Lumbar & Sacral Plexus Anatomy
The lumbosacral plexus provides innervation to the lower extremities (Figure 46-38). The lumbar plexus is formed by the ventral rami of L1-4, with occasional contribution from T12. It lies within the psoas muscle with branches descending into the proximal thigh. Three major nerves from the lumbar plexus make contributions to the lower limb: the femoral (L2-4), lateral femoral cutaneous (L1-3), and obturator (L2-4). These provide motor and sensory innervation to the anterior portion of the thigh and sensory innervation to the medial leg. The sacral plexus arises from L4-5 and S1-4. The posterior thigh and most of the leg and foot are supplied by the tibial and peroneal portions of the sciatic nerve. The posterior femoral cutaneous nerve (S1-3), and not the sciatic nerve, provides sensory innervation to the posterior thigh; it travels with the sciatic nerve as it emerges around the piriformis muscle.
The ventral rami of L1-5 and S1-4 form the lumbosacral plexus, which provides innervation to the lower extremities.
The femoral nerve innervates the main hip flexors, knee extensors, and provides much of the sensory innervation of the hip and thigh (Figure 46-39). Its most medial branch is the saphenous nerve, which innervates much of the skin of the medial leg and ankle joint. The term 3-in-1 block refers to anesthetizing the femoral, lateral femoral cutaneous, and obturator nerves with a single injection below the inguinal ligament; this term has largely been abandoned as evidence accumulated demonstrating the failure of most single injections to consistently affect all three nerves.
A femoral nerve block alone will seldom provide surgical anesthesia, but it is often used to provide postoperative analgesia for hip, thigh, knee, and ankle (for the saphenous nerve) procedures. Femoral nerve blocks have a relatively low rate of complications and few contraindications. Local infection, previous vascular grafting, and local adenopathy should be carefully considered in patient selection.
The femoral nerve provides sensory innervation to the hip and thigh, and to the medial leg via its terminal branch, the saphenous nerve.
With the patient positioned supine, the femoral artery pulse is palpated at the level of the inguinal ligament. A short (5-cm) insulated needle is inserted at a 45° angle to the skin in a cephalad direction (Figure 46-40) until a clear quadriceps twitch is elicited at a current below 0.5 mA (look for patella motion).
Femoral block using nerve stimulation.
A high-frequency linear ultrasound transducer is placed over the area of the inguinal crease parallel to the crease itself, or slightly more transverse (Figure 46-41). The femoral artery and femoral vein are visualized in cross-section, with the overlying fascia iliaca. Just lateral to the artery and deep to the fascia iliaca, the femoral nerve appears in cross-section as a spindle-shaped structure with a “honeycomb” texture (Figure 46-42).
Ultrasound-guided femoral nerve block (in-plane technique).
Femoral nerve block. Ultrasound image of the femoral nerve. FA, femoral artery; FV, femoral vein; FN, femoral nerve; SM, Sartorius muscle; IM, iliacus muscle.
For an out-of-plane technique, the block needle is inserted just lateral to where the femoral nerve is seen, and directed cephalad at an angle approximately 45° to the skin. The needle is advanced until it is seen penetrating the fascia iliaca, or (if using concurrent electrical stimulation) until a motor response is elicited. Following careful aspiration for the nonappearance of blood, 30-40 mL of local anesthetic is injected.
For an in-plane technique, a longer needle may be used. The needle is inserted parallel to the ultrasound transducer just lateral to the outer edge. The needle is advanced through the sartorius muscle, deep to the fascia iliaca, until it is visualized just lateral to the femoral nerve. Local anesthetic is injected, visualizing its hypoechoic spread deep to the fascia iliaca and around the nerve.
The goal of a fascia iliaca block is similar to that of a femoral nerve block, but the approach is slightly different. Without use of a nerve stimulator or ultrasound machine, a relatively reliable level of anesthesia may be attained simply with anatomic landmarks and tactile sensation. Once the inguinal ligament and femoral artery pulse are identified, the length of the inguinal ligament is divided into thirds (Figure 46-43). Two centimeters distal to the junction of the middle and outer thirds, a short, blunt-tipped needle is inserted in a slightly cephalad direction. As the needle passes through the two layers of fascia in this region (fascia lata and fascia iliaca), two “pops” will be felt. Once the needle has passed through the fascia iliaca, careful aspiration is performed and 30-40 mL of local anesthetic is injected. This block usually anesthetizes both the femoral nerve and lateral femoral cutaneous nerves, since the local anesthetic is deposited under the fascia iliaca between the two nerves which run in the same plane between the fascia and underlying muscle.
Lateral Femoral Cutaneous Nerve Block
The lateral femoral cutaneous nerve provides sensory innervation to the lateral thigh (see Figure 46-39). It may be anesthetized as a supplement to a femoral nerve block or as an isolated block for limited anesthesia of the lateral thigh. As there are few vital structures in proximity to the lateral femoral cutaneous nerve, complications with this block are exceedingly rare. The lateral femoral cutaneous nerve (L2-3) departs from the lumbar plexus, traverses laterally from the psoas muscle, and courses anterolaterally along the iliacus muscle (see Figure 46-38). It emerges inferior and medial to the anterior superior iliac spine to supply the cutaneous sensory innervation of the lateral thigh.
The patient is positioned supine or lateral, and the point 2 cm medial and 2 cm distal to the anterior superior iliac spine is identified. A short 22-gauge block needle is inserted and directed laterally, observing for a “pop” as it passes through the fascia lata. A field block is performed with 10-15 mL of local anesthetic, which is deposited above and below the fascia (Figure 46-44).
Lateral femoral cutaneous nerve block.
A block of the obturator nerve is usually required for complete anesthesia of the knee and is most often performed in combination with femoral and sciatic nerve blocks for this purpose. The obturator nerve contributes sensory branches to the hip and knee joints, a variable degree of sensation to the medial thigh, and innervates the adductors of the hip (Figure 46-45). This nerve exits the pelvis and enters the medial thigh through the obturator foramen, which lies beneath the superior pubic ramus. After identification of the pubic tubercle, a long (10-cm) block needle is inserted 1.5 cm inferior and 1.5 cm lateral to the tubercle. The needle is advanced posteriorly until bone is contacted (Figure 46-46). Redirecting laterally and caudally, the needle is advanced an additional 2-4 cm until a motor response (thigh adduction) is elicited and maintained below 0.5 mA. Following careful aspiration for the nonappearance of blood, 15-20 mL of local anesthetic is injected.
Obturator nerve innervation.
Obturator nerve block. Contact pubic tubercle (1), then redirect laterally and caudally (2) until a motor response is elicited.
Posterior Lumbar Plexus (Psoas Compartment) Block
Posterior lumbar plexus blocks are useful for surgical procedures involving areas innervated by the femoral, lateral femoral cutaneous, and obturator nerves (Figure 46-47). These include procedures on the hip, knee, and anterior thigh. Complete anesthesia of the knee can be attained with a proximal sciatic nerve block. The lumbar plexus is relatively close to multiple sensitive structures (Figure 46-48) and reaching it requires a very long needle. Hence, the posterior lumbar plexus block has one of the highest complication rates of any peripheral nerve block; these include retroperitoneal hematoma, intravascular local anesthetic injection with toxicity, intrathecal and epidural injections, and renal capsular puncture with subsequent hematoma.
Lumbar plexus blocks provide anesthesia to the femoral, lateral femoral cutaneous, and obturator nerves.
The lumbar plexus lies in close proximity to several important structures.
Lumbar nerve roots emerge into the body of the psoas muscle and travel within the muscle compartment before exiting as terminal nerves (see Figure 46-38). Modern posterior lumbar plexus blocks deposit local anesthetic within the body of the psoas muscle. The patient is positioned in lateral decubitus with the side to be blocked in the nondependent position (Figure 46-49). The midline is palpated, identifying the spinous processes if possible. A line is first drawn through the lumbar spinous processes, and both iliac crests are identified and connected with a line to approximate the level of L4. The posterior superior iliac spine is then palpated and a line is drawn cephalad, parallel to the first line. If available, ultrasound imaging of the transverse process may be helpful to estimate lumbar plexus depth. A long (10- to 15-cm) insulated needle is inserted at the point of intersection between the transverse (intercristal) line and the intersection of the lateral and middle thirds of the two sagittal lines. The needle is advanced in an anterior direction until a femoral motor response is elicited (quadriceps contraction). If the transverse process is contacted, the needle should be withdrawn slightly and “walked off” the transverse process in a caudal direction, maintaining the needle in the parasagittal plane. The needle should never be inserted more than 3 cm past the depth at which the transverse process was contacted. Local anesthetic volumes greater than 20 mL will increase the risk of bilateral spread and contralateral limb involvement.
Patient positioning and surface landmarks for posterior lumbar plexus block.
The saphenous nerve is the most medial branch of the femoral nerve and innervates the skin over the medial leg and the ankle joint (see Figure 46-39). Therefore, this block is used mainly in conjunction with a sciatic nerve block to provide complete anesthesia/analgesia below the knee.
The saphenous nerve may be accessed proximal to the knee, just deep to the sartorius muscle. A high-frequency linear probe is used to identify the junction between the sartorius, vastus medialis, and adductor muscles in cross-section just distal to the adductor canal. A long needle is inserted from medial to lateral (in-plane) or angled cephalad (out-of-plane) and 5-10 mL of local anesthetic deposited within this fascial plane.
Proximal Saphenous Technique
A short block needle is inserted 2 cm distal to the tibial tuberosity and directed medially, infiltrating 5-10 mL of local anesthetic as the needle passes toward the posterior aspect of the leg (Figure 46-50). Ultrasound may be used to identify the saphenous vein near the tibial tuberosity, facilitating a perivascular technique with infiltration about the vein.
Proximal saphenous nerve block.
Distal Saphenous Technique
The medial malleolus is identified, infiltrating 5 mL of local anesthetic in a line running anteriorly around the ankle (see Ankle Block below).
The sciatic nerve originates from the lumbosacral trunk and is composed of nerve roots L4-5 and S1-3(see Figure 46-38).
Blockade of the sciatic nerve may occur anywhere along its course and is indicated for surgical procedures involving the hip, thigh, knee, lower leg, and foot. The posterior femoral cutaneous nerve is variably anesthetized as well, depending on the approach. If sacral plexus or posterior femoral cutaneous nerve anesthesia is required, the parasacral approach is used (a technique that is beyond the scope of this chapter).
Posterior (Classic or Labat) Approach
The patient is positioned laterally with the side to be blocked in the nondependent position. The patient is asked to bend the knee of the affected leg and tilt the pelvis slightly forward (Sim’s position; Figure 46-51). The greater trochanter, posterior superior iliac spine (PSIS), and sacral hiatus are then identified. A line is drawn from the greater trochanter to the PSIS, its midpoint identified, and a perpendicular line extended in a caudal direction. Next, a line is drawn from the greater trochanter to the sacral hiatus and the intersection point is marked; this is the initial needle insertion point. A long (10-cm) insulated needle is inserted at an angle perpendicular to all planes to the skin (Figure 46-51). The needle is advanced through the gluteal muscles (a motor response of these muscles may be encountered) until plantar- or dorsiflexion is elicited (plantarflexion or foot inversion is preferred for surgical anesthesia). A local anesthetic volume of 25 mL provides surgical anesthesia.
Patient positioning, surface landmarks, and needle positioning for proximal sciatic nerve block (classic approach).
After leaving the sciatic notch, the sciatic nerve descends behind the lesser trochanter to a position posterior to the femur. It can be accessed from the anterior thigh just medial to the lesser trochanter. Lateral or prone positioning may present a challenge for some patients requiring a sciatic nerve block (ie, elderly patients, pediatric patients under general anesthesia). An anterior approach can be technically challenging but offers an alternative path to the sciatic nerve. Before proceeding with this block, which carries a risk of vascular puncture (femoral artery and vein), patient-specific risks should be considered (eg, coagulopathy and vascular grafting). In addition, if combining this block with the femoral nerve block in an unanesthetized patient, performing the sciatic block first is recommended to avoid passing the block needle through a previously anesthetized femoral nerve. A local anesthetic volume of 25 mL provides surgical anesthesia.
With the patient positioned supine, a line is drawn along the inguinal ligament, from the anterior superior iliac spine to the pubic tubercle (Figure 46-52). A second line is drawn parallel to the first that traverses the greater trochanter (intertrochanteric line). Next, these two lines are connected with a third line drawn from the point between the medial one third and lateral two thirds of the first line, at a 90° angle, and extended caudally to intersect with the intertrochanteric line. A long (10- to 15-cm) needle is inserted through this intersection and directly posterior until foot inversion or plantarflexion is elicited (dorsiflexion is acceptable for postoperative analgesia). Often with this approach, the femur is contacted before the needle reaches the sciatic nerve. When this occurs, the needle should be withdrawn 2-3 cm, the patient should be asked to internally rotate the leg, and then the needle should be advanced. If the femur is contacted again, the landmarks may require reassessment. A local anesthetic volume of 25 mL provides surgical anesthesia.
Anatomy and surface landmarks for anterior sciatic nerve block.
With the patient positioned supine and the leg externally rotated, a low-frequency curvilinear transducer is placed transversely over the medial thigh, approximately at the level of the lesser trochanter. The femur, femoral vessels, adductor muscles, and gluteus maximus are identified in cross-section. The elliptical, hyperechoic sciatic nerve is found in the fascial plane between adductors and gluteus muscles, posterior to the femur. Using a long (10-cm) needle, the nerve is approached in-plane (anterior to posterior) or out-of-plane (cephalad to caudad), taking care to avoid femoral vessels, until the needle tip lies in this muscle plane and a local anesthetic injection can be observed as hypoechoic spread surrounding the sciatic nerve.
A subgluteal approach to the sciatic nerve is a useful alternative to the traditional posterior approach. In many patients the landmarks are more easily identified, and less tissue is traversed. With the sciatic nerve at a more superficial location, the exclusive use of ultrasound becomes far more practical, as well. If sciatic nerve block is being combined with a femoral block and ambulation is desired within the local anesthetic duration, consider a popliteal approach (below) that will not affect the hamstring muscles to the same degree, allowing knee flexion to lift the foot with the use of crutches.
With the patient in Sim’s position, the greater trochanter and ischial tuberosity are identified and a line drawn between them (Figure 46-53). From the midpoint of this line, a second line is drawn perpendicularly and extended caudally 4 cm. Through this point a long (10-cm) insulated needle is inserted directly slightly cephalad until foot plantarflexion or inversion is elicited (dorsiflexion is acceptable for analgesia). A local anesthetic volume of 25 mL provides surgical anesthesia.
Patient positioning and surface landmarks for subgluteal sciatic block.
Using the same positioning and landmarks (Figure 46-53), a linear or low-frequency curvilinear (best) ultrasound transducer is placed over the midpoint between the ischial tuberosity and the greater trochanter in a transverse orientation. Both bony structures should be visible in the ultrasound field simultaneously. Gluteal muscles are identified superficially, along with the fascial layer defining their deep border. The triangular sciatic nerve should be visible in cross-section just deep to this layer in a location approximately midway between the ischial tuberosity and the greater trochanter, superficial to the quadratus femoris muscle.
For an out-of-plane ultrasound-guided sciatic block, the block needle is inserted just caudad to the ultrasound transducer and advanced in an anterior and cephalad direction. Once the needle passes through the gluteus muscles with the tip next to sciatic nerve, careful aspiration for the nonappearance of blood is performed and local anesthetic is injected, visualizing spread around the nerve.
For an in-plane technique, the block needle is inserted just lateral to the ultrasound transducer near the greater trochanter. It is advanced through the field of the ultrasound beam until the tip is visible deep to the gluteus maximus, next to the sciatic nerve. Again, local anesthetic spread around the nerve should be visualized.
Popliteal nerve blocks provide excellent coverage for foot and ankle surgery, while sparing much of the hamstring muscles, allowing lifting of the foot with knee flexion, thus easing ambulation. All sciatic nerve blocks fail to provide complete anesthesia for the cutaneous medial leg and ankle joint capsule, but when a saphenous (or femoral) block is added, complete anesthesia below the knee is provided. The major site-specific risk of a popliteal block is vascular puncture, owing to the sciatic nerve’s proximity to the popliteal vessels at this location.
The sciatic nerve divides into the tibial and common peroneal nerves within or just proximal to the popliteal fossa (Figure 46-54). The upper popliteal fossa is bounded laterally by the biceps femoris tendon and medially by the semitendinosus and semimembranosus tendons. Cephalad to the flexion crease of the knee, the popliteal artery is immediately lateral to the semitendinosus tendon. The popliteal vein is lateral to the artery, and the tibial and common peroneal nerves are just lateral to the vein and medial to the biceps tendon, 2-6 cm deep to the skin. The tibial nerve continues deep behind the gastrocnemius muscle, and the common peroneal nerve leaves the popliteal fossa by passing between the head and neck of the fibula to supply the lower leg. The sciatic nerve is approached by either a posterior or a lateral approach. For posterior approaches, the patient is usually positioned prone with the knee slightly flexed by propping the ankle on pillows or towels. For lateral approaches, the patient may be in the lateral or supine position.
The sciatic nerve divides into tibial and peroneal branches just proximal to the popliteal fossa and provides sensory innervation to much of the lower leg.
Nerve Stimulation (Posterior Approach)
With the patient in the prone position, the apex of the popliteal fossa is identified. The hamstring muscles are palpated to locate the point where the biceps femoris (lateral) and the semimembranosus/semitendinosus complex (medial) join (Figure 46-55). Having the patient flex the knee against resistance facilitates recognition of these structures. The needle entry point is 1 cm caudad from the apex. An insulated needle (5-10 cm) is advanced until foot plantarflexion or inversion is elicited (dorsiflexion is acceptable for analgesia). A volume of 30-40 mL of local anesthetic is often required for single-injection popliteal-sciatic nerve block.
Anatomy and sonoanatomy of the sciatic nerve in the popliteal fossa. PA, popliteal artery; PV, popliteal vein; N, sciatic nerve; BFM, biceps femoris muscle; F, femur.
Nerve Stimulation (Lateral Approach)
With the patient in the supine position and the knee fully extended, the intertendinous groove is palpated between the vastus lateralis and biceps femoris muscles approximately 10 cm proximal to the superior notch of the patella. A long (10-cm) insulated needle is inserted at this point and advanced at a 30° angle posteriorly until an appropriate motor response is elicited. If bone (femur) is contacted, the needle is withdrawn and redirected slightly posteriorly until an acceptable motor response is encountered.
With the patient positioned prone, the apex of the popliteal fossa is identified, as described above. Using a high-frequency linear ultrasound transducer placed in a transverse orientation, the femur, biceps femoris muscle, popliteal vessels, and sciatic nerve or branches are identified in cross-section (Figure 46-55). The nerve is usually posterior and lateral (or immediately posterior) to the vessels and is often located in close relationship to the biceps femoris muscle, just deep to its medial edge.
For an out-of-plane technique, the needle is inserted just caudad to the ultrasound transducer and directed anteriorly and slightly cephalad. When the needle is positioned in proximity to the sciatic nerve, and following careful aspiration, local anesthetic injected, observing for spread around the nerve.
For an in-plane technique, the block needle is inserted lateral to the ultrasound transducer, traversing—or just anterior to—the biceps femoris muscle (Figure 46-56). The needle is advanced in the ultrasound plane, while visualizing its approach either deep or superficial to the nerve.
Patient positioning, probe, and needle orientation for popliteal block.
If surgical anesthesia is desired, local anesthetic should be seen surrounding all sides of the nerve, which usually requires multiple needle tip placements with incremental injection. For analgesia alone, a single injection of local anesthetic is acceptable. Ultrasound-guided popliteal sciatic blocks may be performed with the patient in the lateral or supine positions (the latter with leg up-raised on several pillows). These maneuvers are often more technically challenging.
For surgical procedures of the foot, an ankle block is a fast, low-technology, low-risk means of providing anesthesia. Excessive injectate volume and use of vasoconstrictors such as epinephrine must be avoided to minimize the risk of ischemic complications. Since this block includes five separate injections, it is often uncomfortable for patients and adequate premedication is required.
Five nerves supply sensation to the foot (Figure 46-57). The saphenous nerve is a terminal branch of the femoral nerve and the only innervation of the foot not a part of the sciatic system. It supplies superficial sensation to the anteromedial foot and is most constantly located just anterior to the medial malleolus. The deep peroneal nerve runs in the anterior leg after branching off the common peroneal nerve, entering the ankle between the extensor hallucis longus and the extensor digitorum longus tendons (Figure 46-58), just lateral to the dorsalis pedis artery. It provides innervation to the toe extensors and sensation to the first dorsal webspace. The superficial peroneal nerve, also a branch of the common peroneal nerve, descends toward the ankle in the lateral compartment, giving motor branches to the muscles of eversion. It enters the ankle just lateral to the extensor digitorum longus and provides cutaneous sensation to the dorsum of the foot and toes. The posterior tibial nerve is a direct continuation of the tibial nerve and enters the foot posterior to the medial malleolus, branching into calcaneal, lateral plantar, and medial plantar nerves. It is located behind the posterior tibial artery at the level of the medial malleolus and provides sensory innervation to the heel, the medial sole, and part of the lateral sole of the foot, as well as the tips of the toes. The sural nerve is a branch of the tibial nerve and enters the foot between the Achilles tendon and the lateral malleolus to provide sensation to the lateral foot.
Cutaneous innervation of the foot.
Tibial and common peroneal nerve courses.
A complete ankle block requires a series of five nerve blocks, but the process may be streamlined to minimize needle insertions (Figure 46-59). All five injections are required to anesthetize the entire foot; however, many surgical procedures involve only a few terminal nerves, and only affected nerves should be blocked. In addition, unlike a sciatic nerve block, an ankle block provides no analgesia for (below-the-knee) tourniquet pain, nor does it allow for perineural catheter insertion. To block the deep peroneal nerve, the groove between the extensor hallucis longus and extensor digitorum longus tendons is identified. The dorsalis pedis pulse is often palpable here. A short, small-gauge block needle is inserted perpendicular to the skin just lateral to the pulse, bone is contacted, and 5 mL of local anesthetic is infiltrated as the needle is withdrawn. Continuing from this insertion site, a subcutaneous wheal of 5 mL of local anesthetic is extended toward the lateral malleolus to target the superficial peroneal nerve. The needle is withdrawn and redirected from the same location in a medial direction, infiltrating 5 mL of local anesthetic toward the medial malleolus to target the saphenous nerve. The posterior tibial nerve may be located by identifying the posterior tibial artery pulse behind the medial malleolus. A short, small-gauge block needle is inserted just posterior to the artery and 5 mL of local anesthetic is distributed in the pocket deep to the flexor retinaculum. To target the sural nerve, 5 mL of local anesthetic is injected subcutaneously posterior to the lateral malleolus.
Needle placement for ankle block.