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ACEP Now: Vol 43 – No 05 – May 2024

FIGURE 2. Probe placement (probe marker in green) and corresponding ultrasound images for the superomedial (A), superolateral (B), and inferomedial (C) genicular nerves with color Doppler showing the corresponding genicular arteries. (Click to enlarge.)

Step 2

Place the ultrasound system contralateral to the affected knee so that you have a clear line of sight of the ultrasound screen when performing the block. The three locations for anesthetic deposition are the superomedial, superolateral, and inferomedial genicular nerves. The goal of this block is to gently place the needle tip against the bone at each location and spread local anesthetic along the surface of the periosteum toward the nerve.

Step 3

FIGURE 3. Probe and needle placement with corresponding ultrasound images demonstrating needle positioning and post-injection local anesthetic spread between the muscle and bone (red star) at the superomedial (A), superolateral (B), and inferomedial (C) sites. (Click to enlarge.)

For the superomedial and superolateral genicular nerves, the probe is placed longitudinally just proximal to the knee joint to identify where the femoral shaft curves outward toward the condyles. The medial and lateral sides should appear nearly identical. Identification of the genicular arteries with color Doppler can be attempted but can be difficult because of their small size. Anesthetize the skin with 1 mL of 1 percent lidocaine, then advance the block needle down to the bone. Aspirate prior to injecting to prevent inadvertent intravascular injection and inject 3-4 mL of long-acting anesthetic mixed with corticosteroid at each site. Ensure that anechoic fluid spreads along the femur. We prefer an in-plane, superior to inferior approach, but an out-of-plane technique can be performed, as well.⁴

The inferomedial genicular nerve is about 3 cm medial to the tibial tuberosity (similar to the site used for intra-osseous access), where the surface of the tibia forms a shallow bowl. The probe is placed longitudinally and color Doppler can be used to identify the genicular artery. Anesthetize the skin and insert the block needle, using an out-of-plane approach, down to bone, taking care to avoid the artery. An out-of-plane technique is used for this portion of the block because of the shallow depth of the target. After negative aspiration, inject 3-4 mL of long-acting anesthetic mixed with corticosteroid along the bone.

This adapted technique from our anesthesia colleagues does not provide complete surgical anesthesia, but rather significant sensory analgesia. In our experience, patients have been able to ambulate comfortably with no motor deficits in approximately 30 minutes. Also, the very low volumes needed for this block allows for the possibility of performing bilateral blocks with safe anesthetic volumes.