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General Considerations
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The posterior approach to sciatic blockade has wide clinical
applicability for surgery and pain management of the lower extremity. In
contrast to common belief, this block is relatively easy to perform and is
associated with a high success rate when properly
performed.17,18 It is particularly well suited for surgery
on the knee, calf, Achilles tendon, ankle, and foot. It provides complete
anesthesia of the leg below the knee with the exception of the medial strip
of skin, which is innervated by the saphenous nerve (Figure 37–6).
When combined with a femoral nerve or lumbar plexus block, anesthesia of
almost the entire leg can be achieved.
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Distribution of Anesthesia
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Sciatic nerve blockade results in anesthesia of the skin of the
posterior aspect of the thigh, hamstrings, and biceps muscles, part of the
hip and knee joints, and the entire leg below the knee, with the exception
of the skin of the medial aspect of the lower leg (see Figure 37–6).
Depending on the level of surgery, the addition of a saphenous or femoral
nerve block may be required.
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Classic Posterior Approach
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Landmarks for the posterior approach to sciatic blockade are easily
identified in most patients (Figure 37–7). Proper palpation
technique is of utmost importance because the adipose tissue over the
gluteal area may obscure these bony prominences. The landmarks are outlined
by a marking pen:
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1. Greater trochanter
2. Posterior superior iliac spine
3. Needle insertion site 4 cm distal to the midpoint between the two
landmarks
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The patient is in the lateral decubitus position with a slight forward
tilt. The foot on the side to be blocked should be positioned over the
dependent leg so that twitches of the foot or toes can be easily noted.
After cleaning with an antiseptic solution, local anesthetic is infiltrated
subcutaneously at the determined needle insertion site. The anesthesiologist
performing the block should assume an ergonomic position to allow precise
needle maneuvering and monitoring of the responses to nerve stimulation.
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The fingers of the palpating hand should be firmly pressed on the
gluteus muscle to decrease the skin–nerve distance (Figure 37–8).
The skin below the index and middle finger is stretched for greater
precision during block placement. The palpating hand should not be moved
during block placement; even small movements of the palpating hand can
substantially change the position of the needle insertion site because the
skin and soft tissues in the gluteal region are highly movable. The needle
is introduced at an angle perpendicular to the spherical skin plane (Figure
37–8). The nerve stimulator should be initially set to deliver 1.5 mA
current (2 Hz, 100 μsec) to allow detection of twitches of the
gluteal muscles and stimulation of the sciatic nerve.
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As the needle is advanced, the first twitches observed are from the gluteal
muscles. These twitches merely indicate that the needle position is still
too shallow. The goal is to achieve visible or palpable twitches of the
hamstrings, calf muscles, foot, or toes at 0.2–0.5 mA current. Twitches of
the hamstrings are equally acceptable because this approach blocks the nerve
proximal to the separation of the nerve branches to the hamstrings
muscle. Once the gluteal twitches disappear, brisk response of the sciatic
nerve to stimulation is observed (hamstrings, calf, foot, or toe twitches).
After the initial stimulation of the sciatic nerve is obtained, the
stimulating current is gradually decreased until twitches are still seen or
felt at 0.2–0.5 mA current. This typically occurs at a depth of 5–8 cm.
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After negative aspiration for blood, 15–25 mL of local anesthetic is
injected (Figure 37–9). Any resistance to the injection of local
anesthetic should prompt needle withdrawal by 1 mm. The injection is then
reattempted. Persistent resistance to injections should prompt complete
needle withdrawal and flushing to ensure needle patency before
reintroduction.
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The continuous sciatic nerve block is an advanced regional anesthesia
technique, and experience with the single-shot technique is recommended to
ensure its efficacy and safety. Continuous sciatic nerve block was described
by Gross in 1956.19 The current technique used is similar
to the single-shot injection; however, slight angulation of the needle in
the caudal direction is necessary to facilitate threading of the catheter.
Securing and maintenance of the catheter are easy and convenient. This
technique can be used for surgery and postoperative pain management in
patients undergoing a wide variety of lower leg, foot, and ankle surgeries.
Perhaps the single most important indication for use of this block is for
amputation of the lower extremity.
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The continuous sciatic block technique is similar to the single-shot
technique. A standard regional anesthesia tray is prepared and an 8–10 cm
long, insulated stimulating needle (preferably Tuohy-style tip) is used.
Proper positioning at the outset and maintenance of the position during the
continuous sciatic nerve block are crucially important to allow for precise
catheter placement. A slight forward pelvic tilt prevents the “sag” of the
soft tissues in the gluteal area and significantly facilitates block
placement.
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With the patient in the lateral decubitus position and a slight forward
pelvic tilt, the landmarks are identified and marked with the pen. After a
thorough skin cleaning with antiseptic solution, the skin at the needle
insertion site is infiltrated with local anesthetic. A 10-cm long
continuous-block needle is connected to the nerve stimulator (1.5 mA) and
inserted at an angle perpendicular to the skin sphere. The opening of the
needle should face distally (pointing toward the patient's foot) to
facilitate catheter insertion. The initial intensity of the stimulating
current should be 1.0–1.5 mA.
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As the needle is advanced, the first twitches obtained are from the
gluteus muscle. Deeper needle advancement results in stimulation of the
sciatic nerve. The principles of nerve stimulation and needle redirection
are identical with those in the single-shot technique. After obtaining the
appropriate twitches, manipulate the needle until the desired response is
seen or felt using a current of 0.2–0.5 mA. At this point, a bolus of local
anesthetic is injected (20 mL) after negative aspiration for blood. This is
followed by insertion of the catheter 5–10 cm beyond the needle tip
(Figure 37–10). Before administering local anesthesia, the
catheter is checked for inadvertent intravascular placement by a negative
test for blood.
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A number of techniques to secure the catheter to the skin have been
proposed. A benzoin skin preparation, followed by application of a clear
dressing and a cloth tape is a simple and often sufices. The infusion
port should be clearly marked as “continuous sciatic block.”
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Continuous infusion is initiated after an initial bolus of
dilute local anesthetic through the catheter. Ropivacaine 0.2% is
commonly used for this purpose (15–20 mL). Diluted solutions of bupivacaine
or l-bupivacaine are also suitable, but can result in
undesirably greater motor blockade. The infusion is initiated at 10 mL/h or
5 mL/h when a patient-controlled analgesia (PCA) dose is planned (5 mL).20,21
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Described by Mansour in 1993, the parasacral sciatic nerve block
features relatively simple landmarks, and it is well suited for continuous
infusion of local anesthetic.8,22–27 In addition, the
extension of the sciatic nerve block has characteristics of a plexus block.
As such, there is a high success rate, anesthesia of the entire sacral
plexus, and motor blockade of the obturator nerve.23,24,28
Ripart25 reports a 94% success rate in his series of
400 parasacral sciatic nerve block cases.
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The parasacral approach to sciatic blockade has a wide clinical
applicability for surgery and analgesia of the lower extremity,
particularly when combined with a femoral or psoas compartment
block.23,28 The technique is associated with a high
success rate and is particularly well suited for surgery on the popliteal
fossa and the knee.23
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Distribution of Anesthesia
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Parasacral sciatic nerve blockade results in anesthesia of the skin of
the posterior thigh, hamstrings, and biceps muscles; part of the hip and
knee joint; and the entire leg below the knee except the medial cutaneous
skin of the lower leg (see Figure 37–6). Morris23
demonstrated extension of anesthesia to the obturator nerve after sciatic
nerve block, as tested by the presence of adductor muscle weakness on a
numeric scale. However, Jochum29 suggested that the obturator nerve
is only occasionally blocked by the parasacral sciatic nerve block. These
conflicting data may be due to the fact that the parasacral plexus could be
responsible for one third of adductor muscle strength. Depending on the
level of surgery, the addition of a psoas compartment or femoral nerve block
may be required.
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Landmarks for the parasacral approach to sciatic blockade are easily
identified in most patients (Figure 37–11). Proper palpation
technique is of the utmost importance because the adipose tissue over the
gluteal area may obscure these bony prominences (Figures 37–12 and 37–13). The following landmarks are outlined by a marking pen:
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- Posterior–superior iliac spine (PSIS)
- Ischial tuberosity (IT)
- A line between the PSIS and the IT is drawn. The needle insertion point lies
6 cm caudad to the PSIS on this line. The insulated needle is inserted at
this point and advanced in a sagittal plane.
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The patient is positioned in a lateral decubitus position, similar to
the position required for the classic posterior approach to sciatic block
(Figure 37–13). The dependent limb at the knee and hip is kept straight,
and the limb to be blocked is flexed at both the hip and knee. Appropriate
sedation and analgesia are mandatory to ensure the patient's comfort
throughout the procedure. After cleaning with an antiseptic solution, local
anesthetic is infiltrated subcutaneously at the determined needle insertion
site. The practitioner performing the block should assume an ergonomic
position to allow precise needle maneuvering and monitoring of the responses
to nerve stimulation.
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The needle is inserted perpendicular to the skin and advanced slowly
(Figure 37–14). The motor response of the sciatic plexus is
usually obtained at a depth between 6 and 8 cm. The goal is to achieve
visible or palpable twitches of the hamstrings, calf muscles, foot, or toes
at the current intensity of 0.2–0.5 mA. Twitches of the hamstrings are
equally acceptable because this approach blocks the sciatic nerve proximal
to the separation of the nerve branches to the hamstring muscles. The
distal motor response may be either a tibial or a peroneal response with
equal success of sciatic nerve blockade (Figure 37–15). Extension
or flexion of the toes (responses of the common peroneal
nerve and tibial nerve, respectively) rarely occur deeper than 8 cm. It is not necessary
to stimulate both the common peroneal nerve and the tibial nerve; either
response is adequate.30,31
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Once the aforementioned distal motor response is obtained with
low-intensity stimulation (<0.5 mA), the local anesthetic solution
is injected slowly while performing frequent aspiration tests. Twenty to 25
mL of local anesthesia is sufficient to produce sciatic nerve blockade
(Figure 37–16).
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Cuvillon et al.7 compared the parasacral sciatic nerve
block with Winnie's approach, eliciting one or two stimulations. Winnie's approach
with double-injection technique required more time to perform the block
compared to Winnie's single-injection technique and the parasacral method.
Although the onset of sensory and motor blocks were significantly faster
with the double-injection method, the additional time needed to perform the
double-injection block eliminated the advantage of the faster onset.
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Continuous Parasacral Sciatic Nerve Block
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The continuous parasacral sciatic nerve block is similar to the
single-shot injection; however, slight caudad angulation of the needle is
necessary to facilitate threading of the catheter. Securing and maintenance
of the catheter are easy and convenient.32 This technique
can be used for surgery and postoperative pain management in patients
undergoing a wide variety of knee, lower leg, foot, and ankle surgeries.
Perhaps the single most important indication for use of this block is for
cancer surgery of the lower extremity and for knee surgery.
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Several kits for catheter insertion are currently commercially
available. An ideal kit should include an insulated needle with a short
bevel, a stimulating needle for catheter insertion, a catheter, an
electrical wire connection, and an antibacterial filter (Figure
37–17). With the patient in the lateral decubitus position, the landmarks
are identified and marked with a pen. After a thorough skin cleaning with an
antiseptic solution, the skin is infiltrated with local anesthetic at the
needle insertion site. A continuous-block needle is connected to the nerve
stimulator and inserted at an angle perpendicular to the skin sphere. The
opening of the needle should face distally (pointing toward the patient's
foot) to facilitate catheter insertion.33 The initial
intensity of the stimulating current should be 1.0–1.5 mA.
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As the
needle is advanced, twitches of the gluteus muscle are obtained
first. Deeper needle advancement results in stimulation of the piriformis
muscle first, then the sciatic nerve. The principles of nerve stimulation
and needle redirection are identical with those of the single-shot
technique. After obtaining the appropriate twitches, manipulate the needle
until the desired response is seen or felt using a current of 0.2–0.5 mA.
At this point, a bolus of local anesthetic is injected (20 mL) after
negative aspiration for blood (Figure 37–18). This is followed by
insertion of the catheter tip. Before
administering the local anesthetic, the catheter is checked for inadvertent
intravascular placement. If confirmation of catheter placement is desired,
contrast media can be injected through the catheter and radiographic images
can be studied.31 The presence of a spindle 2–3 cm in
length with an oblique orientation crossing the sciatic notch on the
anteroposterior radiograph and/or shadowing of the sacral roots are
considered to indicate injection into the correct plane and adequate
placement of the catheter (Figure 37–16). For continuous infusion,
ropivacaine 0.2% is maintained at 5 mL/h and a PCA dose is often planned
(5 mL, lock-out time 30–45 minutes).
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