Understanding the Anatomy of the Popliteal Fossa
To perform a popliteal nerve block using the landmark technique, a solid understanding of the popliteal fossa's anatomy is critical. The popliteal fossa is the diamond-shaped space behind the knee, containing the sciatic nerve and its bifurcation, along with major blood vessels.
- Lateral border: The tendon of the biceps femoris muscle.
- Medial border: The tendons of the semitendinosus and semimembranosus muscles.
- Inferior border: The popliteal crease, the visible skin fold behind the knee.
- Sciatic nerve: The nerve typically bifurcates into the tibial nerve and the common peroneal nerve approximately 5-12 cm proximal to the popliteal crease. The key to a successful block is targeting the sciatic nerve before this division.
The Landmark Technique for a Popliteal Nerve Block
Patient Positioning and Preparation
Proper patient positioning is essential for accurate landmark identification. The most common position is prone (face down).
- Position the patient prone with the operative leg slightly bent at the knee, often supported by a pillow or towel roll under the ankle, allowing the foot to dangle freely. This makes it easier to observe motor responses.
- Identify the three primary landmarks using palpation:
- The popliteal crease. Draw a line across the skin fold.
- The biceps femoris tendon. Palpate along the lateral border of the fossa.
- The semitendinosus and semimembranosus tendons. Palpate along the medial border of the fossa.
- Have the patient flex their knee against gentle resistance to make the hamstring tendons more prominent and easier to trace.
- Mark the needle insertion point. For the intertendinous approach, mark a spot 7-10 cm superior to the popliteal crease, precisely at the midpoint between the biceps femoris and semitendinosus tendons.
Procedure with a Nerve Stimulator
The landmark technique is performed with the aid of a peripheral nerve stimulator to confirm proximity to the sciatic nerve. An insulated block needle is used to deliver a low electrical current, which causes a motor response (twitch) in the foot when close to the nerve.
- Initial Insertion: After local anesthetic infiltration of the skin, insert the insulated needle at the marked point. The needle is typically advanced at a 45-60° angle in a cephalad (upward) direction.
- Nerve Stimulation: Initially set the nerve stimulator to deliver a current of 1.0–1.5 mA. When advancing the needle, watch for a motor response in the foot.
- Confirming Placement: The goal is to obtain a foot twitch at a low current (0.2–0.5 mA) without eliciting a twitch at the hamstring muscles. The type of twitch indicates which branch of the sciatic nerve has been stimulated:
- Plantarflexion and/or inversion of the foot: Indicates stimulation of the tibial nerve, a good location for injection.
- Dorsiflexion and/or eversion of the foot: Indicates stimulation of the common peroneal nerve.
- Troubleshooting: If no motor response is observed, withdraw the needle and re-insert slightly laterally. If local hamstring muscle twitches occur, redirect the needle away from that muscle. Once the appropriate low-current twitch is achieved, inject the local anesthetic after negative aspiration for blood.
Comparison of Landmark vs. Ultrasound-Guided Techniques
While the landmark technique was once the standard, the introduction of ultrasound has offered significant improvements in safety and efficacy. The table below compares the key differences between these two approaches.
| Feature | Landmark Technique | Ultrasound-Guided Technique |
|---|---|---|
| Equipment Required | Nerve stimulator, insulated needle | Ultrasound machine, linear probe, nerve stimulator (optional) |
| Visualization | Indirect (based on surface anatomy and motor response) | Direct, real-time visualization of the nerve and needle |
| Accuracy | Dependent on correct identification of external landmarks and nerve location relative to them | Higher accuracy due to direct visualization |
| Procedural Speed | Can be faster in experienced hands but may require more time if landmarks are difficult to identify | Can be slower initially but often faster to achieve a successful block |
| Safety Profile | Higher risk of vascular puncture or intraneural injection due to blind needle advancement | Lower risk of complications due to real-time guidance; confirms needle tip position away from vessels and nerves |
| Efficacy | Effective when performed correctly, but relies heavily on patient anatomy | Higher success rates reported in recent studies |
Considerations, Advantages, and Risks
The primary advantage of the landmark technique is that it does not require access to or expertise with ultrasound equipment. However, it does present several disadvantages, including variability in success rates and potential safety concerns compared to ultrasound guidance.
Potential Risks and Complications
- Nerve Injury: Can occur from mechanical trauma, particularly with multiple needle passes. This is a higher risk with the blind nature of the landmark approach.
- Incomplete Block: If the local anesthetic is not placed properly around the nerve, the block may be incomplete. This is more likely if the injection occurs distal to the sciatic nerve bifurcation.
- Local Anesthetic Systemic Toxicity (LAST): Caused by accidental intravascular injection. While rare with this technique, aspiration for blood before injection is critical.
- Vascular Puncture: Although the sciatic nerve is generally superficial to the popliteal vessels, inadvertent puncture is possible, especially if the needle is advanced too deeply.
- Fall Risk: With a complete block, motor function is affected, requiring patient precautions to prevent falls.
Contraindications
- Patient Refusal: The patient must consent to the procedure.
- Infection: An active infection at the injection site is an absolute contraindication.
- Allergy: Allergy to local anesthetics should be ruled out.
- Pre-existing Neuropathy: Caution is advised, as it can be difficult to assess new nerve damage from the block.
- Coagulopathy: Relative contraindication due to bleeding risk.
Conclusion
The landmark technique for a popliteal nerve block, used for decades in regional anesthesia, relies on palpation and nerve stimulation to identify the sciatic nerve in the popliteal fossa. While effective, it is less precise and has a higher risk of complications than the modern, safer, and more reliable ultrasound-guided approach. While many practitioners have transitioned to ultrasound for real-time visualization and higher success rates, understanding the landmark method provides a foundational knowledge of nerve anatomy and procedural technique. For more advanced techniques and protocols, resources like the New York School of Regional Anesthesia are valuable.(https://www.nysora.com/topics/regional-anesthesia-for-specific-surgical-procedures/lower-extremity-regional-anesthesia-for-specific-surgical-procedures/foot-and-anckle/block-sciatic-nerve-popliteal-fossa/)
