Introduction
Lower extremity blocks (LEBs) are common techniques for surgical anesthesia and postoperative analgesia. They are being used more frequently in the setting of outpatient surgery worldwide because of their favorable recovery and side effect profile when compared with general and neuraxial anesthesia. LEBs may decrease the incidence of postoperative pain syndromes, including chronic postamputation phantom limb pain. In addition to surgical applications, LEBs have also been reported to be beneficial in patients with complex regional pain syndrome, chronic cancer pain, peripheral vascular diseases (ischemia, Reynaud’s disease, peripheral embolism), intractable phantom limb pain, and spasticity. Continuous LEBs with perineural catheters can also provide analgesia for an extended period. Moreover, neurolysis of lower extremity peripheral nerves can also be performed with techniques analogous to peripheral nerve blockade.
Ultrasound-guided techniques are becoming more prevalent methods of performing LEBs in many centers. Several reports have suggested that ultrasound guidance results in more precise needle and catheter placement during LEBs than do blocks performed with nerve stimulator or landmark techniques only or with both.
This chapter provides an overview of the relevant anatomy of the lower extremity, technical aspects of performing LEBs, and common indications for their use in clinical practice.
Anatomy
Innervation of the lower extremity is derived from both the lumbar plexus and the sacral plexus, sometimes referred as the lumbosacral plexus.
Lumbar Plexus
The lumbar plexus is made up of the L1 through L5 spinal nerve roots. As the L2, L3, and L4 roots of the lumbar plexus depart from their spinal nerves and emerge from the intervertebral foramen, they enter the posterior third of the psoas muscle. Once in the muscle, these roots then become organized into anterior and posterior divisions. The divisions reunite to form the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, femoral, and obturator nerves ( Fig. 54.1 ). The most significant nerves in the lower extremity are the femoral, lateral femoral cutaneous, and obturator nerves.
The femoral nerve is formed by the posterior divisions of L2-4. The nerve descends from the plexus lateral to the psoas muscle. The femoral nerve innervates the rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis muscles. It also provides cutaneous sensory innervation to much of the anterior and medial aspects of the thigh, as well as the medial part of the leg distal to the knee ( Fig. 54.2 ).
The lateral femoral cutaneous nerve is formed from the L2 and L3 nerve roots and, as its name indicates, is a cutaneous, sensory nerve. It provides sensation to the lateral aspect of the thigh (see Fig. 54.2 ).
The obturator nerve (L2-4) provides sensory innervation to a variable portion of the leg proximal to the knee, as well as motor innervation to the adductor muscles (see Fig. 54.2 ).
The iliohypogastric and ilioinguinal nerves are primarily sensory nerves that arise from L1 and supply innervation to the skin of the suprapubic and inguinal regions.
The genitofemoral nerve arises from the L1 and L2 roots and supplies motor innervation to the cremasteric muscle and additional sensory innervation to the inguinal area.
Sacral Plexus
The sacral plexus is formed from the L4-S3 nerve roots ( Fig. 54.3 ). It is shaped like a triangle pointing toward the sciatic notch, with its base spanning across the anterior sacral foramina. The roots of the sacral plexus lie on the anterior surface of the lateral sacrum and form the sciatic nerve on the ventral surface of the piriformis muscle. The sacral plexus gives rise to one major nerve and six collateral nerve branches. The sciatic nerve exits the pelvis through the greater sciatic notch and then descends between the greater trochanter of the femur and the ischial tuberosity. It runs along the posterior part of the thigh to the lower third of the femur, where it diverges into two divisions: the tibial and common peroneal. The sciatic nerve provides motor and sensory innervation to the hamstring muscles and the entire leg below the knee with the exception of the cutaneous region medially, which is innervated by the saphenous nerve (see Fig. 54.2 ). The posterior cutaneous nerve of the thigh exits the pelvis with the sciatic nerve but then diverges posteriorly to become a cutaneous nerve that is the major sensory nerve from the sacral plexus within the thigh.
Anatomy
Innervation of the lower extremity is derived from both the lumbar plexus and the sacral plexus, sometimes referred as the lumbosacral plexus.
Lumbar Plexus
The lumbar plexus is made up of the L1 through L5 spinal nerve roots. As the L2, L3, and L4 roots of the lumbar plexus depart from their spinal nerves and emerge from the intervertebral foramen, they enter the posterior third of the psoas muscle. Once in the muscle, these roots then become organized into anterior and posterior divisions. The divisions reunite to form the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, femoral, and obturator nerves ( Fig. 54.1 ). The most significant nerves in the lower extremity are the femoral, lateral femoral cutaneous, and obturator nerves.
The femoral nerve is formed by the posterior divisions of L2-4. The nerve descends from the plexus lateral to the psoas muscle. The femoral nerve innervates the rectus femoris, vastus medialis, vastus intermedius, and vastus lateralis muscles. It also provides cutaneous sensory innervation to much of the anterior and medial aspects of the thigh, as well as the medial part of the leg distal to the knee ( Fig. 54.2 ).
The lateral femoral cutaneous nerve is formed from the L2 and L3 nerve roots and, as its name indicates, is a cutaneous, sensory nerve. It provides sensation to the lateral aspect of the thigh (see Fig. 54.2 ).
The obturator nerve (L2-4) provides sensory innervation to a variable portion of the leg proximal to the knee, as well as motor innervation to the adductor muscles (see Fig. 54.2 ).
The iliohypogastric and ilioinguinal nerves are primarily sensory nerves that arise from L1 and supply innervation to the skin of the suprapubic and inguinal regions.
The genitofemoral nerve arises from the L1 and L2 roots and supplies motor innervation to the cremasteric muscle and additional sensory innervation to the inguinal area.
Sacral Plexus
The sacral plexus is formed from the L4-S3 nerve roots ( Fig. 54.3 ). It is shaped like a triangle pointing toward the sciatic notch, with its base spanning across the anterior sacral foramina. The roots of the sacral plexus lie on the anterior surface of the lateral sacrum and form the sciatic nerve on the ventral surface of the piriformis muscle. The sacral plexus gives rise to one major nerve and six collateral nerve branches. The sciatic nerve exits the pelvis through the greater sciatic notch and then descends between the greater trochanter of the femur and the ischial tuberosity. It runs along the posterior part of the thigh to the lower third of the femur, where it diverges into two divisions: the tibial and common peroneal. The sciatic nerve provides motor and sensory innervation to the hamstring muscles and the entire leg below the knee with the exception of the cutaneous region medially, which is innervated by the saphenous nerve (see Fig. 54.2 ). The posterior cutaneous nerve of the thigh exits the pelvis with the sciatic nerve but then diverges posteriorly to become a cutaneous nerve that is the major sensory nerve from the sacral plexus within the thigh.
Nerve Block Techniques
Lumbar Plexus Block
Clinical Applications
A lumbar plexus block (LPB) can provide anesthesia or analgesia to the anterolateral and medial aspects of the thigh, the knee, and the medial portion of the leg below the knee. Common indications include anesthesia and analgesia following total hip arthroplasty, total knee arthroplasty, and anterior cruciate ligament reconstruction, as well as for treatment of chronic hip pain.
Nerve Stimulator– and Surface-Based Techniques
An LPB is a deep block and the needle traverses several layers of structures, including (from posterior to anterior) the posterior lumbar fascia, paraspinous muscle, anterior lumbar fascia, quadratus lumborum muscle, and psoas muscle.
Nerve stimulation is probably the most commonly used method to localize the lumbar plexus during blockade. To perform the block, the patient is placed in the lateral decubitus position with the operative side up ( Fig. 54.4 ). The foot on the side to be blocked is positioned over the dependent leg so that the motor response of the quadriceps muscle can easily be seen. The two surface anatomic landmarks for determining the insertion point for the needle are the iliac crest and the midline spinous processes.
The top of the iliac crest correlates with the L3-4 interspace in most patients. The point 3 to 3.5 cm lateral to the intersection of the iliac crest and the midline spinous processes marks the needle insertion site (see Fig. 54.4 ). After setting the nerve stimulator to an initial current of 1.5 mA (0.1 msec, 2 Hz) and skin preparation, a 4-inch insulated needle is advanced in a posterior-to-anterior manner. As the needle is advanced, local contractions of the paravertebral muscles are commonly elicited and indicate that placement of the needle is too shallow. The needle is advanced further until the transverse process is encountered. Contact with the transverse process is not routinely sought, but when present, it provides a consistent landmark. After contact with the transverse process, the needle is redirected cephalad or caudad and advanced approximately 2 cm beyond the transverse process. Starting from the level of the skin, the lumbar plexus is located between 6.1 and 10.1 cm in men and 5.7 and 9.3 cm in women.
Once a quadriceps muscle–evoked response is obtained between 0.5 and 1.0 mA, 20 to 35 mL of local anesthetic is injected slowly, with frequent aspiration between injection aliquots. Unintentional deep placement of the needle carries a risk for injury to internal organs, such as renal hematoma. Epidural spread of local anesthetic has been reported to occur in as many as 16% to 27% of blocks. Limiting the force of injection decreases the risk for epidural spead.
Ultrasound-Guided Techniques
Because of the deep location of the plexus, ultrasound-guided LPBs have not become as rapidly adopted as ultrasound-guided blocks of more peripheral sites. To obtain adequate views, a low-frequency (4 to 8 MHz) curvilinear probe is used. Perhaps the most common approach is an ultrasound-assisted technique in which the scanning is performed in the parasagittal axis. After preparation of the skin and transducer, the transverse processes are identified first on the parasagittal axis ( Fig. 54.5 ). Starting from the sacrum, the L5 transverse process is identified. Continuing the scan cephalad will allow visualization of the other transverse processes in ascending order. Once the transverse process of L3 is identified, the needle is inserted until contact with the transverse process and then “walked off” until contractions of the quadriceps muscle are elicited. The rest of the injection procedure is similar to the nerve stimulator–guided technique.
More recently, a transverse oblique technique has been suggested in which the stimulating needle is directed into the posteromedial aspect of the psoas muscle. With this technique the roots of the lumbar plexus may be seen exiting through the intervertebral foramen ( Fig. 54.6 ). The needle is advanced in plane, and its position is routinely monitored by nerve stimulation.
Complications
Complications associated with LPB fall into one of three categories: (1) systemic toxicity of local anesthetics, (2) neuraxial spread, and (3) needle misadventures. Epidural spread of the local anesthetic, total spinal anesthesia, and circulatory collapse have been reported. Care should be taken to avoid major vascular structures and the lower pole of the kidney because cases of subcapsular renal hematoma have been reported. The reported cases of delayed retroperitoneal hematoma after LPB in the absence of anticoagulation underscore the need for caution in patients treated with anticoagulants. Although there are no clear guidelines on the safety of LPB in the presence of anticoagulation, it is prudent to consider alternative anesthetic plans for patients with known coagulopathy.
Lateral Femoral Cutaneous Nerve Block
Clinical Applications
Blockade of lateral femoral cutaneous nerve (LFCN) is indicated for anesthesia or analgesia of the anterolateral aspect of the thigh, as well as for the diagnosis and treatment of meralgia paresthetica.
Anatomy
The LFCN is typically blocked 1 to 2 cm medial and inferior to the anterior superior iliac spine (ASIS). At this location the nerve lies beneath the fascia iliaca, just lateral to the sartorius muscle and medial to the tensor fasciae latae muscle (TFLM).
Nerve Stimulator– and Surface-Based Techniques
Surface-based techniques yield variable success rates because of the anatomic variability of the LFCN. Electrical nerve stimulation can be used to elicit paresthesia in the sensory distribution of the nerve to increase the chance of success. With either technique, the needle is inserted 1 to 2 cm medial and inferior to the ASIS and advanced until loss of resistance is felt as the needle penetrates the fascia iliaca. When the technique is aided by nerve stimulation, current intensity is set to 1.0 to 2.0 mA with a pulse duration of 1.0 msec for greater sensitivity.
Ultrasound-Guided Technique
After preparation of the skin and transducer, a high-frequency linear probe is placed inferior to the ASIS, parallel to the inguinal ligament, to identify the sartorius and TFLM first. The LFCN is visualized as a small oval hypoechoic structure beneath the fascia iliaca, just lateral to the sartorius muscle, and medial to the TFLM ( Fig. 54.7 ). A 2-inch, 22-gauge needle is used to approach the nerve, typically in an in-plane technique, and 5 to 10 mL of the local anesthetic of choice is injected.
Complications
Because no large vascular structures or other organs are nearby, an LFCN block is associated with a low risk for complications.
Femoral Nerve Block
Clinical Applications
A femoral block is one of the most commonly used peripheral nerve block techniques in clinical practice. It is often used for anesthesia or postoperative analgesia (or both) in patients undergoing surgery on the anterior aspect of the thigh and the knee, quadriceps tendon repair, and femoral surgery. A continuous femoral nerve block has become a mainstay analgesic technique in patients after knee arthroplasty. It has also been reported to be used for palliative care, and in such cases an indwelling catheter can be left in situ for up to several weeks.
Anatomy
The femoral nerve arises from the L2-4 roots of the lumbar plexus. The nerve travels through the psoas muscle and ends on the anterior surface of the iliopsoas as it passes under the inguinal ligament. At this level the nerve lies beneath the fascia iliaca and lateral to the femoral artery ( Fig. 54.8 ). A femoral nerve block provides sensory anesthesia to the anterior aspect of the thigh, the knee, and the medial portion of the calf and ankle.
Technique
Nerve Stimulator– and Surface-Based Techniques
After skin preparation, the needle is introduced in a sagittal, slightly cephalad plane immediately lateral to the femoral artery at the femoral crease to obtain a femoral nerve response (quadriceps twitch). If a sartorius muscle twitch is obtained first (twitch of the medial aspect of the thigh with no patellar movement), the needle is advanced deeper and more lateral to ensure that the needle is positioned closer to the main trunk of the femoral nerve. A recent report suggests that injection of local anesthetic after obtaining a sartorius motor response results in a successful femoral nerve block. However, a sartorius response can also take place when a sartorius branch is stimulated remotely from the main trunk of the femoral nerve, particularly in patients with less than ideal anatomy (e.g., obese). Once a quadriceps muscle twitch at a current of 0.2 to 0.5 mA is elicited, 10 to 15 mL of local anesthetic is injected to accomplish the block.
Ultrasound-Guided Techniques
A high-frequency linear transducer (8 to 14 MHz) is used in most patients. After preparation of the skin and transducer, the transducer is placed parallel to the inguinal crease to identify the femoral artery and vein. The nerve is imaged just lateral to the femoral artery on the surface of the iliacus muscle, underneath iliacus fascia ( Fig. 54.9 ). A 4-inch nerve block needle can be inserted in an in-plane or out-of-plane fashion, the former being favored by many clinicians. The needle is advanced underneath the iliacus fascia to allow spread of the local anesthetic around the femoral nerve. When adequate needle-nerve imaging is present, it is not necessary to elicit a motor response. However, when a motor response is elicited, the current is reduced to ensure that the response is not present below 0.2 mA because a motor response to a very low current intensity may indicate intraneural placement of the needle. Injection underneath the fascia iliaca (perineural) should not result in significant resistance to the injection. A monitoring algorithm combining nerve stimulation with ultrasound guidance for a femoral nerve block, as well as most other peripheral major nerve blocks, is outlined in Figure 54.10 .
Continuous Femoral Nerve Block
A continuous femoral nerve block is similar to the single-shot technique except that a larger-gauge needle is used to allow insertion of the catheter. Once the tip of the needle is deemed to be in the correct position, the catheter is inserted through the needle approximately 3 cm beyond the tip. After an initial bolus of local anesthetic, a continuous infusion is started at a rate of 5 mL/hr, typically with a patient-controlled bolus of 5 mL/hr. Ultrasound imaging can be used to confirm proper spread of local anesthetic underneath the fascia iliaca in the vicinity of the femoral nerve.
A continuous femoral nerve block is used to provide postoperative analgesia after knee surgery, and catheters may be left in situ for several days. Perineural infusion for up to several weeks has also been reported to be useful in palliative care.
Three-in-One Block
A 3-in-1 block refers to a modification of the standard femoral block in which a larger volume of local anesthetic is injected, with pressure being held distal to the needle injection site. Although this technique is still used in clinical practice, several studies have failed to document reliable spread of the local anesthetic. Usually, the obturator nerve is not blocked with this technique. Technically, a 3-in-1 block is essentially the same as a large-volume femoral block or fascia iliaca block.
Complications
Complications of a femoral nerve block include injury to the nerve itself, vascular injury, and loss of muscle strength. The incidence of transient adverse neurologic symptoms associated with a continuous block is 0.4% to 0.5% with femoral catheters. The rate of vascular puncture during placement of a femoral nerve block has been reported to be as high as 5.6%, but this rate may be decreased with the use of ultrasound. Femoral neuropathy secondary to compression of the femoral nerve by a retroperitoneal hematoma has also been documented. Quadriceps weakness in patients with femoral nerve blocks may lead to falls, which can cause significant morbidity. The frequency of falls in these patients may have been underreported.
The incidence of bacterial colonization associated with continuous femoral nerve blocks was evaluated in 208 patients. Fifty-seven percent were found to have positive bacterial colonization of the catheter 48 hours postoperatively. Three patients had transient symptoms of bacteremia that resolved with removal of the catheter, but there was no report of long-term infectious complications. Two case reports of a psoas abscess requiring drainage and intravenous antibiotic therapy after a continuous femoral nerve block have been described, but there are no case reports of infection after a femoral nerve block performed with a single injection.
Fascia Iliaca Block
A fascia iliaca block is an alternative approach to a femoral nerve block in which local anesthetic is injected underneath the fascia iliaca at a distance from the femoral nerve. The rationale is that because the femoral nerve and LFCN both lie deep to the iliacus fascia, a sufficient volume of local anesthetic deposited beneath the fascia iliaca will spread underneath the fascia and reach these nerves. Ultrasound has been shown to improve the success rate of a fascia iliaca block over the landmark-only approach. Functionally, the technique is similar to a femoral block except that the large volume of local anesthetic used has a greater chance of also blocking the LFCN in addition to the femoral nerve.
Surface Anatomy–Based Technique
The traditional landmark-based technique involves placement of the needle at the lateral third of the distance from the ASIS and the pubic tubercle by using a “double-pop” technique as the needle passes through the fascia lata and fascia iliaca. Block success can be sporadic because false “pops” can occur.
Ultrasound-GUIDED Technique
With the patient in the supine position, a high-frequency linear probe is placed on the femoral crease and the fascia iliaca is identified. The probe is then moved to the medial border of the sartorius muscle ( Fig. 54.11 ). At this position a 4-inch 22-gauge needle is inserted in an in-plane approach, the fascia is pierced, and the local anesthetic is placed below the fascia iliaca, above the iliopsoas muscle (see Fig. 54.11 ). If infiltration is observed within the muscle, the needle should be withdrawn slightly. Proper injection will result in separation of the fascia iliaca from the muscle in a medial-to-lateral direction.
