Patient Positioning for a Femoral Nerve Block
For a femoral nerve block, the standard and most advantageous patient position is supine, or lying flat on their back. In this position, the operative leg—the side where the block is being performed—is slightly abducted (moved away from the body's midline) and externally rotated (turned outward). This specific positioning provides optimal access to the inguinal region, where the femoral nerve is located, and makes the key anatomical landmarks more prominent. A medical provider may place a pillow or towel under the hip to help maintain this posture, particularly in obese patients, to improve access to the area and facilitate palpation. In obese patients, retracting the lower abdomen laterally can also help expose the inguinal region for clearer visualization and access.
Anatomical Landmarks for Guiding the Block
The placement of a femoral nerve block relies on identifying specific anatomical landmarks. The two primary landmarks are the inguinal crease and the femoral artery pulse. The inguinal crease is the skin fold that demarcates the boundary between the abdomen and the thigh. In contrast, the inguinal ligament runs from the anterior superior iliac spine (the bony prominence on the front of the hip) to the pubic tubercle, and the block is typically performed just below this ligament at the femoral crease. The femoral artery runs under this ligament and its pulsations are a key indicator for locating the nerve. A helpful mnemonic for recalling the order of structures in the groin from medial (inside) to lateral (outside) is "VAN": Vein, Artery, Nerve. The femoral nerve lies just lateral to the femoral artery.
Techniques for Positioning the Needle
There are two main methods for guiding needle placement for a femoral nerve block: the traditional landmark-based technique and the more modern, preferred ultrasound-guided technique. Both rely on the same fundamental patient positioning but differ in how the final needle placement is confirmed.
Ultrasound-Guided Technique
With the patient in the supine position and their leg abducted and externally rotated, the clinician places a high-frequency linear ultrasound probe in the inguinal crease, perpendicular to the thigh. The provider then identifies the femoral artery and vein, which appear as hypoechoic (darker) round structures. Lateral to the artery, the femoral nerve is identified as a hyperechoic (brighter) triangular or oval-shaped structure, situated deep to the fascia iliaca and superficial to the iliopsoas muscle. The needle is inserted in-plane with the ultrasound transducer, typically from the lateral side, and advanced toward the nerve. After penetrating the fascia iliaca, which often feels like a distinct 'pop,' local anesthetic is injected to spread around the nerve and confirm placement. This real-time visualization allows for superior accuracy and safety.
Landmark-Based Technique
In this method, the clinician relies solely on anatomical palpation to determine the injection site. After positioning the patient supine, the provider palpates for the femoral artery pulse just below the inguinal crease. The needle is inserted approximately 1–2 cm lateral to the artery's pulse. A nerve stimulator can be used to elicit a motor response (patellar twitch) from the quadriceps muscle, confirming the needle's proximity to the nerve. This method is less common today due to the widespread availability and improved safety profile of ultrasound guidance.
Comparison of Femoral Nerve Block Techniques
| Feature | Ultrasound-Guided Technique | Landmark-Based Technique |
|---|---|---|
| Accuracy | High. Direct visualization of the nerve and spread of local anesthetic. | Variable. Relies on palpation and nerve stimulation. |
| Safety | High. Reduced risk of vascular puncture and nerve injury. | Lower. Increased risk of vascular puncture and intraneural injection. |
| Patient Comfort | Often better. Reduced need for multiple needle passes. | Can be less comfortable due to repeated needle adjustments. |
| Required Skills | Requires training in ultrasound visualization and interpretation. | Requires skill in palpating anatomy and interpreting nerve stimulator feedback. |
| Equipment | Ultrasound machine and probe, standard block needle. | Nerve stimulator, stimulating block needle. |
| Time to Perform | Generally quicker once landmarks are identified. | Can take longer if repeated adjustments are needed to elicit a twitch. |
Key Considerations for Proper Positioning
- Maintain Sterility: Aseptic technique is paramount for any nerve block to prevent infection at the injection site.
- Patient Cooperation: The patient must be able to remain still throughout the procedure to ensure accurate needle placement. Light sedation may be used if needed.
- Monitor for Complications: The patient's vitals (heart rate, blood pressure) should be monitored for signs of local anesthetic systemic toxicity (LAST), which can occur from inadvertent intravascular injection.
Conclusion
In conclusion, mastering where is the positioning for a femoral nerve block is fundamental to delivering safe and effective regional anesthesia. The patient is placed in a supine position with the affected leg slightly abducted and externally rotated. When using the modern, preferred ultrasound-guided method, the needle is inserted lateral to the femoral artery and guided by real-time imaging until it is positioned deep to the fascia iliaca and adjacent to the femoral nerve. While the traditional landmark-based technique is also an option, the enhanced safety and accuracy of ultrasound-guided positioning have made it the standard of care. This meticulous approach to patient and needle positioning ensures successful pain relief for procedures involving the anterior thigh and knee, with minimal risk to surrounding structures.
For additional information on regional anesthesia techniques, including the femoral nerve block, please consult resources like the NCBI Bookshelf's StatPearls article on the subject: Femoral Nerve Block.
