Ultrasound guided Continuous Adductor Canal Block for Analgesia After Total Knee Replacement
Ultrasound-Guided Adductor Canal Block
CLINICAL PERSPECTIVES
Adductor canal block has become an increasingly desirable alternative to femoral nerve block when providing surgical anesthesia and postoperative pain relief for total knee arthroplasty and other surgical procedures just below the knee. Unlike femoral nerve block at the inguinal ligament, which produces both motor and sensory block, adductor canal block preserves quadriceps strength with the only motor nerve blocked being the nerve to the vastus medial muscle. Continuous adductor canal block may provide prolonged postoperative pain relief and allow early ambulation due to the limited motor block associated with this technique.
FIGURE 135.1. Cross-sectional anatomy of the adductor canal. Note the relationship of the saphenous nerve, the femoral artery and vein, and the fascial plane between the vastus medialis muscle and the sartorius muscle.
CLINICALLY RELEVANT ANATOMY
The adductor canal, which is also known as Hunter's or the subsartorial canal, extends from the apex of the femoral triangle to the opening in the adductor magnus muscle, the adductor hiatus. This intramuscular tunnel lies in the medial aspect of the anterior compartment of the thigh and is bounded posteriorly by the adductor longus and adductor magnus muscles, laterally by the vastus medialis muscle, and anteriomedially by the sartorius muscle (Fig. 135.1). Entering the adductor canal through the superior foramen, the saphenous nerve passes though the adductor canal along with the nerve to the vastus medialis, the superficial femoral
artery, and the femoral vein (Fig. 135.2). The saphenous nerve moves superficially as it approaches the knee, exiting the anterior foramen of the canal and piercing the adductor intermuscular septum. The nerve passes over the medial condyle of the femur, splitting into terminal sensory branches (Fig. 135.3). The saphenous nerve is subject to trauma or compression anywhere along its course. The nerve is frequently traumatized during vein harvest for coronary artery bypass grafting procedures. The saphenous nerve also is subject to compression as it passes over the medial condyle of the femur. The femoral artery and vein continue their downward path through the adductor canal, exiting via the adductor hiatus. Cadaver studies suggest that local anesthetic injected into the adductor canal may also anesthetize fibers of the medial femoral cutaneous nerve, the articular branches of the obturator nerve, and the medial retinacular nerve. Local anesthetic injected into the distal adductor canal may also spread inferiorly through the adductor hiatus to reach the popliteal fossa, blocking fibers of the sciatic nerve, helping explain the fact that adductor canal block produces sensory block beyond the distribution of the saphenous nerve (Fig. 135.4). Adductor canal block is analogous to the axillary block in that both the vascular structures and neural elements are contained within the same fascial plane and the extent of the nerves block is dependent on the flow of the local anesthetic onto each nerve.
FIGURE 135.2. Anatomy and contents of the adductor canal.
ULTRASOUND-GUIDED TECHNIQUE
The patient is placed in the supine position with the arms resting comfortably across the chest and the affected lower extremity flexed at the knee externally rotated. A total of 20 mL of local anesthetic is drawn up in a 12-mL sterile syringe
with attention given to the total milligram dose of local anesthetic to avoid local anesthetic toxicity. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. The superior pole of the patella is then identified by palpation. A high-frequency linear ultrasound transducer is placed in a transverse plane over the previously identified point just above the superior pole of the patella, and a sonogram is obtained (Fig. 135.5). The hyperechoic anteromedial border of the femur, as well as the vastus medialis muscle just anteromedial to it, will be visualized (Fig. 135.6). The ultrasound transducer is then slowly moved in a more posteromedial medial direction until the fascial plane between the vastus medialis muscle and the sartorius muscle, which lies posteromedial to the vastus medialis muscle, is visualized (Figs. 135.7 and 135.8). The saphenous nerve lies just in the fascial plane between the vastus medialis muscle and the sartorius muscle. When the fascial plane between the vastus medialis muscle and the sartorius muscle has been identified on ultrasound imaging, the ultrasound transducer is slowly moved in a cephalad direction along the fascial plane between the vastus medialis muscle and the sartorius muscle (Fig. 135.9). Keeping the fascial plane between the vastus medialis muscle and the sartorius muscle in the middle of the image, the
transducer is moved in cephalad direction until the femoral artery and vein are identified (Fig. 135.10). It is just above this level that the adductor canal is injected. The skin is prepared with anesthetic solution, and a 22-gauge, 3½-inch needle is introduced at the lateral border of the ultrasound transducer and advanced medially using an in-plane approach, with the trajectory adjusted under real-time ultrasound guidance until the needle tip is resting within the fascial plane between the vastus medialis muscle and the sartorius muscle in proximity to the femoral artery in a manner analogous to an axillary block (Fig. 135.11). When the tip of the needle is thought to be in satisfactory position, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is correctly placed. There should be minimal resistance to injection. After needle tip placement is confirmed, the remainder of the contents of the syringe is slowly injected. The needle is then removed, and a sterile pressure dressing and ice pack are applied at the injection site.
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