Residency · Residency · Anesthesiology

Ultrasound-Guided Lower Extremity Blocks

Lower Extremity Innervation Overview

Lumbar Plexus (L1-L4)

The lumbar plexus gives rise to three major nerves supplying the lower extremity. The femoral nerve (L2-L4) innervates the anterior thigh, the knee, and the medial leg through its saphenous branch. The lateral femoral cutaneous nerve (L2-L3) provides sensation to the lateral thigh. The obturator nerve (L2-L4) innervates the medial thigh and hip joint, with a variable contribution to knee sensation.

Sacral Plexus (L4-S3)

The sacral plexus contributes the sciatic nerve (L4-S3), the dominant nerve of the lower extremity below the knee. It supplies the posterior thigh and the entire leg below the knee except for the medial saphenous territory. The sciatic nerve divides into the tibial and common peroneal (fibular) nerves, typically in the distal thigh or popliteal fossa. The posterior femoral cutaneous nerve (S1-S3) provides sensation to the posterior thigh.

Key Principle

Complete surgical anesthesia of the lower extremity requires coverage from both the lumbar and sacral plexuses. For knee surgery, a femoral (or adductor canal) block combined with a sciatic block covers most procedures. For ankle and foot surgery, a sciatic block at the popliteal level with or without a saphenous nerve block covers all relevant territories.

<image>Anatomical illustration of the lower extremity showing the cutaneous innervation map. The anterior, medial, lateral, and posterior surfaces of the thigh, knee, leg, and foot are color-coded by nerve territory: femoral (anterior thigh and medial leg via saphenous), lateral femoral cutaneous (lateral thigh), obturator (medial thigh), sciatic-tibial (posterior leg and sole), and sciatic-common peroneal (lateral leg and dorsum of foot). The lumbar and sacral plexus origins are shown in an inset.</image>

Femoral Nerve Block

Anatomy

The femoral nerve lies lateral to the femoral artery, deep to the fascia iliaca, within the femoral triangle at the inguinal crease. The relationship of structures from lateral to medial follows the mnemonic NAVEL: Nerve, Artery, Vein, Empty space, Lymphatics.

Sonoanatomy

A linear high-frequency probe is placed at the inguinal crease in transverse orientation. The femoral artery serves as the central landmark. The femoral nerve appears as a hyperechoic, triangular or ovoid structure lateral to the artery, lying deep to both the fascia lata and fascia iliaca.

Technique

An in-plane approach is used from lateral to medial, advancing the needle tip deep to the fascia iliaca and adjacent to the nerve. A volume of 15-20 mL is typical. A catheter can be placed for continuous analgesia.

Coverage

The femoral nerve block provides analgesia to the anterior thigh (quadriceps innervation), the medial leg below the knee (saphenous nerve), and the anterior knee (articular branches).

Limitations

The major limitation of the femoral nerve block is that it causes quadriceps weakness, creating a significant fall risk, particularly in elderly patients. Multiple studies have demonstrated an association between femoral nerve blocks and increased falls after total knee arthroplasty. This limitation has been the primary driver of the shift toward the adductor canal block.

Adductor Canal Block

Rationale

The adductor canal block was developed as a motor-sparing alternative to the femoral nerve block. It targets the saphenous nerve and articular branches to the knee within the adductor canal while preserving quadriceps strength. The motor branch to the vastus medialis typically joins the femoral nerve proximal to the typical injection site, explaining the motor preservation.

Anatomy

The adductor canal (subsartorial canal) lies beneath the sartorius muscle in the mid-thigh. Its boundaries are the vastus medialis laterally, the adductor longus and adductor magnus medially and posteriorly, and the sartorius forming the roof. The canal contains the saphenous nerve, the nerve to the vastus medialis (in its proximal portion), and the femoral artery and vein.

Sonoanatomy

A linear probe is placed at mid-thigh in transverse orientation. The femoral artery is identified deep to the sartorius muscle within the adductor canal. The saphenous nerve appears as a small hyperechoic structure lateral or anterolateral to the artery.

Technique

An in-plane approach from lateral to medial is used. Local anesthetic is injected deep to the sartorius, around the artery within the canal. A volume of 15-20 mL is typical.

Evidence: Adductor Canal vs. Femoral Block for TKA

Multiple randomized controlled trials have demonstrated that the adductor canal block provides equivalent or near-equivalent analgesia compared to the femoral nerve block for total knee arthroplasty. The critical advantage is significantly better quadriceps strength preservation and reduced fall risk. The adductor canal block is now preferred over the femoral nerve block for TKA by most centers. It may be combined with an iPACK block (infiltration between the popliteal artery and capsule of the knee) for posterior knee coverage.

<image>Side-by-side ultrasound illustration comparing femoral nerve block (left) and adductor canal block (right). The femoral nerve block shows the probe at the inguinal crease with the femoral nerve lateral to the femoral artery beneath fascia iliaca. The adductor canal block shows the probe at mid-thigh with the sartorius muscle as the roof, the femoral artery deep within the canal, and the saphenous nerve adjacent. Below each, a cross-sectional anatomical diagram labels all structures with arrows indicating the motor-sparing advantage of the adductor canal approach.</image>

Popliteal Sciatic Nerve Block

Indications

The popliteal sciatic nerve block is most commonly indicated for foot and ankle surgery. Other indications include below-knee amputation and Achilles tendon repair. It is typically combined with a saphenous or adductor canal block for complete below-knee anesthesia, since the sciatic nerve does not cover the medial ankle and foot.

Anatomy

The sciatic nerve bifurcates into the tibial and common peroneal (fibular) nerves in the popliteal fossa, with the bifurcation typically occurring 5-8 cm above the popliteal crease. The tibial nerve is the larger component, lying deep and medial. The common peroneal nerve is smaller, superficial, and lateral, following the biceps femoris tendon toward the fibular head.

Sonoanatomy -- Posterior Approach

With the patient prone or in lateral decubitus, a linear probe is placed transversely in the popliteal fossa. The popliteal artery and vein serve as deep landmarks. The tibial nerve appears as a hyperechoic, round structure superficial to the popliteal vessels. The common peroneal nerve is hyperechoic and tracks laterally and superficially. Scanning proximally reveals the point where the two nerves converge at the bifurcation.

Sonoanatomy -- Lateral Approach

When the patient cannot be positioned prone, the lateral approach is performed with the patient supine and the leg extended. The probe is placed between the biceps femoris and vastus lateralis, and the sciatic nerve is visualized between the femur (deep) and biceps femoris (superficial).

Technique

An in-plane approach is used. Injection at or just proximal to the bifurcation provides the most reliable blockade of both branches. A volume of 20-30 mL is used, with care to ensure circumferential spread. Injection within the common epineural sheath may facilitate distribution to both the tibial and common peroneal components.

Coverage

The tibial nerve provides coverage of the sole of the foot, posterior leg, and posterior ankle. The common peroneal nerve covers the dorsum of the foot, lateral leg, and anterior ankle. The medial leg and ankle (saphenous nerve territory) are not covered and require a separate saphenous block.

Ankle Block

Five Nerves at the Ankle

Five nerves must be blocked for complete foot anesthesia at the ankle level. The posterior tibial nerve is located posterior to the medial malleolus and supplies the sole of the foot. The sural nerve lies between the lateral malleolus and Achilles tendon, supplying the lateral foot. The saphenous nerve is anterior to the medial malleolus, supplying the medial ankle and foot. The deep peroneal nerve runs between the extensor hallucis longus and tibialis anterior tendons, supplying the first web space. The superficial peroneal nerve is subcutaneous over the anterolateral ankle, supplying the dorsum of the foot.

Technique

The ankle block combines ultrasound-guided perivascular injection for the posterior tibial nerve with subcutaneous infiltration for the saphenous, sural, and superficial peroneal nerves. The deep peroneal nerve is blocked adjacent to the dorsalis pedis artery. Each nerve receives 3-5 mL, for a total of 15-25 mL. A dilute concentration should be used to avoid toxicity when performing five separate injections.

Advantages

The ankle block avoids motor blockade of the leg, making it ambulatory surgery-friendly. It has a low complication rate and is well-suited for forefoot surgery in outpatient settings.

Fascia Iliaca Block

Indications

The fascia iliaca block is commonly used for hip fracture analgesia in the emergency department and preoperative setting, for hip arthroplasty as part of a multimodal regimen, and for anterior thigh and knee surgery.

Mechanism

A large-volume injection deep to the fascia iliaca spreads to block the femoral nerve and lateral femoral cutaneous nerve. Obturator nerve coverage is unreliable with the infrainguinal approach. The supra-inguinal approach, performed above the inguinal ligament, improves coverage of all three nerves.

Technique

The infrainguinal approach uses a linear probe at the inguinal crease. Local anesthetic (30-40 mL) is injected deep to the fascia iliaca, lateral to the femoral nerve. The supra-inguinal approach places the probe above the inguinal ligament in a lateral position, targeting the space between the fascia iliaca and the iliacus muscle with 30-40 mL. The supra-inguinal approach provides better coverage of the lateral femoral cutaneous and obturator nerves.

Clinical Pearls

The adductor canal block has largely replaced the femoral nerve block for total knee arthroplasty because it provides equivalent analgesia with preserved quadriceps function and reduced fall risk. For popliteal sciatic blocks, injecting at the bifurcation within the common paraneural sheath provides the most reliable blockade of both the tibial and common peroneal nerves. A popliteal sciatic block must always be supplemented with a saphenous nerve block (or adductor canal block) for medial ankle and foot surgery. The ankle block is ideal for forefoot surgery in ambulatory patients because it spares proximal motor function entirely. For hip fracture patients in the emergency department, a fascia iliaca block provides excellent analgesia and reduces opioid consumption while awaiting surgical fixation. The supra-inguinal fascia iliaca approach provides more consistent coverage of the lateral femoral cutaneous and obturator nerves compared to the infrainguinal approach.

References

• Jaeger P, Nielsen ZJ, Henningsen MH, et al. Adductor canal block versus femoral nerve block and quadriceps strength: a randomized, double-blind, placebo-controlled, crossover study. Anesthesiology. 2013;118(2):409-415.
• Hadzic A, ed. Hadzic's Textbook of Regional Anesthesia and Acute Pain Management. 2nd ed. McGraw-Hill; 2017.
• Hussain N, Ferreri TG, Prusick PJ, et al. Adductor canal block versus femoral nerve block for total knee arthroplasty: a meta-analysis. Anesthesia & Analgesia. 2016;123(5):1316-1326.
• Tran DQ, Gonzalez AP, Bernucci F, et al. A randomized comparison between bifurcation and prebifurcation subparaneural popliteal sciatic nerve blocks. Anesthesia & Analgesia. 2013;116(5):1170-1175.
• Desmet M, Vermeylen K, Van Herreweghe I, et al. A longitudinal supra-inguinal fascia iliaca compartment block reduces morphine consumption after total hip arthroplasty. Regional Anesthesia and Pain Medicine. 2017;42(3):327-333.

Read this lecture as Markdown