Peripheral Nerve Blocks of the Lower Extremity

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Femoral nerve block provides anesthesia/analgesia to the anterior thigh, including the flexor muscles of the hip and extensor muscles of the knee. Historically this block was also known as the "3-in-1 block," suggesting that the femoral, lateral femoral cutaneous, and obturator nerves could be blocked from a single injection at the femoral crease. However, it has been demonstrated that the femoral and lateral femoral cutaneous nerves can be reliably blocked by a single injection, but the obturator nerve is often missed. The femoral nerve block is an ideal block for surgeries of the hip, knee, or anterior thigh and can be combined with a sciatic nerve block for near complete lower extremity analgesia. Complete analgesia of the leg can be achieved by adding an obturator nerve block.

Dorsal divisions of the anterior rami of L2-L4 form the femoral nerve (largest terminal branch of the lumbar plexus). Femoral nerve travels through the psoas muscle, then descends caudally into the thigh (via the groove formed by the psoas and iliacus muscles), entering the thigh beneath the inguinal ligament. Femoral nerve divides into an anterior and posterior branch after emerging from the ligament. It is usually located lateral and posterior to the femoral artery at this level. The anatomic location of the femoral nerve makes this block one of the easiest to master because the landmarks are easily identified, patient's remains supine, and the nerve depth is superficial.

Sciatic nerve supplies motor and sensory innervation to the posterior aspect of the thigh as well as the entire lower leg (except for sensory to the medial leg below the knee, which is supplied by the saphenous nerve, a terminal branch of the femoral nerve). The sciatic nerve, formed from the ventral rami of spinal nerves L4-S3, forms most of the sacral plexus (L4-S4) and is the largest nerve in the human body. Since the sciatic nerve is so large, it can be blocked at several different locations along the lower extremity.

The sciatic nerve is actually two nerves in close apposition, tibial and common peroneal (fibular) nerves. These nerves usually separate at the mid-thigh (75%), although separation as proximal as the pelvis and as distal as the popliteal crease may occur. Sciatic nerve leaves the pelvis via the greater sciatic foramen, travels under the gluteus maximus, and continues distally toward the posterior thigh between the greater trochanter and the ischial tuberosity. The sciatic nerve supplies motor innervation to the posterior thigh muscles as well as all muscles of the leg and foot. It also provides sensory innervation to the skin of most of the leg and foot (except for medial leg below the knee).

The popliteal block of the sciatic nerve is typically performed at a more distal location immediately cephalad to the popliteal fossa. Posterior or lateral approaches to the sciatic nerve at this level anesthetize the same dermatomes distal to PNB placement. The posterior approach may be technically easier than the lateral block, as the needle depth is shallower, making it more comfortable for the patient. Because the popliteal block of the sciatic nerve is performed more distal than the subgluteal (posterior) or anterior (medial thigh) approaches to the sciatic nerve, attention to both components of the sciatic nerve (tibial and common peroneal) is necessary to ensure adequate anesthesia and analgesia.

In the following descriptions, standard anatomic axes or planes will be used throughout for consistency, with the three primary descriptive planes: horizontal, coronal, and sagittal. Probe angulation is further described in the cephalad-caudad, medial-lateral, and anterior-posterior directions. The most common ultrasound orientation for nerve (and blood vessel) visualization will be in the short-axis (view in transverse or cross section) view. A long-axis (view in longitudinal section) view of nerves may be used to distinguish a putative nerve from other structures, but short axis is most commonly employed during needle placement, whether the needle is introduced either in-plane or out-of-plane with the ultrasound beam.

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