Regional Anesthesia

Lumbosacral Plexus Blocks

Introduction: The nerve supply to the lower extremity is derived from the lumbar and sacral plexuses. The lumbar plexus provides sensory innervation to the anterior thigh and to the medial portion of the lower leg via the saphenous nerve (distal branch of the femoral nerve), as well as the majority of the femur, ischium, and ilium. The sacral plexus provides sensory and motor innervation of the buttock, posterior thigh, and the leg and foot (with the exception of the medial aspect of the foot which is innervated by the saphenous branch of the femoral nerve).


The choice of lumbosacral block techniques depends on the portion of the lower extremity to be blocked. The combination of a proximal lumbar plexus block and a proximal sciatic block will provide complete lower extremity anesthesia and analgesia.



• Infection at the site or sepsis
• Patient refusal


• Anticoagulation
• Preexisting peripheral neuropathy
• Abnormal anatomy
• Uncooperative patient

Relevant Anatomy:

Lumbar Plexus (L1-L5):

The lumbar plexus is most commonly formed from the ventral rami of L1-L4, although frequently a branch of T12, and occasionally a branch from L5 are included. The plexus lies anterior to the transverse processes of the lumbar vertebrae and descends vertically with the psoas muscle. The branches of the lumbar plexus emerging from the psoas muscle are:

• Femoral nerve (L2-4)
• Saphenous nerve (L2-4)
• Obturator nerve (L2-4)
• Lateral femoral cutaneous nerve (L2-3)
• Iliohypogastric nerve (L1)
• Ilioinguinal nerve (L1)
• Genitofemoral nerve (L1-2).

Sacral Plexus (L4, L5, S1-S3):

The sacral plexus is formed as the ventral rami of L4, L5, S1-S3pass together through the pelvis and the greater sciatic foramen. The most important components are the posterior cutaneous and sciatic nerves and their terminal branches which include:

• Posterior femoral cutaneous nerve (S1-S3)
• Sciatic nerve (L4, L5, S1-S3)
• Tibial nerve (L4, L5, S1-S3)
• Common Peroneal Nerve (L4, L5, S1, S2)
• Sural Nerve (L5, S1, S2)


Posterior lumbar plexus (psoas) block:

  • Patient Position: The patient is positioned laterally with the hips flexed and perpendicular to the horizontal plane (similar to the position utilized for an intrathecal injection) with the operative leg uppermost.
  • Needle Insertion Site: There are several proposed needle insertion sites, although the landmarks described by Capdevila et al.[1] use localization of the L4 transverse process thereby reducing the likelihood of excessive needle depth. The intercristal line is identified and drawn. A horizontal line is drawn identifying the midline. A line originating at the PSIS is drawn parallel to midline. The distance between the PSIS and midline is dissected into thirds. The needle insertion site is 1 cm cephalad to the intercristal line at the junction of the lateral one third and medial two-thirds line.
  • With the use of a nerve stimulator, the needle is advanced perpendicular to the skin at the entry site until contact is made with the transverse process of L4. The needle is then withdrawn and “walked off” the transverse process in a caudad direction until a motor response of the lumbar plexus is elicited. A motor response of the quadriceps femoris muscle is ideal, although any motor response of the lumbar plexus may be utilized. Once the desired motor response is obtained, the local anesthetic solution is slowly administered with frequent aspiration for blood or cerebral spinal fluid. For a continuous catheter technique a 20-guage catheter is threaded through an 18-guage insulated needle approximately 4-5 cm into the psoas compartment.
  • Needle Redirection Cues: If contact with the transverse process is not made on the first pass, the needle is redirected, first caudad, then cephalad searching for the transverse process. If the transverse process has not been contacted and the desired motor response has not been elicited, the needle is redirected slightly medial and the above steps are repeated until a lumbar plexus motor response is obtained. Due to an increased incidence of complications, extreme medial redirection of the needle should be avoided. Dural sleeves surround the roots of the lumbar plexus at this level, therefore stimulation at currents less than 0.5 mA could indicate needle placement within the dural sleeve. Injection of local anesthetic within a dural sleeve could cause significant epidural or subarachnoid spread. If a motor response of the hamstring muscles is obtained the needle has been inserted too caudally. Withdraw the needle and reinsert in a more cephalad direction. In some patients the normal kidney may extend down to the level of the L3 vertebrae, therefore it is important to avoid extreme cephalad redirection when the needle insertion site is at the level of L4.

Femoral block:

  • Patient Position: The patient is positioned supine.
  • Needle Insertion Site: With the patient in the supine position, a line representing the inguinal ligament is drawn between the palpated anterior superior iliac spine (ASIS) and the pubic tubercle. A second line is drawn representing the femoral artery. Using a nerve stimulator, a 22-gauge, 3- to 5-cm, short beveled needle is advanced until a motor response of the quadriceps muscle is obtained (patellar snap). After negative aspiration, 10 to 30 mL of local anesthetic solution is injected incrementally through the needle.
  • The use of ultrasound may be useful in patients in whom it is difficult to palpate a femoral pulse, due to weight, anatomic variability, or changes to the needle insertion site (prior radiation or surgery). The femoral nerve can be identified lateral to the artery as a triangular shaped structure.

Sciatic Nerve Block-Classic Approach of Labat:

  • Patient Position: The patient is positioned laterally, with the leg to be blocked flexed and rolled forward so that the heel of the upper (operative) leg rests on the knee of the dependent (nonoperative) leg, which is stretched out in a straight line with the torso.
  • Needle Insertion Site: A line is drawn between the palpated posterior superior iliac spine and the greater trochanter of the femur. This line is bisected with a perpendicular line extending approximately 5 cm caudad. A confirmatory line is drawn between the greater trochanter of the femur and the sacral hiatus, which will cross the perpendicular at a point 3 to 5 cm along the line. This represents the point of needle insertion. Using a nerve stimulator, a 22-gauge, 10- to 12-cm, short-bevel needle is advanced perpendicular to the skin until a motor response of the distal lower extremity is obtained. If bone is contacted, the needle is redirected medially until the nerve is located. When the sciatic nerve is located, 25 to 30 mL of local anesthetic solution is injected incrementally.
  • An alternative approach in the lateral decubitus position is the subgluteal approach, with or without ultrasound guidance. A curvilinear probe is placed just distal to the gluteal cleft and scanned lateral to medial. The sciatic nerve can be identified as a flat hyperechoic structure medial to the greater trochanter and lateral to the hyperechoic border of the ischial tuberosity. A 5 to 10 cm 21 gauge insulated needle is advanced in an out-of-plane approach towards the sciatic nerve (the simultaneous use of a nerve stimulator can help to confirm the location of the nerve) and after negative aspiration 20-30 ml of local anesthetic is injected around the nerve.

Sciatic Nerve Block-Anterior Approach

  • Patient Position: This technique is useful for situations where the patient cannot be positioned for the classic posterior approach because of pain or lack of cooperation. The patient is placed in the supine position.
  • Needle Insertion Site: A line is drawn between the ASIS and the pubic tubercle, representing the inguinal ligament. This line is trisected, and a second parallel line is drawn from the point of the tuberosity of the greater trochanter of the hip. The intersection of this second line with the more medial of the perpendicular lines represents the point of needle entry. Using sterile technique, a 22-gauge, 10- to 12-cm, short-bevel needle is advanced with a slight lateral angulation until the lesser trochanter of the femur is encountered. At this point, the needle is redirected slightly medially, walked off the femur, and advanced until a motor response of the distal extremity is elicited, (approximately 5 cm past bone). A total of 25 to 30 mL of local anesthetic solution is injected incrementally after careful aspiration.

Sciatic Nerve Block-Popliteal Approach:

  • This peripheral nerve block is useful for foot and ankle surgery. It can be performed as a single injection block or a continuous catheter can be placed for longer post-operative analgesia. A saphenous nerve block is required to provide complete anesthesia to the ankle and foot.
  • Patient Position: The patient is placed in the prone or semi-prone position.
  • Needle Insertion Site: The popliteal crease can be easily identified with leg flexion. The popliteal fossa, bounded by semimembranous and semitendinous muscles medially and the biceps femoris muscle laterally, is divided into medial and lateral triangles. The needle insertion site is five to seven centimeters proximal from the crease, just lateral to midline. A four inch insulated needle is advanced at a 45 degree angle until an appropriate nerve twitch is obtained. After negative aspiration 30 to 40 ml of local anesthetic is incrementally injected.
  • The use of ultrasound can help to identity the point of divergence of the sciatic nerve into the peroneal and tibial branches. The ultrasound probe is placed parallel to the popliteal crease and the leg is scanned in a proximal direction. It is often helpful to first identify the popliteal artery and then to locate the nerves just lateral and superficial to artery. Using either an in-plane or out-of-plane approach a 4 inch 21 gauge insulated needle is directed toward the sciatic nerve at, or just proximal to the point of divergence. After negative aspiration, local anesthetic spread can be visualized around the nerve. Nerve stimulation can also be used to confirm the identity of the sciatic nerve prior to local anesthetic deposition.


  • Nerve injury from needle trauma or drug toxicity is an unlikely complication from all blocks. Local anesthetics should not be injected if the patient complains of pain or paresthesia, therefore in adults this block should not be performed under general anesthesia, although sedation is usually required.
  • Bleeding or hematoma formation
  • Infection
  • Local anesthetic toxicity from intravascular injection or rapid uptake of drug.
    Complications specific to psoas blockade: Nerve injury during psoas compartment blockade may be caused by direct needle trauma to the nerve roots. Unlike the relatively minor complications associated with other lower extremity nerve blocks, the risks associated with a psoas block can be quite severe. Due to the proximity of the neuraxis, intrathecal or epidural local anesthetic or catheter placement is a potential complication. Epidural spread of local anesthetic is the most common complication with an incidence ranging from 1.8% to 16%. The factors that may contribute to epidural spread are; a medially directed needle, large volumes of local anesthetic, and the presence of a spinal deformity (scoliosis). Less commonly, intrathecal or subarachnoid injection or catheter placement have been reported, leading to a high spinal. Hypotension is rare due to a unilateral sympathectomy, although if epidural spread occurs it may result in significant hypotension.
  • Because the psoas compartment block results in injection of local anesthetic into or in close proximity to large, richly vascularized, muscles (psoas, quadratus lumborum) there are case reports of severe retroperitoneal or renal capsular hematomas. The majority of these patients have undergone a psoas compartment block while anticoagulated, or have received anticoagulation medication shortly after block placement or in the presence of a continuous psoas catheter. Although larger studies are needed, the American Society of Regional Anesthesia conservatively recommends that patients having a lumbar plexus block be managed in much the same way as those undergoing neuraxial blockade when thromboprophylaxis is ordered.


  1. Capdevila, X., et al., Continuous psoas compartment block for postoperative analgesia after total hip arthroplasty: new landmarks, technical guidelines, and clinical evaluation. Anesth Analg, 2002. 94(6): p. 1606-13, table of contents.
  2. Horlocker TT, Wedel DJ, Rowlingson JC, Enneking FK, Kopp SL, Benzon HT, Brown DL, Heit JA, Mulroy MF, Rosenquist RW, Tryba M, Yuan CS. Regional anesthesia in the patient receiving antithrombotic or thrombolytic therapy: American Society of Regional Anesthesia and Pain Medicine Evidence-Based Guidelines (Third Edition). Reg Anesth Pain Med 2010 Jan-Feb; 35(1):64-101.
  3. Weller, R.S., et al., Extensive retroperitoneal hematoma without neurologic deficit in two patients who underwent lumbar plexus block and were later anticoagulated. Anesthesiology, 2003. 98(2): p. 581-5.
  4. Kopp SL. Posterior Approach to the Lumbar Plexus. In: Hebl JR, Lennon RL. Mayo Clinic Atlas of Regional Anesthesia and Ultrasound-Guided Nerve Blockade. Rochester (MN) and New York: Mayo Clinic Scientific Press and Oxford University Press; 2010
  5. Vloka, JD et al. Anterior Approach to the Sciatic Nerve Block: The Effects of Leg Rotation. Anesth Analg 2001;92:460-2
  6. Chelly, JE et al. A New Anterior Approach to the Sciatic Nerve Block. Anesthesiology 1999;91:1655-60
  7. Paqueron, X. et al. The Lateral Approach to the Sciatic Nerve at the Popliteal Fossa: One or Two Injections? Anesth Analg 1999;89(5):1221
  8. Singelyn et al. Continuous Popliteal Sciatic Nerve Block: An Original Technique to Provide Postoperative Analgesia after Foot Surgery. Anesth Analg 1997;84:383-6
  9. Di Benedetto P et al. A New Posterior Approach to the Sciatic Nerve Block: A Prospective, Randomized Comparison with the Classic Posterior Approach. Anesth Analg 2001;93:1040-4.
  10. Di Benedetto et al. Postoperative Analgesia with Continuous Sciatic Nerve Block after Foot Surgery. Anesth Analg 2002;94:996-1000
  11. Ericksen, ML et al. The Anatomic Relationship of the Sciatic Nerve to the Lesser Trochanter: Implications for Anterior Sciatic Nerve Block. Anesth Analg 2002;95:1071-4.
  12. Liu, SS et al. Continuous Plexus and Peripheral Nerve Blocks for Postoperative Analgesia. Anesth Analg 2003;96:263-72.
  13. Chan VW et al. Ultrasound Examination and Localization of the Sciatic Nerve. Anesthesiology 2006; 104:309-14