Terminal Nerve Blocks of the Lower Extremity

Fig. 25.1

Surface anatomy and landmarks for femoral nerve blocks

The patient lies supine with the legs extended and the hip slightly externally rotated after general anesthesia has been induced. Surface landmarks include:

Inguinal ligament:
- Attached medially to the pubic tubercle (approximately 0.5–1 cm from midline on upper pubis border, depending on the age and size of the child) and laterally to the anterior superior iliac spine (ASIS).
Femoral/inguinal crease:
- Natural oblique skin fold parallel and 0.5–1 cm distal to the inguinal ligament (depending on the age and size of the child); the femoral artery is most superficial here.
Femoral artery pulse:
- The femoral artery lies at the “mid-inguinal point,” at the junction between the medial third and lateral two thirds of the inguinal ligament, although it is most superficial at the femoral crease.
- Located medial to the nerve.

The needle is inserted at the inguinal crease: approximately 0.5–1 cm lateral to the femoral artery and approximately 0.5–1 cm below the inguinal ligament, depending on the age and weight of the child. Ultrasound guidance may identify a more optimal needle insertion site.

25.1.3 Nerve Stimulation Technique

25.1.3.1 Needle Insertion

A flowchart illustrating the needle insertion site and procedures is shown in Fig. 25.2.

Fig. 25.2

Flowchart of needle insertion and procedures for femoral nerve blocks

Insert a 35–50 mm, 22G short-beveled, insulated needle using an angle of approximately 30–45° in a cephalad direction.
Aspirate the needle frequently since the femoral artery is situated close to the nerve.
As the needle advances, loss-of-resistance “pops” may be felt upon penetration of the fascia lata and iliaca, although penetration of the fascia iliaca may sometimes be difficult to feel. These “pops” may be more pronounced if using a lateral needle puncture point (see “Clinical Pearls” below).
In the case of femoral arterial puncture, compress the artery for 5–10 min to prevent hematoma formation. Repeat the procedure in a more lateral direction with care.

25.1.3.2 Current Application and Appropriate Responses

Figure 25.3 illustrates the procedure for employing nerve stimulation techniques for femoral nerve block. See Table 14.3 for expected motor responses during nerve stimulation.

Fig. 25.3

Flowchart of procedure for employing nerve stimulation techniques for femoral nerve blocks

Using nerve stimulation at an initial current of 0.8–1 mA (1 Hz, 0.1–0.2 ms), a quadriceps femoris muscle response (“patellar kick”) is sought. A current threshold of 0.5 mA is used for accurate localization.

25.1.3.3 Modifications to Inappropriate Responses (Table 25.1)

Table 25.1

Responses and recommended needle adjustments for use with nerve stimulation during femoral nerve block

Correct response from nerve stimulation

The most reliable response is a visible or palpable ipsilateral femoral muscle twitch (patella twitch) at 0.3–0.5 mA current, which indicates that one is stimulating the posterior division of the nerve. If twitches of the sartorius muscle occur, the needle may be outside the nerve sheath, and one may be stimulating the proximal branch which supplies the sartorius muscle

Other common responses and needle adjustments

Muscle twitches from electrical stimulation

Iliopsoas or pectineus (direct stimulation of muscle)

Explanation: too superior or deep needle tip placement

Needle adjustment: withdraw needle completely and reinsert

Sartorius (branches of femoral nerve to sartorius)

Explanation: needle tip too anteromedial to main femoral nerve trunk

Needle adjustment: redirect needle laterally and advance 1–3 mm deeper

Bone contact

Hip or superior ramus of pubic bone

Explanation: needle tip too deep

Needle adjustment: withdraw needle to subcutaneous tissue and reinsert

No response

Explanation: needle tip often too medial or lateral

Needle adjustment: withdraw completely and reinsert after checking landmarks

Vascular puncture

Femoral artery or vein

Explanation: needle tip too medial

Needle adjustment: withdraw needle completely and reinsert laterally

An algorithm of modifications in the case of inappropriate responses to nerve stimulation is shown in Fig. 25.4.

Fig. 25.4

Flowchart of modifications to inappropriate responses to nerve stimulation during femoral nerve blocks

Clinical Pearls

For a fascia iliaca block, loss-of-resistance technique is used instead of nerve stimulation since the needle is placed away from the nerve. The puncture point is 0.5–1 cm below the inguinal ligament (depending on the age and size of the child) at the interception of one third lateral and two thirds medial of the inguinal ligament (pubic tubercle-ASIS). The needle is introduced perpendicularly to the skin plane. Two “pops” are felt when the needle traverses the fascia lata and iliacus and enters the iliopsoas muscle.

25.1.4 Ultrasound-Guided Technique

For a summary of ultrasound-guided techniques in femoral nerve blocks, see Fig. 25.5. Major anatomical structures surrounding the femoral nerve as captured by MRI and VHVS images are shown with the corresponding ultrasound image in Fig. 25.6.

Fig. 25.5

Flowchart of ultrasound-guided techniques in femoral nerve blocks

Fig. 25.6

(a) VHVS and MRI images of major anatomical structures surrounding the femoral nerve. (b) Ultrasound image of major anatomical structures surrounding the femoral nerve

Prepare the needle insertion site and skin surface with an antiseptic solution. Prepare the ultrasound probe surface by applying a sterile adhesive dressing to it prior to needling as discussed in Chap. 4.

25.1.4.1 Scanning Technique

A 5–10 MHz hockey stick probe can be used for most children and will allow good axial resolution of the nerve in order to distinguish it from the surrounding structures (vessels and muscles).
Position the probe transverse to the nerve axis in the proximal thigh, approximately 0.5–1 cm inferior to the inguinal ligament and along the inguinal crease. The nerve should appear approximately 0.5–1 cm deep (depending on the size and age of the child) and just lateral to the femoral artery.
Color Doppler may be used to localize the femoral artery and vein.
If the nerve is difficult to characterize, as in obese children, a useful reference landmark is the profunda femoris (deep femoral) artery (Fig. 25.7a). Scan distally to locate the point where profunda femoris artery branches off deep to the femoral artery (Fig. 25.7b). Trace this artery proximally again to its convergence with the femoral artery; the femoral nerve will be located lateral to the artery.

Fig. 25.7

(a) VHVS and MRI images of the profunda femoris artery. (b) Ultrasound image with color Doppler used to localize the profunda femoris artery
Tilting the probe from perpendicular usually helps to improve the visualization of the femoral nerve (see “Clinical Pearls” below).
Color Doppler may be used to localize the profunda femoris artery (Fig. 25.7b).

25.1.4.2 Sonographic Appearance

Short-axis plane below inguinal crease (Fig. 25.6b):
- The nerve lies lateral and deep to the large, circular, and anechoic femoral artery.
- The fascia lata (most superficial) and iliaca (immediately adjacent to the nerve, separating the nerve from the artery) are superficial to the femoral nerve and often appear bright and longitudinally angled. The fascia may be better identified in contrast to the hypoechoic injectate after injection.
- The femoral nerve often appears oval shaped, although it may be triangular and of variable size due to inconsistencies in diameter throughout its course. For example, early division above the inguinal ligament can increase the transverse diameter of the nerve.
- The iliopsoas muscle may be seen deep to the nerve.

25.1.4.3 Needle Insertion

Insert a 35–50 mm, 22G–25G needle either in-plane (IP) (Fig. 25.8) or out of plane (OOP) (Fig. 25.9 to the transverse probe at the location identified by ultrasound (approximately at the inguinal crease).

Fig. 25.8

In-plane needling technique for ultrasound-guided femoral nerve block. Blue rectangle indicates probe footprint

Fig. 25.9

Out-of-plane needling technique for ultrasound-guided femoral nerve block. Blue rectangle indicates probe footprint
For an IP approach, insert the needle in a lateral-to-medial direction from the lateral edge of the probe. This will help to ensure that the needle approaches the nerve before the artery or vein.
For an OOP approach, the needle will be inserted caudad to the probe in one of the following ways:
- After identifying the nerve, center the screen on the needle target area (just lateral to the nerve). At a distance from the probe that is equal to the depth of the target area, insert the needle caudad to the probe at a 45° angle to the skin, and advance the needle until the tip is visualized at the target location. The hypoechoic spread of a test dose of D5W is helpful to locate the needle tip.
- An OOP “walk down” approach with incremental, stepwise angulation of the needle will provide immediate localization of the needle tip as a bright dot, allowing the tip to be followed to the required depth.
Try to localize the needle tip beneath the fascia iliaca.

Clinical Pearls

It is important to ensure that the ultrasound beam is perpendicular to the nerve’s transverse axis to minimize anisotropic effects altering the echogenic properties of the structure. It has been shown that an approximate 10° cephalad or caudad tilt of the transducer can make the nerve isoechoic (similar appearing) to the underlying iliopsoas muscle.

25.1.5 Local Anesthetic Application

Inject 0.2–0.5 mL/kg of 0.25 % bupivacaine or 0.2 % ropivacaine without exceeding the recommended toxic dose of local anesthetic (2 mg/kg for bupivacaine and 3–4 mg/kg for ropivacaine without epinephrine).
Oberndorfer et al. [1] showed that a lower volume of local anesthetic is required when using ultrasound-guided technique as compared to the nerve stimulator approach in children between the ages of 1 and 8 (0.2 mL/kg versus 0.3 mL/kg of 0.5 % levobupivacaine).
Farid et al. [2] described the use of 0.5 mL/kg of 0.2 % ropivacaine to a maximum volume of 40 mL in patients aged 8–16 years (without US).
Sethuraman et al. [3] used 0.2 mL/kg 0.5 % bupivacaine with the total dose not exceeding 1–1.5 mg/kg in their cohort of patients <10 years old undergoing muscle biopsy (without ultrasound).
Perform intermittent injection with interval aspiration.
If a sciatic nerve block will be administered in addition to the femoral block, use less (e.g., two thirds) of the local anesthetic solution in order to avoid local anesthetic toxicity.

Ultrasound Considerations

Performing a test dose with D5W is recommended prior to local anesthetic application to visualize the hypoechoic spread and confirm nerve localization.
Local anesthetic spread can be seen as an expanding hypoechoic area; it should occur beneath the fascia iliaca and should surround the nerve.
The solution may displace the nerve medially toward or laterally away from the artery, depending on the needle approach.

25.1.6 Current Literature in Ultrasound-Guided Approaches

Oberndorfer et al. [1] compared ultrasound guidance to nerve stimulator technique for sciatic and femoral nerve blocks in a randomized study of 46 children. The primary outcome of the study was the duration of the nerve blockade. The study showed that ultrasound-guided blocks lasted longer (508 vs. 335 min) and that a lower volume of local anesthetic (0.2 mL/kg vs. 0.3 mL/kg of levobupivacaine 0.5 %) was required for an adequate analgesia compared to the nerve stimulator technique.

Gurnaney et al. [4] reported a combined use of ultrasound and nerve stimulation in the placement of a femoral perineural catheter in a pediatric patient with variant anatomy. A 15-year-old presented to the operating room for an arthroscopic chondroplasty of medial and lateral condyles and patella and open lateral release of the knee. After induction of general anesthesia, a linear transducer revealed a nerve-like structure lateral to the femoral artery; stimulation of this structure resulted in a motor response of the sartorius muscle. Stimulation of another hyperechoic structure posterior to the iliacus muscle elicited contraction of quadriceps femoris muscle. Twenty milliliters of 0.2 % ropivacaine was then injected through a stimulating nerve catheter.

A recent case study [5] demonstrated the value of a femoral nerve block for rapid analgesia in an emergency setting: a 3-month-old infant presented with a subtrochanteric femoral neck fracture due to non-accidental trauma. The patient received one dose of morphine and two doses of fentanyl prior to the block. Ultrasound was placed 1 cm distal to the inguinal ligament of the limb in question, allowing visualization of the femoral nerve and surrounding structures. Under ultrasound guidance, a 27G needle was inserted in-plane and used to inject 2 mL 0.25 % bupivacaine (1.25 mg/kg) to surround the nerve. After 15 min, pain control was sufficient to place a Pavlik harness on the patient; only one dose of analgesic was required in the 18 h following the block.

In a recent letter, Miller [6] described combined ultrasound-guided femoral and lateral femoral cutaneous nerve block for pediatric patients undergoing surgical repair of femur fractures. The femoral nerve was blocked first using a 50 mm, 22G needle inserted in-plane in a lateral-to-medial direction, followed by injection of ropivacaine 0.2 % (0.2–0.5 mL/kg; max. volume 20 mL). The needle was withdrawn, and the ultrasound transducer was moved laterally along the inguinal ligament until its lateral aspect is in contact with the ASIS; the lateral femoral cutaneous nerve and local anesthetic from the previous injection was visualized. The same needle was inserted in-plane so that the needle tip was placed between the fascia lata and the fascia iliaca. Ropivacaine 0.2 % (0.05–0.2 mL/kg; max. volume 6 mL) was injected and observed to surround the lateral femoral cutaneous nerve.

Miller also provided the first report of ultrasound-guided fascia iliaca compartment block in pediatric patients [7]. With the ultrasound probe positioned in the long axis and located mid-point over the inguinal ligament, lateral to the border of the middle and lateral third of the ligament, the ilium, iliacus muscle, and fascia can be visualized. A 100 mm, 21G or 50 mm, 22G needle was inserted in-plane approximately 1–2 cm below the inguinal ligament, directed cephalad, and advanced until the tip was just below the fascia iliaca and in the iliacus muscle. An appropriate volume of ropivacaine 0.2 % was injected in this area achieving blockade of the femoral, obturator, and lateral femoral cutaneous nerves. Pain control was excellent in all three cases, with no requirement for narcotics in the recovery unit.

Ponde et al. [8] performed a randomized clinical trial to examine success rates of combined femoral and sciatic block for pediatric patients suffering from arthrogryposis multiplex congenital who were undergoing foot surgery. The results demonstrated significantly higher block success rates using ultrasound guidance compared to guidance by nerve stimulation alone. Interestingly, post-surgery analgesia duration was extended by an hour in the ultrasound group, although the clinical significance of this finding is minimal.

Clinical Pearls

Although there is no direct evidence to prove that ultrasound can reduce the risk of inadvertent vessel puncture, this has been the case in our experience. Prevention of intraneural local anesthetic injection or direct intravascular puncture may not be possible in all cases (as in situations where the needle tip is not clearly seen and the nerve may not be visible). However, precise placement of local anesthetic, enabled by ultrasound imaging, may lessen the frequency of injecting large volumes of local anesthetic to ensure adequate spread, as is often the case when using blind techniques.
Our preference is to use an IP technique, which offers the ability to visualize the needle tip and the needle shaft as it advances beneath the fascia iliaca.

25.1.7 Case Study

Case Study: Femoral Nerve Block (Contributed by S. Suresh)

An 8-year-old male, 34.2 kg, with no past medical or relevant family history was presented for physeal-sparing left knee anterior cruciate ligament reconstruction with autogenous iliotibial band and lateral meniscus repair based on MRI of the left knee without contrast. The patient received an in-plane, ultrasound-guided femoral nerve block with a 50 mm 20G needle (10 mL 0.25 % bupivacaine) (see Fig. 25.10), followed by placement of a perineural catheter. Duration of surgery was 2 h, 53 min. Postoperative analgesia reporting was 0/10 (verbal) immediately following surgery and 7/10 (verbal) 30 min after surgery. Postoperatively, the patient received a morphine injection (2 mg) and a continuous infusion of bupivacaine 0.1 % solution.