Although knowledge of classic anatomic descriptions is essential to perform peripheral nerve blocks, it is important to recognize that variations are frequent and that “normal anatomy” is found in only 50% to 70% of cases.
A. Lumbar Plexus (Fig. 11-1)
The lumbar plexus is an anastomotic complex formed by the anterior roots from L1 to L3 and the greater part of L4. It has a triangular form, with its base resting against the lumbar vertebrae and veretrebral transverse process and its apex formed by the union of the third roots with the ascendant rami of the fourth. There is frequent anastomosis between the upper part of the plexus with the subcostal nerve and between the L4 rami and the lumbosacral trunk. As with the brachial plexus, we can describe a pre- or a postfixed plexus with respect to the main anastomosis; however, an important characteristic of lumbar plexus anatomy is its great variability and frequent asymmetry. The best definition is given by Bonniot: “The superior limit corresponds to the first roots where the first collateral of the lumbar plexus originate, and the inferior part is defined by the first roots entirely distributed to the sacral plexus” (1922).
In the majority of cases, the fourth roots give rise to fibers for the three main trunks of the lower limb: the femoral, obturator, and sciatic nerves (lumbosacral trunk). This may explain the effectiveness of the posterior approach of the lumbar plexus at the L4 level for hip surgery.
The lumbar plexus lies anterior to the transverse process between the two parts of the psoas muscle. All of the branches of the lumbar plexus emerge from the psoas muscle and all leave the pelvis, though, not all by the same path. Near the vertebral body, the psoas muscle is divided into two planes: a superficial plane that arises from the lateral faces of the vertebral bodies from T12 to L4 and a deep plane arising from the transverse process of L1 to L5. Within these two planes lie the trunks of the lumbar plexus and the ascending lumbar vein. In addition, the lumbar arteries, which supply the psoas muscle, provide branches for the plexus. The two parts of the muscle then fuse to form the psoas major.
Figure 11-1. Lumbar plexus and sacral plexus nerves.
Besides two small branches—for the quadratus lumborum and the psoas muscles—six branches originate from the lumbar plexus. They are classified either as collateral or terminal branches.
Lumbar Plexus Collateral and Terminal Nerves (Fig. 11-2)
Iliohypogastric Nerve (L1)
The iliohypogastric nerve emerges at the level of the psoas muscle and runs down and laterally along the anterior face of the quadratus lumborum muscle. Close to the iliac crest, it runs between the transversus abdominis and the oblique muscles. It leaves a lateral branch for the anterosuperior part of the buttock, supplying the skin as far as the greater trochanter (55% of cases). Variations are possible, however, including a shorter branch that does not reach the greater trochanter (20%) and a longer branch that extends beyond it (15%). The nerve divides into two branches: an abdominal branch, which supplies the muscles and skin of the inferior part of the abdomen, and an inguinal branch, supplying the skin of the pubis and the scrotum (men) or labium majus (women). The inguinal branch perforates the internal oblique muscle and then runs between the oblique muscles.
Ilioinguinal Nerve (L1)
The ilioinguinal nerve follows the iliohypogastric nerve. Its diameter is inversely proportional to that of the iliohypogastric nerve, and it may be absent in some cases. It gives rise to an abdominal branch, which is frequently anastomosed with the abdominal branch of the iliohypogastric nerve, and the inguinal branch, which supplies the skin of the superior and medial aspects of the thigh and the genital area. It can replace the genital branch of the genitofemoral nerve and all or part of the lateral femoral cutaneous nerve.
Figure 11-2. Lumbar plexus collateral and terminal nerves.
Genitofemoral Nerve (L1-2)
This nerve crosses the psoas muscle and extends along its anterior face before dividing into genital and femoral branches. The genital branch passes through the inguinal canal and supplies the cremaster muscle and the skin of the pubis and scrotum (or labium majus). The femoral branch enters the thigh under the inguinal ligament in the femoral sheath and then supplies the skin of the upper part of the femoral triangle. This branch is probably the most variable of the lumbar plexus. It supplies an important part of the anterior aspect of the thigh. In some cases, it receives an anastomosis from T12, and sometimes it is completely absent, in which case it is replaced by the ilioinguinal and the lateral femoral cutaneous nerve.
Lateral Femoral Cutaneous Nerve (L2-3)
After crossing the psoas muscle, the lateral femoral cutaneous nerve follows an oblique and lateral course between the iliac muscle and the fascia iliaca in the direction of the anterosuperior iliac spine. It provides branches for the peritoneum. It then enters the thigh by passing through or under the inguinal ligament and then divides into a posterior branch, which supplies the skin of the superior and lateral aspects of the thigh, and an anterior branch, which supplies the anterolateral part of the thigh as far as the knee. Variations in the course of this nerve are described in 25% of cases, and it may arise directly from the femoral nerve in 18% of cases. Variability also exists with respect to the superficial territories supplied by this nerve. In 31% of cases, this innervation may extend medially as far as the anterior surface of the thigh. This may represent a possible explanation for failure of anesthesia in this region after femoral block in some patients. The lateral femoral cutaneous nerve is considered by some as a branch of the femoral nerve that leaves the femoral nerve after its origin.
As noted previously, these nerves are highly variable in their courses and their extension territory. These variations explain why anesthesia of the anterior face of the thigh is difficult to anticipate both with a femoral block and with a posterior approach. However, it is important to recognize that all of the cutaneous branches of these nerves are mainly subaponeurotic and therefore can be blocked with a subcutaneous injection at the level of the inguinal crease.
Obturator Nerve (L2-4)
The obturator nerve exits the psoas muscle at the level of the sacroiliac joint. It runs outward and downward close to the ureter and the internal iliac artery. Its position in the pelvis, after it leaves the psoas muscle, explains why extension of anesthesia to this nerve after an anterior approach (three-in-one or iliofascial block) is unlikely. It reaches the obturator groove at the upper part of the obturator foramen and then enters the thigh. It then divides into an anterior and a posterior branch. In 50% of cases, this division occurs in the obturator groove, but it may also occur before or after the groove. In the pelvis, it sometimes provides a ramus for the articular capsule of the hip joint. This represents an additional argument for using a posterior lumbar plexus approach for hip surgery.
This branch runs in front of the adductor brevis and adductor magnus muscles and behind the adductor longus. It innervates the pectineus, adductor longus, gracilis, and adductor brevis muscles. It typically supplies a limited skin territory at the medial surface of the thigh and the knee. However, there are important variations in the location and extent of this area. The frequent anastomosis of the anterior branch with the saphenous and anterior femoral cutaneous nerves in the adductor canal (the subsartorial plexus) is responsible for significant overlaps. It has been established that an effective block of the obturator nerve is associated with no sensory block in 57% of patients in the presence of a reduction of the adduction motor function. For German anatomists the specific sensitive area of the obturator nerve corresponds to a small posteromedial territory at the distal third of the thigh between the adductor gracilis and the adductor magnus. This explains why the determination of the intensity of an obturator block based only on sensory assessment is difficult in the absence of simultaneous assessment of motor function.
This branch runs between the adductor brevis and adductor magnus muscles. It supplies the obturator externus, adductor magnus and adductor brevis muscles, and produces articular branches for the hip and the knee joints.
Accessory Obturator Nerve (L3-4)
This nerve is found in 10% to 12% of cases. It follows the same course as the obturator nerve, except that it enters the thigh over the pubic anterior branch. It supplies the obturator and the pectineus muscles and gives rise to a branch for the hip joint.
Figure 11-3. The femoral nerve to artery relationship is one of the most consistent in the body. The relationship of the nerve lateral to the artery is best approached superior to the extensive branching (arborization) at or just below the inguinal crease.
Femoral Nerve (L2-4) (Fig. 11-3)
The femoral nerve is the largest branch of the lumbar plexus. It exits the lateral side of the psoas muscle and runs in the iliopsoas groove toward the inguinal ligament, beneath which it enters the thigh. In the pelvis, it produces branches for the psoas and iliac muscles. Generally, it divides in four terminal branches at the level of or immediately below the inguinal ligament, although in some cases the division occurs before. In the femoral triangle, there are two planes to be considered. The anterior femoral nerve is located within the anterior plane, which innervates the skin over the front and medial sides of the thigh and also contributes to the subsartorial plexus and branches to the sartorius, pectineus, and sometimes part of adductor longus muscles. The posterior plane consists of the quadriceps femoris and the saphenous nerve. The saphenous nerve also contributes to the subsartorial plexus. It supplies the skin of the medial part of the knee and the anteromedial part of the leg as far as the foot, although in some cases it may extend as far as the base of the first toe. Additionally, the saphenous nerve also plays an important role in the innervation of the knee joint.
There are frequent overlaps concerning the cutaneous regions supplied by some branches of the lumbar plexus, especially the lateral femoral cutaneous and the femoral branch of the genitofemoral nerves, which may explain instances of incomplete superficial blocks.
B. Sacral Plexus (Fig. 11-4)
The sacral plexus is formed by the anterior sacral roots from S1 to S3, and the lumbar roots from L5 are associated with an anastomotic branch from L4 (furcal nerve). The L5 roots and the L4 anastomotic branch form the lumbosacral trunk. The lumbosacral trunk and the sacral roots converge toward the sciatic foramen and merge before entering the buttock. The sacral plexus is shaped like a triangle, with its base lying against the anterior sacral foramina and its vertex corresponding to the anteromedial border of the sciatic foramen. The sacral plexus transverses the sciatic foramen lying anterior to the piriformis muscle and is covered by the pelvic aponeurosis (corresponding to the fascia of the pelvic muscles), which separates it from the visceral structures of the pelvis. In this section only the collateral branches of the sacral plexus for the lower limb are described.
Branches Collateral and Terminal Nerves (Fig. 11-4)
In the buttock, the sacral plexus branches into several nerves supplying the muscles of the region. Some are more relevant for regional anesthesia.
Superior Gluteal Nerve
The superior gluteal nerve leaves the plexus before it crosses the sciatic foramen. It supplies the gluteus medius and gluteus minimus muscles and finishes its course in the tensor fasciae latae muscle. In this regard, it is important to note that it is a branch of the lumbar plexus that provides sensory innervation of the skin over the tensor fasciae latae muscle before it leaves a branch for the hip joint.
Nerve to the Quadratus Femoris and Inferior Gemellus Muscles
This nerve leaves the plexus while crossing the sciatic foramen. In addition to supplying these two muscles, it also produces a small ramus for the hip joint.
Inferior Gluteal Nerve
The inferior gluteal nerve arises directly from L5 to S2. It penetrates the buttock through the lateral border of the sciatic foramen. It may have a common origin with the posterior cutaneous nerve of the thigh, but frequently these two nerves are separated. Beyond the piriformis muscle, the inferior gluteal nerve hooks around the inferior border of the gluteus maximus muscle and supplies it.
Posterior Cutaneous Nerve of the Thigh
This nerve arises directly from S1 to S3. With the inferior gluteal nerve, it penetrates the buttock just beyond the piriformis muscle, and it gives rise to branches for the inferior part of the buttock (inferior gluteal nerve) and the perineal region. It runs between the posterior muscles of the thigh immediately under the fascia lata and gives branches that perforate the fascia and supply the skin of the posterior surface of the thigh as far as the popliteal region. In the popliteal region, it pierces the fascia and divides into two branches: one for the posterior and upper surface of the thigh; and one that follows the small saphenous vein as far as the middle of the calf, where it is anastomosed with the sural nerve.
Figure 11-4. Branches collateral and terminal nerves.
Note that, at the inferior border of the piriformis muscle, the sciatic nerve, the inferior gluteal muscle, and the posterior cutaneous nerve of the thigh are close to each other.
Sciatic Nerve (Fig. 11-5)
The sciatic nerve is the largest nerve in the human body. Although it may be considered the solitary terminal branch of the sacral plexus, at this level it is already composed of two distinct contingents reunited in a common sheath. After passing through the sciatic foramen, it inclines laterally under the gluteus maximus muscle. On its medial side, it is accompanied by the posterior cutaneous nerve of the thigh and the inferior gluteal vessels. It runs anterior to the piriformis muscle, and, at the midpoint between the greater trochanter and the ischial tuberosity, it turns downward toward the thigh. In the thigh, it runs between the biceps femoris muscle (laterally) and the semi-tendinous and semi-membranous muscles (medially) and just behind the adductor magnus. At the vertex of the popliteal fossa or even higher, it separates into tibial and common peroneal nerves. In 15% of cases, division of the sciatic nerve occurs at the level of the piriformis muscle. Several anatomic modalities can be observed, which are classified into three types: (a) both contingents pierce the pyramidal muscle; (b) only the common peroneal contingent pierces the muscle; and (c) one contingent passes over the muscle.
In the buttock, the sciatic nerve leaves a branch for the posterior part of the hip joint capsule. The medial part of the sciatic nerve (tibial contingent) provides a branch that innervates the semi-tendinous and semi-membranous muscles, the long head of the biceps femoris muscle, and the ischiocondylar part of the adductor magnus muscles. In the middle of the thigh, the lateral part of the sciatic nerve (common peroneal contingent) has two branches: one for the short head of the biceps femoris and the other for the posterior and lateral part of the knee joint capsule.
Figure 11-5. This is a fresh tissue dissection of the sciatic nerve from its origin at the sacrum. The pen is at the level of the PSIS which is circled. The needle is 7 cm from that point at the exit of the sciatic nerve. The gluteus maximus has been reflected so the exit of the sciatic nerve is just caudal to the piriformis muscle. As the nerve exits the buttocks it is between the ischial tuberosity and greater trochanter.
Tibial Nerve (Fig. 11-6)
The tibial nerve follows the line of the sciatic nerve through the popliteal fossa and the leg. In the popliteal fossa, it runs laterally to the vessels and follows the median axis of the popliteal fossa. In the leg, it passes downward in an oblique and medial direction, lying between the tibialis posterior muscle, then the flexor digitorum muscles in front and the soleus muscles behind. In the distal third of the leg, the nerve is covered only by skin and fascia. It curves medially behind the medial malleolus and produces two branches: the medial and lateral plantar nerves. In the popliteal fossa, it leaves a branch for the capsule of the knee joint, gives rise to a part of the sural nerve (medial sural cutaneous nerve), and supplies the muscles of the calf. In the leg, it leaves articular branches for the ankle, the tibiofibular joint and bone, and innervates the flexor muscles of the foot and toes. In the ankle and foot region, it supplies the skin of the sole and the intrinsic muscles of the foot.
Figure 11-6. This unaltered fresh tissue dissection is to illustrate the straight course the sciatic nerve takes though the leg. Note that there is no vascular bundle as the artery is much closer to the femur than the sciatic nerve. Just above the knee the branches into the posterior tibial and common peroneal are seen. This branching is usually within 10 cm of the crease of the knee.
Common Peroneal Nerve (Fig. 11-6)
The common peroneal nerve descends along the lateral border of the popliteal fossa and gives a branch to the sural nerve. It winds around the head of the fibula, passes over the neck of the fibula, and then divides into a superficial and a deep branch. The superficial peroneal nerve descends along the lateral aspect of the leg and supplies the skin of the anterior and lateral face of the leg and the dorsum of the foot and lateral peroneal muscles. The deep peroneal nerve supplies the tibialis anterior and extensor muscles. It penetrates the foot between the tendon of the extensor digitorum longus and the extensor hallucis longus muscles, and supplies contiguous sides of the first and second toes.
In the popliteal fossa, the tibial and the common peroneal nerves provide a medial and a lateral sural cutaneous nerve, respectively. The lateral sural nerve has a communicating branch that joins the medial sural cutaneous nerve to form the sural nerve. The lateral sural cutaneous nerve supplies the skin of the lateral aspect of the leg while the sural nerve supplies the skin of the posterolateral part of the leg and the lateral border of the foot.
There are frequent variations: The junction between the medial sural cutaneous nerve and the communicating branch can occur at variable levels, and in some cases can even be absent. In such cases, the communicating branch supplies the lateral malleolus and the external face of the heel. Moreover, the proportion of fibers originating from the medial sural cutaneous nerve and the communicating branch is variable. For this reason, it is probably preferable to consider the sural nerve as an independent nerve formed by the union of two roots: a medial root from the tibial nerve and a lateral root from the common peroneal nerve.
C. Topographic Anatomy
Lumbar Paravertebral Region
This region is limited on its upper margin by the 12th rib, laterally by the external border of the quadratus lumborum muscle, medially by the vertebral body, and on its lower border by the iliac crest. As previously described, the lumbar plexus lies between the two parts of the psoas muscle. After their origin from L1, the iliohypogastric and the ilioinguinal nerves immediately leave the posterior psoas muscle posteriorly. This may explain why a posterior approach of the lumbar plexus at the level of L4-5 may not result in a block of these nerves. The genitofemoral nerve (from L2) has a more prolonged course in the psoas muscle and leaves it anteriorly. Thus, a block of this nerve is probably more likely to be achieved by using a posterior approach. Concerning the three main trunks of the lumbar plexus, dissections have found a fan disposition in the muscle: The lateral femoral cutaneous nerve is lateral, the obturator nerve is medial, and the femoral nerve lies in between, usually in the same plane. However, variations are common. Although the lateral femoral cutaneous and femoral nerves have a constant relationship with one another, the presentation of the obturator nerve is more variable, and frequently it is in a different plane from the one enclosing the other nerves. This variability might explain a limited extension to the obturator nerve, even with a posterior approach.
The L4-5 level is a site frequently chosen for lumbar plexus blocks for two reasons. First, the landmarks are easy to identify, and second, the L4 roots provide fibers for the trunks of both the lumbar and sacral plexus. Moreover, it seems that this approach is associated with a lower rate of complications. One of the landmarks used is the iliac crest, which usually corresponds to the L4-5 space. However, it is important to remember that this line is not an absolute reference: The rate of inaccuracy is close to 50% and corresponds to the normal distribution of an anthropometric variable. Palpation and drawing of the spinal processes are important aids to confirming the correct position of this line. The second line parallel to the spine passes through the posterosuperior iliac spine. Although, the needle is classically introduced at the junction of these two lines, the junction of the lateral one third and medial two thirds between the lines of the spinous process and the posterior iliac spine on the iliac crest line has also been advocated. This modification is important to take into account for two reasons. First, this approach is strictly parallel to the vertebral column, whereas a more lateral approach would require the needle to be redirected medially to reach the plexus (as recommended by Winnie). Such redirection of the needle increases the risk of peridural or spinal placement of the needle. Second, although there is great individual variation in the distance between skin and lumbar plexus (55 mm to 110 mm), the distance between the transverse process and the lumbar plexus is relatively constant at around 18 mm. Therefore, the contact with the transverse process constitutes an important safety marker.
Because of the paravertebral situation of the lumbar plexus, the risk of epidural and even intrathecal extension of a lumbar plexus block cannot be completely eliminated, even with meticulous technique. Moreover, prolongations of the spinal dura mater surrounding the roots may favor an intrathecal diffusion despite a correct approach. This risk must be taken into account when performing this block.
Although posterior lumbar plexus blocks were first performed using a “loss of resistance” technique (psoas compartment block), the use of a nerve stimulator has increased the reliability of this approach. The optimal muscular response is a contraction of the quadriceps with ascension of the patella (Fig. 11-7, green dot). A contraction of the adductor muscles (Fig. 11-7, orange dot) suggests that the needle position is too medial and that the needle should be redirected laterally in search of the proper motor response. In some cases (obese patients) the contraction of the adductor muscles may simulate a stimulation of the femoral nerve. In such patients, only palpation allows the proper diagnostic confirmation. Simultaneous contraction of the quadriceps and adductor muscles suggest that the needle is paravertebral (Fig. 11-7, red dot) with an increased risk of epidural diffusion.
Figure 11-7. The patellar (green dot), adductor muscles (orange dot), and quadriceps (red dot).
The lower part of the kidney corresponds to the middle of L3 at the right and L2-3 vertebral discs at the left. The location of the kidneys explains why renal subcapsular hematomas have been described with a high posterior approach at the L3 level.
Below the inguinal ligament, the femoral nerve runs in the femoral triangle, and it is at this level that the femoral nerve is approached. The femoral triangle is limited laterally by the sartorius muscle and medially by the border of the adductor longus and pectineus muscle. The junction of the sartorius and the adductor longus muscles corresponds to the vertex of the triangle. From the surface downward, there is a subaponeurotic plane, containing lymphatics and vessels, followed by two aponeurotic planes: the fascia lata, stretching between the sartorius and adductor longus muscle, and the aponeurosis of the psoas major, iliacus, and pectineus muscles: the fascia iliaca. Because it is pierced with several openings through which vessels and nerve pass, the fascia lata in the femoral triangle is also known as the cribriform fascia. The femoral nerve is under the fascia iliaca, but the femoral artery and vein lie between the two fasciae. Therefore there is no common vascular–nervous sheath at this level. It is also important to recognize that although the femoral nerve and vessels are clearly separated in the femoral triangle, there are superficial (subaponeurotic) and deep (between fascia lata and fascia iliaca) circumflex iliac vessels that cross over the femoral nerve and can be punctured during the performance of a femoral block.
It is usual to distinguish between the femoral, fascia iliaca compartment, and three-in-one blocks. However, for each of these approaches the goal is to inject the local anesthetic solution under the fascia iliaca using one of two techniques: either with a nerve stimulator to localize the femoral nerve or by feeling the penetration of the two fasciae. Anatomically, these three techniques are similar.
The femoral block is based on a direct localization of the femoral nerve using the nerve stimulator. The approach is most frequently immediately lateral to the femoral artery at the level of inguinal crease. At the level of the inguinal ligament, and especially when the needle is introduced cephalad, the femoral nerve can be missed because the orientation of the nerve at this level is anteroposterior in the direction of the psoas muscle, rather than cephalad to caudal. The appropriate motor response is a contraction of the quadriceps muscle associated with movement of the patella. A contraction of the vastus medialis indicates that the needle needs to be redirected laterally and slightly deeper, whereas a direct contraction of the sartorius muscle suggests a medial redirection of the needle. Again, at times, it may be difficult to distinguish between the contraction of the sartorius muscle and contraction of the vastus medialis muscle.
In the fascia iliaca compartment block, the needle is introduced just below the inguinal ligament immediately to the medial border of the sartorius muscle. A short bevel needle is used to feel the passage of the fasciae lata and iliaca (“double click”). This approach is based on the concept of a diffusion of the local anesthetic solution under the fascia iliaca. Generally, the femoral nerve fans out in its different branches quickly below the inguinal ligament, but there are frequent anatomic variations. Consequently, the risk of femoral nerve injury exists even if the needle is introduced along the medial border of the sartorius muscle.
The three-in-one block is also based on diffusion of the local anesthetic solution. Indeed, Winnie postulated that, with firm compression immediately below the needle and a sufficient volume of local anesthetic, it is possible for the local anesthetic solution to diffuse toward the psoas muscle and consequently produce a block of the femoral, obturator, and lateral femoral cutaneous nerves. However, two studies, one radiographic and one using magnetic resonance imaging, have shown that such diffusion to the psoas compartment is exceptional. Although a diffusion of local anesthetic toward the lateral femoral cutaneous nerve frequently occurs, especially after fascia iliaca block, the block of the obturator nerve is unlikely. Only one study has made a correct evaluation of the block of the obturator nerve after a three-in-one block using electromyographic techniques, and this study confirmed that the obturator nerve is practically never blocked after performing a three-in-one block.
In conclusion, whichever technique is used (femoral, three-in-one, or fascia iliaca compartment blocks) the end point is the same: injection of the local anesthetic under the fascia iliaca. In fact, the most logical approach is to localize the femoral nerve with a nerve stimulator after feeling the two “clicks.” In addition, the “quest” for a complete lumbar plexus block through an anterior approach is unlikely to succeed. With an anterior approach, even in the best cases, it is more likely to obtain a two-in-one block. More frequently, just a femoral block or less (when the territory of the lateral femoral extends until the anterior face of the thigh) can be observed.
In the parasacral region, the sacral plexus leaves the pelvis and enters the buttock through the greater sciatic foramen. From an anesthetic point of view, this region can be compared to the supraclavicular region for the brachial plexus in that, at this level, all the nerves of the plexus are close to each other and, therefore, all can be blocked with a relatively small volume. Furthermore, the possibility of an extension to the obturator nerve has been postulated; however, such an extension seems unlikely considering that there is a pelvic fascia between the sacral plexus and the obturator nerve. Nevertheless, the excellent extension of the block to the sacral plexus territory is real, but this advantage must be balanced by the theoretical risk of injury to the internal iliac vessels, ureter, or rectum if the needle is introduced beyond the plexus.
The landmarks described by Labat and Winnie for the classic approach correspond to the inferior border of the piriformis muscle. At this level, the sacral plexus has already fanned out in its different branches, and only the sciatic nerve, the posterior cutaneous nerve of the thigh, and probably the inferior gluteal nerves can be reliably blocked with such an approach. The superior gluteal nerve and the nerve to the quadratus femoris muscle are unlikely to be blocked with this approach. This distinction may be important to recognize, especially for hip surgery, because these two nerves supply a part of the posterior hip joint capsule. At this level, the sciatic nerve, the inferior gluteal nerve, and the posterior cutaneous nerve of the thigh are close enough together for all to be blocked, providing adequate anesthesia for any surgery being performed from the thigh to the foot, including those requiring the use of a tourniquet for more than 30 minutes. Moreover, if the needle misses the sciatic nerve, its course is stopped by bone. The major risk of this approach is limited to a puncture of the inferior gluteal vessels. However, a more distal approach at the buttock progressively increases the risk of missing the posterior femoral cutaneous nerve. Even though the inferior gluteal and the posterior cutaneous nerves are frequently merged at this level (40% of cases), it is not necessary to block the inferior gluteal nerve to guarantee extension to the posterior femoral cutaneous nerve for two reasons: (a) localization of the sciatic nerve, especially with a two-stimulation technique (tibial and common peroneal responses) is sufficient to ensure anesthesia in this territory; and (b) the inferior gluteal nerve pursues a circular course and can be stimulated superficially as a possible means of extension to the posterior femoral cutaneous nerve. The size of the sciatic nerve and the possibility that its division can occur high, justify the use of a multi-stimulation technique.
Upper Part of the Thigh Region
Only the sciatic nerve can be constantly blocked at this level. Extension to the posterior cutaneous nerve of the thigh is unpredictable. Two approaches are generally described: an anterior and a lateral approach. In all cases, the sciatic nerve is approached at the level of the lesser trochanter. These approaches are frequently associated with a direct contact of the femoral shaft before reaching the nerve. Moreover, the nerve is deep (8 to 12 cm). Therefore, it is generally difficult to reorient the needle to perform a two-stimulation technique. In the anterior approach, because the needle approaching the sciatic nerve comes close to the femoral nerve, it is necessary to immediately switch on the nerve stimulator after passing the skin. One of the problems encountered with the anterior approach is the difficulty in reaching the nerve when it is located under the femoral shaft. It has been shown that when the needle is inserted at the level of the lesser trochanter, an internal rotation of the leg facilitates the approach to the sciatic nerve.
The popliteal region is a triangle, with a base corresponding to the popliteal crease and limited medially by the semi-tendinous and semi-membranous muscles and laterally by the biceps femoris muscle and its tendons. Different approaches to the sciatic nerve have been described, and all can be grouped into posterior and lateral approaches. Of the various posterior approaches to the popliteal fossa, high approaches are most logical. There is no risk of vessel puncture above the adductor hiatus because it is not close to any vessels there; below the adductor hiatus, the risk of puncturing popliteal vessels increases as the approach becomes lower and the distance between the nerve and the vessels decreases. Moreover, the higher is the approach and the closer are the two branches of the sciatic nerve. This is especially important when a perineural catheter is placed. The posterior approaches near the popliteal crease must be avoided because popliteal vessels lie immediately anterior or behind the tibial nerve, thus increasing the risk of vessel puncture. For the lateral approach, there is also a theoretical risk of popliteal vessel puncture, but this risk is limited if the needle is directed at a 25° to 30° angle.
The union of the medial and lateral sural nerves that comprises the sural nerve may occur at different levels in the popliteal region. Consequently, at the level of the popliteal fossa, the block of the sural nerve is sometimes incomplete.
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