Lower Extremity-Blocks of the Lumbar Plexus and Lumbar Plexus Peripheral Nerves
Francis V. Salinas
KEY POINTS
1. Detailed knowledge of the lumbar plexus, and the osseous and articular innervation of the joints of the lower extremity, will allow the operator to select the optimal approach based on the surgical procedure.
2. Ultrasonography has become the predominant technique for localization of the lumbar plexus and its peripheral nerve branches.
3. Successful ultrasound-guided nerve block of the lumbar plexus and lower extremity peripheral nerves relies on visualization and identification of both the neural structures and the fascial compartments that contain the target nerves.
4. The lumbar plexus approach (psoas compartment) blocks all three peripheral nerve branches (femoral, obturator, and lateral femoral cutaneous) within the substance of the psoas muscle.
5. Successful femoral nerve block relies on placement of the local anesthetic deep to the fascia iliaca. Femoral nerve block provides effective postoperative analgesia after major knee surgery and provides effective preoperative analgesia for hip fractures.
6. Ultrasound-guided subsartorial femoral triangle block is a relatively new technique, but has gained widespread adoption for total knee arthroplasty because of its analgesic efficacy and minimum effect on quadriceps motor function. It has been has referred to as adductor canal block, but the correct anatomic location for block placement is within the subsartorial apex of the femoral triangle
I. Anatomy
CLINICAL PEARL
The clinically relevant branches of the lumbosacral plexus for lower extremity block are derived from the ventral rami of the L2-S4 spinal nerves. Anatomically, the lumbar and sacral plexus are connected through L4 as it bifurcates to join with L5 and form the lumbosacral trunk (Fig. 10.1). In contrast to the brachial plexus, there is no technique that allows the entire lumbosacral plexus to be anesthetized with a single injection. Therefore, for functional purposes, the lumbar and sacral plexus are distinct entities and must be blocked separately to provide complete unilateral lower extremity anesthesia and/or analgesia.
A. Lumbar plexus
1. The clinically relevant motor and sensory innervation of the lumbar plexus arises from the ventral rami of the L2-L4 spinal nerve roots, which give rise to the lateral femoral cutaneous nerve (LFCN), femoral nerve (FN), and obturator nerve (ON). Shortly after exiting from their respective intervertebral foramina, the L2-L4 nerve roots lie within a fascial plane within the posterior aspect of the psoas major muscle (PMM) (Fig. 10.1).
 FIGURE 10.1 Overview of the lumbosacral plexus. The origin of the lumbosacral plexus is broader than the brachial plexus in the cervical region. The ventral rami of the L2-L4 spinal nerve roots emerge from their respective intervertebral foramina and travel within a fascial plane located between the posterior third and anterior two-thirds of the psoas major muscle (PMM). Within the substance of the PMM, the ventral rami form the peripheral nerves of the lumbar plexus. From a medial to lateral orientation, the obturator nerve is located most medial, the lateral femoral cutaneous nerve most lateral, and the femoral nerve located in between (see inset). The sacral spinal nerve roots (S1-S4) give rise to the sacral plexus and sciatic nerve, and require a separate injection. |
2. The origins of the larger anterior PMM include the lateral surfaces of the lumbar vertebral bodies and their associated intervertebral discs, whereas the origins of the smaller posterior PMM are the ventral and lower surfaces of the respective transverse processes. The intervertebral foramina lie anterior to the transverse processes and posterior to the anterior muscular attachments to the vertebral bodies. Therefore, the nerve roots enter the PMM between their respective anterior and posterior segments.
3. Within the PMM, the ventral rami divide into anterior and posterior branches, which reunite to give rise to the individual peripheral nerves of the lumbar plexus. The lumbar plexus descends vertically within the substance of the psoas, and at the L4 and L5 levels, the terminal nerves have been formed.
4. Based on anatomic dissections and computed tomography imaging, the terminal nerves are arranged in a medial to lateral topographic arrangement, with the ON most medial, the LFCN most lateral, and the FN located in between1,2 (Fig. 10.1). Although all the three terminal nerves are located within the PMM, anatomic studies have demonstrated that the ON may be separated from the FN and LFCN by a muscular fold more than 50% to 60% of the time, which may potentially lead to incomplete blockade of the lumbar plexus.1,2
B. Femoral nerve
1. The FN is derived from dorsal divisions of the ventral rami of the L2-L4 spinal nerve roots. It is the largest and most commonly blocked peripheral nerve branch of the lumbar plexus. Within the pelvis, the FN supplies muscular branches to the iliacus muscle and pectineus muscle (PM), as well as an articular branch to the hip joint.3 The FN continues distally to enter the base of the femoral triangle (FT) in the proximal aspect of the anterior thigh by passing deep to the inguinal ligament (Fig. 10.2). The borders of the FT include the inguinal ligament (base), the sartorius muscle (lateral), and adductor longus muscle (AL) (medial). The iliacus muscle and PMM form the floor of the FT laterally and the PL and AL medially, while the roof is the overlying fascia lata. The apex of the FT is the intersection of the medial borders of the sartorius muscle and AL. In contrast, the medial border of the sartorius muscle, but the lateral border of the AL corresponds to the apex of the iliopectineal fossa (IPF), which is a proximal subset of the FT.4
2. The FN and femoral vessels continue distally from the base to the apex of the IPF. At the level of the inguinal ligament, the FN is within the IPF and is located 1 to 2 cm lateral to the femoral artery (FA), with the femoral vein (FV) located immediately medial to the FA (Fig. 10.2). As the FN continues further distally to the level of the inguinal crease, it is just lateral or posterolateral to the FA. Within the FT, the FN is located deep to both the fascia lata and fascia iliaca. In contrast, the femoral vessels (enveloped by the femoral sheath) are located deep to the fascia lata, but are superficial to the fascia iliaca. Thus, the fascia iliaca physically separates the FN (located within the muscular fascia iliaca compartment) from the femoral vessels (located within the vascular fascial compartment of the femoral sheath) (Fig. 10.3).
3. The topography of the FN demonstrates a relatively flat cross-sectional diameter with a mean medial to lateral width of 9 to 11 mm and a mean anterior to posterior height of 1.3 to 2.3 mm.5 The FN is composed of multiple fascicles supplying muscular and cutaneous branches to the anterior compartment of the thigh, articular branches to the hip and knee joints, and cutaneous branches to the medial aspect of the lower leg (Fig. 10.4). Fascicular branches innervating the vastus medialis muscle (VMM), vastus intermedius muscle, and vastus lateralis muscle are positioned in the central and dorsal portion of the FN. The fascicular branches innervating the rectus femoris (laterally located), PMs (medially located), and the cutaneous nerves to the anterior and medial thigh are located on the peripheral aspects of the FN. The fascicular branch supplying the sartorius muscle is typically located
on the ventral aspect of the FN, but may be in a lateral, medial, or central position within the FN.5 The saphenous nerve (SN) is consistently medial to the nerve branch to vastus medialis muscle (N-VMM) and together, they continue distally with the FA and FV (as a neurovascular bundle) toward the apex of the FT4 (Fig. 10.5).
on the ventral aspect of the FN, but may be in a lateral, medial, or central position within the FN.5 The saphenous nerve (SN) is consistently medial to the nerve branch to vastus medialis muscle (N-VMM) and together, they continue distally with the FA and FV (as a neurovascular bundle) toward the apex of the FT4 (Fig. 10.5).
 FIGURE 10.2 Fresh cadaver dissection of the femoral nerve (FN) within the iliopectineal fossa (IPF) and femoral triangle (FT). The inguinal ligament is retracted cephalad to show the course of the FN in the base of the FT. Several centimeters caudad to the inguinal ligament, the FN assumes a more superficial location within the FT. At the level of the inguinal crease, note that the FN lays just anterior to the iliacus muscle and just lateral to the femoral artery. The FN has a flattened appearance grossly and has a wider medial to lateral dimension compared to its anterior to posterior dimensions. |
4. In the distal part of the FT, the N-VMM lies between the sartorius muscle and VMM. The posterior branch of the medial femoral cutaneous lies along the posterior side of the sartorius muscle and communicates with the SN and anterior branch of the ON forming the subsartorial plexus superficial to the vasoadductor membrane (VAM). These three
nerves lie deep to the sartorius muscle and lateral to the FA within the subsartorial apex (Scarpa) of the FT.
nerves lie deep to the sartorius muscle and lateral to the FA within the subsartorial apex (Scarpa) of the FT.
 FIGURE 10.3 Axial illustration of the anatomic relationship of the femoral nerve (FN) to the surrounding perineural structures within the iliopectineal fossa (IPF). The FN lays directly anterior to the iliacus muscle and just posterior (deep) to the fascia iliaca. The FN is contained within the fascia iliaca compartment, just lateral to the femoral artery (FA). The FA (contained within the femoral sheath) is deep to the fascia lata but superficial to the fascia iliaca and is in a separate fascial compartment from the FN. This illustration most closely represents the ideal two-dimensional ultrasound short-axis image of the FN-FA anatomic relationship observed at the inguinal crease. |
5. At the apex of the FT, the neurovascular bundle dives deep to the sartorius muscle into the groove of the FT. The SN and N-VMM exit the apex of the FT, but only the saphenous enters the adductor canal (AC) in conjunction with the superficial femoral artery (SFA) and superficial femoral vein (SFV). The AC is the neurovascular pathway from the apex of the FT to the adductor hiatus. The SFA exits the adductor hiatus and dives posterior in to the popliteal fossa to become the popliteal artery. Within the AC, the neurovascular bundle is sandwiched between the Al and AM posteromedially, the VMM anterolaterally, and the VAM anteromedially.4,6 Within the AC, the SN is initially located lateral to the SFA, but as it continues distally, the SN assumes a position anterior and then medially located to the SFA within the distal AC.
6. Although the anatomic data is conflicting (because of differences in dissection technique),4,6,7 the N-VMM is consistently located in a separate myofascial tunnel running alongside, but superficial to the AC. It gives rise to muscular branches that supply both the VMM and then continues further distally to supply the anterior and medial capsule of the knee joint and the medial retinaculum4,6 (Fig. 10.6).
7. Both the SN and muscular branches from N-VMM give rise to branches that form a deep plexus lying between the SFA and femur. This deep plexus gives rise to anterior and medial genicular nerves that supply the deep anteromedial aspect of the knee joint.4,6
8. At the distal end of the AC, the SN pierces the VAM and emerges subcutaneously between the sartorius and gracilis muscles. As the SN continues further distally toward the joint line of the knee, it further divides into infrapatellar and sartorial branches. The infrapatellar branch provides cutaneous sensory innervation around the anterior aspect of the knee and an articular branch to the medial aspect of the knee joint. The sartorial branch continues
distally in the subcutaneous tissue of the lower leg, and continues further distally passing anteromedial to the medial malleolus to provide cutaneous innervation to the anteromedial lower leg and medial aspect of the foot. The sartorial branch also provides articular branches to the medial ankle and talocalcaneonavicular joint.8
distally in the subcutaneous tissue of the lower leg, and continues further distally passing anteromedial to the medial malleolus to provide cutaneous innervation to the anteromedial lower leg and medial aspect of the foot. The sartorial branch also provides articular branches to the medial ankle and talocalcaneonavicular joint.8
 FIGURE 10.4 The tip of the clamp is over the inguinal ligament, whereas the middle of the clamp is under the proximal femoral nerve (FN). Distally, at least four individual fascicular components (branches) of the FN (black arrowheads) are visible. At this level, the branches of the femoral nerve will likely have arborized to supply their respective muscles of the quadriceps muscle group. |
C. Obturator nerve
1. The ON is formed within the substance of the PMM from the anterior divisions (ADs) of the ventral rami of the L2-L4 spinal nerves. The ON is the most medial branch of the lumbar plexus within the PMM. It emerges from the posterior border of the PMM and descends along the lateral wall of the pelvis toward the superior part of the obturator foramen (Fig. 10.1). The ON then enters the adductor compartment of the proximal thigh by passing through the obturator foramen. After emerging through the obturator foramen and just inferior to the superior pubic ramus, the ON continues distally in an interfascial plane anterior to the obturator externus muscle (OE) and deep to the PMs9 (Fig. 10.7 A, B).
2. The ON has AD and posterior division (PD) that provide muscular branches to the adductor muscles of the thigh, articular branches to the hip and knee joints, and variable cutaneous sensory
branches to the posteromedial distal thigh. Anatomic studies have demonstrated a considerable degree of variability in the anatomy of the ON. It may divide into its respective AD and PD within the pelvis (23%) as it enters the obturator foramen, within the obturator foramen (52%), or may emerge united from the obturator foramen and divide in the proximal medial thigh (25%).9 One to three articular branches innervate the hip joint: they typically arise from the proximal ON. However, the hip joint may also be supplied directly from both the AD and PD.9,10
branches to the posteromedial distal thigh. Anatomic studies have demonstrated a considerable degree of variability in the anatomy of the ON. It may divide into its respective AD and PD within the pelvis (23%) as it enters the obturator foramen, within the obturator foramen (52%), or may emerge united from the obturator foramen and divide in the proximal medial thigh (25%).9 One to three articular branches innervate the hip joint: they typically arise from the proximal ON. However, the hip joint may also be supplied directly from both the AD and PD.9,10
 FIGURE 10.5 The course of the individual fascicular components of the femoral nerve is illustrated. The arrows indicate muscular branches to the vastus muscles. Note the two more medially located fascicular components (under the tip of the dissecting needle), accompanied by the femoral artery coursing caudad to the apex of the femoral triangle. The sartorius muscle has been retracted laterally. |
 FIGURE 10.6 The contents of the adductor canal (AC). The sartorius muscle has been retracted medially. The nerve branch to the vastus medialis muscle (V-MMN) continues alongside and superficial to the AC then provides muscular branches to VMM. The saphenous nerve (SN) continues distally within the AC, initially lateral to the superficial femoral artery, courses anteriorly and then medially prior to exiting through the adductor hiatus. |
 FIGURE 10.7 A: Cadaver dissection demonstrating the anatomy of the obturator nerve. Exposure of the structures before transection and reflection of the pectineus muscle from the superior pubic ramus. An ultrasound-guided injection into the fascial between the pectineus and obturator externus muscles was performed with 15 mL of methylene blue prior to transection. OC indicates the orifice of the obturator canal (depiction of the deeper position). B: After reflection of the PM, the anterior (#a) and posterior (#p) divisions of the obturator nerve exiting the OC, and are visibly stained in their extrapelvic trajectory anterior to the obturator externus muscle (OE). Note that the deep surface of the PM and the superficial surface of the OE are stained by methylene blue. AB, adductor brevis muscle; AL, adductor longus muscle; PM, pectineus muscle. (From Nielsen TD, et al. A cadaveric study of ultrasound-guided subpectineal injected spread around the obturator nerve and its articular branches. Reg Anesth Pain Med. 2017;42(3):357-361.) |
3. The AD courses between the AL and the adductor brevis (AB), whereas the PD courses between the AB and adductor magnus (AM). Most commonly, the AD provides 2 to 3
branches (95%) to the AL, AB, and gracilis muscle. Less commonly, the AD may provide a fourth branch (5%) to the PM.9 The AD provides a variable degree of cutaneous sensory distribution to the distal medial thigh.11 At the lower border of the AL, the distal continuation of the AD occasionally communicates with the medial cutaneous and saphenous branches of the FN to form a subsartorial plexus that supplies the skin on the medial side of the thigh.
branches (95%) to the AL, AB, and gracilis muscle. Less commonly, the AD may provide a fourth branch (5%) to the PM.9 The AD provides a variable degree of cutaneous sensory distribution to the distal medial thigh.11 At the lower border of the AL, the distal continuation of the AD occasionally communicates with the medial cutaneous and saphenous branches of the FN to form a subsartorial plexus that supplies the skin on the medial side of the thigh.
4. The PD commonly separates into two muscular branches providing innervation to the AB and AM. Less commonly, the PD may provide additional branches that supply the OE and AL. The posterior branch descends between the AB and AM and perforates the AM distally at its opening. It then enters the distal aspect of the AC and courses with the SFA through the adductor hiatus to enter the popliteal fossa. Within the popliteal fossa, the posterior branch anastomoses with branches of the tibial nerve to supply the posterior aspect of the knee joint.4,6
5. An accessory obturator nerve (AON) may be present in 10% to 30% of patients and is derived from the ventral rami of the L3 and L4 spinal nerves or directly from the ON.12 The AON courses along the posterior aspect of the external iliac artery and descends caudally over the superior pubic ramus giving off branches to the pectineus and hip joint. The presence of an AON may have clinical consequences for hip surgery. Consequently, achievement of complete ON block, especially to the hip joint, may require an approach that consistently blocks the AON, when present.
D. Lateral femoral cutaneous nerve
1. The LFCN is a purely sensory nerve formed from the PDs of the ventral rami of the L2 and L3 spinal nerves (Fig. 10.8A). It emerges from the lateral border of the PMM coursing obliquely across the iliacus muscle (deep to the fascia iliaca) toward the anterior superior iliac spine (ASIS) (Fig. 10.8A). The LFCN continues distally, most commonly, deep to the inguinal ligament. However, it may also pass through a split in the inguinal ligament, and in cases of nerve entrapment, through an iliac bone canaliculus.13
2. The course of the LFCN as it enters the thigh, particularly in relation to the inguinal ligament and ASIS demonstrates considerable variability.13,14 It is typically located within 2 to 3 cm medial to the ASIS, although it may be located up to 7 cm medial, or even lateral to ASIS. The LFCN most commonly enters the thigh as a single branch (72% of cases), although it can divide into 2 to 5 branches proximal to the inguinal ligament. As the LFCN enters the thigh, it is most commonly located superficial to the sartorius muscle and deep to the fascia iliaca. It may also pass through the muscle (in up to 22% of cases), and rarely it may even pass superficial to the fascia lata.
3. The LFCN provides cutaneous sensory innervation of the lateral thigh from the area of the greater trochanter as far distal as the lateral aspect of the knee, but may also extend to the medial aspect of the thigh and distally to the patella (Fig. 10.8B).14,15
II. Drugs. To select the optimal local anesthetic agent, the operator must decide whether the aim is postoperative analgesia (in the context of concomitant general anesthesia or spinal anesthesia, as well as the presence of continuous perineural catheter) or surgical anesthesia. Tables 10.1 and 10.2 provide suggested duration(s) of anesthesia and analgesia, but may vary significantly with type and dose of local anesthetic, local anesthetic adjuvants (e.g., epinephrine or dexamethasone) interindividual patient variability, and block location. Surgical anesthesia for lower extremity surgery will require additional neural blockade (sciatic nerve) based on the surgical considerations (i.e., location of surgery and tourniquet requirements). ON or LFCN blocks are rarely indicated for surgical anesthesia.
TABLE 10.1 Local anesthetic choices for posterior lumbar plexus (psoas compartment) block | |||||||||||||||
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TABLE 10.2 Local anesthetic choices for femoral nerve block | |||||||||||||||
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III. Approaches and techniques
CLINICAL PEARL
The term “approach” refers to the anatomic location where the lumbar plexus or its peripheral nerve branches are targeted (e.g., posterior paravertebral for psoas compartment vs. inguinal perivascular for FN). The term “technique” refers to the modality for nerve localization (e.g., ultrasound guidance [USG], peripheral nerve stimulation [PNS], loss of resistance, or a combined technique) utilized for a given approach.
In addition, continuous lumbar plexus and lower extremity perineural catheter techniques provide superior postoperative analgesia compared to traditional systemic opioid-based therapy and comparable postoperative analgesia to lumbar epidural infusions after major lower extremity surgery.16
USG has become the predominant technique for lower extremity peripheral nerve localization. USG has not only been shown to increase the onset of complete sensory block but also decreases block performance time, block onset time, and local anesthetic requirements compared to PNS.17 There is no evidence to support the routine use of concurrent PNS to supplement a primary USG (“dual-guidance”) technique, especially when the target nerve is well visualized. However, dual guidance may be useful in two specific circumstances: (1) when target nerve visualization is difficult in specific circumstances, such as increased body habitus, deep nerve location (lumbar plexus-psoas compartment approach), or anatomic variation and (2) evoked motor responses at current output ≤0.2 mA is highly suggestive of intraneural needle placement, and should prompt slight withdrawal or repositioning of the needle tip prior to local anesthetic injection; however, stimulatory thresholds >0.2 mA do not offer a fail-safe guarantee that the needle tip is extraneural.
A. Lumbar plexus (psoas compartment approach) block
CLINICAL PEARL
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