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Pro and Con: How Important Is the Exact Location of Adductor Canal and Femoral Triangle Blocks?

Woodworth, Glenn E. MD^*; Arner, Andrew MD^†; Nelsen, Sylvia PhD^‡; Nada, Eman MD, PhD^§; Elkassabany, Nabil M. MD, MSCE^∥

Author Information

From the ^*Department of Anesthesiology and Perioperative Medicine, Oregon Health & Science University, Portland, Oregon

^†Department of Anesthesiology and Perioperative Medicine, Oregon Health & Science University, Portland, Oregon

^‡Department of Biomedical and Diagnostic Sciences, School of Dentistry, Oregon Health & Science University, Portland, Oregon

^§Department of Anesthesiology, Renaissance School of Medicine, Stony Brook University, Stony Brook, New York

^∥Department of Anesthesiology and Critical Care, University of Pennsylvania, Philadelphia, Pennsylvania.

Accepted for publication August 4, 2022.

Funding: Support was provided solely from institutional and/or departmental sources.

The authors declare no conflicts of interest.

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Address correspondence to Glenn E. Woodworth, MD, Department of Anesthesiology and Perioperative Medicine, Oregon Health & Science University, 3181 SW Sam Jackson Park Rd, Mail Code UHS-2, Portland, OR. Address e-mail to [email protected].

Anesthesia & Analgesia 136(3):p 458-469, March 2023. | DOI: 10.1213/ANE.0000000000006234

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KEY POINTS

Question: Is there an optimal location for performing anterior thigh nerve blocks for perioperative analgesia of the knee to provide perioperative analgesia following total knee arthroplasty?
Findings: Although cadaveric studies and the anatomy suggest that different locations for local anesthetic injection during true adductor canal or distal femoral triangle block could produce different effects, the limited available clinical evidence does not clearly support a clinically meaningful difference.
Meaning: Further research is required to clarify the optimal location of a peripheral nerve block for perioperative analgesia following total knee arthroplasty.

In this Pro-Con commentary article, we discuss what anatomical, cadaveric, and clinical evidence support the location of “adductor canal” block matters in providing analgesia for total knee arthroplasty (TKA). To frame the debate, it is essential to understand: (1) surgical tissue disruption from TKA and the innervation of the knee, (2) potential block locations, (3) the key anatomical relationships of the thigh, and (4) the fascial compartments of the thigh.

SURGICAL TISSUE DISRUPTION DURING TKA AND INNERVATION OF THE KNEE

TKA usually involves a midline incision that begins 5 cm proximal to the patella and extends distally to the tibial tuberosity. Further dissection and a deeper incision medial to the patella through the medial retinacular complex are used to expose the joint. Table 1 summarizes the surgical dissection, the structures disrupted, and their innervation.^¹^,^²

POTENTIAL BLOCK LOCATIONS

Multiple anterior thigh block locations have been proposed to provide perioperative analgesia for TKA. Clinical studies often use surface landmarks (eg, anterior superior iliac spine [ASIS]), inguinal ligamen (IL)t, inguinal skin crease, and base of the patella (proximal border of the patella) to guide placement, whereas cadaveric studies use additional internal anatomical landmarks (eg, iliopectineal fossa, femoral triangle, and adductor canal). Unfortunately, many early studies “incorrectly” identified block locations as within the “adductor canal,” when they used surface landmarks to define the location of the block.^³ More recent clinical studies correlate sonographic landmarks with internal landmarks (eg, apex of the femoral triangle) to identify the block location.^⁴

To identify the potential proximal to distal locations in the anterior thigh, we divided the thigh into regions based on the surface and muscular anatomy: distal iliopectineal fossa, distal femoral triangle, and adductor canal (Figure 1). The IL stretches between the ASIS and pubic tubercle and forms the proximal base of the femoral triangle. The inguinal skin crease (tethering of the FL and soft tissue) is 4 to 6 cm distal and parallel to the IL.^⁵ The lateral border of the femoral triangle is the medial border of the sartorius muscle (SAM), and its medial border is the medial border of the adductor longus muscle (ALM). The apex of the femoral triangle is formed by the intersection of the medial borders of the sartorius and ALMs. The femoral triangle contains a smaller triangle that forms the iliopectineal fossa. The apex of this triangle is the intersection of the medial border of the SAM with the lateral border of the ALM. Between the apices of the iliopectineal fossa and femoral triangle, the femoral artery (FA) is posterior to the SAM. Blocks in this region deep to the SAM are commonly referred to as within the femoral triangle^³; however, the medial border of the SAM is the lateral border of the femoral triangle. Thus, distal femoral triangle blocks (FTBs) placed posterior to the SAM and medial to the FA are not technically within the femoral triangle. For simplicity, and to be consistent with previous literature, we describe blocks in this region as distal FTBs, but we realize a more anatomically correct term would be beneficial. The apex of the femoral triangle defines the proximal border of the adductor canal.^¹ The distal border of the adductor canal is where the femoral vessels pass through the adductor hiatus of the adductor magnus muscle (AM) to enter the popliteal fossa. We provide estimates of the proximal to distal length of each region and distances between anatomical landmarks in Figure 1.^⁴^,^⁵^,^{^6–9}

KEY ANATOMICAL RELATIONSHIPS IN THE THIGH

Proximal Iliopectineal Fossa

In this region, 1 to 3 intermediate femoral cutaneous nerves (IFCNs) branch off the femoral nerve (FN) to travel over the medial border of the SAM (some branches may pierce the sartorius) and then travel anterior to the muscle in fat filled tunnels formed by duplicature of the FL before piercing the FL to reach the subcutaneous tissue^² (Figure 2). Near the inguinal skin crease, the nerves to the vastus lateralis, rectus femoris, and vastus intermedius muscles branch off to enter their respective muscles. FN block is performed in the proximal iliopectineal fossa near the inguinal skin crease.^⁵ Because FN block causes quadriceps weakness, there has been a shift toward more distal nerve blocks in the anterior thigh to decrease motor weakness but still provide analgesia.^¹^,^¹⁰^,^¹¹

Distal Iliopectineal Fossa

In the distal iliopectineal fossa, other FN branches, including the nerve to the VM, the medial femoral cutaneous nerve (MFCN), and the saphenous nerve (SN), are close to the lateral aspect of the FA.^² However, the nerve to the vastus medialis (NVM) enters its own fascial compartment, formed by splitting of the fascia, investing the VM to envelop the nerve and separate it from the saphenous nerve and FA. This fascial compartment is on the surface of the VM.^³^,^{^12–15} This separation of the NVM and its terminal branches from the SN and FA is maintained further distally into the adductor canal.^¹

Distal Femoral Triangle

Lateral to the distal femoral triangle, the SAM is anterior to the FA. This region is about 10 cm in length. The midthigh, determined by the midpoint from the ASIS to the base of the patella (proximal pole), lies reliably within this region, and not within the adductor canal.^¹ Even after publication of a review by Bendtsen et al^¹ that clarified the anatomy, articles still mistakenly refer to blocks placed in the midthigh as adductor canal blocks when they should be referred to as distal FTBs. In the distal iliopectineal fossa or proximal portion of the distal femoral triangle, the MFCN crosses obliquely over the FA to its medial side (Figure 2). The anterior branch of the MFCN then quickly travels around the medial border of the SAM or pierces the sartorius to travel a short distance in a fatty tunnel duplicature of the FL before becoming subcutaneous to innervate the skin over the medial thigh.^²^,^¹⁶ The posterior branch continues medial to the FA and follows the medial border of the SAM.^² It eventually pierces the FL and innervates the skin over the distal medial thigh, potentially reaching the region where most surgical incisions for TKA take place.^²^,^¹⁶ It is unclear whether the MFCN provides innervation to the patella or knee joint.^² Some authors have reported that the posterior branch continues with the FA into the adductor canal but do not clearly indicate in which fascial compartment it lies.^¹²^,^¹⁴ It is more likely that the nerve travels in the subsartorial compartment, which is separated by fascia from the femoral vessels and SN (see below).^¹^,^¹³

Table 1. - Summary of Surgical Disruption With TKA and Innervation of the Knee

Surgical incision or dissection	Structure disrupted	Innervation
Midline skin incision from 5 cm proximal to the patella to the anterior tibial tubercle	Skin	Intermediate femoral cutaneous nerve (branch of femoral nerve)
		Medial femoral cutaneous nerve (branch of femoral nerve)
		Possible lateral femoral cutaneous nerve and infrapatellar branch of saphenous nerve
Medial parapatellar arthrotomy	Medial retinacular complex	Medial retinacular nerve (terminal branch of nerve to the vastus medialis)
	Synovial membrane
	Synovial membrane	Possible contribution of anterior branch of obturator nerve
	Medial joint capsule	Possible contribution of anterior branch of obturator nerve
Intra-articular posterior knee elements	Menisci and ligaments	Popliteal plexus derived from the:
	Posterior joint capsule
	Cruciate ligaments
	Cruciate ligaments	Tibial nerve
	Infrapatellar fat pad	Posterior branch of the obturator nerve

Abbreviation: TKA, total knee arthroplasty.

Figure 1.:
Locations and definitions for potential peripheral nerve blocks in the thigh. The sartorius muscle has been removed‚ with only the proximal and distal ends displayed. We divided the thigh into zones based on anatomical landmarks. To provide a sense of scale, we added average measurements obtained from published studies to estimate the distances. The IL attaches to the ASIS and pubic tubercle. It forms the base of the femoral triangle (the entire triangle is outlined in bold black). The lateral border of the femoral triangle is the medial border of the sartorius muscle. The medial border of the femoral triangle is bounded by the medial border of the ALM. The inguinal skin crease is 4–6 cm distal to the IL. The region from the inguinal skin crease to the apex of the iliopectineal fossa is the mid-femoral triangle. In the distal aspect of this region, the sartorius muscle is just beginning to become anterior to the femoral artery. The region between the apex of the iliopectineal fossa and the apex of the femoral triangle defines the distal femoral triangle (the femoral artery is reliably deep to the sartorius muscle). The apex of the femoral triangle is the most distal aspect of the femoral triangle and the cephalad margin or beginning of the true adductor canal. It is defined as the location where the medial border of the sartorius muscle crosses the medial border of the ALM. The distal border of the adductor canal is the adductor hiatus, where the femoral artery descends through the adductor hiatus to enter the posterior compartment of the leg (popliteal fossa) and is renamed the popliteal artery. ALM indicates adductor longus muscle; AM, adductor magnus muscle; ASIS, anterior superior iliac spine; IL, inguinal ligament; Pec, pectineus muscle; RFM, rectus femoris muscle; VM, vastus medialis muscle.

Figure 2.:
Medial and intermediate cutaneous nerves of the thigh. A, The skin and subcutaneous tissues are removed, exposing the FL that wraps the entire thigh. The IFCN pierces the FL as 2 separate branches close together anterior to the SAM. The MFCNp pierces the FL on the distal medial thigh. The MFCNa pierces the FL anterior to the SAM somewhat distal to the piercing of the SAM by the IFCN branches. After the IFCN and MFCN branches have pierced the FL, they ramify and innervate the subcutaneous tissue and skin. B, The IFCN pierces the IPF as one nerve trunk just distal to the IL before it splits in 2 branches. Sometimes, the most lateral of the IFCN branches pierces the SAM. In this case, both branches wind around the medial margin of the SAM. C, The MFCN pierces the IPF more distal and at a deeper level compared with IFCN. The MFCN is adjacent to the SN and the MVN anterolateral to the FA until the FA dives deep to the medial margin of the SAM. At that level, the MFCN starts crossing anterior to the FA and splits in 2 branches (MFCNp and MFCNa). The MFCNp follows the medial margin of SAM, while the MFCNa pierces the SAM. The blue arrow indicates the level of the distal FTB corresponding to the midpoint between the anterior superior iliac spine and the base of the patella (blue line). This is consistently proximal to the apex of femoral triangle and the proximal border of the VAM, indicated by the green arrows. VAM is the roof of the adductor canal. Distal FTB anesthetizes the SN and MVN anterolateral to the FA. The red arrow indicates the level of the proximal FTB corresponding to the intersection of the FA and the medial border of the SAM. It anesthetizes the MFCN, SN, and MVN anterolateral to the FA. Reprinted with permission from Regional Anesthesia and Pain Medicine and Bjørn et al.^² Modified excerpt from VH Dissector with permission from Touch of Life Technologies Inc (www.toltech.net). Built on real anatomy from the National Library of Medicine Visible Human Project. ALM indicates adductor longus muscle; FA, femoral artery; FL, fascia lata; FN, femoral nerve; FTB, femoral triangle block; IFCN, intermediate femoral cutaneous nerve; IL, inguinal ligament; IM, iliac muscle; IPF, iliopsoas fascia; MFCN, medial femoral cutaneous nerve; MFCNa, anterior branch of the medial femoral cutaneous nerve; MFCNp, posterior branch of the medial femoral cutaneous nerve; MVN, medial vastus nerve; PM, pectineus muscle; RFM, rectus femoris muscle; SAM, sartorius muscle; SN, saphenous nerve; VAM, vastoadductor membrane; VM, medial vastus muscle.

A key anatomical feature of the distal femoral triangle and adductor canal is the presence of fascia that extends from the fascia of the vastus medialis to the fascia of the adductor muscles, effectively forming a distinct triangular subsartorial canal that contains the FA and some of the nerves.^⁶ The walls of the subsartorial canal wall are formed by the VM (anterolateral wall), ALM or magnus muscles (posteromedial wall), and a fascial roof (vastofemoral fascia in the distal femoral triangle and vastoadductor membrane [VAM] in the adductor canal). The vastofemoral fascia proximal to the adductor canal is contiguous with the distal thickened fascia, which is the VAM, the roof of the true adductor canal. Some studies have shown the vastofemoral fascia and the VAM to be discontinuous or have fenestrations, but this is likely due to dissection technique, with most specimens demonstrating a continuous fascial membrane perforated only by vessels and cutaneous nerves.^¹³^,^¹⁷ The SAM is anterior to this fascia and separated from it by a thin layer of loose fat and connective tissue.^¹^,^¹³^,^¹⁷ Thus, in the thigh, there is a thin space between the SAM and the vastofemoral fascia and VAM. For clarity, we termed the region between the SAM and the vastofemoral and adductor membrane fascia the “subsubsartorial compartment” and the region posteromedial to the vastofemoral and adductor membranes the “subsartorial canal.” We refer to the portion of the subsartorial canal proximal to the adductor canal as the “vastofemoral canal.”

Adductor Canal

Although there is agreement on the proximal and distal borders of the canal, it is unclear in many publications whether nerves described as “within the adductor canal” are deep to the VAM and within the true adductor canal or simply within the proximal to distal borders of the adductor canal but outside of the walls of the adductor canal. The adductor canal is approximately 8 cm long.^⁴^,^⁹ As the SN and femoral vessels enter the adductor canal, the SN is lateral to the FA. Within the adductor canal, the SN moves to a more dorsal position, relative to the artery where it (or some of its branches) may pierce the VAM.^⁶^,^¹⁸ More commonly, the nerve exits the adductor canal at its distal opening, the adductor hiatus.^¹⁹^,^²⁰ Toward the distal adductor canal, branches of the SN may communicate with the posterior branch of the MFCN and the anterior branch of the obturator nerve via a subsartorial plexus, which is superficial to the adductor canal and in the subsartorial compartment.^¹^,^¹² Distal to the adductor canal, the infrapatellar branch of the SN pierces the SAM, where it then crosses the patellar ligament to form the patellar plexus.^¹⁸

The NVM has been described as traveling with the SN into the region of the adductor canal (even though it is in its own fascial tunnel and is separated from the femoral vessels and SN). Some have proposed that the NVM is a more important contributor to the sensory innervation to the knee than the SN. This is due to the sensory fibers to the knee contained within the NVM and its terminal branch, the medial retinacular nerve.^¹^,^¹²^,^¹⁵^,^{^21-24}

Figure 3.:
Fascial compartments within the distal femoral triangle and adductor canal. 1) Subsartorial compartment deep to the SAM and superficial to the vastofemoral fascia and vastoadductor membrane (orange line). 2) Lateral compartment lateral to the artery but outside the neurovascular sheath (purple circle) and deep to the vastofemoral fascia and vastoadductor membrane. 3) Within the neurovascular sheath containing the SN, the FA, and the FV. The neurovascular sheath is adherent to the vastofemoral and vastoadductor membranes (orange line), the fascia covering the vastus medialis (green line), and the fascia covering the adductor muscles (blue line). The NVM (yellow asterisk) is separated by a fascial covering from the lateral compartment. AM indicates adductor magnus muscle; FA, femoral artery; FV, femoral vein; NVM, nerve to the vastus medialis; SAM, sartorius muscle; SN, saphenous nerve; VM, vastus medialis muscle.

Other important nerves in this region include the posterior branch of the MFCN and the obturator nerve. Some studies on adductor canal block describe the MFCN (most likely the posterior branch) to be within the adductor canal without specifying whether it was superficial or deep to the VAM.^¹² Others describe it as deep to the VAM,^¹⁴ although this is very unlikely.^³

The anatomy of the obturator nerve branches with respect to the fascial roof of the adductor canal is unclear.^¹^,^⁶^,^¹²^,^²¹^,^²⁵ Most likely, the posterior branch of the obturator nerve travels on the surface or within the AM and passes through the adductor hiatus without entering the true adductor canal. The nerve then anastomoses with the popliteal plexus and innervates the posterior knee capsule.^¹³^,^¹⁴^,^²⁶^,^²⁷

Fascial Compartments Within the Distal Femoral Triangle and Adductor Canal

In the thigh, the FA and vein are covered by a delicate connective tissue sheath to form a neurovascular compartment.^¹³^,^{^28-30} The neurovascular sheath has also been found to be adherent to the walls of the subsartorial canal and can affect bulk flow of injectates in the canal.^¹⁷^,^{^28-30} Thus, from the apex of the iliopectineal fossa to the distal border of the adductor canal, there are 5 potential compartments in which injections could occur: (1) the subsartorial compartment anterior to the vastofemoral and adductor canals, (2) a lateral compartment lateral and outside the neurovascular sheath but within the subsartorial canal, (3) within the neurovascular sheath, (4) a medial compartment that is medial to the neurovascular sheath but within the subsartorial canal, and (5) a compartment outside and deep to the neurovascular sheath between the vastus medialis and adductor muscles (Figure 3).

PRO—THE LOCATION FOR INJECTION MATTERS

The choice of proximal to distal location to place local anesthetic during nerve blocks in the anterior thigh should be influenced by the course of the nerves that become separated from each other by tissue or fascial planes. Fascia planes serve as a barrier to local anesthetic diffusion^³¹^,^³² and may also affect the spread of the injectate.^¹⁴^,^³³

Two recent meta-analyses demonstrated superior analgesia with FN block compared with more distal, motor-sparing approaches in the distal femoral triangle or adductor canal.^{^34-36} This provides some evidence that proximal FN branches, including the femoral cutaneous nerves and nerves to the muscles via their genicular branches, play a role in pain following TKA. More distal block locations may not include these nerves.

Bjørn et al^² injected 10 mL of 0.5% ropivacaine lateral to the FA at the apex of the iliopectineal fossa. Only 4 of 20 volunteers demonstrated complete cutaneous anesthesia in the area of a hypothetical TKA incision. The addition of an IFCN block increased the number of subjects with complete cutaneous anesthesia to 15 of 20. These results suggest that a 10-mL block at the apex of iliopectineal fossa will not include the IFCNs that innervate a portion of a TKA incision. Of the patients receiving a 10-mL distal FTB (described as midthigh), 3 of 20 demonstrated cutaneous anesthesia in the territory of the MFCN compared to 18 of 20 for those receiving a block at the apex of the iliopectineal fossa. This demonstrates that 10-mL blocks distal to the iliopectineal fossa may not include the MFCN or IFCN. Despite the elegant work by Bjørn et al,^² the relative contribution of the femoral cutaneous nerves to pain following TKA compared to other sources of pain is not clear (eg, bone, ligament, or muscle disruption).

Grevstad et al^³⁷ injected 10 to 30 mL of 1% lidocaine in the midthigh of volunteers without further specifying the exact location (most likely the block was in the distal femoral triangle). Vastus medialis function based on electromyography was affected but did not demonstrate a difference in motor strength compared to control. The differential sensitivity of motor and sensory nerves may still allow for a potential analgesic effect on mixed motor/sensory nerves. Inclusion of the NVM during anterior thigh nerve blocks might explain the analgesia to the knee in clinical studies that exceeds what would have been expected from only a SN block.^²⁵ Some advocate a separate local anesthetic injection within the fascial covering of the NVM be performed along with distal femoral triangle or adductor canal blocks to avoid relying on diffusion through the fascia to block this nerve.^²⁴ It is possible the NVM may be excluded with adductor canal blocks compared to blocks in the distal femoral triangle, but this remains unproven.

One potential problem with adductor canal blocks is the possible tracking of local anesthetic following the artery through the adductor hiatus to reach the sciatic nerve causing weakness of the lower extremity. Gautier et al^³⁸ reported a case of a 20-mL adductor canal block, which resulted in decreased sensation in the lateral aspect of the leg and foot. They were able to inject 20 mL of radiopaque contrast through the catheter and use computed tomography to demonstrate contrast around the sciatic nerve in the popliteal fossa. They followed this report with a prospective study in which they injected 6 patients with 18 mL of 1% mepivacaine and 2 mL of radiopaque dye in the distal adductor canal.^³⁹ Although dye was detected in “the popliteal fossa and vicinity of the popliteal vessels and sciatic nerve” by imaging, sensory testing in the peroneal and tibial nerve territories was “similar to the contralateral side” or “slightly diminished” in all patients. Motor weakness of the foot was not detected in any patient. Goffin et al^⁴⁰ injected 20 mL of ropivacaine mixed with dye into the distal adductor canal of 8 cadavers. In all specimens, some dye contacted the sciatic nerve. These results are complemented by a cadaver study that injected 10 mL of dye into the adductor canal 1 to 2 cm proximal to the adductor hiatus or 2 to 3 cm proximal to the apex of the femoral triangle.^⁴¹ Following dissection, all adductor canal injections resulted in extension of the dye into the popliteal fossa; however, the sciatic nerve was stained in only 1 of 10 specimens. None of the injections in the distal femoral triangle spread dye into the popliteal fossa. A prospective, randomized trial injected 42 patients with either 20 or 40 mL of 0.375% ropivacaine during “adductor canal” block performed “with the FA immediately under the sartorius.”^⁴² This injection is likely in the distal femoral triangle. They demonstrated >50% of patients had diminished sensation in the distribution of the peroneal nerve, with a trend toward a greater proportion of patients with diminished sensation in the higher volume group. These studies suggest adductor canal blocks may reach the sciatic nerve and the risk is increased with higher-volume injections.

The configuration of the fascia can also have important implications for needle tip placement and subsequent injection of local anesthetic during a block. In the anatomy section, 5 possible compartments were identified (Figure 3). The subsartorial compartment may be a bit of a strong description as the medial and lateral boundaries are not well defined. The existence of this compartment has been demonstrated with spread of dye injected in unembalmed cadavers superficial to the VAM.^¹³ Ultrasound-guided injections in this space can be detected by a characteristic initial injection underneath the SAM that readily spreads superficial to both the medial and lateral sides of the FA.^¹³ As noted above, some of the cutaneous branches of the FN pass through this space before piercing the SAM or winding around it. Some studies have also identified branches of the SN passing through the VAM to enter this space while others dispute this finding.^⁶^,^¹²^,^¹⁹^,^²⁰ Theoretically, intentional injection into this compartment could provide some analgesia to the knee by blockade of nerves that travel in through this compartment; however, it would likely miss nerves that lie below the vastofemoral fascia or VAM (eg, SN, possibly NVM).

The fascia forming the vastofemoral and adductor canals is far more restrictive than the subsartorial compartment and can significantly influence proximal and distal spreads of local anesthetic injections in the canal. Andersen et al^¹³ noted that resin injected into the subsartorial compartment did not spread as far proximally and distally compared to resin injected into the adductor canal. This is consistent with the hypothesis that the loose connective tissue of the subsartorial compartment allows for broader medial-lateral spread of injectate compared to the more restricted space of the subsartorial canal. Tran et al^⁴³ injected aqueous dye in the proximal adductor canal with the dye staining the deep surface of the SAM. This suggests the VAM may not completely contain injected anesthetic within the adductor canal.^⁴³ This could be explained by leakage back through the needle puncture site through the VAM, tracking of injectate along vessels or nerves that penetrate the membrane, or fenestrations in the membrane. These holes serve as conduits for bulk flow or diffusion. It is also possible the resolution of current ultrasound machines does not allow for precise determination of the location of the needle tip during injections or movement of the needle tip between compartments during injection.^⁴⁴ Our observations of clinical injections visualized with ultrasound in this region have often noted spread into more than one compartment along the needle path. Multicompartment spread may confound clinical studies attempting to identify optimal needle tip placement for blocks. Unfortunately, clinical studies comparing subsartorial compartment spread to vastofemoral or adductor canal block have not been performed.

Injections of local anesthetic within the lateral compartment of the adductor canal spread to the interior of the neurovascular sheath to reach the SN but may not reach the NVM. Johnston et al^³³ studied injections of 20 mL of methyl cellulose and ink in the distal femoral triangle and adductor canal using ultrasound guidance in 4 fresh cadavers. In all 4 injections in the distal femoral triangle, the dye stained the NVM, while none of the injections in the distal adductor canal stained the nerve. Clinical research demonstrating SN block with “presumed” needle tip placement in the lateral compartment seem to show consistent SN block. This is not surprising given the thinner nature of the fascia forming the neurovascular sheath, which may not form much of a diffusion barrier. Intentionally piercing the neurovascular sheath may improve SN block with lower local anesthetic concentrations or volumes; however, it may also increase the likelihood of SN or vascular injury and may be unnecessary.

Finally, we must consider the effects of volume on the spread of injections in various compartments and the ability to reach proximal or distal neural elements. Several studies have examined the spread of dyes and resin after injections at various points from proximal to distal in the thigh in cadavers.^⁷^,^¹³^,^¹⁴^,^³³^,^⁴⁰^,^⁴⁵^,^⁴⁶ One additional study evaluated spread of local anesthetic in a patient with magnetic resonance imaging,^²⁶ and one evaluated spread with fluoroscopy.^⁴⁷ The results of spread in those studies in which the actual location of the injection inferred based on descriptions of the blocks are summarized in Figure 4. In addition, Jæger et al^⁴⁸ performed a dose (volume) finding study with an injection of 1% lidocaine in the distal femoral triangle. Magnetic resonance imaging scans were used to determine proximal and distal spreads. They calculated that the minimum effective dose volume 95% to reach the end of the adductor canal was 20 mL. Approximately 50% of subjects given 10, 15, or 20 mL had evidence of spread to the apex of the iliopectineal fossa; however, the proximal spread between the groups was not statistically significant. Although the study was not designed or powered to determine a difference in maximal voluntary quadriceps contraction between the groups, there was a nonsignificant trend of decreased maximal contraction in the 20-mL group. In another dose finding study, Johnston et al^⁴⁹ calculated the minimum effective dose volume 50% and the minimum effective dose volume 95% of 0.5% ropivacaine injected in the midadductor canal to produce a 30% reduction in quadriceps strength as 46.5 mL and 50.3 mL, respectively.

Figure 4.:
Summary of cadaver studies evaluating the spread of injections in the thigh lateral to the femoral artery. Because the descriptions of the injection locations varied and used different definitions, we inferred the actual injection location based on ultrasound images in the study or described anatomical landmarks before plotting them according to our anatomical locations in the thigh.^⁷ ^, ^¹³ ^, ^²⁶ ^, ^³³ ^, ^⁴⁰ ^, ^⁴⁵ ^, ^⁴⁶ All described injections were in the lateral compartment of the vastofemoral or adductor canals. Red dots indicate the location of the injection. Solid arrows indicate proximal or distal spread described in the studies. Dashed arrows indicate inadequately described spread. Lack of an arrow in a proximal or distal direction indicates spread in that direction was not described in the study.

In summary, the Pro authors conclude that injection site and volume do matter. The more proximal the injection site or the larger the volume of injectate, the greater the risk of proximal spread and motor strength reduction. However, more proximal injection and larger injectate volume may increase the inclusion of MFCN, IFCN, and NVM. More distal injections in the adductor canal may decrease the inclusion of these nerves and increase the risk of spread to the sciatic nerve. We conclude that 15 mL in the distal femoral triangle appropriately balances the risks and benefits, but may miss blocking some of the femoral cutaneous nerves.

CON—THE “EXACT” LOCATION OF INJECTION OF LOCAL ANESTHETIC MAY NOT BE CLINICALLY RELEVANT

The concept of selectively blocking branches of the FN for motor sparing led to the development of the adductor canal block. A combination of adductor canal block, local anesthetic infiltration in the posterior capsule or ultrasound-guided Injection between the Popliteal Artery and Capsule of the Knee (iPACK), and multimodal analgesia usually provide reasonable postoperative pain control that allows early rehabilitation and discharge.^⁵⁰ Proponents of adductor canal block propose better motor sparing and possibly better analgesia due to popliteal spread with resultant additional blockade of some sciatic nerve articular branches.^¹³^,^³⁹^,^⁴¹ While blocks at different locations may be anatomically different as previously mentioned, we would like to draw the attention of the readers to a few points.

First, there is probably no “clinically meaningful” difference between distal femoral triangle and adductor canal blocks. This has been shown in studies comparing efficacy of proximal and distal locations of the adductor canal block after TKA.^⁵¹^,^⁵² Not only was the analgesic effect the same, but the incidence of quadriceps weakness was similar between the 2 groups. Preserving quadriceps strength after knee procedures is the key tenant of femoral triangle or adductor canal blocks as it allows the patient to participate in rehabilitation and minimizes the risk of falls. Although adductor canal block is shown to be more motor sparing compared to FTB,^¹⁰ significant quadricep weakness is occasionally reported in both locations.^⁴⁷^,^{^53-55} Although a secondary outcome, Mariano et al^⁵² reported similar incidence of bolus-induced motor weakness in both distal femoral triangle and distal adductor canal block. Another study reported similar timed up and go in the first 48 hours after TKA when perineural catheters were inserted in the distal femoral triangle and the adductor canal.^⁵⁶ The same study also reported similar pain level, pain relief, interference of functional activities and interpersonal relationships by pain, and opioid consumption between the groups. On the other hand, some studies showed superior analgesia during knee surgery from injections in the femoral triangle compared to the adductor canal.^{^57-59} Two studies showed better pain scores in the femoral triangle injection group.^⁵⁷^,^⁵⁹ Although this difference was statistically significant, it was not clinically important as both groups were comfortable, and pain was mild. Abdallah et al^⁵⁸ also found that patients who had a proximal block had improved pain scores for the first 6 hours and used less opioids, compared to the midthigh and distal adductor canal blocks. Meanwhile, there was no difference in the quadricep strength for all locations. Additionally, the proximal location used in this study was very high up in the thigh close to the FN and could block more of its branches, which may explain why it was superior in terms of analgesia. There was no difference in any of the outcomes between the 2 more distal locations when compared to 1 another. It seems clear that blocks performed at different levels result in an equivalent analgesic benefit after knee surgery. This leads to the question whether the interest in the exact location of block placement stems from clinical observation or pure academic interest.

Second, while femoral triangle and the adductor canal may have separate anatomical definitions as 2 different regions within the subsartorial canal, local anesthetic injected will spread between the 2 locations. Conventionally, the block performed at the midthigh level (FTB) is done where the FA is approximately in the middle and underneath the SAM. At this level, the local anesthetic is injected deep to the fascia that forms the roof of the subsartorial canal. The fascia forming the roof of the vastofemoral canal can be traced all the way to the where it joins the VAM, which forms the roof of the adductor canal. Local anesthetic injected in this area has been shown to spread distally more consistently than it does proximally.^⁴⁸ Ishiguro et al^⁷ concluded in their cadaveric study that no boundary was found between the apex of the femoral triangle and the adductor canal, and they also demonstrated in a case series that local anesthetic can spread distally from the distal femoral triangle to the adductor canal. This supports a lack of clinical utility in distinguishing the exact location of injection of the local anesthetic in the distal femoral triangle or adductor canal.

Third, although some authors argue that the exact location of the block may affect the inclusion of the NVM, Gervstad et al^³⁷ found that 20 mL of local anesthetic would block the NVM in 84% of patients, and 30 mL would block it in 100% of patients, sparing the NVM most of the time when a midthigh block was used.

Table 2. - Pro-Con Summary—Is There an Optimal Location for Peripheral Nerve Block for Perioperative Analgesia for TKA?

Pro	Con
The NVM, medial femoral cutaneous nerve, and intermediate femoral cutaneous nerve are described to contribute to knee pain following TKA, but have varying courses in the thigh, become separated by fascial planes from the saphenous nerve at varying points in the thigh—the anatomy supports different potential clinical effects with different injection locations or spread of local anesthetic.	There is insufficient evidence to determine whether adductor canal blocks spare the NVM and what the clinical relevance of this would be.
15-mL injections in the distal femoral triangle reliably fill the adductor canal.	Comparison of current studies is confounded by varying local anesthetic doses, volumes, and injections, making determination of an optimal location of peripheral nerve block difficult.
Volumes greater than 15 mL are more likely to cause quadriceps weakness or reach the sciatic nerve.	There is insufficient clinical evidence to support the superiority of adductor canal or distal femoral triangle blocks over one another.
10-mL injections in the adductor canal do not block the medial femoral or intermediate femoral cutaneous nerves.

Abbreviations: NVM, nerve to the vastus medialis; TKA, total knee arthroplasty.

We cannot discount the value of cadaveric studies in proposing different actions of different peripheral nerve blocks. However, we have to interpret the results of these studies with caution when it comes to translating the results to clinical practice for 2 reasons: (1) the pattern of dye spread in cadavers may be different from local anesthetic and other contrast materials in patients;^⁶¹ or (2) other mechanical factors in patients may play a role in the spread of local anesthetic after TKA, including the use of a tourniquet and physical therapy after surgery.^⁶² Passive leg mobilization has been shown to alter the distribution of local anesthetic in different fascial planes.^⁶²^,^⁶³

CONCLUSIONS

Table 2 summarizes the Pro and Con argument of an optimal location for peripheral nerve block for perioperative analgesia following TKA. The SN, NVM, MFCN, IFCN, and obturator nerve have varying courses and locations where they become separated by fascia from the SN. There is clinical evidence that low-volume FTB will miss the IFCN and MFCN; however, it has not been determined whether higher-volume (15–20 mL) injections in the distal femoral triangle will reach these nerves. There is insufficient clinical evidence to determine whether adductor canal blocks spare the NVM and what the clinical relevance of this would be. There is only limited clinical evidence that blocks in the adductor canal increase the risk of spread to the popliteal plexus and sciatic nerve, which may contribute both a positive and negative side effect. Subsartorial compartment blocks have been insufficiently studied. Although 15-mL injections in the distal femoral triangle may balance the risks and benefits of anterior thigh blocks, future studies will be necessary to determine whether there is a clinically meaningful difference in analgesia between iliopectineal, distal femoral triangle, adductor canal, and subsartorial compartment blocks and the optimal location and volume of injection to maximize benefits and minimize side effects.

DISCLOSURES

Name: Glenn E. Woodworth, MD.

Contribution: This author helped conceive the article; acquire, analyze, and interpret data; and draft and revise the article.

Name: Andrew Arner, MD.

Contribution: This author helped acquire, analyze, and interpret data; and draft and revise the article.

Name: Sylvia Nelsen, PhD.

Contribution: This author helped acquire, analyze, and interpret data; and revise the article.

Name: Eman Nada, MD, PhD.

Contribution: This author helped acquire, analyze, and interpret data; and draft and revise the article.

Name: Nabil M. Elkassabany, MD, MSCE.

Contribution: This author helped conceive the article; acquire, analyze, and interpret data; and draft and revise the article.

This manuscript was handled by: Michael J. Barrington, MB BS, FANZCA, PhD.

^GLOSSARY

ALM: adductor longus muscle
AM: adductor magnus muscle
ASIS: anterior superior iliac spine
FA: femoral artery
FL: fascia lata
FN: femoral nerve
FTB: femoral triangle block
FV: femoral vein
IFCNs: intermediate femoral cutaneous nerves
IL: inguinal ligament
IM: iliacus muscle
iPACK: injection between the popliteal artery and capsule of the knee
IPF: iliopsoas fascia
MFCN: medial femoral cutaneous nerve
MFCNa: anterior branch of the medial femoral cutaneous nerve
MFCNp: posterior branch of the medial femoral cutaneous nerve
MVN: medial vastus nerve
NVM: nerve to the vastus medialis
Pec: pectineus muscle
PM: pectineus muscle
RFM: rectus femoris muscle
SAM: sartorius muscle
SN: saphenous nerve
TKA: total knee arthroplasty
VAM: vastoadductor membrane
VM: vastus medialis muscle

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Pro and Con: How Important Is the Exact Location of Adductor Canal and Femoral Triangle Blocks?

SURGICAL TISSUE DISRUPTION DURING TKA AND INNERVATION OF THE KNEE

POTENTIAL BLOCK LOCATIONS