14.2 Lower Extremity & Truncal Blocks: Femoral, Sciatic, TAP & ESP Blocks
Key Takeaways
- The lumbar plexus (L1-L4) innervates the anterior thigh and medial leg (femoral, lateral femoral cutaneous, obturator), while the sacral plexus (L4-S4) forms the sciatic nerve, innervating the posterior thigh, posterior knee capsule, and entire lower leg/foot (except the medial saphenous strip).
- Femoral nerve block (L2-L4) provides potent analgesia for femur fractures and anterior knee surgery but induces profound quadriceps motor weakness, resulting in knee buckling and substantial postoperative fall risk.
- Adductor canal block (ACB) targets the saphenous nerve and nerve to vastus medialis within the subsartorial canal, delivering sensory analgesia to the knee while sparing rectus femoris, vastus lateralis, and vastus intermedius motor function, facilitating early ambulation after total knee arthroplasty (TKA).
- Sciatic nerve block via subgluteal or popliteal fossa approaches anesthetizes the Tibial and Common Peroneal nerves; popliteal block is the gold standard for foot/ankle surgery and below-knee amputations.
- Transversus Abdominis Plane (TAP) block deposits local anesthetic between the internal oblique and transversus abdominis muscles, providing somatic analgesia (T7-L1) for abdominal wall incisions without visceral peritoneal coverage; Erector Spinae Plane (ESP) and Quadratus Lumborum (QL) blocks provide both multi-dermatomal somatic and visceral analgesia via paravertebral and sympathetic spread.
Last updated: August 2026
14.2 Lower Extremity & Truncal Blocks: Femoral, Sciatic, TAP & ESP Blocks
Regional anesthesia for lower extremity and abdominal surgery requires targeting two distinct neural networks: the Lumbar Plexus (L1-L4) and the Sacral Plexus (L4-S4) for lower extremity innervation, and the thoracolumbar intercostal/spinal rami within specific musculofascial planes for truncal analgesia. Modern regional practice emphasizes motor-sparing techniques (e.g., adductor canal blocks) to promote early rehabilitation and planes with dual somatic/visceral coverage (e.g., ESP and QL blocks).
1. Lumbar & Sacral Plexus Neuroanatomy
+-------------------------------------------------------------------------+
| LOWER EXTREMITY PLEXUS NEUROANATOMY |
+-----------------------+-----------------------+-------------------------+
| Nerve Branch | Spinal Origins | Sensory & Motor Coverage|
+-----------------------+-----------------------+-------------------------+
| **Iliohypogastric** | L1 | Hypogastric / iliac skin|
| **Ilioinguinal** | L1 | Inguinal / groin / labia|
| **Genitofemoral** | L1 - L2 | Scrotum / mons / cremast|
| **Lat. Fem. Cutaneous**| L2 - L3 | Lateral thigh sensation |
| **Femoral Nerve** | L2 - L4 (Post. Div.) | Anterior thigh; quads; |
| | | medial leg (saphenous) |
| **Obturator Nerve** | L2 - L4 (Ant. Div.) | Medial thigh adductors; |
| | | medial knee capsule |
| **Sciatic Nerve** | L4 - S3 | Posterior thigh; ham- |
| (Tibial + Common Per) | | strings; leg/foot distal|
+-----------------------+-----------------------+-------------------------+
[CROSS-SECTION OF INGUINAL CREASE (NAVEL)]
Lateral Medial
[ NERVE ] [ ARTERY ] [ VEIN ] [ EMPTY ] [ LYMPH ]
Femoral Nerve Femoral Artery Femoral Vein (Canal) (Nodes)
|
+---> Deep to Fascia Lata AND Fascia Iliaca
2. Femoral Nerve Block vs. Adductor Canal Block (ACB)
The Femoral Nerve Block (L2-L4)
- Anatomy: Emerges from the lateral border of the psoas major muscle, traverses beneath the inguinal ligament, and enters the femoral triangle lateral to the femoral artery (NAVEL landmark).
- Fascial Organization: Lies deep to both the Fascia Lata and the Fascia Iliaca. The ultrasound needle must traverse both fascial layers with a distinct "pop" to deposit local anesthetic adjacent to the hyperechoic, wedge-shaped femoral nerve.
- Sensory Coverage: Anterior thigh, anterior knee joint capsule, medial femur, and the medial lower leg/foot (via the terminal saphenous branch).
- The Critical Fall Risk Hazard: Femoral block produces complete motor paralysis of the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius). Patients experience knee buckling upon standing, dramatically increasing postoperative fall risk and precluding early physical therapy.
+-------------------------------------------------------------------------+
| FEMORAL NERVE BLOCK vs ADDUCTOR CANAL BLOCK |
+--------------------+------------------------+---------------------------+
| Feature | Femoral Nerve Block | Adductor Canal Block (ACB)|
+--------------------+------------------------+---------------------------+
| **Target Site** | Femoral triangle | Midthigh subsartorial |
| | (Inguinal crease) | (Hunter's) canal |
| **Nerves Blocked** | Main Femoral trunk: | Saphenous nerve AND Nerve |
| | Motor branches to all | to Vastus Medialis |
| | 4 quadriceps heads | (Pure sensory + 1 motor) |
| **Quadriceps | **Profound / Complete | **Minimal / Motor-Sparing |
| Weakness** | (High Fall Risk)** | (Preserves rectus/vastus) |
| **Analgesia** | Anterior knee + femur | Medial knee capsule + |
| | | medial lower leg/saphenous|
| **Ambulation** | Delayed; requires knee | **Early ambulation on POD0|
| | immobilizer | (Gold standard for TKA)** |
+--------------------+------------------------+---------------------------+
The Adductor Canal Block (Subsartorial / Hunter's Canal)
- Anatomical Boundaries:
- Roof (Anteromedial): Sartorius muscle.
- Lateral Wall: Vastus medialis muscle.
- Floor / Posterior Wall: Adductor longus (proximally) and Adductor magnus (distally).
- Contents: Saphenous nerve (terminal sensory branch of femoral nerve), Nerve to vastus medialis (provides major sensory articular innervation to the anterior/medial knee capsule), Femoral Artery, and Femoral Vein.
- Sonographic Landmark: Place linear high-frequency probe transversely at midthigh level. Locate the triangular Sartorius muscle sitting atop the Femoral Artery. The saphenous nerve is visualized as a hyperechoic structure anterolateral to the artery beneath the vastoadductor membrane.
- Motor-Sparing Advantage: ACB preserves motor innervation to the Rectus Femoris, Vastus Lateralis, and Vastus Intermedius, maintaining active knee extension while delivering potent postoperative analgesia for Total Knee Arthroplasty (TKA).
[ADDUCTOR CANAL CROSS-SECTION (MIDTHIGH)]
[ SARTORIUS MUSCLE ] (Roof)
|
v
[ VASTUS MEDIALIS ] (Saphenous Nerve) [ ADDUCTOR LONGUS/MAGNUS ]
(Lateral Wall) | (Postero-Medial Floor)
v
[ FEMORAL ARTERY / VEIN ]
3. Sciatic Nerve Blocks: Subgluteal & Popliteal Approaches
The sciatic nerve (L4-S3) is the largest nerve in the human body. It provides motor innervation to the posterior thigh hamstrings and all muscles below the knee, and sensory innervation to the posterior knee joint, posterior lower leg, and the entire foot/ankle (except the medial strip supplied by the saphenous nerve).
[POPLITEAL SCIATIC NERVE BIFURCATION]
[ SCIATIC NERVE ] (L4-S3)
|
(5 to 10 cm above popliteal crease)
/ \
/ \
v v
[ TIBIAL NERVE ] [ COMMON PERONEAL NERVE ]
• Larger, Medial • Smaller, Lateral
• Posterior to • Travels along Biceps Femoris
Popliteal V/A to fibular neck
• Plantarflexion • Dorsiflexion & Eversion
& Inversion • Foot drop if injured!
1. Subgluteal Approach
- Landmarks: Needle inserted midway between the Greater Trochanter of the femur and the Ischial Tuberosity of the pelvis, traversing deep to the Gluteus Maximus and Quadratus Femoris muscles.
- Indications: Above-knee or high posterior thigh surgery, total hip arthroplasty (posterior capsule), and combined with femoral/lumbar plexus block for complete lower limb anesthesia.
2. Popliteal Fossa Approach
- Anatomy: In the popliteal fossa, the sciatic nerve bifurcates into the Tibial Nerve (medial, larger; supplies plantarflexion and sole of foot) and Common Peroneal (Fibular) Nerve (lateral, smaller; supplies dorsiflexion, eversion, and dorsum of foot) approximately $5 - 10 \text{ cm}$ superior to the popliteal flexion crease.
- Sonographic Technique: Identify the bifurcation by scanning proximally from the popliteal crease until the two distinct oval nerves merge into a single large hyperechoic sciatic nerve trunk inside the common paraneural sheath (Vloka's sheath). Inject $15 - 20 \text{ mL}$ of local anesthetic beneath the paraneural sheath (circumferential spread around both components).
- Indications: Foot, ankle, distal Achilles tendon surgery, and below-knee amputations (co-administered with a saphenous/ACB block for the medial malleolus).
4. Transversus Abdominis Plane (TAP) Block
The TAP block deposits local anesthetic into the neurovascular fascial plane between the Internal Oblique (IO) and Transversus Abdominis (TA) muscles.
[ANTEROLATERAL ABDOMINAL WALL CROSS-SECTION (TAP)]
Skin / Subcutaneous Fat
|
v
[ EXTERNAL OBLIQUE MUSCLE (EO) ]
|
v
[ INTERNAL OBLIQUE MUSCLE (IO) ]
|
>>> [ TAP FASCIAL PLANE ] <<< (T7-L1 Intercostal / Spinal Nerves)
|
v
[ TRANSVERSUS ABDOMINIS MUSCLE (TA) ]
|
v
[ Transversalis Fascia / Peritoneum ]
Neuroanatomy & Analgesic Distribution
- Target Nerves: Anterior rami of spinal nerves T7 through L1 (lower intercostal, subcostal T12, iliohypogastric L1, and ilioinguinal L1 nerves) as they travel within the IO-TA fascial plane.
- Analgesic Profile: Provides potent SOMATIC analgesia to the anterior abdominal wall skin, subcutaneous tissue, rectus abdominis muscle, and parietal peritoneum.
- Indications: Transverse lower abdominal incisions (Pfannenstiel / Cesarean delivery), laparoscopic port site incisions, open appendectomy, open/laparoscopic ventral hernia repair, and open laparotomy.
Critical Clinical Limitation: Visceral Pain Sparing
- NCE Board Rule: The TAP block provides ZERO VISCERAL ANALGESIA.
- Mechanism: It does not block pelvic/abdominal autonomic visceral afferent C-fibers, mesenteric traction, or visceral peritoneal stretch.
- Clinical Consequence: For abdominal or pelvic surgery, TAP blocks relieve incisional (somatic) pain but must be combined with systemic multimodal non-opioid/opioid analgesia to control visceral cramping, uterine contractions, or bowel manipulation pain.
5. Erector Spinae Plane (ESP) & Quadratus Lumborum (QL) Blocks
+-------------------------------------------------------------------------+
| ESP vs QL vs TAP BLOCKS COMPARISON |
+--------------------+------------------------+---------------------------+
| Block Type | Anatomical Target | Analgesic Profile |
+--------------------+------------------------+---------------------------+
| **TAP Block** | Fascial plane between | • **Somatic ONLY (T7-L1)**|
| | IO and TA muscles | • NO visceral analgesia |
| | | • Abdominal wall incisional|
| **Erector Spinae | Deep to Erector Spinae | • **BOTH Somatic AND |
| Plane (ESP)** | muscle onto Transverse | Visceral** analgesia |
| | Process (bony stop) | • Multi-level paravertebral|
| | | and sympathetic spread |
| **Quadratus | Anterolateral / deep to| • **BOTH Somatic AND |
| Lumborum (QL)** | QL muscle (QL1,2,3) | Visceral** analgesia |
| | | • Extensive T7-L1/L2 spread|
+--------------------+------------------------+---------------------------+
Erector Spinae Plane (ESP) Block Mechanics
- Technique: High-frequency linear probe placed in a parasagittal orientation $2 - 3 \text{ cm}$ lateral to the spinous process. Identify the rounded, hyperechoic Transverse Process with acoustic shadow, covered by the Erector Spinae Muscle (spinalis, longissimus, iliocostalis).
- Injection: Needle advanced in-plane until the tip contacts the transverse process (bony backstop ensures zero pleural puncture risk). Local anesthetic ($20 - 30 \text{ mL}$) is injected deep to the erector spinae muscle, lifting the muscle off the bone.
- Mechanism of Dual Coverage: The injected fluid spreads craniocaudally across $4 - 6$ dermatomal levels and diffuses anteriorly through the intertransverse connective tissue into the thoracic paravertebral space, blocking both the dorsal rami (posterior back muscles/skin), ventral rami (intercostal nerves / somatic chest & abdominal wall), and the rami communicantes / sympathetic chain (visceral analgesia).
- Clinical Utility: Thoracotomy, rib fractures, mastectomy, laparoscopic cholecystectomy (T7-T9 level), and spine surgery.
Loading diagram...
Test Your Knowledge
A 64-year-old female undergoes an elective left Total Knee Arthroplasty (TKA). The surgical team requests a regional anesthetic plan that optimizes postoperative pain control while facilitating immediate postoperative physical therapy and early ambulation on the day of surgery. Which block is the most appropriate choice?
A
B
C
D
Test Your Knowledge
A CRNA performs bilateral ultrasound-guided Transversus Abdominis Plane (TAP) blocks for a patient undergoing an open abdominal hysterectomy. In the PACU, the patient reports severe deep, cramping pelvic pain despite excellent blunting of skin incision tenderness. What is the pharmacological and anatomical explanation for this finding?
A
B
C
D
Test Your Knowledge
When performing an ultrasound-guided popliteal sciatic nerve block for complex ankle reconstruction, where is the optimal anatomical target for local anesthetic deposition to achieve rapid and complete block of both the foot and ankle?
A
B
C
D
Test Your Knowledge
An Erector Spinae Plane (ESP) block performed at the T5 transverse process level provides both multi-dermatomal somatic analgesia of the chest wall and visceral analgesia of the thoracic cavity. What anatomical pathway allows this dual somatic and visceral blockade?
A
B
C
D