Anatomical and Technical Considerations of "Dual Subsartorial Block" (DSB), A Novel Motor-sparing Regional Analgesia Technique for Total Knee Arthroplasty

Background: The modernization of arthroplasty has paved the way for the resurgence of ultrasound-guided regional analgesia (RA) techniques. The evolution of newer RA techniques aids in reducing postoperative pain considerably as well as facilitates early ambulation and discharge. “Dual Subsartorial Block (DSB)” is recently described as a novel procedure-specifi c, motor-sparing, and opioid-sparing RA technique for total knee arthroplasty (TKA) surgery. This review article highlights the innervations covered by DSB based on the anatomical considerations and its suitability for providing analgesic coverage in TKA with medial approaches. Methodology: We describe anatomical considerations based on the available literature about the anatomy related to the femoral triangle, adductor canal, and subsartorial region. The technical consideration of the DSB is based on our observations of the ongoing study on patients undergoing TKA with medial approaches. However, other details of the study are not part of this article. Results: After studying the anatomical and technical aspects of the DSB, it is possible to cover almost all procedure-specifi c innervations of TKA surgeries with the DSB. Our observations and statistical analysis found DSB as a procedure-specifi c, motor-sparing, and opioid-sparing RA technique. Discussion: We describe the complex anatomy of the femoral triangle and adductor canal block along with sonoanatomical variations of various subsartorial regions. We also elaborate on the technical details, analgesic coverage, and possible complications of DSB. Review Article Anatomical and Technical Considerations of "Dual Subsartorial Block" (DSB), A Novel Motor-sparing Regional Analgesia Technique for Total Knee Arthroplasty Kartik Sonawane1*, Hrudini Dixit2, Tuhin Mistry1 and J. Balavenkatasubramanian3 1Junior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India 2Fellow in Regional Anesthesia, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India 3Senior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd., Coimbatore, Tamil Nadu, India Received: 10 June, 2021 Accepted: 29 June, 2021 Published: 01 July, 2021 *Corresponding author: Dr. Kartik Sonawane, Junior Consultant, Department of Anesthesiology, Ganga Medical Centre & Hospitals, Pvt. Ltd, Coimbatore, Tamil Nadu, India, Email:


Introduction
Total Knee Arthroplasty (TKA) is one of the most commonly performed elective orthopedic surgery in the modern world.
This surgery offers the patient a life-changing experience due to increased joint mobility and painless ambulation. However, one deterrent which precluded a patient from undergoing TKA is the signifi cant immediate postoperative pain. Thanks to the regional analgesia (RA) resurgence aided by ultrasound, which enables a considerable reduction in immediate postoperative pain. Most arthroplasty surgeons are keen to mobilize the patient soon after the surgery, necessitating the evolution of new RA techniques that would aid early ambulation with optimal analgesia. Dual Subsartorial Block (DSB) is a procedure-specifi c technique for TKA that would aid in early ambulation due to its motor-sparing and opioid-sparing effect [1]. The DSB covers almost all the innervations of the pain-generating components of the anterior and posterior knee joint. Therefore, knowing the anatomy of the knee joint, types of the subsartorial blocks and their neural components, pain generators before and after TKA surgeries, and innervation of those pain generators is essential to understand the technical aspect of the DSB.
The knee joint is complexly innervated by the main branches of the lumbar and sacral plexus. These include the femoral nerve (anterior knee), obturator nerve (posteromedial knee), and sciatic nerve (posterior knee) [2][3][4]. The blockade of all three nerves provides complete analgesia in their respective territories. However, it is associated with unwanted motor blockade of quadriceps (femoral nerve), adductors (obturator nerve), and hamstring muscles (sciatic nerve). This causes the risk of falls, delayed mobility, and prolonged hospital stay [5]. For these reasons, such blocks are not recommended in enhanced recovery after surgery (ERAS) protocol for knee surgery.
Many alternative motor-sparing regional analgesia techniques have been described to overcome these unwanted side effects. However, the ideal RA technique should cover all the essential innervations of the knee joint involved in each surgical step without causing motor blockade to support early mobility and discharge. Considering all these requirements, we had designed DSB in 2018 and practicing it in all patients undergoing TKA with medial approaches since then. We named it "Dual Subsartorial Block," as it is given at two different locations under the sartorius muscle.
We hypothesize that the DSB is motor-sparing, procedurespecifi c, and opioid-sparing RA technique for TKA. This technical report highlights the innervations covered by DSB based on the anatomical considerations and its suitability in providing analgesic coverage in all TKA procedures with medial approaches. Motor-sparing and procedure-specifi c RA techniques are the future of RA practices, which induces the analytical thought process in applying everchanging knowledge of anatomy, techniques, and methodologies for providing the best patient care. Therefore, this review article mainly focuses on the anatomical and technique aspect of DSB that will make the readers understand the intricacies of this novel motorsparing technique.

Relevant anatomy
The "DSB" combines two subsartorial blocks, the distal femoral triangle block and the adductor canal block. This combination is given to block most of the innervations of the pain generating structures involved in TKA surgery. Various literatures have made the demarcation between the femoral triangle and adductor canal clear [6][7][8][9]. It is essential to understand the extent and content of both these prominent landmarks of the thigh.

Femoral triangle:
The femoral triangle (FT), also called Scarpa's triangle, is a subfascial space below the inguinal ligament that lies in the upper third of the thigh [10]. The  opening covered by the cribriform fascia [11]. The FT fl oor is formed by the pectineus and ALM medially and iliopsoas muscle laterally [11].
There are two intersecting points of the medial and lateral walls of the FT.

Adductor canal
The Adductor Canal (AC), also known as Hunter's canal or the subsartorial canal, is a musculoaponeurotic tunnel in the middle third of the thigh. It is about a 15 cm long tunnel that serves as a passageway for structures moving between the anterior thigh and posterior leg [14]. It extends from the apex of the FT above to the tendinous opening in the adductor magnus muscle (AMM), called adductor hiatus, below. The AC is triangular in a cross-section bounded anterolaterally by VMM, posteromedially by ALM proximally and AMM distally, and medially by the vasoadductor membrane (VAM), a strong fi brous membrane joining anterior and posterior walls ( Figure  2). The AC roof is formed by the skin, subcutaneous tissue, and STM (lying above the VAM). The subsartorial plexus lies in this region sandwiched between STM and VAM. (Figure 2) It is formed by an infrapatellar branch from the SN, anterior division of obturator nerve, medial femoral cutaneous nerve, and NVM. It supplies the skin over the anteromedial aspect of the knee joint, retinaculum, collateral ligaments, and capsule of the knee joint [15].
The AC consists of three foramina: superior, anterior, and inferior. The femoral vessels and the SN enter AC through the superior foramen. The SN and descending genicular vessels exit AC through the anterior foramen, piercing the VAM. Finally, the femoral vessels exit AC via the inferior foramen, also called the adductor hiatus (a gap between the oblique and medial head of AMM). After exiting through this hiatus, femoral vessels become popliteal vessels [16].

Subsartorial region and blocks: [Figure 3]
The apex of the FT that separates the FT from the AC usually lies distal to the mid-thigh point [6] It can be identifi ed under ultrasound where medial borders of ALM and STM overlie (Figure3C), forming a fi gure of "3," also called a "kissing sign." The area just proximal to the apex of FT is the distal femoral triangle, and the area just distal to the apex is the proximal adductor canal. The mid-thigh level block is called the femoral triangle block, as this level usually lies at the distal part of FT.
The sartorius muscle is a common landmark for both femoral triangle and adductor canal blocks while using ultrasound. Therefore, the area under the sartorius in these regions can also be called a subsartorial area. Thus, the femoral triangle block and adductor canal block can be considered variants of the subsartorial blocks. The regional block effect in these subsartorial areas differs from each other due to the varied neurovascular anatomy. The variants of subsartorial block are,

A. Femoral triangle block
It is given at the distal-most part of FT, just (1-2 cm) proximal to the apex of FT. Sonoanatomy of this region shows ALM posteromedially, VMM anterolaterally, and STM medially ( Figure 3B). In this region, there is no VAM like in AC. The SN lies just lateral to the FA, whereas the NVM lies lateral to SN in the intermuscular fascial planes between STM and VMM. The required local anesthetic (LA) volume for this block is 10-20cc.
A higher volume (more than 40cc) causes proximal spread that may include the femoral nerve and its branches. The drug injected into the distal FT spreads distally into the AC along with the vessels, and most of the drug spreads distally under the STM but above the VAM in the AC region. The distal spread of the drug below STM and above VAM involves the subsartorial plexus, which is sandwiched between STM and VAM [17][18][19][20].

C. Mid-adductor canal block
It is given distal to the proximal adductor canal, where the ALM is replaced by AMM posteromedially. In this region, the SN pierces VAM along with the descending genicular artery (branch of FA) and lies below STM and above the VAM ( Figure   3E). Due to the presence of VAM, the lower border of sartorius appears bilayered. The NVM lies above VAM in its fi xed location between STM and VMM. The required LA volume for this block is 10-20cc. A drug injected into the mid-adductor canal region below the VAM travels along with the femoral vessels and fi nally enters the popliteal region to involve the popliteal plexus.

Dual Subsartorial Block (DSB)
The DSB is a hybrid form of subsartorial block, in which the combination of two subsartorial blocks (distal femoral triangle and adductor canal block) is used to cover all procedure-specifi c innervations of pain generators involved in TKA surgery. In this block, the local anesthetic drug is deposited under the sartorius muscle at two different places, so it is termed as a "Dual Subsartorial Block." The DSB is given immediately after the surgery with two different injections at two different locations.

Indications and contraindications
The DSB is a newly introduced motor-sparing RA technique for TKA surgeries. This block can be used for any knee surgery with a medial approach (medial parapatellar, mid-vastus, or subvastus). This block may not work in TKA surgeries with other approaches. Similarly, it may not provide complete analgesia in revision TKA surgeries or knee debridement surgeries as the incision is not always medial parapatellar. The DSB can be an essential component of multimodal analgesia protocol to provide postoperative analgesia without causing any motor blockade. This technique should not be used as a supplementary RA technique in the presence of other RA techniques like femoral nerve block or epidural analgesia. In such a scenario, the motor-sparing advantage of this block will be lost.

Choice of local anesthetics
The DSB requires a larger volume (20-30 mL) of LA solution to adequately cover all the innervation of the postoperative

Landmark and patient positioning
The DSB is performed with the patient in the supine position. The hip is abducted, and the thigh is externally rotated to facilitate ultrasound probe and needle placement. If nerve stimulation is to be used simultaneously, thigh exposure is required to observe the motor response. In addition, exposure of the entire thigh will help appreciate the distance from the groin to the knee to determine the middle portion of the thigh. On moving the probe more distally, it can be seen that the SN moves from lateral to anterior of the artery and becomes superfi cial by piercing the thigh's deep and superfi cial fascia.

Probe position and sonoanatomy
During its course, the SN lies initially between the STM and VMM, then between STM-ALM, STM-AMM, and fi nally between STM and gracilis muscle [8]. Throughout the adductor canal region, the NVM can be identifi ed as a hyperechoic structure above the VAM between STM and VMM.

Goal
During the fi rst injection, the goal is to place the needle tip immediately adjacent to the NVM and SN in the intermuscular plane between the VMM and STM. During the second injection, the goal is to place the needle tip below the VAM, distal to the apex of the FT, immediately adjacent to the FA.

Technique
The DSB can be given by following 3 simple steps,

1.
Identifying the apex of the femoral triangle 2.
First injection (distal femoral triangle block)

Identifying the apex of the femoral triangle:
The

Second injection (adductor canal block): [Figure 4C]
The high-frequency linear ultrasound probe is placed just   a disappearing spread of drug and compression of the vessel ( Figure 4C). Such that injecting the drug compresses the femoral artery, and when the injection is stopped, the drug seems to disappear, thus restoring the femoral vessels to their original position. This block can be given at any distance distal to the apex of FT. However, we recommend giving this block in the proximal adductor canal area to avoid proximity to the sterile surgical site.

Distribution of analgesia
The fi rst injection (distal femoral triangle block) will target SN and NVM directly ( Figure 4A). Next, the distal spread of the drug under STM but above the VAM targets the subsartorial plexus, which is sandwiched between VAM and STM in the adductor canal region ( Figure 4C).
In the second injection (adductor canal block), no nerves are targeted. Simply depositing the drug perivascularly (around FA) under the VAM is suffi cient to obtain the desired outcome.
A drug injected into the adductor canal below VAM will travel along the femoral vessels and enter the adductor hiatus to reach the posterior aspect of the knee joint. It blocks the popliteal plexus formed by the articular branches from the posterior division of the obturator nerve, tibial, common peroneal, and sciatic nerve. Larger LA volumes (30-40 ml) in the proximal AC (Hi-PAC block) [27] or distal AC have the potential to involve the sciatic nerve indirectly (required for below-knee surgeries) [28,29].  Figure 5). Unless the surgical incision involves these spared areas, the lack of analgesia in its distribution is of little clinical consequence. The DSB does not cover tourniquet pain, which is mainly infl ammatory pain due to muscle ischemia rather than pain due to pressure over the skin. If given precisely with the recommended LA concentrations and volumes, the DSB may not cause any motor blockade and provide optimal analgesia by blocking the procedure-specifi c target nerves.
Since DBS is a new technique, it is pertinent to compare it with the existing RA techniques for TKA surgeries (Table 1).
It is also essential to assess the possible complications and measures to improve its safety profi le ( Table 2). More analysis and discussions are required in the future on these two crucial aspects of this block.

Summary
Many available RA options are known to provide the best perioperative analgesia for TKA surgeries. Among these options, it is more suitable to use a motor-sparing modality in this modern era as a part of ERAS protocol to promote early mobilization and reduce hospital stay. Furthermore, after studying the anatomical technical aspect of the DSB, it is almost possible to cover all procedure-specifi c innervations of TKA surgeries with the DSB (Table 3).
Motor-sparing and procedure-specifi c RA techniques are the future of RA practices, which induces the analytical thought process in applying ever-changing knowledge of anatomy, techniques, and methodologies for providing the best patient care. Therefore, to understand DSB better, knowing its anatomical and technical aspects is very important.
This novel technique can potentially provide excellent analgesia and aid in early ambulation by covering all the procedure-specifi c innervations. However, the analgesic effi cacy, motorsparing, and opioid-sparing effect of DSB need more extensive sample-sized comparative studies.