In the emergency department, ultrasound-guided femoral nerve blocks can provide rapid and definitive control of acute pain resulting from traumatic injury to a lower extremity. Additional benefits include reduced risk of opioid-associated complications such as nausea, respiratory depression, and delirium.¹ Femoral nerve blocks are also an attractive alternative to procedural sedation, which requires additional monitoring and personnel (both of which may be limited in the ED setting). Peripheral nerve blockade in the acutely injured patient may blunt the systemic inflammatory stress response and reduce the associated risks of thromboembolism and immunosuppression. Additionally, there is increasing interest in the role of early peripheral nerve blocks in preventing the “wind-up” phenomenon of central sensitization that can lead to chronic pain syndromes and post-traumatic stress disorder.^2,3 Despite the strong evidence supporting their efficacy, safety, and ease of execution, femoral nerve blocks remain relatively underutilized in the ED.⁴

Clinical Indications

The ultrasound-guided femoral nerve block (USFNB) is typically used for fractures of the femur and injuries to the patella and its tendinous attachments. When combined with a lateral femoral cutaneous block, the femoral nerve block provides surgical anesthesia to the anterior and lateral thigh sufficient for deep wound exploration or incision and drainage of an abscess. In particular, several studies suggest that USFNB is superior to traditional intravenous or intramuscular analgesia. Advantages include more rapid onset of analgesia, reduced total pain scores, and reduced risk of oversedation and oxygen desaturation.^1,5,6 Recently, Beaudoin et al. demonstrated the ability of emergency physicians to learn and successfully use USFNB for pain control due to hip fractures in ED patients.⁷

Caution should be taken in patients clinically judged to be at high risk for compartment syndrome of the thigh, and a discussion with consulting orthopedic surgical and/or pain services should occur before performing the procedure. The compartments of the lower leg are innervated via the sciatic nerve, and blockade of the femoral nerve should not alter the clinical presentation of compartment syndrome of the lower leg.⁸

Anatomy

The femoral nerve is one of the three major branches of the lumbar plexus that arises from the first through fourth lumbar ventral rami (L1-L4). The lumbar plexus passes from the spinal roots through the psoas muscle, then descends in a groove between the iliacus and psoas muscles and branches into three major divisions before entering the thigh: the femoral, lateral femoral cutaneous, and obturator nerves. At the level of the inguinal ligament, the femoral nerve passes anterior to the psoas muscle and lateral to the femoral artery, then divides into its superficial and deep branches that ultimately supply sensation to the femur, hip joint, anteromedial thigh, knee, and the medial side of the leg from the knee to the foot.⁹