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.  

 

16  

If  the  nerve  can  be  followed  further  distally,  it  can  be  seen  to  

split  into  its  common  peroneal  branch  laterally  and  Mbial  branch  

medially.    

17  

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.  

19  

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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