Technique

Pre-Block Assessment: As with any nerve block, assess for distal motor and sensory function along with a distal pulse check.

Patient Positioning: Patient positioning for the adductor canal is relatively easy compared to the ergonomic challenges of some other blocks. Place the patient in a “frog-leg” position (see Figure 1), with the machine on the side of the affected leg and the proceduralist on the opposite side of the bed. (As with any bedside ultrasound procedure, position the machine on the opposite side of the bed from the side of the procedure. This allows a clinician to observe the ultrasound screen comfortably.) While local anesthetic systemic toxicity is rare, cardiac monitoring is technically considered a best practice.¹

Figure 5: The hyperechoic needle is positioned correctly as the operator administers local anesthetic (A) just above the saphenous nerve (yellow). This in-plane approach provides better visualization of the needle during the procedure.

Ultrasound Landmarks: Position the probe in transverse at the junction between the middle and distal third of the anteromedial thigh. Identify the femoral artery in this area and direct attention to the superomedial aspect of the artery (see Figure 2), which is where the saphenous nerve passes deep to the sartorius muscle. Do not expect the nerve to appear perfectly circular in this region (see Figure 3). Rely on the anatomical consistency that the saphenous nerve will run with the neurovascular bundle of the distal femoral artery and vein in this fascial plane (see Figure 4). Once anesthetic is placed superomedially to the femoral artery and deep to the posterior fascia of the sartorius muscle, the saphenous nerve will become more visible as a result of hydrodissection.

Performing the Block: Using an in-plane approach, direct the needle toward the plane of the vascular bundle while maintaining visualization of the needle tip (see Figure 5). Once the needle pierces through the posterior fascia of the sartorius muscle (you may feel a popping sensation), administer anesthetic. If injected at the correct location, hydrodissection will occur, which, as noted above, often allows for better identification of the nerve. The anesthetic will bathe and envelop the nerve bundle. Avoid injecting directly into the nerve so as to avoid potential nerve injury.

Summary

The adductor canal block is a useful nerve block to facilitate the management of lower extremity soft tissue injuries or infections in the emergency department. It is a valuable addition to other peripheral nerve blocks used commonly by emergency clinicians.