Chapter 24: Femoral Nerve Block

GENERAL CONSIDERATIONS

The FN block is a well-established regional anesthesia technique. It is the single most powerful analgesic method to treat pain after major knee surgery, either as a single injection or continuous block. However, an FN block invariably results in quadriceps muscle paresis, which may impede early active mobilization and ambulation. The protocols for enhanced recovery after surgery include early mobilization as a requirement, and therefore, an FN block may interfere with this goal. Alternatively, more distal interventional analgesia techniques with less impairment of ambulation may be better suited for some patients and surgeries. These options include blocks of the distal branches of the FN at different levels in the subsartorial space, pericapsular or soft tissue infiltration with an LA. Lower doses and concentrations of LAs for FN block and periarticular infiltration of LAs can also be used.

Regardless, the FN block is still widely used in patients with hip fractures both as an analgesic modality in the emergency department and to facilitate patient positioning for spinal anesthesia. In clinical situations where early mobilization is not required, the femoral block is the most effective and consistent interventional analgesic method. Finally, an FN block is often used as the sole anesthetic for quadriceps muscle tear and tendon rupture repairs, evacuation of the knee hematoma after total knee replacement surgery, and for surgery on the patella.

Limitations

The FN block has been associated with a risk of postoperative falls in the ward, due to the quadriceps muscle weakness. Protocols for specifying the risk and risk preventions are necessary whenever lower extremity nerve blocks are used, particularly for femoral and sciatic blocks. The incidence of FN injury reported in the literature is lower than that of upper extremity nerve blocks. However, the disability associated with FN injury is significant. Therefore, we advise strict adherence to triple monitoring (i.e., ultrasound [US], nerve stimulation, and opening injection pressure).

ANATOMY

The FN originates from the dorsal divisions of the ventral rami of the L2-L4 lumbar nerves. Approximately at the level of the fifth lumbar vertebral body, the FN exits the psoas muscle in a medial-to-lateral direction deep to the iliac fascia. It continues caudally and enters the anterior compartment of the thigh passing deep to the inguinal ligament, anterior to the iliopsoas muscle, and lateral to the femoral artery and vein (Figure 24-1). At the femoral triangle, the nerve divides quickly into multiple terminal branches. Deep branches innervate the anterior aspect of the hip, femur, and knee; muscular branches innervate the iliacus, psoas major, pectineus, rectus ...