Saphenous Nerve Block
The saphenous nerve block is a useful adjunct to the variety of sciatic blocks that are described. It covers the strip of innervation along the medial portion of the calf. The saphenous nerve is a terminal portion of the femoral nerve. It is purely sensory in nature. The motor innervation of the lower leg is primarily produced by the distal branches of the sciatic nerve. The saphenous nerve branches from the larger femoral nerve about half the distance of the femurs total length. It then runs medially passing through the facial plane between, and deep to the junction of the vastus medialis and sartorius muscles. Variations exist from patient to patient and the nerve may track below or pierce the sartorius muscle. As a sensory nerve, it cannot elicit a motor response with a nerve stimulator, but a sensation of pulsing at (low current intensities) can be sought. Increasing nerve stimulator current output will produce painful sensations in the saphenous distribution. Several techniques are described to block the saphenous nerve both with and without ultrasound guidance. The ultrasound technique (favored by the author) is accomplished at the inner thigh.
Begin this block by placing a high frequency linear probe set to about 3-5 cm of depth. Place the probe on the anterior thigh and identify the large vastus medialis muscle in short axis. This initial view should image the large rectus femoris vastus medialis muscles of the thigh. These should appear as large easily identifiable marbled ovals at the top of the screen. The femur may be visible as a crescent moon shape at the bottom of the image. Slowly position the probe medially until the next marbled oval comes into view. This smaller oval is the sartorius muscle. The nerve lies between these two muscles or deep to the sartorius. The nerve is most often not visible as it is rather small and the scan can be somewhat deep. Better quality scans can reveal its location, and in some cases, an additional nerve segment on the opposite side of the artery. The saphenous nerve can vary in presentation from patient to patient. It can run deep to the sartorius muscle, between the vastus medialis and sartorius muscles, or penetrate the sartorius muscle. At the level of the mid thigh, it is likely seen lateral and anterior to the artery.
After obtaining the proper short axis views, and prep solution has been placed, generously raise a skin wheal and infiltrate the predicted needle path. Introduce a long block needle in-plane to visualize the needle as it descends to a resting position just below the sartorius muscle, inside the adductor canal. Then reposition the needle between the vastus medialis and sartorius muscles and deposit an additional 5-10 mLs after aspiration. It should be noted that the saphenous nerve runs in close proximity to the inferior genicular artery and the great saphenous vein, so careful aspiration and attention to the first few mLs of solution is important.
Article on this subject by the author, and link to it’s location in the peer reviewed journal (CRNAToday):
THE EVOLUTION OF THE ADDUCTOR CANAL BLOCK:
The emerging technique for motor sparing analgesia to the knee.
The adductor canal block has recently gained attention from the anesthesia and orthopedic community. The desire to produce analgesia without the loss of motor control to the thigh seems to be beneficial. Some of the benefits regarding this technique would be shorter hospital stays, more efficient and earlier rehabilitation, pain control, and retained patient ability to report pain in neighboring distributions that can be involved when attempting to block the femoral nerve at the inguinal crease. This technique also embraces the relatively new regional philosophy of selectivity, or blocking only the area involved in the surgery. This article will review what we know about this relatively novel technique. The original search keywords used for this work were “adductor canal block”. When this yielded only a handful of articles, the search words “saphenous nerve block” were added. Only articles less than 5 years old were included, for the exception of one to show historical foundation.
One of the earliest works leading to the development of the adductor canal block, was done by Van Der Wal and associates in Canada (1). In their 1993 work, they first established the blocks clinical viability utilizing cadavers. They described what they called a “subsartorial approach” to the saphenous nerve blockade. While their efforts were originally to describe an alternative to traditional landmark approaches to the saphenous nerve blocks, primarily for foot and ankle procedures, it’s likely that this laid the groundwork for access to the saphenous nerve now being sought to provide analgesia to the knee.
Relevant block anatomy:
The relevant anatomy for the adductor canal block focuses on the specific target of the saphenous nerve. The femoral nerve originates from the L-2 to L-4 spinal nerves. These L2 through L-4 nerves join together as a structure known as the lumbar plexus. This plexus passes through the psoas compartment made of the psoas muscle posteriorly and the quadratus lumborum anteriorly. The femoral nerve emerges from under the inguinal canal and bifurcates into two smaller peripheral nerves in the anterior thigh (24). Near the location of the superior 1/3rd of the thigh, the femoral nerve continues it’s path deep into the leg to innervate the rectus femoris, vastus lateralis, vastus medialis, vastus intermedius and sartorius. This group is primarily responsible for extending the lower leg, and lifting the thigh forward and transferring weight during ambulation. The femoral nerve also sends off a pure sensory nerve called the saphenous nerve in the anterior thigh. The saphenous nerve separates from the femoral nerve and continues its path toward the knee accompanying the femoral artery and vein. This trio travels through the adductor canal. This canal is made up of the sartorius muscle superficially, and the vastus intermedius muscle as the deep component. The side wall completing a triangle is the adductor longus (see figure 1). Within these three structures, a tunnel or canal is formed allowing for passage of the saphenous nerve, and the distal femoral artery and vein. The saphenous nerve is responsible for the sensory innervation of the anterior knee. It sends a sensory branch inferiorly to the region under the patella called the infra patellar nerve. With this information it seems logical that by blocking only the saphenous nerve it is possible to cause analgesia to the anterior knee while preserving the anterior thigh motor control.
There are a few techniques described for blocking the saphenous nerve selectively. Advancing a needle blindly into the adductor canal was the first landmark based technique, using the middle third of the inner thigh as the starting point. Today, with the advances of ultrasound guidance, this is not recommended. The adductor canals relevant structures can be easily viewed with ultrasound. It seems sensible to begin by slightly externally rotating the thigh. Select a linear probe and set the depth to around 4 cm. Begin the scan by placing the probe in the midpoint of the patella and inguinal crease, on the medial side of the thigh. The probe should be positioned to allow for a short axis view of the distal femoral artery. If you are in doubt of the artery's identity, you may track the femoral artery path starting at the inguinal crease as you would a femoral block, or femoral vessel cannulation. Once you are satisfied with the arteries identity, place the adductor canal in view, with distal femoral artery in short axis visible. The Doppler modes can be used to confirm the structures pulsatile flow. The sartorius and adductor longus should be easily visible (figure 1). Prep the needle insertion region with the institutions antiseptic solution. For awake patients, advance a small gauge needle in plane through the projected needle path and deposit a fast acting local anesthetic solution such as lidocaine or mepivacaine. Once the needle path has been localized advance a long blunt block needle under the artery. It's unlikely that the saphenous nerve will be visible, but tilting the probe about 20-30 degrees away from the head can help. Once the needle has arrived deep to the artery, aspirate and begin to deposit the local anesthetic block solution. Redirect the needle, in plane as necessary to fill the adductor canal. This volume will vary depending on a variety of factors, but one study showed that 30mLs can fill the canal.
The adductor canal block is in its infancy compared to other well established regional techniques. 2013 offered the greatest number of research articles on some various aspects of the block. Unlike other traditional techniques that seek to cause a sensory as well as a motor blockade, the adductor canal block attempts to spare the motor block of the neighboring distributions in an attempt to offer selective analgesia and strength preservation. A frequently asked question is "Can the nerve stimulator be used as a secondary identifier for the saphenous nerve, either in combination or solo for saphenous nerve localization?". The answer seems to be that it can. Although the saphenous nerve is a purely sensory nerve, the nerve stimulator is not without merit. However, sensory nerves do not elicit a motor twitch, so how then can this be explained. This is because although they have a higher Rheobase and Chronaxie, they will elicit an appreciable response in awake patients. Clinical electrical current settings for traditional motor twitch elicitation are between 0.2-1.0 mA. This avoids undesired sensory responses traditionally associated with stimulator settings above 1.0. These increased current settings above 1.0 mA produce sensory pulsations. These can be useful for sensory nerve localization (25). It should be noted that as current settings exceed 1.0 mA, there is an increased likelihood that the patient will complain of painful nerve pulsations. Combining the nerve stimulator with ultrasound for nerve identity then seems of some use, although not one study included in this article employed this technique. The majority of the recent studies (2-15, 17) are optimistic that this relatively novel technique shows promise to accomplish the goal of knee analgesia and thigh strength preservation. Although its too early to be certain of its success, many of the various aspects regarding this technique have been cursorily reviewed. The information currently available is overwhelmingly positive, with only one article showing no real benefit over IV medications following knee arthroscopy. It seems likely that the future holds promise for this emerging technique.
This image shows proper probe position for performance of the saphenous nerve block in the adductor canal.
This video shows in-plane needle approach for the location of the adductor canal. An initial “pop” is felt and seen on the monitor as the fascia is breached.
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