Motor-Sparing Effect of Adductor Canal Block for Knee Analgesia: An Updated Review and a Subgroup Analysis of Randomized Controlled Trials Based on a Corrected Classification System

Objective: Discrepancies in the definition of adductor canal block (ACB) lead to inconsistent results. To investigate the actual analgesic and motor-sparing effects of ACB by anatomically defining femoral triangle block (FTB), proximal ACB (p-ACB), and distal ACB (d-ACB), we re-classified the previously claimed ACB approaches according to the ultrasound findings or descriptions in the corresponding published articles. A meta-analysis with subsequent subgroup analyses based on these corrected results was performed to examine the true impact of ACB on its analgesic effect and motor function (quadriceps muscle strength or mobilization ability). An optimal ACB technique was also suggested based on an updated review of evidence and ultrasound anatomy. Materials and Methods: We systematically searched studies describing the use of ACB for knee surgery. Cochrane Library, PubMed, Web of Science, and Embase were searched with the exclusion of non-English articles from inception to 28 February 2022. The motor-sparing and analgesic aspects in true ACB were evaluated using meta-analyses with subsequent subgroup analyses according to the corrected classification system. Results: The meta-analysis includes 19 randomized controlled trials. Compared with the femoral nerve block group, the quadriceps muscle strength (standardized mean difference (SMD) = 0.33, 95%-CI [0.01; 0.65]) and mobilization ability (SMD = −22.44, 95%-CI [−35.37; −9.51]) are more preserved in the mixed ACB group at 24 h after knee surgery. Compared with the true ACB group, the FTB group (SMD = 5.59, 95%-CI [3.44; 8.46]) has a significantly decreased mobilization ability at 24 h after knee surgery. Conclusion: By using the corrected classification system, we proved the motor-sparing effect of true ACB compared to FTB. According to the updated ultrasound anatomy, we suggested proximal ACB to be the analgesic technique of choice for knee surgery. Although a single-shot ACB is limited in duration, it remains the candidate of the analgesic standard for knee surgery on postoperative day 1 or 2 because it induces analgesia with less motor involvement in the era of multimodal analgesia. Furthermore, data from the corrected classification system may provide the basis for future research.


Introduction
Femoral nerve block (FNB) has previously been the mainstay for postoperative analgesia following knee surgery for years [1]. However, quadriceps weakness, which is unfavorable for rehabilitation and might delay early ambulation, is a major concern of FNB [2]. Recently, an alternative, adductor canal block (ACB), has been introduced as a motor-sparing nerve block for knee surgery and gained attention from anesthesiologists and orthopedists. ACB has been claimed to provide adequate knee analgesia mainly by blocking the saphenous nerve (SN). As such, it could provide some benefits, including improved mobility and reduced risk of falls [3][4][5][6][7][8][9].
Previously, SN block approached with surface landmarks has been established for analgesia for procedures below the knee [10]. Because of its small size and the lack of a motor component in the SN, conventional SN block has inconsistent success. Then, it has evolved to ultrasound guidance to aid in precision and avoid unintentional puncture of major vessels [11,12]. Manickam et al. described the ultrasound-guided SN block in the adductor canal with the needle entry point at 2-3 cm proximal to the adductor hiatus [13]. Lund et al. introduced ACB as halfway between the iliac spine and the patella [8]. Until now, there have been debates about the nomenclature and techniques regarding ACB.
Recently, there has been growing evidence indicating that ACB is as effective as FNB in providing postoperative analgesia following knee surgery. Many studies revealed that ACB could preserve better quadriceps strength and improve mobilization ability without compromising pain control [8,14,15]. ACB also reduced the risk of falls compared with FNB [16][17][18]. However, other studies demonstrated that FNB had denser analgesia, less analgesic requirement, and greater satisfaction compared with ACB [19]. Additionally, there was no significant difference in quadriceps strength deficits after FNB or ACB [20][21][22].
These discrepancies might result from the different approaches of ACB, or more precisely, from the non-unified definition of ACB. Landmark guidance might risk injection outside the anatomical adductor canal. While searching for studies on ACB, there are many ways to perform ACB according to the injection site, including anatomically defined femoral triangle (FT) and proximal or distal adductor canal [23][24][25][26][27]. In several studies, ACB has been performed at the midpoint of the thigh (the midpoint between the anterior superior iliac spine and the base of the patella) [8,9,15,[28][29][30][31][32][33][34][35]. However, some researchers have challenged this definition because the midpoint of the thigh most likely falls within the FT [36]. Although the FT is connected to the adductor canal, the nerves to block differ between these two structures, which may cause clinically meaningful differences [37][38][39]. It is reasonable that the block level can affect the analgesic outcomes, mobilization, and motor-sparing effect. Hence, anatomically classifying published ACB studies before comparing the analgesic effects or other outcomes among different approaches is important. In this study, we will describe the relevant anatomy of ACB, clarify the definition of ACB, develop a nomenclature system to include various approaches, investigate the analgesic and motor-sparing effects by meta-analysis and subgroup analyses based on this corrected classification, and detail the possible complications.

Applied Anatomy
The relevant anatomy is shown in Figure 1. A comprehensive understanding of the anatomy of the FT and adductor canal is necessary to examine the clinical effects of ACB.

Applied Anatomy
The relevant anatomy is shown in Figure 1. A comprehensive understanding of the anatomy of the FT and adductor canal is necessary to examine the clinical effects of ACB. A. The boundaries of the FT and the AC. The base, the medial border, and the lateral border of the FT are the inguinal ligament, medial margin of the adductor longus muscle, and the medial margin of the sartorius muscle. The AC extends from the apex of the FT, (A). The boundaries of the FT and the AC. The base, the medial border, and the lateral border of the FT are the inguinal ligament, medial margin of the adductor longus muscle, and the medial margin of the sartorius muscle. The AC extends from the apex of the FT, defined as the intersection between the medial border of the sartorius muscle and the medial border of the adductor longus muscle. The AC ends at the adductor hiatus of the adductor magnus. (B). The contents of the FT and the AC. The femoral neurovascular bundle is in the FT at the inguinal ligament level and dives subsartorially at the distal FT level (the level of mid-way between ASIS and the patella base). SN is anterolateral to FA and lies between STM and VMM. NVM is lateral to SN, and also lies between STM and The FT is an area in the proximal part of the anterior thigh. The base of the triangle is the inguinal ligament. The medial border is the medial margin of the adductor longus muscle, and the lateral border is the medial margin of the sartorius muscle. The roof is the fascia lata. The floor is formed medially by the adductor longus and the pectineus and laterally by the iliopsoas muscle. The apex of the triangle is continuous with the proximal end of the adductor canal [37,38,40]. The FT contains neurovascular structures, from lateral to medial, including the femoral nerve, femoral vessels, and lymphatics. The femoral neurovascular bundle is in the FT at the inguinal ligament level and dives subsartorially at the distal FT level (the level of mid-way between ASIS and the patella base) [41].
The femoral nerve divides into branches in the FT. The motor branches supply the quadriceps femoris and the hip flexor, whereas the sensory branches innervate the anterior and medial aspect of the thigh (the medial and intermediate cutaneous nerve of thigh), the anteromedial side of the knee (SN, nerve to vastus medialis (NVM)), and the medial side of the leg and foot (SN) [42]. At the distal FT level, the SN and NVM lie subsartorially and outside the FT [41].

Adductor Canal (Hunter's Canal)
The adductor canal (AC) is an aponeurotic tunnel in the thigh, deep to the sartorius, and serves as a passageway that allows structures to move from the FT to the popliteal fossa. It extends from the apex of the FT, defined as the intersection between the medial border of the sartorius muscle and the medial border of the adductor longus muscle, to the adductor hiatus of the adductor magnus. The boundaries of the AC are the sartorius anteromedially, the vastus medialis anterolaterally, and the adductor longus (in the upper part) or the adductor magnus (in the lower part) posteromedially [36,41]. The roof of the entire AC is the vastoadductor membrane (VAM), and the space between the sartorius muscle and VAM is called the subsartorial compartment [36,[41][42][43][44][45]. Superficial to the VAM is the subsartorial plexus, supplying the medial part of the knee [41]. The AC is also known as the subsartorial canal or Hunter's canal [42]. However, the term "subsartorial canal" may cause confusion. Subsartorial space is a space under the sartorius muscle and extends beyond the vastoadductor membrane [41]. The subsartorial space is distinct from the true adductor canal. We should not regard the "subsartorial canal" as the true adductor canal because the term would cause misunderstanding.
In the AC, the nerve without conflict is the SN. The SN enters the AC anterolateral to the femoral artery and crosses the femoral artery anteriorly from the lateral to the medial Healthcare 2023, 11, 210 5 of 32 direction [46]. In the mid-to-distal third of the AC, the SN penetrates the VAM and leaves the canal with the saphenous branch of the descending genicular artery. The SN is a sensory nerve, supplying the anteroinferior and medial aspects of the knee and the medial side of the leg and ankle [11,47].
Other structures, for example, the NVM, the medial cutaneous nerve of the thigh, and the anterior or posterior branch of the obturator nerve have been reported to pass through the AC; however, the precise locations still remain controversial [3,7,36,46,48,49]. In an anatomic study, Burckett et al. reported that the NVM descends superficially to the VAM; other studies found that the NVM lies inside the fascia, covering the vastus medialis outside the AC [41,46,50,51]. The NVM not only gives branches to the vastus medialis but also innervates the anteromedial capsule superior to the patella and the medial retinaculum [46,52,53]. Therefore, the NVM is also a vital nerve to target when treating pain for patients undergoing surgery involving the medial region of the knee and the distal portion of the femur.

Knee Sensory Innervation
The sensory innervation of the knee can be divided into the anterior and posterior groups. In the anterior group, the cutaneous layer is supplied by the medial femoral cutaneous nerve, the infrapatellar branch of the SN medially, and the lateral femoral cutaneous nerve and the common peroneal nerve laterally. The anterior knee joint capsule is supplied by the branches of the NVM, intermedialis, and lateralis. Furthermore, the genicular nerves, common peroneal nerve, and recurrent peroneal nerve terminate in the anterior aspect of the knee capsule [41,53,54].
The posterior group consists of articular branches from the posterior division of the obturator nerve and the tibial nerves, supplying the posterior knee capsule [55].

Corrected Classification System
A study discussing the precise location of the AC, which involved healthy volunteers, was published in 2017 [36]. However, after that, some studies still defined the mid-thigh needle entry point as one of the approaches of ACB. In addition to the inconsistent ACB approaches, the definition of "mid-thigh" also varies; for example, midway between the anterior superior iliac spine (ASIS) and patella, midway between the greater trochanter and patella, and midway between the inguinal ligament and patella. There is still no consensus on its terminology after a rapid increase in the number of studies. The possible reason for the divergent ACB definition is that some studies defined it according to the surface landmark in the early period, and some studies defined it according to sonoanatomy recently (Table 1). Therefore, we provided a corrected classification system with a unified nomenclature to anatomically categorize the various existing ACB approaches into three groups: femoral triangle block (FTB), proximal ACB (p-ACB), and distal ACB (d-ACB). This classification is according to the anatomy of the AC and the FT. Table 1. Summary of reported randomized control trials using different approach to the adductor canal block (total ACB related studies number = 130 # ).

Description Regarding the Needle (Catheter) Tip Position Corrected Classification Rationale Number
Approach by surface landmarks Halfway between ASIS and patella FTB This position has been proven to be proximal to the adductor canal [36]. 27 Halfway between greater trochanter to patella ACB (not sure proximal or distal) Adductor canal is located at the middle one-third of the distance from the base of the patella to the lower border of the greater trochanter [26]. 4 Halfway between inguinal ligament and patella ACB (not sure proximal or distal) 1. Adductor canal is located at the middle one-third of the distance from the base of the patella to the lower border of the greater trochanter [26].
2. The lower border of greater trochanter approximates the level of inguinal crease [56].

12
Halfway between inguinal ligament and medial condyle ACB (not sure proximal or distal)

Femoral Triangle Block (FTB)
FTB refers to an injection at the midway between the ASIS and the base of the patella. Its injection site is subsartorial and anterolateral to the femoral artery and proximal to the apex of the FT, which blocks the SN, the NVM, and the medial femoral cutaneous nerve [41].
The advantages of FTB include that local anesthetic administration may be proximal enough to reliably block the SN and NVM and provide a better analgesic effect while minimizing the spread to the popliteal fossa [46]. Its disadvantages include that local anesthetic may spread to the proximal FT and involve the femoral nerve branches, which may cause quadriceps weakness.
During literature research, the ACB technique has been described as an injection at the "mid-thigh" level in many randomized controlled trials (RCTs). As early as 2014, Bendtsen et al. stated that using the term "ACB" with the mid-thigh needle entry point was a misnomer [26,43,57]. In 2017, a study involving healthy volunteers proved that the midpoint of the thigh, defined as halfway between the ASIS and the base of the patella, mostly resides in the FT [36]. The mean distance from the midpoint of the ASIS and patella to the proximal end of the AC is 4.6 cm [27,36,41]. However, there had been other descriptions of the "mid-thigh" level regarding ACB, such as the midpoint between the greater trochanter and patella, or the midpoint between the inguinal ligament and the patella. Most studies had not described the mid-thigh level clearly. Without a unified definition of the mid-thigh level in these studies, whether the block level was in the FT or in the AC cannot be determined. Hence, the results of the existing studies on ACB should be explained carefully.

Proximal ACB (p-ACB)
Instead of using surface landmarks to determine the injection site of ACB, ultrasound guidance is used to better identify the location of the AC. Under ultrasound guidance, the apex of the FT is identified as the intersection of the medial borders of the sartorius muscle and the adductor longus muscle. This apex is the proximal end of the AC. The distal end of the AC is where the femoral artery becomes deep and passes through the AC hiatus on its way to the popliteal fossa [46,58]. p-ACB targets the AC at its entrance just distal to the apex of the FT.
The potential advantages of p-ACB include a comparable analgesic effect with that of FNB or FTB without increasing quadriceps muscle weakness following knee surgery. Some studies have recently discussed the clinical effect of p-ACB with a precise location under ultrasound guidance. We will discuss the clinical efficacy of p-ACB through a systemic review and meta-analysis in the paragraph below.
p-ACB may have a more significant analgesic effect than d-ACB, whereas it also likely preserves quadriceps motor function. This description can be demonstrated by the cadaveric studies that have found that a dye injected into the proximal AC can spread to the SN throughout the AC and the branches of NVM, including the posteromedial branch and intra-articular branches. The posteromedial branches of the NVM, which terminate as the superior medial genicular nerve and the branch to the vastus medialis obliquus, innervate the anteromedial knee joint and muscles. Even though the posteromedial branches of the NVM are involved, the anterior branches of the NVM were not stained, which would likely preserve greater vastus medialis activity, resulting in the quadriceps motor-sparing feature of the ACB [50,58,59]. In contrast, the NVM was barely stained by dye injection in d-ACB because of the two layers of the distal roof of the AC (VAM and the aponeurosis of vastus medialis obliquus). Containing the posteromedial branches of the NVM makes p-ACB a more effective pain control technique for knee surgery [52,59].

Distal ACB (d-ACB)
The point of injection in d-ACB is approximately 1-2 cm proximal to the adductor hiatus. This injectate at the distal part of the AC may spread to the popliteal fossa and block the popliteal plexus, which is formed by the tibial nerve and posterior obturator nerve, innervating the posterior aspect of the knee. A cadaveric study showed that the 10 mL of injection in the distal part of the AC spread into the popliteal fossa [58]. The possibility of local anesthetics spreading to the SN and the popliteal fossa implies that d-ACB provides analgesia to the posterior knee compartment, but also causes foot drop [14,60,61]. Studies comparing different injection locations for ACB (p-ACB and d-ACB) in terms of analgesic and motor-sparing effects will be discussed in the following section (see Section 3.2 The effect of p-ACB versus d-ACB).

ACB Techniques
Ultrasound guidance is now considered the gold standard for peripheral nerve blocks [62]. Ultrasound-guided FTB, p-ACB, and d-ACB were described according to the unified nomenclature systems. Usually, a high-frequency linear transducer is adequate for most ACB or FTB. To optimize sonoanatomy visualization, appropriately rotating and tilting the probe and adequate pressure are recommended. Based on the physical ergonomics, keeping the eye, hand, needle, probe, and ultrasound instrument all in the same plane facilitates needle visualization [63]. The patient is placed in the supine position with the hip externally rotated and the knee slightly flexed ( Figure 2A). This external rotation can make the SN lie lateral to the femoral artery in the ultrasound image. Thus, the needle trajectory is shortened without passing through the vastus medialis muscle. Operators stand on the same side and lateral to the leg, hold the needle with the dominant hand, and position the ultrasound machine on the opposite side of the patient ( Figure 2B) [64]. Following physical ergonomics can protect operators from injuries, improve needle visualization, and reduce the risk of patient injury. After patient positioning, we suggest the scanning steps as follows: • Mark the location midway between the anterior superior iliac spine (ASIS) and the base of the patella. Place the transducer transversely on the marker of the thigh to obtain a short-axis view. The femoral vessels will be identified beneath the sartorius muscle. The artery can be distinguished by color doppler flow imaging or by compression sign ( Figure 3A,B).

•
Slide the probe along the medial border of the sartorius muscle to visualize the intersection of the medial borders of the sartorius muscle and the adductor longus muscle. This point means the start of the AC ( Figure 3C-F). • Slide the probe caudally until the femoral artery goes deep into the echogenic adductor magnus tendon and then passes through the adductor hiatus. This point is the end of the AC ( Figure 3G,H).
Healthcare 2023, 11, x FOR PEER REVIEW 10 of 35  The position of the probe of ultrasound-guided FTB and ACB, and the matching ultrasound images are shown in Figure 3.

Ultrasound-Guided FTB
• After identifying the AC, place the probe halfway between the base of the patella and the ASIS for a transverse view. The superficial femoral artery (SFA) is identified underneath the sartorius muscle. The SN is usually visible as a hyperechoic structure anterolateral to the artery typically [7,65].

•
Once the neurovascular bundle is seen, adjust the probe so that the bundle is on the medial side of the ultrasound screen. The needle is placed lateral to the SN and femoral artery using an in-plane technique. A periarterial injection of local anesthetics is performed at this level, involving the SN, NVM, and medial and intermediate femoral cutaneous nerves [41]. It may also affect the motor branches of the femoral nerve [7].

Ultrasound-Guided p-ACB
• At the distal FT, move the probe caudally about 1-2 cm beyond the apex of the FT. At this level, SN can usually be visualized laterally to the SFA with the thigh in external rotation, and injection between the SFA and SN helps achieve a successful ACB with 2-5 mL of local anesthetics.

Ultrasound-Guided d-ACB
As for d-ACB, (7) place the probe in the distal third of the thigh, where the SFA is also seen beneath the sartorius muscle. Then, the artery is identified diving posteriorly through the adductor hiatus when tracking down using the probe. [13] The point of injection of d-ACB is approximately 1-2 cm proximal to the adductor hiatus.

Systematic Review of RCTs on the Divergent Definition of ACB
Systematic review and meta-analyses have compared mixed ACB (including both with and without clear disclosure of the anatomical definition of the AC) with FNB in knee surgery; however, there are still no conclusive results regarding the motor-sparing effects and analgesic effects without applying a corrected classification system. Some studies showed that patients receiving ACB or FNB have similar clinical efficacy, including pain scores and opioid consumption. Meanwhile, patients receiving ACB have better quadriceps strength, mobilization, and ambulation [66][67][68][69][70][71][72][73][74][75]. Some systematic reviews and meta-analyses have compared ACB with FNB in patients undergoing anterior cruciate ligament reconstruction (ACLR), and the results are similar to those shown in total knee arthroplasty (TKA) [76,77]. In contrast, Ramizadeh et al. showed no differences in quadriceps muscle strength between ACB and FNB at 24 h after surgery [19], and three studies have reported no significant difference in quadriceps muscle strength at 48 h after surgery [78][79][80].
Among several meta-analyses comparing ACB with FNB, one of the most important limitations is the lack of a uniform block level among ACB approaches, which may cause considerable heterogeneity and unreliable results. Based on the studies on the location of the AC from Wong et al. [36], the biases observed in previous studies misinterpreting mid-thigh injection as ACB make the motor-sparing effect of "true" ACB to be spuriously underestimated, and the analgesic effect of "true" ACB to be spuriously overestimated.
In this study, we distinguish FTB, p-ACB, and d-ACB by analyzing the block location according to the ultrasound description in the index studies previously claimed by the author to be ACB. Based on this corrected classification system, we reclassified these previously-claimed ACB approaches into FTB, p-ACB, and d-ACB, and conducted a metaanalysis to survey the clinical effect of these three techniques. Regarding the fact that the injection site of FTB is proximal to true ACB, and it may involve the medial femoral cutaneous nerve, SN, and NVM, our hypotheses are as follows: (1) the motor-sparing effect of "mixed ACB with the exclusion of FTB" is more significant than that of FNB; (2) d-ACB may have less analgesic effect than p-ACB; (3) true ACB provides quadriceps motor-sparing effect and less analgesic effect than FTB.

Materials and Methods
Our systematic review was registered with PROSPERO, the international prospective register of systematic reviews of the National Institute for Health Research (accessed on 30 October 2020. CRD42020219432). This study followed the Preferred Reporting Items for Systemic Reviews and Meta-analyses guidelines [81].

Search Strategy
Databases including the Cochrane Library, PubMed, Web of Science, and Embase, were searched using the following terms: "Adductor canal" or "Adductor canal block" to identify potentially eligible studies evaluating the efficacy of ACB versus other analgesia methods in patients undergoing knee surgery until February 2022 with the exclusion of non-English studies.

Inclusion and Exclusion Criteria
Inclusion criteria: (1) Study type: clinical RCTs (2) Subjects: patients who underwent knee surgeries without limitations (3) Interventions: comparison of ACB with other methods for knee surgeries, including local infiltration, periarticular local anesthetic infiltration, FNB, interspace between the popliteal artery and the capsule of the posterior knee block, and epidural analgesia, after knee surgery.
Any disagreement in study selection was discussed by two reviewers or arbitrated by a third reviewer. Exclusion criteria: (1) Non-English articles (2) No full text available Risk and bias were assessed using the Cochrane Collaboration tool [82]. We used a random-effects model to represent between-studies heterogeneity. Software of Review Manager (RevMan, version 5.4; The Nordic Cochrane Center, The Cochrane Collaboration, Copenhagen, Denmark) was used for meta-analysis.

Results
Through our systematic database research, 1266 records (after eliminating duplicates) were identified ( Figure 4). After removing the studies that did not meet the inclusion criteria, we reviewed 130 full-text articles and found that 66 studies used an unclear or inconsistent approach to ACB ( Table 1). The variants of ACB intervention were described according to the surface landmark as "halfway between the ASIS and patella" in 27 RCTs, "halfway between the greater trochanter and the patella" in four RCTs, "halfway between the inguinal ligament and the patella" in 12 RCTs, and "halfway between the inguinal ligament and the medial condyle" in two RCTs. According to ultrasound images, ACB interventions were described as follows: "SFA covered by the medial border of the sartorius muscle" in five RCTs, "FA covered by the midpoint of the sartorius muscle" in four RCTs, "FA underneath the sartorius muscles" in 10 RCTs, and "cavity surrounded by the medial femoris, ALM, and sartorius muscle" in five RCTs.

The Clinical Effect of Mixed ACB versus FNB
Data from eight studies were pooled to evaluate the quadriceps muscle strength a receiving mixed ACB or FNB [17,78,79,89,100,101,131,141]. Among these eight stud one study misinterprets the FTB as ACB [89]. Two studies used an undetermined de tion of ACB [17,79]. FTB or proximal ACB cannot be determined because of the lac ultrasound images or cited articles. The outcomes showed that the quadriceps mu strength was more preserved in the mixed ACB group than in the FNB group in the 24 and 48 postoperative hours. This result reached statistical significance but with h heterogeneity ( Figure 5). The cephalad spreading of local anesthetics may anesthetize motor fibers of the femoral nerve. Compared with FNB, ACB blocks the more distal and the motor-sparing effect becomes more significant. Further studies with a uni ACB technique are needed to explore the motor-sparing effect of ACB and FNB after k surgery. As shown in Table 2, we checked the original grouping of the published literature and examined whether they should be anatomically re-classified as FTB, p-ACB, or d-ACB in these 66 reported studies.

Greenky 2020 [132] ACB
The probe was placed anteromedially on the middle and distal third of the thigh. /The femoral artery and sartorius muscle formed the roof of the adductor canal.
FTB or proximal ACB /Whether the medial borders of sartorius and adductor longus meet cannot be identified from the sonographic image.
Kampitak 2020 [133] ACB Halfway between the ASIS and the base of the patella.  Probe was placed on the anteromedial part of the thigh at the level of the midpoint, between the greater trochanter of the femur and the proximal edge of patella.
ACB /Adductor canal is located at the middle one-third of the distance from the base of the patella to the lower border of the greater trochanter.

The Clinical Effect of Mixed ACB versus FNB
Data from eight studies were pooled to evaluate the quadriceps muscle strength after receiving mixed ACB or FNB [17,78,79,89,100,101,131,141]. Among these eight studies, one study misinterprets the FTB as ACB [89]. Two studies used an undetermined definition of ACB [17,79]. FTB or proximal ACB cannot be determined because of the lack of ultrasound images or cited articles. The outcomes showed that the quadriceps muscle strength was more preserved in the mixed ACB group than in the FNB group in the first 24 and 48 postoperative hours. This result reached statistical significance but with high heterogeneity ( Figure 5). The cephalad spreading of local anesthetics may anesthetize the motor fibers of the femoral nerve. Compared with FNB, ACB blocks the more distal site, and the motorsparing effect becomes more significant. Further studies with a unified ACB technique are needed to explore the motor-sparing effect of ACB and FNB after knee surgery. The results of the mobilization ability between the mixed ACB group and the FNB group were available in six studies and were reported as Time Up and Go (TUG) tests ( Figure 6). [17,32,34,93,101,131]. Among these six studies, two studies misinterpret the FTB as ACB [32,34]. One study used an undetermined definition of ACB [17]. FTB or proximal ACB cannot be determined because of the lack of ultrasound images or cited articles. After excluding the above three studies using the undetermined definition of ACB, we performed a subgroup analysis (Figure 7) [17,32,34]. The FNB group was inferior to the mixed ACB group or "the ACB group with the exclusion of FTB or studies with an undetermined definition" 24 and 48 h after surgery (Figures 6 and 7), and this result was highly heterogeneous. The results of the mobilization ability between the mixed ACB group and the FNB group were available in six studies and were reported as Time Up and Go (TUG) tests ( Figure 6). [17,32,34,93,101,131]. Among these six studies, two studies misinterpret the FTB as ACB [32,34]. One study used an undetermined definition of ACB [17]. FTB or proximal ACB cannot be determined because of the lack of ultrasound images or cited articles. After excluding the above three studies using the undetermined definition of ACB, we performed a subgroup analysis (Figure 7) [17,32,34]. The FNB group was inferior to the mixed ACB group or "the ACB group with the exclusion of FTB or studies with an undetermined definition" 24 and 48 h after surgery (Figures 6 and 7), and this result was highly heterogeneous.
FTB as ACB [32,34]. One study used an undetermined definition of ACB [17]. FTB or proximal ACB cannot be determined because of the lack of ultrasound images or cited articles. After excluding the above three studies using the undetermined definition of ACB, we performed a subgroup analysis (Figure 7) [17,32,34]. The FNB group was inferior to the mixed ACB group or "the ACB group with the exclusion of FTB or studies with an undetermined definition" 24 and 48 h after surgery (Figures 6 and 7), and this result was highly heterogeneous.  The analgesic effect between true ACB and FNB for knee surgery was insufficient to perform analysis [17]. Theoretically, placing the block more proximally may have an additional analgesic benefit [41]. The injection site and craniocaudal spread of the injectate were correlated with safety and efficacy. However, continuity between the AC and FT has been debated [37][38][39]142]. Clinically, a large volume of local anesthetic in the distal true AC made no difference in pain scores but resulted in quadriceps weakness (46 mL produced a 30% reduction in quadriceps strength following ACB in 50% of the patients, but without any complication on drop foot was mentioned in this study) [143]. Further welldesigned studies, focusing on low volumes of local anesthetics and using the corrected classification system, are required to compare the analgesic effect between true ACB and FNB in major and minor knee surgeries, respectively.

The Effect of p-ACB versus d-ACB
In 2020, a meta-analysis comparing p-ACB with d-ACB for TKA concluded that both techniques had similar analgesic efficacy [144]. However, this result should be explained carefully due to a lack of disclosure of the exact injection level in all studies in regard to the adductor canal. Based on the corrected classification, insufficient studies were eligible to be pooled to evaluate the clinical effect of the proximal true ACB and the distal true ACB.
In our opinion, p-ACB is a better choice than d-ACB for a safe and effective block after knee surgery because of the nature of d-ACB, such as smaller branches and the variable position of involved nerves. The saphenous nerve consistently exists only in the The analgesic effect between true ACB and FNB for knee surgery was insufficient to perform analysis [17]. Theoretically, placing the block more proximally may have an additional analgesic benefit [41]. The injection site and craniocaudal spread of the injectate were correlated with safety and efficacy. However, continuity between the AC and FT has been debated [37][38][39]142]. Clinically, a large volume of local anesthetic in the distal true AC made no difference in pain scores but resulted in quadriceps weakness (46 mL produced a 30% reduction in quadriceps strength following ACB in 50% of the patients, but without any complication on drop foot was mentioned in this study) [143]. Further well-designed studies, focusing on low volumes of local anesthetics and using the corrected classification system, are required to compare the analgesic effect between true ACB and FNB in major and minor knee surgeries, respectively.

The Effect of p-ACB versus d-ACB
In 2020, a meta-analysis comparing p-ACB with d-ACB for TKA concluded that both techniques had similar analgesic efficacy [144]. However, this result should be explained carefully due to a lack of disclosure of the exact injection level in all studies in regard to the adductor canal. Based on the corrected classification, insufficient studies were eligible to be pooled to evaluate the clinical effect of the proximal true ACB and the distal true ACB.
In our opinion, p-ACB is a better choice than d-ACB for a safe and effective block after knee surgery because of the nature of d-ACB, such as smaller branches and the variable position of involved nerves. The saphenous nerve consistently exists only in the proximal adductor canal (SN pierces VAM at mid-AC). These features make the clinical effect of d-ACB unpredictable. A letter from Gautier et al. described the occurrence of impaired dorsiflex and plantar flexion of the foot after d-ACB with 20 mL local anesthetics injected via the catheter [145]. The spread to the popliteal fossa with apparent contact with both divisions of the sciatic nerve was evidenced by the contrast medium. Although d-ACB may cover the posterior knee pain by blocking the sciatic and obturator nerves, it may also cause foot drop if the catheter tip is placed proximal to the adductor hiatus [145]. The most important way to achieve motor-sparing is to minimize injectate volume to prevent injectate from spreading to other unwanted targets. Therefore, choosing p-ACB allows ACB to be accomplished with the smallest volume, thus sparing motor function. Although current studies are insufficient to prove it, a more consistent conclusion will be achieved if this corrected classification system can be emphasized and applied in future studies.

The Effect of True ACB versus FTB
Five studies compared pain scores between the FTB and the true ACB group 24 h after knee surgery [24,91,111,129,130]. No significant difference in pain scores 24 h after surgery was found between the two groups ( Figure 8). Three studies compared TUG test scores 24 h after surgery between the two groups, and the true ACB group had preserved more mobilization ability (Figure 9) [24,129,142]. In these trials, they used inconsistent outcome measures, and quadriceps strength could not be analyzed. Pain scores from day 0 to day 1 between the two groups cannot be analyzed because of insufficient data. Further research comparing true ACB with FTB is needed to evaluate their motor-sparing effect and analgesic effects.

The Effect of True ACB versus FTB
Five studies compared pain scores between the FTB and the true ACB group 24 h after knee surgery [24,91,111,129,130]. No significant difference in pain scores 24 h after surgery was found between the two groups ( Figure 8). Three studies compared TUG test scores 24 h after surgery between the two groups, and the true ACB group had preserved more mobilization ability (Figure 9) [24,129,142]. In these trials, they used inconsistent outcome measures, and quadriceps strength could not be analyzed. Pain scores from day 0 to day 1 between the two groups cannot be analyzed because of insufficient data. Further research comparing true ACB with FTB is needed to evaluate their motor-sparing effect and analgesic effects.  In brief, based on the corrected classification system, our meta-analysis can conclude that the quadriceps muscle strength and mobilization ability are more preserved in the mixed ACB group than the FNB group 24 h after knee surgery. Compared with the FTB group, the true ACB group preserves more mobilization ability 24 h after knee surgery. Still, more high-quality studies with large sample sizes are needed to explore the clinical effects of FTB, p-ACB, and d-ACB for knee surgeries in the future. The included studies had a low risk of bias after the assessment. Funnel-plot analyses were not performed because they should be used only when there are more than 10 studies included in the metaanalysis [146].

The Effect of True ACB versus FTB
Five studies compared pain scores between the FTB and the true ACB group 24 h after knee surgery [24,91,111,129,130]. No significant difference in pain scores 24 h after surgery was found between the two groups ( Figure 8). Three studies compared TUG test scores 24 h after surgery between the two groups, and the true ACB group had preserved more mobilization ability (Figure 9) [24,129,142]. In these trials, they used inconsistent outcome measures, and quadriceps strength could not be analyzed. Pain scores from day 0 to day 1 between the two groups cannot be analyzed because of insufficient data. Further research comparing true ACB with FTB is needed to evaluate their motor-sparing effect and analgesic effects.  In brief, based on the corrected classification system, our meta-analysis can conclude that the quadriceps muscle strength and mobilization ability are more preserved in the mixed ACB group than the FNB group 24 h after knee surgery. Compared with the FTB group, the true ACB group preserves more mobilization ability 24 h after knee surgery. Still, more high-quality studies with large sample sizes are needed to explore the clinical effects of FTB, p-ACB, and d-ACB for knee surgeries in the future. The included studies had a low risk of bias after the assessment. Funnel-plot analyses were not performed because they should be used only when there are more than 10 studies included in the metaanalysis [146]. In brief, based on the corrected classification system, our meta-analysis can conclude that the quadriceps muscle strength and mobilization ability are more preserved in the mixed ACB group than the FNB group 24 h after knee surgery. Compared with the FTB group, the true ACB group preserves more mobilization ability 24 h after knee surgery. Still, more high-quality studies with large sample sizes are needed to explore the clinical effects of FTB, p-ACB, and d-ACB for knee surgeries in the future. The included studies had a low risk of bias after the assessment. Funnel-plot analyses were not performed because they should be used only when there are more than 10 studies included in the meta-analysis [146].

Complications
Complications related to ACB may include injection pain, infection at the injection site, bleeding, nerve injury, allergic reaction, and local anesthetic systemic toxicity [147]. For ACB or FTB, there is a concern about an intravascular injection of local anesthetics because of the proximity of the femoral vessels [148]. With ultrasound guidance and a negative aspiration test, the risk of nerve injury, local toxicity, and vessel puncture can be reduced. Besides, the half-the-air setting can help identify the correct location using test volume injection other than local anesthetics and prevent incidental intraneural injection [149,150].
If local anesthetics spread to the proximal FT or popliteal fossa, it will block the femoral nerve or sciatic nerve, resulting in quadriceps weakness or foot drop [5,61,145,151]. Chen et al. described a case of quadriceps weakness after a mid-thigh injection of 20 mL of 0.5% ropivacaine [152]. Besides the injection pressure and anatomical variation, the volume may have contributed to the spread of the anesthetics to the proximal motor branches. An editorial commentary shared a case of delayed onset of foot drop after a true mid-AC injection via the catheter (initial injection of 20 mL of 0.2% ropivacaine, then 10-mL demand with 60-min lockout at 6 mL/h infusion). They proposed that the intercompartmental spread to the posterior compartment of the thigh muscle groups may have been created by the injection pressure [153]. Although true ACB is considered a sensory block, patients should be monitored for the occurrence of quadriceps weakness or foot drop. These complications will increase the risk of falls and delay functional recovery and rehabilitation.

Local Anesthetic Volume and Concentration
As of now, the optimal ACB regimen achieving adequate analgesia while avoiding side effects remains undefined. When reading relevant publications, we should be aware of the technique classified. A review article in 2016 reported that single-shot or catheter loading volumes of 15-30 mL at a ropivacaine concentration of 0.2% or 0.5% reveal no clinically significant influence on quadriceps strength [7]. This result should be taken cautiously because the authors mistook FTB as true ACB. Recently, a dose-finding study showed that the ED 50 of 0.5% ropivacaine required to produce a 30% reduction in quadriceps strength in 50% of the patients was 46 mL when using sonographic landmarks to perform p-ACB [143]. In our clinical practice, injectate as low as 2-5 mL was enough to achieve adequate p-ACB.
Delayed quadriceps weakness has been reported in FTB using 8 mL/h of 0.2% ropivacaine of continuous infusion after TKA. In this case, contrast injection under fluoroscopy disclosed proximal spread approaching the common femoral nerve [151]. A recent multivariate analysis in 2020 revealed a 9% prevalence of quadriceps weakness after single-shot d-ACB (mean volume: 21.7 ± 5.3 mL). This study observed quadriceps weakness associated with females and incremental dosing of ACB volume per unit of body mass index (BMI) [154]. Higher BMI may prevent the spread of anesthetic agents to the FT in some manner. No studies have been conducted to determine the lowest effective concentration. According to the aforementioned evidence, a low volume of anesthetics is favored to prevent motor weakness.

Continuous versus Single-Shot ACB
The five latest meta-analyses comparing continuous mixed ACB with single-shot mixed ACB revealed that continuous mixed ACB provides better analgesia and results in a shorter length of hospital stay. This may not be a firm conclusion because of a lack of the use of a corrected classification system of ACB, insufficient data, small sample sizes of published trials, and high heterogeneity. Although continuous ACB can conquer the limitation of the short duration of single-shot ACB, it has introduced some disadvantages, such as technical difficulties, catheter obstruction, migration, leakage of local anesthetics, patient education needs, and infection. For carefully selected patients and experienced anesthesiologists, continuous ACB may be an effective postoperative pain management strategy [155][156][157][158][159].

Perineural Adjuvant of FTB
Numerous published RCTs showed the effect of adjuvants added to local anesthetics for ACB, such as dexamethasone, clonidine, buprenorphine, and dexmedetomidine. Most relevant studies actually used FTB as their ACB model. Adding 4 or 8 mg of dexamethasone to bupivacaine (0.25% or 0.5%) or 0.5% ropivacaine increased the duration of FTB by 5-8 h compared with placebo [110,117,160]. An adjunct of 0.50 µg/kg of dexmedetomidine prolongs the pain-free period by approximately 8 h compared with plain ropivacaine [99] and reduces the total consumption of morphine in the initial four-hour period [96]. An RCT involving healthy volunteers used clonidine as an adjuvant; however, clonidine (150 µg/mL) did not prolong the pain-free duration [97]. The addition of 200 µg of buprenorphine to 30 mL of 0.25% bupivacaine reduced the opioid consumption in the first 24 h after TKA [161]. Various adjuvants for prolonging peripheral nerve blocks have been studied, but the Food and Drug Administration has not yet approved any one of them. The following are some side effects and toxicity correlated with perineural adjuvants: pruritus, nausea and vomiting (opioid), hypotension (clonidine), and bradycardia (dexmedetomidine). Therefore, any perineural adjuvants should be used with caution and concern about side effects and potential toxicity [162].

ACB for Anterior Cruciate Ligament (ACL) Surgery
The best regional analgesic technique for patients undergoing ACL reconstruction (ACLR) remains controversial. Recent suggestions regarding ACB for ACL surgery in four systematic reviews and meta-analyses differ. Articles included in these meta-analyses had mixed ACB definitions, including FTB and proximal true ACB. Two meta-analyses reported that mixed ACB for pain control after ACLR is categorized as weak strength of evidence of benefit [163,164]. However, Sehmbi et al. found that mixed ACB provides modest analgesic benefits for minor arthroscopic knee surgery. The other two meta-analyses concluded that mixed ACB has an analgesic effect similar to that of FNB and spares more quadriceps muscle strength than FNB after ACLR [76,77]. However, these are not definitive conclusions because of the lack of using a corrected classification system, limited sample size, and heterogeneity, and most trials focused on pain severity instead of functional outcomes.

Conclusions
This review used a corrected classification system to categorize the FTB, p-ACB, and d-ACB. Several studies misinterpreting mid-thigh level injection as ACB should be corrected to FTB. After clarifying the anatomy and definition of these blocking techniques, the results of this subgroup meta-analysis showed that: (1) the mixed ACB group has a better motor-sparing effect (quadriceps muscle strength and TUG test) than the FNB group 24 h after knee surgery; (2) true ACB provides an analgesic effect similar to that of FTB and preserves more mobilization ability (TUG test) than FTB. FTB may be an alternative blocking method to FNB or ACB for analgesia after knee surgery. These clinical effects of the nerve blocks are also influenced by adjuvants, the volume and concentration of local anesthetics, and surgery. On postoperative day 0, FNB may be better than FTB or ACB because pain management is the major issue right after surgery. The well-explained risk of falls, careful patient education, and communication with the surgeon are essential before performing nerve blocks. On postoperative days 1-2, ACB is recommended for its motor-sparing effect during rehabilitation. Based on the accumulated evidence, physicians may consider all aforementioned factors to choose the most suitable nerve block technique for multimodal analgesic plans for patients undergoing knee surgery. Institutional Review Board Statement: Ethics approval is not required because this study retrieved data from already published studies.