The anatomy of the lumbar plexus is described in detail in Chapter 8 (Fig. 3–1). The terminal nerves of the lumbosacral plexus relevant for innervating the lower extremity include the lateral cutaneous nerve of the thigh, the femoral nerve, the obturator nerve, and the sciatic nerve. The lateral cutaneous nerve of the thigh and the femoral nerve leave the lumbar plexus along the posterolateral border of the psoas major muscle; the obturator nerve emerges from the medial border of the psoas muscle at the pelvic brim and crosses in front of the sacroiliac joint. 1 The sacral plexus provides sensorimotor innervation to the posterior thigh, most of the lower extremity, the entire foot, and parts of the pelvis. It is formed by the union of the anterior primary rami of the spinal nerves of L4, L5, S1, S2, S3, and S4 (lumbosacral plexus, Fig. 3–2). The sacral plexus lies deep within the pelvis between the piriformis muscle posteriorly and the pelvis fascia anteriorly (Fig. 3–3). The sigmoid colon, ureter, internal iliac artery, and vein lie anterior to it. The superior gluteal artery and vein lies between the lumbosacral trunk and the first sacral nerve, and the inferior gluteal artery and vein lie between the second and third sacral nerves. The nerves forming the sacral plexus converge as they descend towards the lower part of the greater sciatic foramen and unite within the pelvis to form the sciatic nerve (Fig. 3–4). The sciatic nerve is the largest (thickest) nerve of the body and exits the pelvis through the greater sciatic foramen, between the piriformis and the superior gemellus muscles (Fig. 3–5), to enter the “subgluteal space” between the greater trochanter and ischial tuberosity (Figs. 3–6 and 3–7). 2,3 Sciatic nerve and piriformis muscle anomaly are seen in 16.2% (95% CI: 10.7–23.5%) of individuals. 4 The entire sciatic nerve or one of its components (tibial or common peroneal) may rarely exit the pelvis by passing through or above the superior border of the piriformis muscle. 4 The sciatic nerve, after it emerges from the pelvis, descends along the back of the thigh, lying deep to the semitendinosus and biceps femoris muscles, to about its lower third (Figs. 3–8 and 3–9), where it bifurcates into its two branches: the tibial and common peroneal (fibular) nerves. This bifurcation may take place at any point between its origin at the sacral plexus and the lower third of the thigh or at a variable distance from the popliteal crease. 5 The tibial and common peroneal nerves may also arise separately from the sacral plexus.
FIGURE 3–4
Anatomical illustration (dorsal view) showing the sciatic nerve as it exits the pelvis through the greater sciatic foramen. Note the relation of the superior and inferior gluteal nerves, posterior cutaneous nerve of the thigh, nerve to obturator internus, and pudendal nerve to the sciatic nerve as they exit the greater sciatic foramen.
FIGURE 3–6
Multiplanar 3-D anatomy (rendered from the Visible Human Server) of the sciatic nerve at the subgluteal space. Note the reference marker (green crosshair) has been placed over the sciatic nerve in the transverse view and its corresponding position in the sagittal and coronal images can be seen. AM, adductor magnus; VL, vastus lateralis; IT, ischial tuberosity; QF, quadratus femoris; GM, gluteus maximus; GS, gemellus superior; GI, gemellus inferior; BF, biceps femoris; OI, obturator internus; PF, piriformis.
FIGURE 3–8
Multiplanar 3-D anatomy of the sciatic nerve at the midthigh. AL, adductor longus; AM, adductor magnus; BF, biceps femoris; GM, gluteus maximus; RF, rectus femoris; SM, semimembranosus; SR, sartorius; ST, semitendinosus; VI, vastus intermedialis; VL, vastus lateralis; VM, vastus medialis.
FIGURE 3–9
Multiplanar 3-D anatomy of the sciatic nerve at or close to the apex of the popliteal fossa. AM, adductor magnus; AL, adductor longus; BF, biceps femoris; GR, gracilis; SM, semimembranosus; SR, sartorius; ST, semitendinosus; VI, vastus intermedialis; VL, vastus lateralis; VM, vastus medialis; RF; rectus femoris.
The femoral nerve is the largest branch of the lumbar plexus and originates from the posterior divisions of the anterior primary rami of the L2, L3, and L4 spinal nerves. It descends through the fibers of the psoas muscle and exits the lateral border of the inferior part of the psoas muscle in the retroperitoneal space. It then descends between the psoas and the iliacus muscle deep to the fascia iliaca. It enters the femoral triangle of the thigh behind the inguinal ligament, lying lateral to the femoral artery and in a groove between the iliacus and psoas muscles (Fig. 3–10 and 3–11). In between the inguinal ligament and the inguinal crease, the femoral vein, femoral artery, and the femoral nerve have a “VAN” (vein, artery, nerve) relation from the medial to lateral side (Fig. 3–11). The femoral artery and vein are enclosed by the femoral sheath and lie deep to the fascia lata (deep fascia of the thigh), and the femoral nerve lies outside the femoral sheath and deep to both the fascia lata and fascia iliaca on the anteromedial aspect of the iliopsoas muscle (Fig. 3–12). The femoral nerve divides into its anterior and posterior branch after a short course of about 2 cm below the inguinal ligament or at the level of the inguinal crease. 6
FIGURE 3–12
Fascial anatomy in relation to the femoral nerve at the level of the inguinal crease. Note both the femoral artery and vein lie deep to the fascia lata and are enclosed by the femoral sheath, and the femoral nerve lies outside the femoral sheath and deep to both the fascia lata and iliaca.
Position:
Patient: Supine with the ipsilateral leg slightly abducted and externally rotated and the knee slightly flexed.
Operator and ultrasound machine: The operator stands on the side of the intervention and faces the patient’s head. The ultrasound machine is placed on the same side between the operator and the patient’s head. Alternatively, the operator may choose to position the ultrasound machine based on his or her “handedness.” Right-handed operators who hold the ultrasound transducer with their left hand and carry out needle interventions with their right hand should stand on the right side of the patient and position the ultrasound machine on the contralateral side and directly in front. This is vice versa for left-handed operators.
Transducer selection: High-frequency (15-8 or 17-5 MHz) linear array transducer.
Scan technique: For a transverse scan of the femoral nerve, place the ultrasound transducer parallel to the inguinal ligament and approximately 1 cm proximal to the inguinal crease (Figs. 3–15 to 3–17). Gently slide the transducer in a medial to lateral direction until a cross-sectional view of the femoral artery is obtained. The femoral vein lies medial to the femoral artery, and the femoral nerve is lateral to the artery (Fig. 3–18). The femoral vein is compressible, but the femoral artery may not be easily compressible. Color or Power Doppler should be used to differentiate the femoral artery from the vein as part of one’s scan routine (Fig. 3–19). The femoral nerve is most commonly seen on the anteromedial surface of the iliopsoas muscle (Fig. 3–18).
Sonoanatomy: The femoral nerve is typically identified on the anteromedial surface of the psoas muscle as a flat, hyperechoic, and elliptical-shaped structure (Fig. 3–18). Outlines of the fascia iliaca, with the femoral nerve lying deep to this fascia, may be visualized in some individuals (Fig. 3–18).
Clinical Pearls: The femoral nerve is markedly anisotropic in the inguinal region. 7 Therefore, it may be necessary to gently tilt or rotate the transducer during the ultrasound scan before it can be clearly delineated. It is our experience that the position of the femoral nerve, relative to the femoral artery, in the femoral triangle is quite variable. Therefore, we prefer to look for the femoral nerve on the anteromedial surface of the iliopsoas muscle rather than immediately lateral to the femoral artery during the scan. Also in order to locate the femoral nerve before it divides into its anterior and posterior branches, it is preferable to start the ultrasound scan immediately below the inguinal ligament rather than at the inguinal crease. The profunda femoris artery, which is the largest branch of the femoral artery, can be a useful clue as to the level at which the ultrasound scan is being performed. If the profunda femoris artery is seen adjacent (lateral) to the femoral artery in the ultrasound image (Fig. 3–20), it indicates that the ultrasound scan is being performed too low and below the division of the femoral nerve because the profunda femoris artery is generally given off from the femoral artery, about 4 cm below the inguinal ligament.
The obturator nerve is a branch of the lumbar plexus and formed by the anterior division of the anterior primary rami of the L2, L3, and L4 spinal nerves. It exits the pelvis and enters the thigh through the obturator canal. It then divides into its anterior and posterior divisions, usually lateral and distal to the pubic tubercle (Fig. 3–1). 8 The anterior division courses distally, lying between the adductor brevis and the adductor longus muscles, and the posterior division passes distally between the adductor brevis and adductor magnus muscles (Figs. 3–21 and 3–22).
Position:
Patient: Supine with the ipsilateral leg straight and slightly externally rotated at the hip. This position allows optimal visualization of the obturator nerve and its branches. 8
Operator and ultrasound machine: The operator stands on the ipsilateral side of the scan or intervention and faces the patient’s head. The ultrasound machine is placed on the ipsilateral side directly in front of the operator. Alternatively, the operator may choose to position the ultrasound machine depending on his or her “handedness.” Right-handed operators who hold the ultrasound transducer with their left hand and carry out needle interventions with their right hand should stand on the right side of the patient and position the ultrasound machine on the opposite side of the patient. This is vice versa for left-handed operators.
Transducer selection: High-frequency (15-8 or 17-5 MHz) linear array transducer.
Scan technique: The transducer is placed in the transverse orientation 2 cm distal to the pubic tubercle on the medial aspect of the thigh (Figs. 3–25 and 3–26). Alternatively start the ultrasound scan by placing the transducer parallel to the inguinal ligament and over the inguinal crease. 9 Then slide the transducer medially until the pectineus is visualized on the lateral aspect of the ultrasound screen. 9 At this point, the adductor muscles (longus, brevis, and magnus) are visualized adjacent to the pectineus (Fig. 3–27). Because the anterior and posterior divisions of the obturator nerve are flat and small nerves, 8 it is easier to identify them in their respective intermuscular fascial planes by sliding the transducer proximally and distally analogous to the trace back technique. Slightly tilting or rotating the transducer may also help improve visualization. If one traces the two divisions of the obturator nerve proximally, they are seen to come together to form the common obturator nerve. 8 Color or Power Doppler ultrasound can also be used to identify the obturator artery that accompanies the common obturator nerve. 8
Sonoanatomy: The common obturator nerve or its divisions (anterior and posterior) are not readily identified as discrete nerves on ultrasound imaging, as they are small and flat nerves. 8 Unlike other peripheral nerves, the anterior and posterior divisions of the obturator nerve appear as two flat and hyperechoic structures in the intermuscular fascial planes between the adductor muscles (Fig. 3–28).
Clinical Pearls: The anterior division travels in the intermuscular plane between the adductor longus and adductor brevis muscles. The posterior division travels in the plane between the adductor brevis and adductor magnus muscles. The typical appearance on a transverse sonogram would include the pectineus muscle on the lateral aspect of the screen and the three adductors muscles on the medial aspect, with the adductor longus being most superficial, the adductor brevis in the middle, and the adductor magnus deepest, respectively (Figs. 3–27 and 3–28). Small branches of the obturator vessels accompany the divisions of the obturator nerve in the intermuscular plane and can be identified using Color or Power Doppler ultrasound. 8 However, to what extent this is reliable in locating the nerves is yet to be determined, as the position of the obturator vessels relative to the nerves is variable.
The lateral cutaneous nerve of the thigh, also called the lateral femoral cutaneous nerve of the thigh, innervates the skin on the lateral aspect of the thigh. It is a branch of the lumbar plexus and formed within the psoas muscle by the fusion of the posterior divisions of the L2 and L3 spinal nerves. It exits the psoas muscle from its lateral border, in the retroperitoneum, at about its middle and travels across the iliacus muscle obliquely lying deep to the fascia iliaca (Fig. 3–1). It enters the thigh medial to the anterior superior iliac spine (ASIS) lying under the lateral edge of the inguinal ligament (Figs. 3–29 and 3–30). It then crosses over the sartorius muscle in a medial to lateral direction. The course of the lateral cutaneous nerve of the thigh is highly variable. It is found most commonly 10 to 15 millimeters medial to the ASIS but can be located as far medially as 46 millimeters. 10 Its depth in relation to the soft tissues in the region, the sartorius, and the inguinal ligament is also highly variable. Five different variations have been identified: type A, posterior to the ASIS, across the iliac crest; type B, anterior to the ASIS and superficial to the origin of the sartorius muscle but within the substance of the inguinal ligament; type C, medial to the ASIS, ensheathed in the tendinous origin of the sartorius muscle; type D, medial to the origin of the sartorius muscle located in an interval between the tendon of the sartorius muscle and thick fascia of the iliopsoas muscle deep to the inguinal ligament; and type E, most medial and embedded in loose connective tissue, deep to the inguinal ligament, overlying the thin fascia of the iliopsoas muscle, and contributing the femoral branch of the genitofemoral nerve. 11
FIGURE 3–30
Transverse anatomical section of the upper thigh and lower abdomen a few centimeters distal to the anterior superior iliac spine showing the anatomy related to the lateral femoral cutaneous nerve (the nerve is not seen in this image), which usually lies on the anterior surface of the sartorius muscle or in the groove between the sartorius and the iliacus muscles at this level. IO, internal oblique muscle; TA, transversus abdominis muscle.
Position:
Patient: Supine position
Operator and ultrasound machine: The operator may stand on the ipsilateral side of the intervention and face the patient’s head. The ultrasound machine is placed on the same side between the operator and the patient’s head. Alternatively, the operator may choose to position the ultrasound machine depending on his or her “handedness.” Right-handed operators who hold ultrasound transducer with their left hand and carry out needle interventions with their right hand should stand on the right side of the patient and position the ultrasound machine on the opposite side of the patient. This is vice versa for left-handed operators.
Transducer selection: High-frequency (15-8 or 17-5 MHz) linear array transducer.
Scan technique: The transducer is placed with one edge on the ASIS. The medial edge of the transducer is rotated slightly caudally such that the transducer is parallel to the inguinal ligament. Slide the transducer medially along the inguinal ligament (Figs. 3–32 to 3–34). The ASIS appears as a hyperechoic line with an acoustic shadow. Immediately medial to the ASIS is the iliacus muscle. At the level of the inguinal ligament, the lateral cutaneous nerve can be visualized deep to the fascia lata just medial to the ASIS. 12 The transducer can be slid distally approximately 5 cm caudad to the ASIS and rotated to a transverse orientation relative to the femur. At this location, the lateral cutaneous nerve of the thigh is located on the sartorius muscle or in the groove between the sartorius and the iliacus muscles (Fig. 3–35).
Sonoanatomy: The lateral cutaneous nerve of the thigh is a small nerve that may appear as a hypoechoic to hyperechoic structure. At the level of the inguinal ligament, it lies medial to the ASIS and deep to the fascia iliacus. It then courses distally in the groove between the sartorius and iliacus, crossing over the anterior surface of the sartorius (Fig. 3–35) to the lateral aspect of the sartorius muscle.
Clinical Pearls: The lateral cutaneous nerve of the thigh is a small nerve and can be best visualized using a high-frequency linear transducer. The “trace back” technique is important and useful to confirm the identity of the nerve. The important landmarks here are the medial edge of the ASIS, the groove between the satorius and iliacus, and the anterior surface of the sartorius. The nerve can usually be located at one of these areas and “traced back” to confirm its identity along the course. Injection of a small volume of normal saline around the nerve can be used to delineate its course (hydrolocation). It is common to see the injectate spread along its course proximally under the inguinal ligament and under the fascia iliaca within the pelvis.
FIGURE 3–33
Figure showing the position and orientation of the ultrasound transducer during a transverse ultrasound scan for the lateral femoral cutaneous nerve at the inguinal region. Note the ultrasound transducer is positioned a few centimeters distal and medial to the anterior superior iliac spine.
Distal to the inguinal crease, the femoral nerve divides into its terminal branches. 6 The saphenous nerve is a branch of the anterior division of the femoral nerve and supplies the skin on the medial aspect of the leg and foot up to the ball of the big toe. It travels with the femoral artery within the anterior fascial compartment of the thigh under the sartorius muscle (subsartorial), and local anesthetic injected into this intermuscular space produces saphenous nerve block. 13 The “subsartorial canal” is also referred to as the adductor canal or Hunter’s canal and is located on the medial aspect of the middle one-third of the thigh (Fig. 3–36). The adductor canal is triangular in cross-section (Figs. 3–37 and 3–38) and extends from the apex of the femoral triangle, above, to the tendinous opening in the adductor magnus muscle (adductor hiatus), below. The anterior wall of the adductor canal is formed by the vastus medialis muscle; the posterior wall or floor is formed by the adductor longus, above, and the adductor magnus, below; and the roof or medial wall is formed by a strong fibrous membrane underlying the sartorius muscle (Figs. 3–37 and 3–38).
The adductor canal contains the following structures: femoral artery and vein, saphenous nerve, anterior and posterior division of the obturator nerve, and nerve to vastus medialis (Fig. 3–38). The femoral vein lies posterior to the femoral artery in the upper part of the adductor canal and lateral to the artery in the lower part of the canal (Fig. 3–39). The saphenous nerve crosses the femoral artery anteriorly from a lateral to medial direction. The “subsartorial plexus” of nerves lie on the fibrous roof of the adductor canal deep to the sartorius muscle (Fig. 3–38) and are formed by branches from the medial cutaneous nerve of the thigh, saphenous nerve, and anterior division of the obturator nerve. It supplies the neighboring skin and overlying fascia lata. The femoral artery exits the adductor canal through the adductor hiatus and continues as the popliteal artery. At the adductor hiatus, the saphenous nerve leaves the femoral artery and travels along the lower edge of the aponeurosis of the canal and is closely related to the saphenous branch of the descending genicular artery. 14 The saphenous nerve then courses distally along the medial side of the knee deep to the sartorius and pierces the fascia lata, between the tendons of the sartorius and gracilis muscles.
Position:
Patient: Supine position with the ipsilateral hip slightly externally rotated and knee slightly flexed.
Operator and ultrasound machine: The operator may choose to position the ultrasound machine based on his or her “handedness.” Right-handed operators who hold ultrasound probes with their left hand and carry out needle interventions with their right hand should stand on the right side of the patient and position the ultrasound machine on the opposite side of the patient. This is vice versa for left-handed operators.
Transducer selection: High-frequency (15-8 MHz) linear array transducer. A curved array low-frequency (5-2 MHz) transducer can also be used if one wishes to visualize the sciatic nerve, which is located at a depth, at the same time.
Scan technique: The ultrasound transducer is placed on a medial aspect of the thigh of the middle third of the thigh (midfemoral region) in the transverse orientation (Figs. 3–42 to 3–46). The reference structure to identify is the femoral artery in the transverse view. Thereafter slide the transducer along the medial border of the sartorius to visualize the artery at its most superficial location and just proximal to the point where the femoral artery passes the adductor hiatus to become the popliteal artery. 15 The sartorius is typically triangular/elliptical in shape when imaged transversely. Beneath the sartorius, the femoral artery and veins can be imaged and followed until they pass through the adductor hiatus.
Sonoanatomy: The saphenous nerve is a small nerve and may not be visualized as a discrete structure in all individuals at the adductor canal. When visualized, it is seen as a hyperechoic structure that is closely related to the femoral artery (Fig. 3–47).
Clinical Pearls: Because the saphenous nerve is a small nerve, the trace back technique 16 is useful for locating it. It can also be followed distally where it lies between the sartorius and the gracilis muscles and with the saphenous branch of the descending genicular artery. When there is difficulty visualizing the saphenous nerve, imaging the most superficial portion of the distal adductor canal and using a periarterial injection deep to the sartorius, medial to the artery is adequate for a successful saphenous nerve block. 15
FIGURE 3–45
Figure highlighting the anatomical structures that are imaged during a transverse ultrasound scan at the level of the midthigh using a low-frequency transducer. Note that the sciatic nerve is also included in the highlighted area and can be visualized during the midthigh (midfemoral) scan.
FIGURE 3–47
Transverse sonogram showing the boundaries and contents of the adductor canal. FA, femoral artery; FV, femoral vein.
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