Version May 2024
INTRODUCTION — The adductor canal block (ACB) is an interfascial plane block performed in the thigh. It anesthetizes multiple distal branches of the femoral nerve including the saphenous nerve and branches of the mixed sensory and motor nerves to the quadricep, and potentially branches of the obturator nerve. ACB is used for anesthesia and/or analgesia for surgery of the knee, medial lower leg and ankle. This topic will discuss the anatomy, ultrasound imaging, and injection techniques for performing ACB.
General considerations common to all peripheral nerve blocks, including patient preparation and monitoring, use of aseptic technique, localization techniques, drug choices, contraindications, and complications, are discussed separately. (See "Overview of peripheral nerve blocks".)
Ultrasound guidance for peripheral nerve blocks is also discussed in detail separately. (See "Ultrasound for peripheral nerve blocks".)
ANATOMY — The adductor canal (also known as the Hunter's canal) is an aponeurotic space between the distal femoral triangle and the adductor hiatus in the adductor magnus muscle (figure 1A-B).
●Boundaries – The adductor canal is bordered by the vastoadductor membrane (VAM) anteromedially, fascia of the vastus medialis (VM) anterolaterally, the fascia of the adductor longus or magnus posteromedially (image 1) [1,2]. An aponeurotic structure called the VAM forms the roof of the adductor canal and can often be seen sonographically deep to the sartorius muscle as an echogenic fascial plane. The proximal end of the adductor canal can be sonographically identified as the point where the medial border of the sartorius muscle intersects with the medial border of the adductor longus muscle (image 2) [1,2]. This convergence defines the transition point between the femoral triangle proximally and the beginning of the adductor canal distally.
●Nerves in the adductor canal
•Saphenous nerve is a sensory branch of the posterior division of the femoral nerve (figure 2) and reliably travels within the adductor canal adjacent to the superficial femoral artery (SFA) [1]. It is part of the patellar plexus. In the proximal adductor canal, the saphenous nerve usually lies with the femoral artery in a thin fascial layer, usually anterior to femoral artery (lateral on ultrasound imaging if leg is externally rotated). As it runs distally, the nerve courses to the medial side of the SFA where it exits the adductor canal through the VAM with the saphenous branch of the descending genicular artery [1,3].
The saphenous nerve supplies innervation to the patella and subsartorial plexus, the anterior inferior knee capsule [3], and the skin of the anteromedial portion of the lower leg and ankle, but likely not the foot (figure 3) [4].
•Nerve to the vastus medialis – The nerve to the vastus medialis (NVM) is a branch of the posterior division of the femoral nerve and further divides into multiple branches. The posteromedial branch of the NVM courses distally deep to the VAM encased in a thin layer of the VM fascia, which is separate from the contents of the adductor canal but immediately adjacent to it (image 3 and figure 1B) [1]. The posteromedial branch of the NVM then becomes the superior medial genicular nerve as it descends distally and provides sensory innervation to the knee. Both branches are typically blocked with an ACB.
The fascial layer separating the NVM from the adductor canal is likely permeable and thus clinically irrelevant when performing an ACB, as a cadaver study demonstrated that injection of 10 mL of dye in the proximal adductor canal stained both the posteromedial branch of the NVM and the superior medial genicular nerves [1].
The NVM likely innervates the knee joint capsule [5], medial retinaculum [5], and periosteum [6], important structures involved in pain after knee arthroplasty.
•Obturator nerve – The obturator nerve enters the thigh through the obturator foramen and then divides into anterior and posterior branches [3]. The genicular nerve is the distal branch of the posterior obturator nerve, and may lie within the adductor canal, though anatomy is variable. Some cadaver studies have found a branch of the obturator nerve lying in the adductor canal [1,5,7,8], while others have not [1,6]. A cadaver study of proximal adductor canal injection with 10 mL of dye found staining of the genicular branch of the obturator nerve as it descended into the distal adductor canal, without staining of the distal branches of the anterior obturator nerve [1]. In another cadaver study of distal adductor canal injection of 10 mL of dye, the genicular branch was also stained [7].
The sensory distribution of the obturator nerve is highly variable, and debate surrounds its contribution to the knee joint [1,3,9].
●Vascular structures – The femoral artery and vein traverse the canal and more distally become the popliteal artery and vein as they pass through the adductor hiatus located between the adductor magnus muscle and the femur.
ANATOMIC CLINICAL CORRELATIONS — ACB may be used for pain control of surgical procedures involving the knee (eg, total knee arthroplasty [TKA] [10,11], anterior cruciate ligament repair [12,13]) and the medial aspect of the lower leg and ankle (eg, ankle fusion [14]) representing the saphenous nerve distribution (figure 4).
Proximal versus distal adductor canal block — ACB can be performed with an injection in the proximal, mid, or distal adductor canal. Some individuals have advocated for renaming of this block to proximal subsartorial, mid-subsartorial, and distal subsartorial blocks [15]. Experts agree that the nomenclature in the literature is inconsistent, and although important to describe the block location thoroughly when performing research studies, the practical placement of this block is best individualized based on the patient's sonoanatomy.
Several studies have found that the incidence of quadriceps motor block is similar whether an ACB is placed at or proximal to the midpoint of the adductor canal, versus the distal adductor canal [16-20]. Whether analgesia differs with proximal distal block is less clear. Randomized trials of single-injection block for anterior cruciate ligament reconstruction surgery [16] and of continuous block for analgesia after total TKA [17,18] have found a slight analgesic benefit of proximal or midpoint ACB. In contrast, a randomized trial of 90 patients who underwent TKA found similar pain scores and opioid consumption on postoperative day one after proximal versus distal catheter placement for continuous ACB [19].
The authors prefer to place an ACB single injection or catheter at the midpoint of the thigh (mid-subsartorial, superficial femoral artery [SFA] under the middle third of the sartorius muscle) because the saphenous nerve is more superficial, and therefore an easier ultrasound target; the nerve to the vastus medialis is more reliably blocked at the mid-thigh; and in the case of continuous block, the catheter would be placed and dressed further from the surgical site.
SINGLE INJECTION ADDUCTOR CANAL BLOCK — ACB is performed with ultrasound guidance.
Patient positioning and marking — Position the patient supine with slight external rotation of the leg. Mark the location halfway between the anterior superior iliac spine (ASIS) and base of the patella on the patient's thigh (picture 1). Sterilely prep and drape a large area around the marked halfway point (picture 2).
Ultrasound equipment — The authors use a high frequency (13 to 6 MHz) linear ultrasound transducer with a footprint of 3 to 5 cm. Set the ultrasound depth at about 4 cm, with a high frequency preset.
Ultrasound imaging — (movie 1)
●Step one – Begin scanning with the ultrasound transducer on the anterior thigh at the marked point (picture 3). Locate the femur as a bright white (hyperechoic) crescent shaped structure to help determine the proper depth of the superficial femoral artery (SFA), located medially (image 4).
●Step two – Scan medially to locate the SFA (image 5). This artery is large, pulsatile, and easy to find about 3 to 4 cm deep in a patient with normal body mass index.
Tip: Scan proximal to distal if necessary to find the SFA – A common mistake made by providers performing this block is to start scanning too distally on the leg. As the SFA courses distally in the leg it passes through the adductor hiatus as it becomes the popliteal artery and dives deep (posteriorly), where it may not be visualized with a linear transducer, particularly in patients with large thigh circumference. If you are unable to locate the SFA, scan at the inguinal crease on the proximal thigh to find the SFA and then follow it distally into the adductor canal.
●Step three – Center the sartorius muscle superficial to the SFA in the middle of the ultrasound screen. At this point, the roof of the adductor canal can be visualized and consists of the vastoadductor membrane (VAM) on the deep surface of the sartorius muscle (image 6). The saphenous nerve and the nerve to the vastus medialis (NVM) appear as small round hyperechoic structures; the saphenous nerve is located within the, "true," adductor canal, whereas the NVM is located in a separate fenestrated sheath outside the canal (image 3).
Tip: The, "true," anatomic adductor canal is located at the approximate location of the mid-thigh, often just distal to the halfway point between the ASIS and patella [2]. It is sonographically located by scanning medially past the SFA to locate the medial border of the sartorius muscle. Then scan distally, following the medial border of the sartorius muscle as it approaches the medial border of the adductor longus muscle (image 2). It should be noted that the definition of this "true," landmark likely has no clinical relevance with regard to analgesia when local anesthetic (LA) volumes of 10 to 15 mL are used for the halfway point between the ASIS and patella ACB.
Performing the block — (movie 1)
●Step four – Insert a 20 to 22 gauge, 4 inch (10 cm) needle using an in-plane approach (picture 4). The authors insert the needle towards the anterolateral side of the SFA where the saphenous nerve is reliably located. A distinct, “pop,” will be felt when the needle is advanced through the VAM. We place the needle tip adjacent to the SFA at the 9 o’clock position as it is visualized on the ultrasound screen (image 7).
●Step five – After negative aspiration, inject 10 to 15 mL of LA in 5 mL increments, with gentle aspiration between injections, while visualizing spread of LA around the artery (image 8). Note: the superficial femoral vein is often hidden because of transducer compression, and perforating arteries can be present in this area (eg, the descending genicular artery which reliably branches off the SFA at the mid-thigh). To avoid intravascular injection during bolus administration, the authors strongly recommend continuous ultrasound visualization of LA spread, aspiration before injecting and every 5 mL, color Doppler use to help identify vessels, and the use of epinephrine in the LA solution as a vascular marker.
Drug choice
Choice of local anesthetic — The authors use 0.2% ropivacaine or 0.25% bupivacaine for single injection ACB. In studies of ACB, bupivacaine 0.25 or 0.5% or ropivacaine 0.2, 0.5 or 0.75% have been used [21-24].
Volume of local anesthetic solution — We inject 10 to 15 mL of LA for ACB to maximize analgesic efficacy while minimizing the potential for motor block. When choosing the volume of LA to inject for ACB, the total dose of LA to be administered for the block and for the other blocks must be considered, to reduce the risk of LA systemic toxicity. ACB is often used along with other nerve blocks (eg, periarticular injection [PAI]/local infiltration analgesia [LIA], interspace between popliteal artery and posterior capsule of the knee [IPACK] block). The range of typical LA bolus volume in studies of ACB is from 5 to 20 mL [1,21,22,25-27].
A dose finding study of LA volume for ACB in 40 volunteers found that a 20 mL bolus injected at the midpoint between the base of the patella and the ASIS filled the adductor canal to its most distal point in at least 95 percent of subjects [28]. Filling of the adductor canal was used as a surrogate for a successful block based on the assumption that all nerves in the adductor canal would be anesthetized (including the obturator nerve) if the canal was filled with LA. All tested LA volumes (5 to 20 mL) resulted in sensory block of the saphenous nerve. Quadriceps weakness occurred after 7 of 40 blocks; there was no correlation between the volume injected (range 5 to 20 mL) and proximal spread to the femoral triangle or motor block.
In another dose finding study of the volume of LA that would result in motor block after ACB, the effective dose (ED)50 for 0.5% ropivacaine (ie, volume that resulted in 30 percent reduction in quadriceps strength in 50 percent of patients) was 46.5 mL (95% CI, 45-50.4 mL), and the ED95 was 50.3 (95% CI, 48.7-67.3 mL) [29]. However, the primary outcome for this study was motor block and the volumes used in this study were much larger than those that are used clinically. Quadriceps weakness is unpredictable, and has occurred after blocks using much lower volumes of LA.
CONTINUOUS ADDUCTOR CANAL BLOCK — Continuous ACB with a perineural catheter can be used to prolong analgesia beyond the duration of a single-shot block. Prolonged analgesia may be desirable after knee surgery, particularly for major surgery (eg, total knee arthroplasty [TKA]), which results in moderate to severe pain for many patients.
Securing the catheter — If the catheter is placed preoperatively, make sure to keep the insertion site and catheter dressing out of the sterile surgical field. We typically place the catheter on the skin and cover it with clear plastic adhesive dressings proximally. If necessary, tunnel the catheter after placement to a location outside the surgical field. The catheter must be secured in a way that avoids having the catheter and its dressing inadvertently removed when the surgical drapes are being removed.
Catheter placement technique — The technique for catheter placement is similar to the technique used for single-shot injections, using ultrasound guidance as described above. A 19 or 20 gauge Tuohy needle is used, rather than a single injection block needle. A single or multi orifice catheter is inserted either over or through the needle, and typically advanced 4 to 6 cm into the adductor canal.
The technique used when placing an adductor canal catheter may affect the risk of catheter migration. In one trial 67 patients who underwent TKA were randomly assigned to continuous ACB with a catheter inserted parallel to the saphenous nerve versus perpendicular to the nerve [30]. With parallel placement, catheter migration occurred less often (14.7 versus 27 percent), pain scores were lower on day 2 (median 3.5 versus 6/10), and knee range of motion was better. For parallel placement, the needle was inserted in plane with the transducer in a sagittal orientation; for perpendicular placement the needle was inserted in plane with the transducer axis perpendicular to the femur. Further study is required before recommending a specific catheter placement technique with the goal of avoiding catheter migration.
We inject several mL of LA through the needle under ultrasound guidance to confirm correct placement of the needle tip, visualize insertion of the catheter, and then inject the rest of the bolus through the catheter in divided doses while visualizing spread of LA.
Infusion drug dose — After injecting a bolus as described above for single injection block in the post-anesthesia care unit, infusion is started with 0.1 to 0.2% bupivacaine or ropivacaine, using continuous infusion or programmed intermittent bolus. The authors use a continuous infusion of 0.1% bupivacaine at 10 mL per hour, whereas another contributor uses programmed intermittent bolus.
●Continuous infusion – 5 to 10 mL per hour [23].
●Programmed intermittent bolus – 8 mL bolus once per hour to 15 mL bolus every three hours.
The literature comparing continuous infusion with programmed intermittent bolus is scant. In one randomized trial of 110 patients who had continuous ACB for analgesia after TKA, postoperative morphine consumption was similar in patients who received ropivacaine 0.2% by continuous infusion at 7 mL per hour versus 21 mL every three hours by programmed intermittent bolus [31]. Postoperative motor strength was similar in the two groups.
SIDE EFFECTS AND COMPLICATIONS — ACBs are generally very safe. Complications common to all peripheral nerve blocks (eg, nerve injury, bleeding, local anesthetic systemic toxicity [LAST], infection) are discussed in detail separately (see "Overview of peripheral nerve blocks", section on 'Complications'). Several complications are of particular concern with ACB.
Motor block — Motor weakness associated with ACB and other lower extremity blocks may impact rehabilitation after surgery. ACB has become the first choice block for most total knee arthroplasties because it results in less quadricep weakness than femoral block [11]. However, several cadaver studies have found that dye injected into the adductor canal can spread to the femoral nerve proximally (causing quadriceps weakness) or to the popliteal sciatic nerve distally (causing foot drop) [1,7,25] (see "Anesthesia for total knee arthroplasty", section on 'Motor block and patient falls'). The presumed mechanism for distal spread is that local anesthetic (LA) travels along the sheaths of the superficial femoral artery (SFA) and superficial femoral vein through the adductor hiatus towards the popliteal fossa.
Local anesthetic systemic toxicity — Given the proximity of two large blood vessels (SFA and superficial femoral vein) in the adductor canal, as well as perforating arteries, there is potential inadvertent intravascular injection and LAST, and also for hematoma formation.
If ACB is performed for knee surgery in addition to local infiltration analgesia (also known as periarticular injection) and/or infiltration between popliteal artery and posterior knee capsule (IPACK) block, the total dose of LA should be calculated to avoid excessive dosing and to minimize the risk of LAST. (See "Anesthesia for total knee arthroplasty", section on 'Regional anesthesia techniques for postoperative pain control'.)
Myotoxicity — LA-induced myotoxicity has rarely been reported after peripheral nerve block. There are several reported cases of presumed myositis of the quadriceps muscle associated with continuous ACB [32]. In these cases, the patients had prolonged weakness after ACB, with the clinical course, neurophysiologic studies, and magnetic resonance imaging consistent with myositis. Muscle biopsies were not performed.
SUMMARY AND RECOMMENDATIONS
●Anatomy
•The adductor canal block (ACB) is an interfascial plane block performed at the level of the midthigh. The ACB anesthetizes the saphenous nerve, nerve to vastus medialis (NVM), and branches of the obturator nerve, and is used for anesthesia and/or analgesia for surgery of the knee, medial lower leg, and ankle (figure 4). (See 'Anatomic clinical correlations' above.)
•The adductor canal (also known as the Hunter's canal) is an aponeurotic space between the distal femoral triangle and the adductor hiatus in the adductor magnus muscle. The saphenous nerve, the NVM, and in some cases a branch of the obturator nerve, travel within the adductor canal. The femoral artery and vein also traverse the adductor canal (figure 1A-B). (See 'Anatomy' above.)
●Single injection ACB technique – ACB is performed with ultrasound guidance as follows, with further explanation above (movie 1) (see 'Single injection adductor canal block' above):
•Use a high frequency (13 to 6 MHz) linear ultrasound transducer, with ultrasound depth set to about 4 cm.
•Position the patient supine with slight external rotation of the leg.
•Begin ultrasound scanning at a point midway between the anterior superior iliac spine (ASIS) and the base of the patella (picture 1).
•Identify in sequence the femur (to help determine depth of superficial femoral artery [SFA]) (image 4), the SFA and vein, then the muscular borders of the adductor canal (sartorius, vastus medialis, and adductor longus/magnus muscles) (image 2 and figure 1B).
•Insert a 20 to 22 gauge block needle using an in plane approach (picture 4), placing the needle tip adjacent to the femoral artery at the 9 o'clock position (image 7).
•After negative aspiration, inject 5 to 20 mL of local anesthetic (LA) in 5 mL increments, with gentle aspiration between injections, while visualizing spread of LA around the artery (image 8).
●Continuous ACB technique – Continuous ACB is performed with ultrasound guidance as described for single injection block, using a Touhy needle with a 19 or 20 gauge catheter inserted through it, 4 to 6 cm into the adductor canal. (See 'Continuous adductor canal block' above.)
●Drug choice, concentration and volume (see 'Drug choice' above and 'Infusion drug dose' above)
•For single injection ACB – 10 mL of (0.25 or 0.5%) bupivacaine or (0.2 or 0.5%) ropivacaine with a recommended maximum volume of up to 15 mL.
•For continuous ACB – Bolus injection as for single injection ACB, followed by continuous infusion of 0.1 to 0.2% bupivacaine or ropivacaine at 5 to 10 mL per hour or programmed intermittent bolus, 8 mL every hour to 15 mL every three hours.
●Side effects and complications (see 'Side effects and complications' above)
•Motor block – ACB has become the first choice block for most total knee arthroplasties because it results in less quadriceps weakness than femoral block. However, quadriceps weakness has been reported even with low volume ACB, and may impact immediate rehabilitation after surgery. (See 'Motor block' above.)
•Local anesthetic systemic toxicity – Local anesthetic systemic toxicity (LAST) is a concern with ACB because of the large blood vessels and perforating arteries in the adductor canal, and because ACB is often used along with other blocks and/or local infiltration analgesia. Doses should always be injected in increments, with aspiration between injections, and the total expected dose of LA via all methods of administration should be calculated. (See 'Local anesthetic systemic toxicity' above.)
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