Sciatic Nerve Blocks

The posterior femoral cutaneous nerve
accompanies the sciatic nerve medially along with the inferior gluteal
artery, and is reliably blocked using this approach.

This block is commonly performed in
combination with a lumbar plexus block for hip procedures. The depth at
which the sciatic nerve is often located is often the same as the one
of the femoral nerve. Consequently, we first perform the lumbar plexus
block.

The sciatic nerve usually innervates no
muscles in the gluteal region. Direct gluteal stimulation on
advancement of the needle indicates that the sciatic nerve is still
deep to the needle tip.

Pelvic splanchnic and distal sympathetic
block are possible given their close proximity to the block site
leading to possible urinary retention.

In morbidly obese patients it is sometime
necessary to use a 15-cm needle. However, we recommend to first start
with a 10-cm needle, unless there is clear evidence that a 15-cm is
required.

Close pudendal nerve proximity may lead
to genital tingling during needle positioning, and anesthesia following
the block. If genital tingling is elicited during needle placement, the
needle should be repositioned more laterally and superficially.

This approach is useful for hip
procedures as it allows for the concurrent block of additional sacral
branches involved in hip joint innervation. Two of these nerves that
exit through the greater sciatic foramen are the superior gluteal nerve
(L4-S1, coursing superior to the piriformis muscle) and the nerve to
the quadratus femoris muscle (L4-S1, coursing anterior to the sciatic
nerve).

Weakness in leg adduction with this block
is due to block of the sciatic branch to the hamstring portion of the
adductor magnus rather than obturator block.

Limit advance of the needle to 2 cm beyond the border of the sacrum to minimize the chance of pelvic organ damage.

A pillow may be placed between the legs at the level of the knee.

Appropriate positioning is critical to establish the proper site for the introduction of the needle.

Already at this level, the sciatic nerve
is separated into the common peroneal and the tibial nerves, and the
posterior femoral cutaneous nerve of the thigh has branched.

The stimulation of the sciatic nerve is almost always preceded by the stimulation of the gluteus maximus.

A bone contact usually indicates that the needle is too lateral.

Stimulation of the piriformis muscle indicates that the needle is too cephalad.

A motor response at the level of the toes increases the likelihood of success.

When patients complain of pelvic
discomfort, it suggests that the needle is too anterior and is going
through the greater sciatic notch.

Because the sciatic nerve is found at a
depth of 8 to 13 cm, no redirection of the needle should be attempted
after it passes the skin to avoid bending the needle.

This approach can be uncomfortable for
the patient and therefore requires appropriate local anesthesia with a
38-mm needle and an appropriate sedation.

This approach is not recommended in anticoagulated patients.

A new posterior approach has been
described in adults: The patient is positioned either prone or in the
lateral position. The site of introduction of the needle is 10 cm
lateral from the midpoint of the intergluteal sulcus.

Positioning of the patient is important,
as with any regional anesthesia technique. The patient is placed in
true lateral position (not Sim’s). Both hips and knees remain slightly
flexed and the buttocks are at a right angle with the bed. This
position aligns the midline of the patient with the horizontal plane of
the bed making it easier to judge the angle of insertion of the needle.

The prone position can be used but it is usually unnecessary and more time consuming.

The sciatic nerve enters the upper
gluteal area describing a downward curve from medial to lateral before
running down parallel to the midline. Thus, it is recommended to
attempt the block in the lower three-fourths of the buttocks because in
the upper fourth the nerve is located less than 10 cm from the midline.

The differences between male and female
pelvises lie in the shape and diameters of the inner pelvis and not in
the total width (bicrestal diameter), which is virtually the same in
both sexes. The female inner pelvis is wider while the male bones are
thicker. In fact, different hormone-dependent patterns of fat
deposition in both sexes explain the perceived differences in pelvis
width.

Deposition of fat in the buttocks does
not affect the position of the sciatic nerve with respect to the
midline but it does affect its depth and its relative position with
respect to the lateral side of the body. In other words, the nerve’s
relative position in the buttocks changes, but not its absolute
position with respect to the midline, which remains fixed at 10 cm.

The gluteus maximus muscle is the only
muscle that covers the sciatic nerve superficially in most of the
gluteal region. In the uppermost area of the buttocks the small
piriformis muscle is also superficial to the nerve.

In our experience any response that can
be elicited from stimulation of the sciatic nerve (e.g., dorsal or
plantar flexion, eversion and inversion) provides a similar success
rate provided the response is obtained at low output.

When reposition of the needle is
necessary it is important to realize that small changes in the angle of
insertion of the needle have a profound impact in the position of the
tip of the needle when dealing with deep structures like the sciatic
nerve. A modest 10° correction angle for example has been calculated to
move the tip of the needle approximately 1.6 cm at a depth of 9 cm.

The gluteal approach as compared with
other sciatic approaches (classic, high lateral) reduces the risk of
misplacements or dislocation of the catheter after surgery and reduces
the risk of vascular puncture.

This block is usually performed in combination with a single or continuous lumbar plexus or femoral block.

A variant of the gluteal approach is the
subgluteal approach (SG). A line is drawn perpendicularly and extending
caudally for 4 cm in the middle of the ischial tuberosity–greater
trochanter line. The end of this line represents the site of
introduction of the needle (SG) (Fig. 12-7).

Gluteal or subgluteal approach is preferred in obese patients, because at this level the sciatic nerve is the most superficial.

Compared with the classic posterior
approach of Labat, this approach, as well as subgluteal, is less
painful because the needle is introduced in the groove between the
biceps femoris and lateral border of quadriceps femoris muscles.

If the femur is contacted, the needle needs to be withdrawn and redirected medially (toward sacrum).

If direct stimulation of gluteus maximus
muscle is noticed (too shallow), needle should be advanced a few
millimeters until distal response with foot movement is recorded.

Hamstring contraction represents either a
direct muscle contraction or the stimulation of tibial nerve, except
the short head of biceps femoris, which is innervated by common
peroneal nerve (and not considered part of the hamstring muscles). This
response is not reliable for the block below the knee. The needle
should be readjusted to elicit a distal (toes) response.

If patient reports painful paresthesia
along the leg, the needle should be withdrawn and redirected to achieve
painless stimulation with movement of the toes with a current of or
below 0.5 mA.

In this approach, the nerve stimulator
has a dual function. First, it helps prevent damage to the femoral
nerve during the first 3 to 5 cm of needle introduction, and second, it
helps to localize the sciatic nerve. If, during the introduction of the
needle, it is found that the needle is directly on the femoral nerve
(presence of movement of the patella with a current of 0.5 mA), the
needle is directed slightly medially before continuing with any further
advancement (Fig. 12-11).

Except for morbidly obese patients
(>100 kg), it is rarely necessary to introduce the 150-mm needle. In
most cases, the nerve is found at an approximate depth of 10 to 12 cm.

Introduction of an insulated needle
through the quadriceps is easy. However, if at a depth of 10 to 12 cm,
the introduction of the needle becomes more difficult, this is a sign
that the needle is very close to the femur. In such cases, it should be
withdrawn to the skin and redirected slightly more medially.

The onset time for a sciatic nerve block
is usually longer than other blocks. It usually requires 20 to 30
minutes or even longer.

Using the anterior sciatic approach also
allows one to block the femoral nerve through the same skin
introduction point. However, to avoid any risk of injury of the femoral
nerve by the insulated needle on its way to the sciatic nerve, it is
recommended to first block the sciatic nerve and then proceed to block
the femoral nerve while retrieving the needle. This technique offers
the advantage of only one stick. However, with such a distal approach
to the femoral nerve, it is not possible to also block the femoral
cutaneous nerve or the obturator (three-in-one block).

Beck first described an anterior
approach. The Chelly and Delaunay approach and the Beck approach use
different anatomic landmarks, but both lead to the same site (Fig. 12-12) and both approach the sciatic nerve above the lesser trochanter.

Van Elstraete et al. described new
landmarks (2.5 cm medial from the femoral artery and 2.5 cm distal from
the inguinal crease). In this case the 150-mm insulated needle
connected to a nerve stimulator is introduced at a 10° to 15° angle
relative to the vertical plane. This latter approach offers the
advantage of being based only on a landmark

found below the femoral crease. Van Elstraete appears to approach the sciatic nerve below the lesser trochanter.

Anterior approach to the sciatic nerve is
especially interesting in morbidly obese and trauma patients who cannot
be moved. However, in patients weighing over 100 kg, the 150-mm needle
may be too short. In these cases, it is possible to approach the
sciatic nerve using the lithotomy approach described by Raj, with the
patient supine and the hip and the knee flexed. This gluteal approach
is based on the identification of the ischial tuberosity and greater
trochanter. The middle of the ischial tuberosity–greater trochanter
line represents the site of introduction of the needle. The needle is
introduced perpendicular to the skin, and stimulation of the sciatic
nerve is produced within 4 to 12 cm (depending on the size of the
thigh) (Fig. 12-13).

This is one of the easier blocks to perform.

This block can also be performed with a 50-mm needle with the patient placed in the lateral position.

The anatomic landmarks can be easily identified by asking the patient to perform an active flexion of the leg.

The best positioning of the leg and foot
can be achieved by either positioning the patient with the foot outside
of the stretcher or placing a pillow under the leg.

Minimal discomfort should be experienced
by the patient because only interstitial fat lies between the skin and
the nerve. If the patient complains of pain, this suggests that the
needle is going through the semi-tendinous/semi-membranous tendons
rather than being medial to them.

It is possible to perform the same block using the lithotomy approach.

For surgery performed at the level of the
knee, especially with the use of a tourniquet placed at the thigh, a
femoral or a lumbar plexus block is required to complete the
anesthesia. For surgery below the knee, especially when a tourniquet is
placed at the level of the calf or below, a saphenous nerve block is
sufficient.

There are at least three techniques that
can be used to perform a saphenous nerve block at the level of the knee
and with the patient in the supine position. First, a 22-gauge, 38-mm
blunt needle mounted on a 10-mL syringe (containing 10 mL of local
anesthetic solution) is introduced perpendicular to the skin and
through the sartorius muscle, 3 to 4 cm cephalad from the medial
femoral condyle. Within 3 cm, a loss of resistance is felt. After
negative aspiration for blood, 10 mL of local anesthetic is injected
slowly, with aspiration for blood after 5 mL. Second, a 50-mm insulated
needle connected to a nerve stimulator (2 mA, 2 Hz, 0.1 ms) is
introduced perpendicular to the skin, 3 cm distally to the medial
femoral epicondyle. Within 2 to 3 cm, the patient should report an
electrical paresthesia in the medial aspect of the leg. The needle
position is adjusted to maintain the paresthesia with a current of less
than 0.4 mA. After negative aspiration for blood, 6 to 8 mL of local
anesthetic is injected slowly. Third, a 25-gauge, 38-mm needle is
introduced 3 cm below the tibial plateau and directed from the middle
to the medial border of the leg. Ten milliliters of local anesthetic
solution is then injected subcutaneously.

This block can be performed at any level
of the popliteal fossa. However, the more cephalad from the popliteal
crease, the closer are the common peroneal and tibial nerves.
Conversely, the more distal, the more separated the nerves become.
Indeed, the tibial nerve runs in the middle of the popliteal fossa
while the common peroneal nerve runs in a lateral direction toward the
head of the fibula.

Variants of the proposed technique
include using the perpendicular line in the middle of the popliteal
crease as a starting point. If no motor response is elicited, the
needle is directed more laterally.

The deep femoral vessels become the
popliteal vessels after passing through the adductor hiatus. The
popliteal vessels, which are initially medial and anterior and
separated from the sciatic nerve, run medial and anterior to the tibial
nerve at the lower part of the popliteal fossa. Consequently, the risk
of vessel puncture is minimal if the needle is kept in the lateral
upper quadrant of the popliteal fossa or if the needle is introduced
above the adductor hiatus (above the popliteal fossa). However, this
latter approach requires a 100-mm needle to be introduced deeper and
through the hamstring muscles, which is more painful to patients.

If the medial aspects of the lower leg
and ankle are involved in the surgery, the addition of a saphenous
nerve block will be necessary to provide complete anesthesia. This can
be achieved by blocking the saphenous nerve selectively above or below
the knee or by performing a femoral nerve block. The latter is
especially recommended if thigh coverage will be used during surgery.

In most individuals the division of the
sciatic nerve into the common peroneal nerve and the tibial nerve
occurs below the suggested level of needle insertion. However, if this
block is performed closer to the knee, a double stimulation and
injection technique accounting separately for both branches may be
necessary to achieve a complete block in the sciatic nerve territory of
the lower leg.

If the local biceps femoris muscle twitch
cannot be elicited, the needle can be inserted in a perpendicular
direction until the femur is contacted. Then the needle is withdrawn
and reinserted in 10° to 15° increments posterior to the horizontal
plane until a sciatic nerve response is obtained.

Plantar flexion (tibial nerve response)
as the elicited motor response may yield a more complete block and a
faster onset time than dorsiflexion (peroneal nerve response).

This approach is a great alternative to
the posterior popliteal and the classic approaches to the sciatic nerve
if the patient cannot be positioned in prone or lateral decubitus
position.

Slightly supporting the knee by placing a small towel under the popliteal fossa allows for easier needle operation.