Ultrasound-guided quadratus lumborum block is a recently described fascial plane block where local anesthetic is injected adjacent to the quadratus lumborum muscle with the goal of anesthetizing the thoracolumbar nerves. The objective of this article is to review the relevant anatomy, potential mechanisms, approaches, and techniques and summarize the clinical evidence for quadratus lumborum block.

Anatomical Concepts

Muscles

Quadratus lumborum is a posterior abdominal wall muscle that originates from the posteromedial iliac crest and inserts into the medial border of the twelfth rib and the transverse processes of the first to fourth lumbar vertebrae. The lateral free border of quadratus lumborum is angled from craniomedial to caudolateral (fig. 1A). The quadratus lumborum and psoas major muscles traverse posterior to the lateral and medial arcuate ligaments of the diaphragm, respectively (fig. 1B). Posterior to the quadratus lumborum muscle lies the erector spinae muscle group, consisting of the multifidus, longissimus, and iliocostalis (figs. 2 and 3).

Fig. 1.

(A) A posterior schematic illustration of the musculature of the posterior abdominal wall. The quadratus lumborum muscle originates from medial border of the twelfth rib and lumbar vertebrae transverse processes and inserts into the posteromedial iliac crest. (B) An anterior schematic illustration of the musculature of the posterior abdominal wall. On the left, the psoas muscle has been removed to reveal the ventral rami of the spinal nerve roots and branches passing anterior to the quadratus lumborum muscle. ES, erector spinae; LD; latissimus dorsi; QL, quadratus lumborum; TP, transverse process. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Fig. 1.

(A) A posterior schematic illustration of the musculature of the posterior abdominal wall. The quadratus lumborum muscle originates from medial border of the twelfth rib and lumbar vertebrae transverse processes and inserts into the posteromedial iliac crest. (B) An anterior schematic illustration of the musculature of the posterior abdominal wall. On the left, the psoas muscle has been removed to reveal the ventral rami of the spinal nerve roots and branches passing anterior to the quadratus lumborum muscle. ES, erector spinae; LD; latissimus dorsi; QL, quadratus lumborum; TP, transverse process. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Fig. 2.

A schematic illustration of cross-section at L4 level showing the quadratus lumborum muscle with the different layers of the thoracolumbar fascia. On the left, the two-layer model is depicted, where the purple dashed line represents the anterior layer of the thoracolumbar fascia, and the green dashed line represents the transversalis fascia. On the right, the three-layer model is depicted, where the purple dashed line represents the middle layer of the thoracolumbar fascia, and the green dashed line represents the anterior layer of the thoracolumbar fascia. The blue dashed line represents the posterior thoracolumbar fascia. IL, iliocostalis; LD, latissimus dorsi; Lo, longissimus; Mu, multifidus; PM, psoas major; QL, quadratus lumborum; TLF, thoracolumbar fascia. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Fig. 2.

A schematic illustration of cross-section at L4 level showing the quadratus lumborum muscle with the different layers of the thoracolumbar fascia. On the left, the two-layer model is depicted, where the purple dashed line represents the anterior layer of the thoracolumbar fascia, and the green dashed line represents the transversalis fascia. On the right, the three-layer model is depicted, where the purple dashed line represents the middle layer of the thoracolumbar fascia, and the green dashed line represents the anterior layer of the thoracolumbar fascia. The blue dashed line represents the posterior thoracolumbar fascia. IL, iliocostalis; LD, latissimus dorsi; Lo, longissimus; Mu, multifidus; PM, psoas major; QL, quadratus lumborum; TLF, thoracolumbar fascia. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Fascia

The quadratus lumborum muscle is surrounded by a fibrous composite of aponeurotic and fascial tissue: the thoracolumbar fascia. The thoracolumbar fascia is part of a myofascial girdle that surrounds the lower torso and is important for posture, load transfer, and stabilization of the lumbar spine. The thoracolumbar fascia comprises multilayered fascia and aponeuroses, with two proposed models.

The two-layered model incorporates a posterior layer surrounding the erector spinae muscles and an anterior layer lying between the erector spinae and the quadratus lumborum muscles.1  In the two-layered model, the fascia on the anterior aspect of quadratus lumborum is the transversalis fascia, a fascial structure that is embryologically independent from the thoracolumbar fascia. The transversalis fascia lines the peritoneal surface of the transversus abdominis muscle and wraps the anterior aspect of the investing fascia (epimysium) of both quadratus lumborum and psoas muscles. The two-layered model follows the embryological development of the trunk muscles, where quadratus lumborum and psoas muscles belong to the hypaxial muscle compartment, anterior to the transverse processes, whereas the erector spinae muscles belong to the epaxial muscle compartment, posterior to the transverse processes.1  We illustrate the two-layered model of the thoracolumbar fascia at L4 schematically in figure 2.

In the three-layered model, the posterior thoracolumbar fascia layer surrounds the erector spinae muscles, the middle layer passes between the erector spinae muscles and quadratus lumborum, and the anterior layer lies anterior to both quadratus lumborum and psoas muscles. We illustrate the three-layered model of the thoracolumbar fascia at the L4 level schematically in figure 2.

Cranially, the anterior layer of the thoracolumbar fascia (the transversalis fascia in the two-layer model) divides into two layers. One layer is continuous with the endothoracic fascia in the thorax, and the other layer blends with the diaphragm at the arcuate ligaments. Caudally, this fascial layer is continuous with the fascia iliaca.2–4 

The three-layered model is the most commonly used. Regardless of which model is accepted, the fascial planes in the abdominal compartment follow the quadratus lumborum and psoas muscles through the medial and lateral arcuate ligaments and the aortic hiatus of the diaphragm, forming the endothoracic fascia. This provides a potential pathway for spread of injectate, such as local anesthetic, from the abdominal to the thoracic cavity and paravertebral space, thereby achieving clinical effect.2–4 

Lateral Raphe and Lumbar Interfascial Triangle

The paraspinal retinacular sheath is the deep lamina of the posterior layer of the thoracolumbar fascia extending from the spinous to transverse processes.1  The lateral raphe is a dense connective tissue complex formed where the abdominal myofascial structures (aponeurotic sheaths of the transversus abdominis and internal oblique muscles) join the paraspinal retinacular sheath at the lateral border of the paraspinal muscles. At this point, the myofascial structures separate into two laminae, which join the anterior and posterior paraspinal retinacular sheath layers. This creates the lumbar interfascial triangle, situated along the lateral border of the paraspinal muscles from the twelfth rib to the iliac crest (fig. 3).5  The lumbar interfascial triangle provides a theoretical pathway for injectate spread deep to the thoracolumbar fascia.

Fig. 3.

A schematic illustration of cross-section at L4 level showing the detailed descriptions of the three-layered model of the thoracolumbar fascia and its sublayers (left) and the anatomical relations of the three approaches to quadratus lumborum block (right) at L4 level. The dark blue dashed line represents the superficial lamina of the posterior layer of the thoracolumbar fascia encircling the latissmus dorsi and erector spinae muscles. The light blue dashed line represents the deep lamina of the posterior layer of the thoracolumbar fascia, also termed as paraspinal retinacular sheath, encircling the erector spinae muscles. The red dashed lines represent the epimysium-investing fascia of quadratus lumborum and psoas major muscles. The white (gray dashed) layer represents the aponeurosis of the internal oblique and transversus abdominis muscles. IL, iliocostalis; LD, latissimus dorsi; Lo, longissimus; Mu, multifidus; PM, psoas major; PTLF, posterior thoracolumbar fascia; QL, quadratus lumborum. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Fig. 3.

A schematic illustration of cross-section at L4 level showing the detailed descriptions of the three-layered model of the thoracolumbar fascia and its sublayers (left) and the anatomical relations of the three approaches to quadratus lumborum block (right) at L4 level. The dark blue dashed line represents the superficial lamina of the posterior layer of the thoracolumbar fascia encircling the latissmus dorsi and erector spinae muscles. The light blue dashed line represents the deep lamina of the posterior layer of the thoracolumbar fascia, also termed as paraspinal retinacular sheath, encircling the erector spinae muscles. The red dashed lines represent the epimysium-investing fascia of quadratus lumborum and psoas major muscles. The white (gray dashed) layer represents the aponeurosis of the internal oblique and transversus abdominis muscles. IL, iliocostalis; LD, latissimus dorsi; Lo, longissimus; Mu, multifidus; PM, psoas major; PTLF, posterior thoracolumbar fascia; QL, quadratus lumborum. Reprinted with permission, Cleveland Clinic Center for Medical Art & Photography © 2018. All Rights Reserved.

Vascular Structures

The abdominal branches of the lumbar arteries arise from the abdominal aorta and run laterally and posterior to the quadratus lumborum muscle. One exception is the fourth lumbar artery that may be located anterior to the quadratus lumborum.6 

Viscera

Intraabdominal viscera are located in close proximity to where quadratus lumborum block is performed. However, the transversalis fascia separates the muscle layers from the retroperitoneal abdominal contents. The kidney lies anterior to the quadratus lumborum muscle and is separated from it by para- and perinephric fat, the posterior layer of renal fascia, and the transversalis fascia.

Neural Structures

The iliohypogastric and ilioinguinal nerves (ventral ramus of L1 with occasional contributions from T12, L2, and L3) depart through the proximal and lateral aspect of the psoas major muscle and traverse the ventral surface of quadratus lumborum (fig. 1B).7,8  In four cadaver studies, the iliohypogastric and ilioinguinal nerves were consistently involved in spread of injectate.4,9–11  In published cases the reported dermatomal sensory blockade frequently includes the T12–L2, indicating consistent involvement of iliohypogastric and ilioinguinal nerves using different approaches.12–21  The lateral femoral cutaneous, obturator, and femoral nerves exit the psoas major muscle at more caudal levels (fig. 1B).22–24  The dorsal rami of the spinal nerves traverse the medial aspect of the middle thoracolumbar fascia posterior to the quadratus lumborum muscle and then enter the erector spinae muscles.

Spread of Injectate and Mechanisms of Action

Endothoracic Fascia Pathway

It is plausible that local anesthetic injected anterior to the quadratus lumborum muscle and posterior to the transversalis fascia will spread to the thoracic paravertebral space, posterior to the medial and lateral arcuate ligaments of the diaphragm, along the endothoracic fascia to block the somatic nerves and thoracic sympathetic trunk of the lower thoracic levels (fig. 1B). In addition to existing supporting anatomical2,25  and clinical literature,3  two recent cadaveric studies support this mechanism of action in anterior quadratus lumborum block.4,10  Dam et al.4  replicated an anterior quadratus lumborum block in a cadaveric model at the iliac crest (L4) and L2 and documented thoracic paravertebral spread involving somatic nerves and the thoracic sympathetic trunk to the T9–T10 level. Similarly, Elsharkawy et al.10  replicated a subcostal anterior quadratus lumborum approach at L1–2 level using a parasagittal oblique approach and found cranial spread to involve T7–12. In contrast, Sondekoppam et al.11  demonstrated, with an anterior approach at L3, dye spread in the lateral part of the thoracic paravertebral space (T11–12) with no clear craniocaudal spread. Adhikary et al.9  performed anterior quadratus lumborum blocks at L3–L4 levels and also demonstrated no evidence of thoracic paravertebral spread. However, Elsharkawy et al.10  investigated the posterior quadratus lumborum approach at the L3–4 level found staining up to T10 inside the lateral thoracic paravertebral space (table 1). Overall, local anesthetic injected between the transversalis fascia and the quadratus lumborum muscle may spread to the thoracic paravertebral space, and the vertebral level of injection will influence the extent of cranial spread.

Table 1.

Cadaveric Studies Conducted Investigating Quadratus Lumborum Block

Cadaveric Studies Conducted Investigating Quadratus Lumborum Block
Cadaveric Studies Conducted Investigating Quadratus Lumborum Block

Involvement of the Lumbar Spinal Nerve Roots and Branches

Cadaveric data also support a possible mechanism of action via direct spread of dye to the roots and branches of the lumbar plexus.4,9–11  L1–3 nerve roots were consistently involved in an anterior quadratus lumborum cadaveric injection study at the L3–4 level.26  Three cadaveric studies9–11  also demonstrated involvement of the upper lumbar plexus nerves and subcostal nerve after anterior quadratus lumborum block. Elsharkawy et al.10  and Dam et al.4  did not observe direct lumbar plexus involvement within the psoas major muscle in all approaches. It is notable that the spread of local anesthetic in living humans may be different from that of cadavers, and cadaveric evidence may not forecast clinical outcomes. Thus, careful analysis of translational data is needed.

Peripheral Sympathetic Field Block

Recent evidence suggests that rather than being a “passive” scaffold, fascial tissue is more complex, with rich vascular and sensory innervation. The thoracolumbar fascia has extensive sensory innervation by both A- and C-fiber nociceptors and mechanoreceptors.27  Sympathetic nerve fibers related to the abdominal branches of the lumbar arteries, located posterior to the quadratus lumborum muscle, innervate the thoracolumbar fascia.28  Because these nerves have a strong vasomotor component, blocking these sympathetic afferents could theoretically induce changes to both the local circulation and the general autonomic tone.28  This might potentially contribute to the analgesic efficacy of posterior quadratus lumborum block.29 

Approaches, Sonography, and Technical Performance

We have proposed that quadratus lumborum block be named based on the anatomical location of needle tip placement in relation to the quadratus lumborum muscle (fig. 3).30  Therefore we recommend the following terminology: lateral, posterior, and anterior quadratus lumborum block approaches. Figure 4 illustrates examples of in-plane approaches with anterior–posterior, posterior–anterior, and caudal–cranial trajectories.

Fig. 4.

Photographs and ultrasound images for different quadratus lumborum block approaches. (A–C) Lateral or posterior quadratus lumborum blocks. Transverse transducer and anteroposterior needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with a solid arrow indicating the needle trajectory for a lateral quadratus lumborum block and the dashed line indicating the needle trajectory for a posterior quadratus lumborum block approach. The red-/blue-shaded area represents the spread of the local anesthetic. (D–F) Anterior quadratus lumborum block: transverse oblique paramedian approach. Transverse transducer and posteroanterior needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with an arrow indicating the needle trajectory. The blue-shaded area represents the spread of the local anesthetic. (G–I) Anterior quadratus lumborum block: subcostal approach. Parasagittal oblique transducer and caudal-to-cranial needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with an arrow indicating the needle trajectory. The blue-shaded area represents the spread of the local anesthetic. EO, external oblique; ES, erector spinae; IO, internal oblique; PM, psoas major; QL, quadratus lumborum; TA, transversus abdominus; TP, transverse process.

Fig. 4.

Photographs and ultrasound images for different quadratus lumborum block approaches. (A–C) Lateral or posterior quadratus lumborum blocks. Transverse transducer and anteroposterior needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with a solid arrow indicating the needle trajectory for a lateral quadratus lumborum block and the dashed line indicating the needle trajectory for a posterior quadratus lumborum block approach. The red-/blue-shaded area represents the spread of the local anesthetic. (D–F) Anterior quadratus lumborum block: transverse oblique paramedian approach. Transverse transducer and posteroanterior needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with an arrow indicating the needle trajectory. The blue-shaded area represents the spread of the local anesthetic. (G–I) Anterior quadratus lumborum block: subcostal approach. Parasagittal oblique transducer and caudal-to-cranial needle trajectory are shown. The external image and ultrasound images show the ultrasound probe position with an arrow indicating the needle trajectory. The blue-shaded area represents the spread of the local anesthetic. EO, external oblique; ES, erector spinae; IO, internal oblique; PM, psoas major; QL, quadratus lumborum; TA, transversus abdominus; TP, transverse process.

Needle Length and Gauge

Typical needle length would range from 80 to 150 mm depending on patient body habitus. The exact gauge would depend on single injection technique versus continuous technique.

Injectate

Local anesthetic dosage in the range of 0.2 to 0.4 ml/kg of 0.2 to 0.5% ropivacaine31–33  or 0.1 to 0.25% bupivacaine34–36  per side is recommended. The operator will need to adjust dosage to ensure toxic thresholds are not exceeded, particularly when bilateral blocks are performed. There is no comparative data on the efficacy of adjuvants in quadratus lumborum blocks; however, the use of epinephrine may have benefits in reducing the rate of absorption and in detecting and limiting inadvertent intravascular injection. Standard safety precautions for performing regional anesthetic blocks should be followed.

Positioning

The patient can be positioned supine with a lateral tilt, lateral, sitting or prone, largely depending on physician preference, patient mobility, and planned needle trajectory. For example, a posterior–anterior trajectory will require the patient to be lateral, prone, or in the sitting position.

Lateral Quadratus Lumborum Block

This can be performed using an in-plane approach, with a needle insertion lateral (anterior) to the ultrasound transducer with an anterior-to-posterior needle trajectory. Local anesthetic is deposited at the lateral border of quadratus lumborum muscle after the needle tip penetrates the transversus abdominis aponeurosis.37  Lateral quadratus lumborum block has been shown to be opioid-sparing compared with placebo for post–cesarean section analgesia.31,32 

Posterior Quadratus Lumborum Block

This can be performed using an in-plane approach using an anterior-to-posterior or posterior-to-anterior trajectory. Local anesthetic is injected on the posterior surface of quadratus lumborum muscle, between the quadratus lumborum and erector spinae muscles. This approach was used in two randomized controlled trials that documented an opioid-sparing effect of quadratus lumborum block compared with placebo34  or transversus abdominis plane block35  after caesarean section.

Anterior Quadratus Lumborum Block

This can be performed using an in-plane approach, with a needle insertion medial to the ultrasound transducer, using a posterior-to-anterior trajectory.38  Alternatively an in-plane approach, with an anterior-to-posterior trajectory can be used. A further option is with the subcostal oblique anterior approach; the needle insertion is caudal to the transducer, and the trajectory is in-plane, caudal–lateral to cranial–medial. The point of injection of local anesthetic lies in the tissue plane between the quadratus lumborum and psoas muscles.38–40  These variants differ in the needle trajectory used (anterior-to-posterior; posterior-to-anterior; caudal-to-cranial) but have the same plane of injection.

According to cadaveric and clinical reports, it is reasonable to conclude that different quadratus lumborum blocks have different mechanisms of action. Anterior quadratus lumborum block injectate may spread to the lumbar nerve roots and branches in addition to the thoracic paravertebral space.4,9–11  Posterior quadratus lumborum blocks appear to demonstrate their clinical effect by injectate spread along the middle thoracolumbar fascia intertransverse area.10,26  Lateral quadratus lumborum blocks are associated with injectate spread to the transversus abdominis muscle plane and to subcutaneous tissue,26  although clinical reports refer to a more extensive distribution.31,32,35,41  At present, there is insufficient evidence to recommend one approach and transducer positioning over another for individual patient populations and specific surgical types.

Sonography

Because identifying the quadratus lumborum muscle is critical to performing quadratus lumborum block, it is important to note the following landmarks and relationships: the aponeuroses of the abdominal wall muscles (external oblique, internal oblique, transversus abdominis) are located posterolateral to the quadratus lumborum muscle (fig. 2); the quadratus lumborum muscle is often hypoechoic relative to psoas major muscle, which is found anteromedially; and the lumbar transverse processes are apparent with their hyperechoic curved appearance (fig. 4E).

A curvilinear low-frequency transducer is often required, facilitating tissue penetration of ultrasound and a wide field of view. The transducer is placed in transverse orientation at the posterior or midaxillary line at the L2–L4 level with the objective of imaging the quadratus lumborum and erector spinae muscles, together with a transverse process forming the “shamrock sign” (fig. 4E).42  A variation is maintaining the transducer in transverse orientation but placing it more medially, approximately 3 cm lateral to the L2 spinous process (fig. 4D). This has been referred to as the transverse oblique paramedian placement, and the image is enhanced with medial rocking of the transducer and slight caudal rotation of the lateral aspect of the transducer.38  A further option is to place the transducer in a parasagittal oblique plane tilted medially (subcostal oblique anterior approach) at the level of the twelfth rib approximately 6 to 8 cm from the spinous process (fig. 4G). This view corresponds to the transverse oblique paramedian view but with the transducer rotated to a parasagittal oblique plane. With the subcostal oblique anterior approach, the quadratus lumborum muscle can be imaged at its point of insertion on the lower border of the twelfth rib, and the erector spinae muscle is seen posterior (superficial) to the quadratus lumborum muscle. The psoas major muscle, diaphragmatic zone of apposition, the kidney and perinephric fat and renal fascia are anterior (deep) to the quadratus lumborum muscle. The needle trajectory for the anterior approach to quadratus lumborum block is deep and close to the abdominal and retroperitoneal viscera; therefore a high level of vigilance and technical competency is required.

Indications and Clinical Relevance

To date, two randomized controlled trials demonstrated that quadratus lumborum block reduces cumulative opioid consumption for 48 h after caesarean section.31,32  Two further randomized controlled trials have demonstrated that posterior quadratus lumborum block has an opioid-sparing effect after caesarean section (table 2).34,35  Posterior quadratus lumborum block is associated with reduced postoperative pain scores after laparoscopic gynecological surgery33  and reduced rescue analgesia requirements after lower abdominal surgery.36  Successful use of quadratus lumborum block with all approaches has been published in case reports for the following surgical procedures: proctosigmoidectomy,18  hip surgery,12  above-knee amputation,43  abdominal hernia repair,44  breast reconstruction,13  colostomy closure,14  radical nephrectomy,15  lower extremity vascular surgery,45  total hip arthroplasty,16,17,20  laparotomy,19  and colectomy.46  Several other case reports with a variety of indications for quadratus lumborum block document sensory blockade to include the T7–L2 dermatomes (table 3).9,19,20,24,44,47–66 

Table 2.

Randomized Controlled Trials with Quadratus Lumborum Block Interventions

Randomized Controlled Trials with Quadratus Lumborum Block Interventions
Randomized Controlled Trials with Quadratus Lumborum Block Interventions
Table 3.

Case Reports of Quadratus Lumborum Block

Case Reports of Quadratus Lumborum Block
Case Reports of Quadratus Lumborum Block

Contraindications

Because of the ongoing investigative nature of this technique, empirical contraindications apply. Absolute contraindications include local infection, allergy to local anesthetics, and a known bleeding diathesis because it is a deep block. Relative contraindications include anatomical abnormalities, hemodynamic instability, and known neurologic disorders.

Complications

Local Anesthetic-related Complications

Quadratus lumborum block may result in local anesthetic distribution to the lumbar plexus and prolonged motor block, delaying mobilization and hospital discharge. Lower-limb weakness has been reported after use of all quadratus lumborum block approaches.67,68  Hypotension has been reported, which may be related to local anesthetics spread to the paravertebral space.53  Because of the doses used and the vascularity of the area, local anesthetic systemic toxicity is a potential risk,69,70  although peak concentrations of local anesthetic are lower after quadratus lumborum blocks than transversus abdominis plane blocks.71 

Needle Trauma

The proximity of quadratus lumborum block to the pleura and kidney in the subcostal anterior approach presents a risk because of direct needle trauma.10  The risks of bleeding complications are not yet known. There is no evidence for the stratification of the risk of bleeding based on quadratus lumborum approaches. The posterior and lateral quadratus lumborum block approaches use a fascial plane through which the abdominal branches of the lumbar arteries course. The anterior quadratus lumborum block is a deep block, close to the lumbar plexus and risks retroperitoneal spread of hematoma. Therefore, the authors recommend that, for all approaches, the American Society of Regional Anesthesia and Pain Medicine guidelines for deep peripheral blocks be followed.

Conclusions

An understanding of the relevant anatomy and technical aspects of quadratus lumborum block are essential for its effective and safe use. Cadaveric studies demonstrate that the iliohypogastric and ilioinguinal nerves are consistently involved. Quadratus lumborum block approaches are named in reference to injection location in relation to the quadratus lumborum muscle. Current indications are based on few existing randomized controlled clinical trials and case reports (tables 2 and 3). Although the evidence base is weak and still growing, the data thus far suggest that quadratus lumborum block potentially results in extensive sensory blockade (T7–L2). Quadratus lumborum block may lead to dermatomal coverage required for abdominal surgery and hip surgery, representing an avenue for future research.

Acknowledgments

The authors acknowledge Jens Børglum, M.D., Ph.D., Department of Anesthesia and Intensive Care Medicine, Zealand University Hospital, University of Copenhagen, Denmark, and Rafael Blanco, M.D., Department of Anesthesia, Corniche Hospital, Abu Dhabi, United Arab Emirates, for their significant contributions in the development of the presented theories.

Research Support

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

Competing Interests

Dr. Elsharkawy has received unrestricted educational funding from PAJUNK, Norcross, Georgia, and is a consultant for PACIRA, Parsippany-Troy Hills, New Jersey. Those companies had no input into any aspect of this article. The other authors declare no competing interests.

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