Anatomy of the 
Adductor Canal

The adductor canal is a musculoaponeurotic, triangular shaped tunnel lying posterior to the sartorius muscle that begins at the distal femoral triangle (see illustration), extending distally to the adductor hiatus (a location demarcated by the point where the superficial femoral artery (SFA) exits the AC and descends posteriorly to become the popliteal artery)

• Superficial femoral artery (SFA) and vein (SFV)
  • • The SFA descends distally in the leg and travels from the medial edge of the sartorius muscle to the lateral edge of sartorius until it reaches the distal opening of the AC or the adductor hiatus before becoming the popliteal artery in the posterior knee.
• Medial Vastus Nerve or Nerve to the vastus medialis (NVM):
  • • This nerve is a branch of the femoral nerve that innervates the vastus medialis muscle (one of the four quadriceps muscles) and has a significant contribution to the sensory articular innervation of the anteromedial joint capsule.
    • The NVM exits the femoral triangle in a self-contained sheath outside the AC distal to the femoral triangle or can be located within the AC.
    • The NVM is located between the vastus medialis and sartorius muscle just distal to femoral triangle or within the body of the vastus medialis muscle depending on the distal location from the femoral triangle.
    • Ultrasound identification of this nerve can be difficult in the mid-distal portions of the AC, since this nerve begins to branch in the mid-adductor canal region to innervate the vastus medialis muscle and anteromedial joint capsule.
    • The medial retinacular nerve is a distal branch of the medial vastus nerve that innervates the medial retinaculum of knee joint capsule.
• The neurovascular structures contained in the AC are surrounded by (3) muscle groups to form the triangular-shaped AC tunnel:
  • • Vastus medialis (anterolaterally), adductor longus (posteromedially), and sartorius (superiorly)

Dermatome mapping for the saphenous nerve and the osteotome distribution are shown in the following illustrations:

Scanning Technique

Background

Historically, local anesthetic placement within in AC was described as a midway point between the anterior superior iliac spine (ASIS) and the patella. However, cadaveric dye studies have demonstrated that blocking at this level can often corresponds to area within distal femoral triangle, leading to proximal spread and resultant quadricep weakness from a femoral nerve block. Current recommendations for identifying the ideal location for the placement of local anesthetic for the AC block is to use ultrasound and identify an area distal to the apex of the femoral triangle and within the proximal or mid AC regions.

• A high frequency linear array transducer (5-12 MHz) is used for this block. For large habitus patients or scanning depths > 4-5 cm, a low frequency 1-5 Mhz probe can facilitate proper imaging for this block.

• Proximal AC (Figure B):
  • Area where the saphenous nerve, SFA and SFV descend posteriorly into the AC and represents an ideal location to perform an ACB, as efficacy can be achieved with less local anesthetic.
  • To identify the proximal AC with ultrasound (US), scan from the FT distally to the “intersection” of the medial borders of the sartorius and adductor longus muscle.

• • The SFA is located inferiorly and on the medial upslope to the sartorius muscle on ultrasound, with the saphenous nerve (green arrow) being lateral from the 9-12 o’clock position relative to the SFA (Figure B). The SFV is located posterior to the SFA and is often compressed by the probe during this procedure.
• Mid-Adductor (Figure C): The SFA is located at a midway point and inferior to the sartorius muscle on ultrasound and represents an anatomical half-way point between the femoral triangle and adductor hiatus.
• Distal Adductor (Figure D): An area immediately distal to the adductor hiatus.
  • Cadaveric and clinical studies have demonstrated that a block done in this region may result in a failure to anesthetize proximal sensory branches of subsartorial plexus and vastus medialis nerve.
  • The saphenous nerve (green arrow) ascend superficial and exits the AC region where it lies between the sartorius (1) and gracilis (5) muscles in Figure D.

• A distinctive “pop” is felt when the needle pierces the AC. Inject normal saline to observe anterior elevation of the vastoadductor membrane and sartorius muscle while witnessing anechoic spread of fluid over the superior aspect of the SFA. Once proper location is confirmed, continue to inject local anesthetic (15-20 ml) observing expansional spread of the subsartorial space.
• A small aliquot (5-7 ml) of local anesthetic is injected in and around the medial vastus nerve which lies lateral between the sartorius and vastus medialis muscle.
  • The medialis vastus nerve is often contained within in its own fascial sheath between the sartorius and vastus medialis muscle. Blockade of this nerve as a distinctive block, has demonstrated enhanced analgesic coverage of the knee joint.

• Adductor Canal: 15-20 ml
• Vastus Medialis nerve: 5-7 ml

• Rotating the transducer slightly to create an oblique orientation can facilitate can help facilitate catheter placement into the adductor canal region.
• Inability to advance a catheter into the AC indicates the needle tip has not completely penetrated the vastoadductor membrane or the needle is abutting against structures within the canal. A change in needle direction/angle or slight advancement under ultrasound guidance can alleviate the obstruction and facilitate catheter advancement.
• Catheters should be advanced under ultrasound guidance NO further than 3 cm into the canal.
• Ultrasound verification of the catheter tip (above or adjacent to the SFA) is accomplished with visualization of fluid or air expansion in the AC.
• Dermabond placed at the catheter entry point can reduce backflow leakage if a catheter through the needle system is utilized.
• An infusion rate of 6-8 ml/hr of analgesic strength local anesthetic can be initiated to maintain the block postoperatively. The infusion rate is adjusted as needed for insufficient analgesia or if an unexpected femoral block occurs with prolonged infusions.

• LAST (Local Anesthetic Toxicity)
• Infection
• Hematoma
• Nerve injury
• Femoral nerve block with resulting quadricep weakness

• Needle insertion parallel to the transducer enhances tip visualization when entering the AC to reduce saphenous nerve injury. This is best accomplished by inserting the needle several centimeters lateral to the transducer.
• Elevation of the posterior sartorius sheath and vastoadductor membrane with visualization of anechoic spread of fluid, superior and lateral to the SFA, during injection is of paramount importance. An injection of local anesthetic between these two structures or outside the AC region could result in a block failure.
• A pre-procedure scan identifying the NVM with a separate, targeted injection can improve the quality of the AC block with minimal reduction in quad strength.
• A nerve stimulator can be used by setting the pulse duration to 1 mS and an amperage of 1 mV. The patient will describe a tingling sensation on the medial aspect of their calf when the needle tip approaches the saphenous nerve.

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2. Bendtsen TF, Moriggl B, Chan V, Pedersen EM, Børglum J (2014) Defining adductor canal block. Reg Anesth Pain Med 39(3):253– 254. https://doi.org/10.1097/AAP.0000000000000052
3. Bendtsen TF, Moriggl B, Chan V, Børglum J. Basic topography of the saphenous nerve in the femoral triangle and the adductor canal. Reg Anesth Pain Med. 2015;40:391–392.
4. Bendtsen TF, Moriggl B, Chan V, Børglum J. The optimal analgesic block for total knee arthroplasty. Reg Anesth Pain Med. 2016;41(6):1-9.
5. Kendir S, Bilge Ipek Torun, Akkaya T, Comert A, Tuccar E, Tekdemir I. Re-defining the anatomical structures for blocking the nerves in adductor canal and sciatic nerve through the same injection site: An anatomical study.
6. Hadzic A, ed. Hadzic’s Textbook of Regional Anesthesia and Acute Pain Management. 2^nd New York, NY; McGraw Hill Education, 2017.