An adductor canal block is a form of regional anesthesia, which can be thought of as a targeted numbing injection. It is used to block pain signals in a specific part of the leg, most often for surgeries involving the knee. This procedure involves delivering medication to numb the saphenous nerve, a nerve that provides sensation to parts of the knee, lower leg, and foot.
Purpose of an Adductor Canal Block
The primary purpose of an adductor canal block is to provide pain management following knee surgery, such as a total knee replacement or an anterior cruciate ligament (ACL) repair. Its advantage lies in its precision. The block specifically targets sensory nerves, which transmit pain, while having minimal impact on motor nerves that control muscle movement.
This selective numbing is a benefit when compared to other types of nerve blocks, like the femoral nerve block. A femoral block, while effective for pain, often causes weakness in the quadriceps muscle at the front of the thigh. This muscle weakness can make it difficult for a patient to walk safely and can delay participation in physical therapy after surgery.
By preserving quadriceps strength, the adductor canal block allows patients to get up and move around sooner and more safely. This early mobilization is a component of a faster recovery. It helps reduce the risk of post-surgical issues such as joint stiffness and may contribute to a shorter stay in the hospital recovery unit.
The Procedure
The procedure for an adductor canal block begins with positioning the patient, who is typically lying down with the leg turned slightly outward to give the anesthesiologist access to the inner thigh. Before the injection, monitors are placed to track blood pressure, heart rate, and oxygen levels, and the patient often receives IV sedation to ensure comfort. The skin over the mid-thigh area is then thoroughly cleaned with a sterile solution to prevent infection.
An important part of the procedure is the use of an ultrasound machine. The anesthesiologist uses the ultrasound transducer on the thigh to get a clear, real-time image of the underlying anatomy, including muscles, the femoral artery, and the adductor canal. This imaging allows for precise identification of the saphenous nerve, which appears as a small, bright, round structure on the screen, usually located next to the artery.
Once the exact location is confirmed, the anesthesiologist will often numb the skin with a small amount of local anesthetic so that the main injection is more comfortable. A specialized needle is then guided, under continuous ultrasound visualization, through the sartorius muscle until its tip is positioned within the adductor canal near the saphenous nerve. After confirming the correct placement, the local anesthetic is slowly injected to surround the nerve, and the needle is removed.
Effects and Duration
After the block is administered, the primary effect is numbness in the areas supplied by the saphenous nerve. This typically includes the front and inner parts of the knee, extending down the inner side of the lower leg, and sometimes to the inner ankle and foot. This numbness effectively controls pain from the surgical site, and a significant feature of this block is that patients should still be able to contract their quadriceps muscle.
The pain relief from a single-injection adductor canal block is temporary. The numbing effect typically lasts between 12 and 24 hours, though in some cases it can persist for longer. The specific duration depends on the type and amount of local anesthetic used, such as bupivacaine or ropivacaine.
As the anesthetic begins to wear off, sensation will gradually return to the leg. This transition is managed with other forms of pain relief, such as oral medications, to ensure the patient remains comfortable. In some situations, a catheter may be placed in the adductor canal during the procedure to provide a continuous infusion of anesthetic for several days.
Potential Risks and Side Effects
An adductor canal block has potential risks, though serious complications are uncommon. More frequent and minor side effects can include some bruising, soreness, or tenderness at the injection site on the thigh. Occasionally, the block may not be fully effective, resulting in what is known as a block failure, where the desired level of numbness isn’t achieved.
Though rare, more significant risks include infection at the injection site, bleeding or a hematoma (a collection of blood under the skin), and nerve damage, which is usually temporary. In some instances, the local anesthetic can spread beyond the adductor canal and cause some temporary quadriceps weakness, which could increase the risk of a fall.
A rare but serious complication is local anesthetic systemic toxicity (LAST), which occurs if the anesthetic accidentally enters the bloodstream and can affect the heart and brain. Symptoms of LAST include a metallic taste in the mouth, ringing in the ears, or numbness around the lips. The use of ultrasound guidance to visualize the needle and surrounding blood vessels significantly reduces the likelihood of these more serious complications.