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The Adductor Canal  Block

T-drop1he Adductor Canal block is another block of the femoral nerve,  further down the thigh so that much of the motor innervation of the quadriceps group has already departed the nerve, well above the position of the local anesthetic. This preserves much of the quadriceps strength making early ambulation and rehab safer.

The femoral nerve is commonly approached just at or below the level of the inguinal ligament. Blocking it at the inguinal crease can provide post-operative analgesia to the entire front of the upper thigh down to and including the patella as well as the medial side of the lower leg to approximately the medial malleolus.

The Adductor Canal block of the femoral nerve occurs more distally and therefore covers a lesser amount of the leg. The Adductor Canal block can be useful for procedures of the distal thigh and femur, knee and lower leg on the medial side. It can be combined with a sciatic block to provide complete coverage of the distal thigh, knee nad lower leg and foot.  It is especially well suited to combination with the Anterior Approach to the sciatic at the same level as the Adductor Canal block (and with the same prep and needle). More on the Anterior Approach to the sciatic can be found here

Anatomy in Brief

The femoral nerve is actually many separate nerve fibers traveling together.  They arise from the Lumbar Plexus which consists of spinal nerves from 12th thoracic through 5th lumbar levels.

At or above the level of the pelvis, the Lumbar Plexus gives off several branches including the iliohypogastric, ilioinguinal, genitofemoral, the obturator and accessory obturator nerve, and the lateral femoral cutaneous nerve. After these nerves diverge from the plexus more proximally, the femoral nerve proper emerges from the pelvis passing beneath the inguinal ligament and with the femoral artery and vein follows a course along the medial aspect of the thigh, beneath the sartorius muscle. While the roof of the canal is formed by the sartorius, the vastus medialis and adductor longus and magnus form the other walls. Other names found for this anatomic tunnel are “subsartorial canal” and “Hunter’s canal”.

As the femoral nerve travels along the canal, it gives off sensory and motor branches to the surrounding structures. There are multiple innervation points and redudancy for motor units of the quadriceps.

The femoral nerve exits the canal at the hiatus, several centimeters above the medial femoral condyle. The femoral artery passes medially and deep to the femur to form the popliteal artery at this point, while the terminal branch of the femoral nerve, the saphenous nerve, exits the hiatus to become superficial following the great saphenous vein down the medial side of the lower leg to the medial malleolus.  

 

 

 

 

 

 

 

 

 

 

 

 

The adductor canal block should take place about midway down the thigh, placing local anesthetic around the femoral nerve before it has exited from the canal. Since the adductor canal is formed by the fascial planes of 3 muscles, the space can be treated as a compartment containing the femoral neurovascular bundle, and the injection of the local anesthetic solution will probably be effective if it is injected as a single bolus in the space. However it is important to observe the spread of the local solution using ultrasound to insure that local spreads completely around the artery. It may be necessary to reposition the needle at least once during the injection to assure adequate distribution.

The Block in Brief

The Adductor Canal Femoral Nerve Block

 

Comments

Patient Position

Supine

Operative leg slightly flexed at knee and externally rotated.

Ultrasound Probe

8 – 14 mHz Linear

Mid-range

Needle

3.5 - 4”

Tuohy 20G 3.5” is good choice.

Needle Approach

In-Plane or Out-of-Plane

No obvious advantage to either.

Local Solution Volume

20 – 30 ml

Half of solution on each side of artery.

Key Anatomy Landmark

Femoral Artery beneath the Sartorius muscle

Femoral Nerves will be medial and lateral to the artery

Miscellaneous

Look for the subfascial grouping of femoral nerves within the superficial iliopsoas muscle lateral and posterior to artery.

Danger of Patient Falling while block working

A Nerve Block Warning Protocol

In addition to the sensory/pain block, the femoral nerve block can also cause motor block of the quadriceps muscles in the front of the thigh.  The quadriceps group is essential for stabilizing the knee during weight-bearing so all patients who receive a femoral block should be forcefully warned of the greatly increased risk of falling if they attempt to stand or walk using the blocked leg.  This cannot be over-stressed.  Every year many patients fall because of nerve block and some of them have quite serious injuries with life-changing sequelae.

Please consider adopting or developing a formal protocol of warning patients of the dangers attendant on nerve blocks as well as an adhesive label placed directly on the effected limb warning the patient and those coming in contact with the patient that a nerve block has been performed, to take precautions to protect the limb, exercise caution to prevent falls, and the approximate date and time that the block should wear off along with instructions to remove the warning label when the block has dissipated.

This type of comprehensive Nerve Block Warning Protocol costs very little and really works to avoid many potentially serious block-related falls and accidents.

Below you will see an example of a warning label that can be inexpensively reproduced and placed on patients. At our institution we place the label directly on the outer layer of dressing on the limb with the block.  We place an arbitrary date and time of 24 hours on the presumed length of the block with instructions that it may have a longer or shorter duration.

blocksticke0925r

 

The Adductor Canal Block Procedure

With the patient supine, flex the operative leg at the knee slightly and externally rotate the into a stable position. After prepping the leg and applying sterile ultrasound gel on the medial aspect about midway down the thigh, Place the ultrasound probe transversely along the medial aspect, about midway down the thigh.  Have the depth of the survey set to 4 -5 cms. You are trying to identify  the femoral artery. The femoral vein will be just beneath the artery and will be easily compressed by downward pressure on the ultrasound probe.

If you have difficulty finding the femoral artery, first try moving the probe circumferentially around the thigh while keeping it oriented transversely to the long axis of the leg. If you continue having trouble locating the vessels, try turning on the color or power Doppler to hi-light the flow in the vessels and increase the survey depth.  If you are unable to find the artery at the midpoint of the thigh, move your probe to the inguinal crease, locate the artery there and follow the superficial branch distally to the midpoint o fthe thigh. 

After identifying the femoral artery the femoral nerves will be seen on either side of the artery as bright, echodensities. just inferior and parallel to the inguinal ligament. Locate the femoral artery, note the position of the femoral veins. 

adductir canal external

Inject half the local anesthetic solution on each side of the artery at the points of the bright densities.

Ignore the bright density seen beneath the vessel unless you can positively identify it as a nerve. Echodensities (bright areas) see below vessels usually artifacts called “acoustic enhancement" which makes things on the far side of liquids, like blood, appear brighter because of the increase in the speed of sound through liquid versus tissue. (It's obscure, just take my word for it.)

 

 

 

 

 

 

 

 

 

 

 

As I posited on the home page, I would expect single shot adductor blocks to have much shorter durations than single shot femoral blocks performed at the inguinal crease because the local anesthetic solution bolusses placed int he adductor canal will be below the tourniquet site most of the time. This will spread the solution over a larger area increasing the surface area for absorption. In addition, early active ambulation will increase the blood and interstitial movement in the area of the local solution deposition speeding its movement and absorption away from the femoral nerve, shortening the block duration.

The answer to this is to place a perineural catheter into the block site after the initial single shot nerve block and attach a continous infusion pump to it or bolus inject the catheter periodically to extend the duration of the block. 

Thanks for visiting Neuraxiom.

Jack

May 25, 2013

 

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