the popliteal area can
be obtained from the Sim’s posiMon or from a number of other
posiMons, depending on the clinical situaMon. The images for this
secMon were obtained from the model in the supine posiMon and
her lower leg on a specially designed pillow that allows for the
ultrasound transducer probe to be placed on the posterior aspect of
the leg in the popliteal fossa.
(To prepare this image, the model was posiMoned in the Sim’s
posiMon, and the HFL-‐38 ultrasound probe was used.)
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The sciaMc nerve enters the popliteal fossa and is situated posterior
to the femoral artery aaer it penetrates the adductor hiatus and
becomes the popliteal artery. It varies from one paMent to another,
but the sciaMc nerve is usually a solitary nerve high up in the
popliteal fossa and splits into the Mbial nerve and common peroneal
(someMmes referred to as the common fibular) nerve at around 7 to
9 cm above the popliteal crease. This may be an overesMmaMon of
the distance. The nerve probably splits closer to 3 to 5 cm above the
crease. The medial and lateral sural nerves also originate from the
Mbial and common peroneal nerves, respecMvely, in the popliteal
fossa, but these are seldom seen with rouMne ultrasound scanning.
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Sonoanatomy of the scia2c nerve (popliteal nerve) in the
popliteal fossa behind the knee.
Med = medial; Lat = lateral; SM/ST M = semimembranosus and
semitendinosus muscles; PV = popliteal vein; PA = popliteal
artery; SN = scia@c nerve (popliteal part of…). 3.5 cm is the
distance in cen@meters from the skin.
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If the nerve can be followed further distally, it can be seen to
split into its common peroneal branch laterally and Mbial branch
medially.
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Sonoanatomy of the scia2c (popliteal) nerve in the popliteal
fossa showing the early spli?ng of the nerve.
Med = medial; Lat =lateral; ST/SM M = semimembranosus and
semitendinosus muscles; CPN = common peroneal nerve (also
called common fibular nerve); TN = @bial nerve. 4.0 cm is the
depth in cen@meters from the skin.
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Sonoanatomy of the scia2c nerve (popliteal nerve) in the
popliteal fossa a@er the scia2c nerve has split.
Lat = lateral; Med = medial; CPN = common peroneal nerve; TN
= @bial nerve.
Note that the circumneural sheath (or paraneural sheath) as
described by Karmakar and colleagues cannot clearly be seen
with regular ultrasound equipment as used here.
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Thank you for your attention and we look forward to seeing
you again soon in another talk by André Boezaart in this
series on static sonoanatomy for regional anesthesia and
acute pain medicine topics.
Please be sure to view and listen to the lecture series on the
Must-Know Anatomy for Regional Anesthesia and Acute Pain
Medicine, and watch the movies on all the Dynamic
Sonoanatomy for RA & APM.
All material is protected by copyright to RAEducation.com
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This lecture series was adapted from:
“The Anatomical Foundations of Regional Anesthesia and
Acute Pain Medicine: Macroanatomy, Microanatomy,
Sonoanatomy and Functional Anatomy”
By: André P. Boezaart
Illustrated by: Mary K. Bryson
Published by: Bentham Science (eBooks)
(http://ebooks.benthamscience.com/index.php)
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Other lectures in this series on static sonoanatomy:
1. Sonoanatomy of the posterior triangle of the neck
2. Sonoanatomy of the infraclavicular area
3. Sonoanatomy of the nerves in the axilla and around the
elbow and wrist
4. Sonoanatomy of the nerves in the anterior upper thigh
5. Sonoanatomy of the sciatic nerve
6. Sonoanatomy of the abdominal wall and TAP
7. Sonoanatomy of the thoracic paravertebral space
Also please see other lecture series (Visit RAEducation.com):
1. Dynamic sonoanatomy movie lecture series (14 movies)
2. Must-know anatomy for RA and APM series (18 lectures)
3. Controversial issues in RA & APM series (12+ lectures)
4. High yield continuous nerve blocks movie series (5+
movies)
5. Vintage block movies (Moore) (6 movies)
6. Block movies pre-ultrasound (Boezaart) (16 movies)
7. Functional anatomy movie series (percutaneous nerve
stimulation on painted model) (17 movies)
8. Bigeleisen sagittal section movies (2 movies)
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