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.

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.

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.

In the buttock, the sacral plexus branches into several nerves supplying the muscles of the region. Some are more relevant for regional anesthesia.

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.

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.

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.