Ultrasound-guided anterior approach to sciatic nerve block: a comparison with the posterior approach.

Abstract

Although the anterior approach to the sciatic nerve block has rarely been performed due to lack of reliable surface anatomical landmarks and technical difficulty, ultrasound guidance may make performance of this approach easier. In this study, we evaluated the clinical use of the ultrasound-guided anterior approach to sciatic nerve block and compared this approach with the posterior approach in adults.
One hundred patients undergoing minor knee surgery were randomly divided into two groups to receive anterior and posterior (subgluteal) approaches to sciatic nerve block, using 1.5% mepivacaine 20 mL with epinephrine combined with femoral and lateral femoral cutaneous nerve blocks. Both approaches to sciatic nerve block were performed using a low-frequency, 5 to 2 MHz, curved array transducer. Measurements included block execution time, depth and size of the nerve, needle depth, onset time of sensory and motor blockade, and duration of the block.
The sciatic nerve was located significantly deeper and the needle depth was significantly greater in patients undergoing the anterior approach compared with the subgluteal approach. Both approaches were similar for execution time of sciatic nerve block, but the former took less time than the latter to perform all combinations of blocks. Although sensory block in the posterior femoral cutaneous nerve was achieved less often with the anterior approach compared with subgluteal approach (14.9% and 68.1%, respectively; P < 0.001), there were no differences in success rate, onset time or duration of blockade of the peroneal and tibial nerves between the two groups.
The anterior approach to sciatic nerve block is performed as easily and successfully as the posterior approach using ultrasound guidance.

Full text

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Ultrasound-Guided Anterior Approach to Sciatic Nerve
Block: A Comparison with the Posterior Approach
Junichi Ota, MD
Shinichi Sakura, MD
Kaoru Hara, MD
Yoji Saito, MD
BACKGROUND: Although the anterior approach to the sciatic nerve block has rarely
been performed due to lack of reliable surface anatomical landmarks and technical
difficulty, ultrasound guidance may make performance of this approach easier. In
this study, we evaluated the clinical use of the ultrasound-guided anterior
approach to sciatic nerve block and compared this approach with the posterior
approach in adults.
METHODS: One hundred patients undergoing minor knee surgery were randomly
divided into two groups to receive anterior and posterior (subgluteal) approaches to
sciatic nerve block, using 1.5% mepivacaine 20 mL with epinephrine combined with
femoral and lateral femoral cutaneous nerve blocks. Both approaches to sciatic nerve
block were performed using a low-frequency, 5 to 2 MHz, curved array transducer.
Measurements included block execution time, depth and size of the nerve, needle
depth, onset time of sensory and motor blockade, and duration of the block.
RESULTS: The sciatic nerve was located significantly deeper and the needle depth
was significantly greater in patients undergoing the anterior approach compared
with the subgluteal approach. Both approaches were similar for execution time of
sciatic nerve block, but the former took less time than the latter to perform all
combinations of blocks. Although sensory block in the posterior femoral cutaneous
nerve was achieved less often with the anterior approach compared with subglu-
teal approach (14.9% and 68.1%, respectively; P ? 0.001), there were no differences
in success rate, onset time or duration of blockade of the peroneal and tibial nerves
between the two groups.
CONCLUSION: The anterior approach to sciatic nerve block is performed as easily and
successfully as the posterior approach using ultrasound guidance.
(Anesth Analg 2009;108:660–5)
Sciatic nerve block is used to provide anesthesia or
analgesia in patients undergoing surgery of the lower
extremities. The anterior approach to the sciatic nerve
is performed with the patient in supine position and,
thus, can be advantageous in a situation in which
femoral nerve block is combined. However, since the
sciatic nerve runs along the posterior thigh and behind
the femur, the anterior approach using surface ana-
tomical landmarks has been associated with technical
difficulty1,2and is considered an advanced nerve
block technique.3
Recently, there has been an increase in interest in
the use of ultrasound for peripheral nerve blocks,4,5
including applications using the posterior approach
to sciatic nerve block at the subgluteal level.6,7The
results of a volunteer study showed that the sciatic
nerve can be identified at the lesser trochanter level
anteriorly,8where the anterior approach to the
sciatic nerve block is also performed. However, no
literature has adequately described successful use of
the ultrasound-guided anterior approach to sciatic
nerve block. At this level, the sciatic nerve is situ-
ated more deeply from the anterior aspect of the
thigh and is less visible with ultrasound than the
sciatic nerve in the subgluteal space, making ultra-
sound guidance with the anterior approach difficult.
In this report, we describe clinical use of the
ultrasound-guided anterior approach to sciatic nerve
block. In addition, we compared the quality of the block
and execution time of the anterior approach with the
posterior (subgluteal) approach under ultrasound guid-
ance in patients undergoing minor knee surgery.
METHODS
After IRB approval and written informed consent,
100 patients (ASA I and II) undergoing minor knee
From the Department of Anesthesiology, Shimane University
School of Medicine, Izumo City, Japan.
Accepted for publication July 22, 2008.
Funding and study equipment support was provided from
departmental and institutional sources and SonoSite Japan, KK
(Tokyo, Japan).
Presented, in part, at the Annual Meeting of the American
Society of Anesthesiologists, San Francisco, CA, October 13–17,
2007.
Reprints will not be available from the author.
Address correspondence to Shinichi Sakura, MD, Department of
Anesthesiology, Shimane University School of Medicine, 89-1,
Enya-cho,IzumoCity,693-8501,
ssakura@med.shimane-u.ac.jp.
Copyright © 2009 International Anesthesia Research Society
DOI: 10.1213/ane.0b013e31818fc252
Japan.Address e-mail to
Vol. 108, No. 2, February 2009
660

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surgery, including arthroplasty, menisectomy or me-
niscal repair, were enrolled in the study. Patients with
a history of diabetes mellitus, neurologic disease,
infections at the site of injection or those who could
not tolerate positioning for the anterior approach were
excluded.
All patients fasted for approximately 8 h before
entering an operating room, where an IV infusion of
acetated Ringer’s solution was initiated at a rate of 1 to
3 mL ? kg?1? h?1. Standard noninvasive monitors
were applied, and oxygen was administered via a
facemask. Fentanyl 50 to 100 ?g with or without
midazolam 1 to 2 mg was given IV for anxiolysis as
necessary, while ensuring that patients remained re-
sponsive to verbal cues. Patients were randomly di-
vided by envelope into two groups to receive anterior
and posterior (subgluteal) approaches to sciatic nerve
block combined with femoral and lateral femoral
cutaneous nerve blocks. Both approaches to sciatic
nerve block were performed using a hand-held ultra-
sound device (MicroMaxxUltrasound System, Sonosite,
Bothell, WA) with a low-frequency, 5 to 2 MHz, curved
array transducer (C60e).
Patients in the anterior approach group were
placed in supine position with the hip and knee on the
operated side flexed and the leg externally rotated at
approximately 45 degrees. The ultrasound transducer
was first positioned perpendicular to the skin approxi-
mately 8 cm distal to the inguinal crease (Fig. 1). The
location was then scanned by sliding and tilting the
transducer until a clear transverse image of the hy-
perechoic sciatic nerve located posterior and medial to
the lesser trochanter was obtained (Fig. 2A). After skin
sterilization with an iodine-containing solution and
skin infiltration with 1% mepivacaine, a short bevel
100-mm, 21-gauge insulated nerve block needle
(Stimuplex A, B. Braun Melsungen AG, Germany)
connected to a nerve stimulator (B. Braun Melsungen
AG, Germany) was inserted parallel and inline with
the ultrasound transducer covered with a sterile plas-
tic cover and gel from anteromedial to posterolateral
of the thigh while the sciatic nerve was kept in the
middle of the ultrasound screen. The needle was
advanced slowly under real-time ultrasound guidance
until it was in close proximity to the nerve. A nerve
stimulator with a pulse duration of 0.1 ms and stimu-
lating frequency of 2 Hz was then turned on to elicit
foot plantarflexion or dorsiflexion. The needle was
further adjusted as needed to evoke a motor response
at 0.7 mA or less. A local anesthetic solution of 20 mL
of 1.5% mepivacaine with 1:400,000 epinephrine was
then injected incrementally. The needle-tip was repo-
sitioned so that a circumferential spread of the solu-
tion could be produced (Fig. 2B).
Patients in the posterior approach group were
placed laterally with the side to be anesthetized up-
permost and the hip and the knee on the operated side
flexed at approximately 45 degrees. The ultrasound
transducer was positioned perpendicular to the skin
on the line connecting the ischial tuberosity and
greater trochanter (Fig. 3), and a clear transverse
image of the hyperechoic sciatic nerve between the
Figure 1. Ultrasound transducer and block needle position
for anterior approach to the sciatic nerve block.
Figure 2. The ultrasound images of the sciatic nerve obtained
with the anterior approach before (A) and during (B) the
block are shown in short axis (transverse view). ALM ?
adductor longus muscle; AMM ? adductor magnus muscle;
GMM ? gluteus maximus muscle; LT ? femur (lesser
trochanter); VLM ? vastus lateralis muscle; arrows ? sciatic
nerve; triangles ? needle; LA ? local anesthetic.
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ischial tuberosity and greater trochanter was obtained
(Fig. 4). After skin sterilization with an iodine-
containing solution and skin infiltration with 1%
mepivacaine, the block was conducted with a short
bevel 100-mm, 21-gauge insulated nerve block needle
inserted parallel and inline with the ultrasound trans-
ducer from posterolateral to anteromedial. Injection of
local anesthetic solution was the same as with the
anterior approach. If the sciatic nerve was not visual-
ized with ultrasound, the patient was excluded from
the study.
After sciatic nerve block, patients in both groups
were placed in a supine position with both legs
extended. Femoral nerve and lateral femoral cutane-
ous nerve blocks were also then performed under
real-time ultrasound guidance using a high-frequency,
13 to 6 MHz, linear array transducer (HFL38, Sonosite,
Bothell, WA) as described elsewhere.9,10Local anes-
thetic solutions used for the former and latter blocks
were 0.5% ropivacaine 15 to 20 mL and 1% mepiva-
caine 5 to 10 mL, respectively. All nerve blocks were
performed by anesthesiologists (the authors) who
have a similar amount of experience in each block and
technique used in the study. During surgery, no
additional local anesthetics were administered, but
patients were sedated as requested with bolus IV
injections of midazolam 1 to 2 mg or a continuous IV
infusion of propofol 2 to 6 mg ? kg?1? h?1. If surgical
anesthesia was deemed inadequate during surgery,
bolus IV injections of fentanyl 50 ?g were given at the
discretion of the attending anesthesiologist. If general
anesthesia was required, the patient was excluded
from the study.
Measurements included the depth and the size of
the sciatic nerve, the needle depth (distance from the
skin to the needle tip that was confirmed to be in
contact with the sciatic nerve both with the ultrasound
image and nerve stimulation), block execution time for
the sciatic nerve block (time from the insertion of the
block needle to the end of local anesthetic injection
and withdrawal of the needle), block execution time
for all the blocks (time for sciatic, femoral, lateral
femoral cutaneous nerve blocks from the start of
positioning a patient for the first [sciatic nerve] block
to the end of the third [lateral femoral cutaneous
nerve] block), time required for onset of sensory and
motor blocks of the sciatic nerve and duration of
blockade of the sciatic nerve (time to complete recov-
ery of sensation on the dorsal aspect of the foot and
motor recovery of foot movement). Measurements of
sizes and distances were conducted on the screen by
an internal measuring program of the ultrasound
device.
Sensory and motor blockade on the operated limb
were evaluated every 5 min after injection of local
anesthetic for 30 min, and again immediately and
every 2 h after the completion of surgery. Sensory
examination was conducted on the plantar aspect of
the foot (tibial nerve), the dorsal aspect of the foot
(superficial peroneal nerve), the posterolateral area of
the leg (sural nerve), and the posterior area of the
thigh (posterior cutaneous nerve of thigh) by pinprick
Figure 3. Ultrasound transducer and block needle position
for posterior approach to the sciatic nerve block.
Figure 4. The ultrasound image of the sciatic nerve obtained
with the posterior (subgluteal) approach before (A) and
during (B) the block are shown in short axis (transverse
view). GMM ? gluteus maximus muscle; GT ? greater
trochanter; IT ? ischial tuberosity; QFM ? quadratus femo-
ris muscle; arrows ? sciatic nerve; triangles ? needle; LA ?
local anesthetic.
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(18G). Postoperative sensory examination was not
conducted in the sural nerve and posterior cutaneous
thigh nerve that were covered with a dressing. Sen-
sory block was considered complete when the patient
did not feel pinprick sensation. Motor block was
considered complete when the patient could neither
planarflex nor dorsiflex the foot. All data were col-
lected by an investigator who was not blinded to
group assignment.
Sample size was determined by a power analysis
based on the ability to detect a difference of 25% in the
rate of sensory block with ? set at 0.2 and ? set at 0.05.
A minimal sample of 82 patients (41 in each group)
met these criteria. To accommodate for patient drop-
outs, 100 patients were enrolled in the study. Data
were presented as mean ? sd unless otherwise stated.
Continuous variables were compared with Student’s
t-test. The ?2 analysis was used to compare the
number of patients whose sensory and motor block-
ade was completed 10, 20, and 30 min after the block.
A P value ?0.05 was considered to be statistically
significant.
RESULTS
Of 100 patients enrolled in the study, 94 (47 in each
group) completed the study. Five patients were ex-
cluded because the sciatic nerve could not be identi-
fied with ultrasound; the remaining one patient in the
anterior approach group was excluded from study
due to a protocol violation (Table 1). Both groups were
similar in sex, age, weight, height and physical status
(Table 2).
The sciatic nerve was located significantly deeper in
the anterior approach group than in the posterior
approach group (Table 3). The size of the sciatic nerve
was similar on the ultrasound screen in both groups.
Although entire block needles could rarely be seen
while being advanced in either group, the needle tip
was visualized in most cases or inferred by the move-
ment of the tissue around the needle. When the
contact between the nerve and the needle tip was
confirmed using both ultrasound and nerve stimula-
tion with similar minimum electric currents between
the two groups, the depth of the needle was signifi-
cantly larger in the anterior approach group than in
the posterior approach group. The anterior and posterior
approaches were similar for block execution time for
sciatic nerve block, but the former took less time than the
latter to complete all the three blocks including chang-
ing positions. The circumferential spread of local
anesthetic was observed after injection in all patients
except 5 (4 and 1 patients in the anterior and posterior
approach groups, respectively).
Onset of sensory blockade of the superficial pero-
neal, sural and tibial nerves was similar between
groups (Table 4). Within 30 min after sciatic nerve
block, complete sensory blockade was obtained in at
least 1 of the 3 territories of the sciatic nerve in every
patient except 3 (2 and 1 patients in the anterior and
posterior approach groups, respectively). Both groups
were similar for the percentage of patients who had
complete sensory blockade in all territories of the
sciatic nerve tested at 30 min. Sensory blockade in the
Table 1. Patients Whose Sciatic Nerve was Not Identified
Subject
1
2
3
4
5
Age (yr)
57
60
69
39
59
Height (cm)
153
154
168
172
158
Weight (kg)
60
52
63
79
50
BMI (kg/m2)
25.6
21.9
22.3
26.7
20.0
Sex
F
M
M
M
F
Approach
Anterior
AnteriorfPosterior
Posterior
Posterior
Posterior
Technique
NS
Ultrasound
NS
NS
NS
Subjects 1, 3, 4, 5 received sciatic nerve block using the approach as initially assigned with the help of nerve stimulation (NS). Subject 2 received sciatic nerve block using the posterior approach
with ultrasound after the identification of the sciatic nerve for the anterior approach failed.
BMI ? body mass index.
Table 2. Patient Demographics
Anterior
approach
(n ? 47)
38 ? 20
24/23
163 ? 10
61 ? 11
22.7 ? 2.5
33/14
Posterior
approach
(n ? 47)
38 ? 19
26/21
164 ? 10
61 ? 14
22.7 ? 4.0
24/23
Age (yr)
Sex (F/M)
Height (cm)
Weight (kg)
BMI (kg/m2)
ASA status (I/II)
Values are expressed as the mean ? SD or absolute numbers.
There were no differences between groups.
BMI ? body mass index.
Table 3. Ultrasound Measurements and Block Characteristics
Anterior
approach
(n ? 47)
36 ? 38
5.9 ? 1.1
16.5 ? 4.2
5.8 ? 1.4
7.5 ? 1.2
0.39 ? 0.12
Posterior
approach
(n ? 47)
31 ? 25
3.4 ? 0.7
14.9 ? 3.0
5.2 ? 2.4
6.1 ? 1.1
0.40 ? 0.11
Recognition time (s)
Nerve depth (cm)*
Nerve width (mm)
Nerve thickness (mm)
Needle depth (cm)*
Minimum stimulating current
(mA)
Execution time for SNB (min)
Execution time for all (min)*
Block duration (h)
5.0 ? 1.8
20.6 ? 4.3
9.3 ? 3.5
6.0 ? 3.0
25.0 ? 5.6
9.5 ? 3.9
Values are expressed as the mean ? SD.
Execution time for SNB ? time for sciatic nerve block from the insertion to the withdrawal of
the block needle; Execution time for all ? time for sciatic, femoral, lateral femoral cutaneous
nerve blocks from the start of positioning a patient for the first (sciatic nerve) block to the end
of the third (lateral femoral cutaneous nerve) block.
* P ? 0.05 between groups.
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posterior femoral cutaneous nerve was achieved sig-
nificantly less often in patients in the anterior ap-
proach group than those in the posterior approach
group. Onset of motor blockade was also similar
between the two groups. Two (1 in each group) of the
five patients in whom the circumferential spread of
the local anesthetic was not observed did not show
sensory blockade in any territories examined within
30 min. Intraoperative fentanyl (50–200 ?g) injections
were required by 7 patients in each group because of
tourniquet pain and slight knee pain in two and five
patients in each group, respectively. Block resolution
was similar between groups. No neurological compli-
cations, such as prolonged sensory or motor dysfunc-
tion, were observed postoperatively.
DISCUSSION
In this report, we describe clinical use of the ante-
rior approach to sciatic nerve block under real-time
ultrasound guidance and show that the technique can
be used as easily and successfully as the posterior
approach for minor knee surgery. As expected, the
location of the sciatic nerve for the anterior approach
was deeper than that for the posterior approach.
However, the ultrasound image of the sciatic nerve
was obtained in a similar percentage of patients and
within a similar length of time with either approach.
In addition, time for executing sciatic nerve block was
similar between approaches, resulting in less time for
the combination of blocks required for knee surgery
when the anterior approach was chosen. The resultant
quality of sensory and motor blockade was also simi-
lar except that the anterior approach rarely blocked
the posterior area of the thigh.
The anterior approach to sciatic nerve block was
first described by Beck11in 1963, who used the greater
trochanter as an anatomic landmark. His technique
involved addressing the problem of not being able to
always identify the landmark and, thus, several
groups of researchers have later reported new tech-
niques using different anatomical landmarks, such as
the anterosuperior iliac spine and pubic symphysis
tubercle12or the inguinal crease and femoral artery.13
However, even with landmarks that can be easily
identified, the location of the sciatic nerve varies
among individuals. In addition, since the sciatic nerve
in the anterior approach is deeply located, the block
needle has to travel a long distance, and the needle tip
can easily deviate from the target nerve. In contrast,
ultrasound visualization enabled us to guide the
needle to the nerve before a nerve stimulator was
turned on despite that visualizing the entire needle
was not possible.
A similar rate of visualization of the sciatic nerve
between the two approaches was unexpected, since
the nerve in the anterior approach is located signifi-
cantly deeper. There were only two patients in whom
the sciatic nerve, using the anterior approach, was not
identified. However, the high rate of visualization
may be explained by our use of relatively young and
non-obese subjects enrolled in the study. In elderly
patients, muscles can be atrophic, and the fascia may
not be distinguishable with ultrasound.14,15In obese
patients, the nerve would have been located deeper
and might have been less clearly visualized.16
The results showing that there were no differences
in the onset of sensory and motor blockade of the
sciatic nerve after the block between two approaches
suggest that we can select the approach interchange-
ably for minor knee surgery. Although sensory block
in the posterior femoral cutaneous nerve, which runs
parallel to the sciatic nerve in the gluteal region, was
rarely achieved after the anterior approach, this does
not constitute a disadvantage for knee surgery in
which a thigh tourniquet is used. In our study, the
number of patients with intolerable tourniquet pain
who required fentanyl was similar between groups.
This result corresponds to clinical data showing that
tourniquet pain is not affected by the presence of a
posterior femoral cutaneous nerve block.17
There are some limitations to this study. First,
investigators who collected data were not blinded to
group assignment since most of the measurements
had to be taken during and immediately after sciatic
nerve block. Second, all blocks were conducted by
anesthesiologists (the authors) who were well experi-
enced in ultrasound-guided peripheral nerve blocks,
including the techniques used in the study. Thus,
visualization of the nerve and resultant sensory and
motor blockade may be poorer in a daily clinical
situation.
In conclusion, the present study showed that the
anterior approach to sciatic nerve block is performed
as easily and successfully as the posterior approach
under ultrasound guidance in patients undergoing
knee surgery.
Table 4. Evaluation of Sensory and Motor Blockade
Group
10
min
20
min
30
min
Sensory blockade
Superficial
peroneal nerve
Sural nerve
Anterior
Posterior
Anterior
Posterior
Anterior
Posterior
Anterior
Posterior
4.3%
2.1%
2.1%
2.1%
2.1%
0%
0%
2.1%
66.0%
72.3%
44.7%
51.1%
34.0%
25.5%
10.6%
42.6%*
95.7%
93.6%
78.7%
89.4%
51.1%
51.1%
14.9%
68.1%*
Tibial nerve
Posterior femoral
cutaneous
nerve
Motor blockade Anterior
Posterior
6.4%
0%
36.2%
36.2%
59.6%
78.7%
Percentage of patients with complete sensory blockade of the superficial peroneal, sural,
tibial, and posterior femoral cutaneous nerves and complete motor blockade of the sciatic
nerve 10, 20, and 30 min after sciatic nerve block using the anterior and posterior
approaches.
*P ? 0.05 compared with the anterior approach group.
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