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Abstract
Background and objectives The erector spinae plane (ESP) block is a relatively new interfascial block technique. Previous cadaveric studies have shown extensive cephalocaudal spread with a single ESP injection at the thoracic level. However, little data exist for lumbar ESP block. The objective of this study was to examine the anatomical spread of dye following an ultrasound-guided lumbar ESP block in a human cadaveric model.
Methods An ultrasound-guided ESP block was performed in unembalmed human cadavers using an in-plane approach with a curvilinear transducer oriented longitudinally. 20 mL of 0.166% methylene blue was injected into the plane between the distal end of the L4 transverse process and erector spinae muscle bilaterally in four specimens and unilaterally in one specimen (nine ESP blocks in total). The superficial and deep back muscles were dissected, and the extent of dye spread was documented in both cephalocaudal and medial–lateral directions.
Results There was cephalocaudal spread from L3 to L5 in all specimens with extension to L2 in four specimens. Medial–lateral spread was documented from the multifidus muscle to the lateral edge of the thoracolumbar fascia. There was extensive dye in and around the erector spinae musculature and spread to the dorsal rami in all specimens. There was no dye spread anteriorly into the dorsal root ganglion, ventral rami, or paravertebral space.
Conclusions A lumbar ESP injection has limited craniocaudal spread compared with injection in the thoracic region. It has consistent spread to dorsal rami, but no anterior spread to ventral rami or paravertebral space.
- lower extremity
- nerve block
- regional anesthesia
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Introduction
The erector spinae (ES) plane (ESP) block is a relatively new interfascial block technique that has gained interest given its reported efficacy in early studies. Since the first description of its use to treat neuropathic thoracic pain in 2016,1 the use of ESP for postoperative pain control has expanded to include a wide variety of clinical applications. In the thorax and abdomen, ESP has been used for pain control after thoracotomy,2 video-assisted thoracic surgery,3 rib fractures,4 thoracic outlet decompression,5 cardiac surgery,6 mastectomy,7–9 ventral hernia repair,10 11 and even chronic shoulder pain.12 More recently, the technique has been applied to lumbar and low abdominal surgeries, including spine,13 14 hip,15 femur fracture,16 and gynecological surgeries.17
The exact mechanism of the ESP block is poorly defined with conflicting results throughout the clinical and anatomical literature. Some studies assert that ESP injection results in anterior diffusion of anesthetic into the paravertebral space with subsequent blockade of the dorsal and ventral roots of the spinal nerves. This mechanism is supported by studies reporting anterior dermatomal sensory blockade in patients.1 14 18 19 Cadaveric studies are conflicting with some demonstrating spread to the paravertebral space and ventral rami,17 20–23 while others do not show any spread beyond the dorsal ramus.24
The objective of this observational study was to describe the anatomical distribution of an ESP injection at the fourth lumbar (L4 level) in a cadaveric model.
Methods
Four unembalmed human cadavers were obtained from the Center for Procedural Innovation at the Mayo Clinic in Scottsdale, Arizona, USA. These anatomic specimens had no prior back or pelvic surgery. They were initially frozen at −10°F and then were thawed and kept at room temperature for 48–72 hours prior to injection and dissection.
ESP block
The ESP block was performed with the cadavers in the prone position, using a Philips ClearVue 850 Ultrasound System (Philips, Andover, Massachusetts, USA). A low frequency C5-2 curvilinear ultrasound transducer (5–2 MHz) was placed in a longitudinal parasagittal orientation at the L4 transverse process. This was identified by first using the ultrasound probe to locate the sacrum in the sagittal plane, sliding cranially to the L4 spinous process and then laterally to identify the end of the L4 transverse process. The level and ESP location were confirmed by two anesthesiologists with regional anesthesia expertise (MH, RC). A 22-gauge, 8 cm echogenic needle (Pajunk Sonoplex II, Geisingen, Germany) was inserted in-plane in a cranial-to-caudal direction and was placed deep to the ESP on the tip of the L4 transverse process, as described in several studies.20 21 23–25 The ESP was confirmed under ultrasound using 1–2 mL of 0.9% normal saline. Once confirmed, 20 mL of methylene blue 0.166% was injected into the space (figure 1).
Ultrasound image of erector spinae block performed at L4 transverse process (TP).
Initially, one cadaver was injected on the left side only to test whether the dye spread across the midline. Once the unilateral extent of dye spread was verified with dissection, the left and right sides of the remaining four cadavers were injected, for a total of nine injections.
Anatomic dissection
The cadavers were dissected by the anatomist (NRL) in the prone position at least 2 hours after injection. A midline skin incision was made at T10 and extended inferiorly to the sacrum. The skin and superficial fascia were reflected laterally to expose the superficial musculature and posterior layer of the thoracolumbar fascia (TLF). The latissimus dorsi muscle, serratus posterior inferior muscle, and TLF were incised near the midline and reflected laterally to expose the underlying ES muscles of the lumbar region (longissimus and iliocostalis) and the ES aponeurosis (the common tendon of the ES and multifidus muscles). Longissimus and the ES aponeurosis were incised near the midline and reflected laterally to examine the multifidus muscle. The multifidus muscle was reflected laterally to examine the intertransversarii and psoas major muscles. A laminectomy was performed at the levels of dye spread to identify the paravertebral space and dorsal and ventral primary rami.
The extent of craniocaudal dye spread was documented relative to vertebral levels, and the lateral extent of the dye spread was documented in relation to the ES muscles. The intramuscular plane of the spread was noted (ie, if the dye was restricted to the ES muscles or penetrated deeper into multifidus), as was the dye spread onto the dorsal and ventral primary rami.
Results
Lumbar ESP injections were performed on nine sides from five cadavers using sonographic guidance without difficulty. The spread of the test injectate confirmed needle placement deep to the ES muscles in the ESP prior to dye injection in all specimens. Extensive dye spread was seen in the deep back musculature (figure 2). The longissimus and multifidus muscles were dyed in all injections (100%); the iliocostalis was dyed in six of the nine injections (67%). In one specimen, a thin line of dye was noted on the posterior aspect of the quadratus lumborum muscle at the level of injection (L4), but this muscle was not stained in any of the other injections.
Cadaver image of dye spread from L2 to L5. Black line approximates midline (spinous processes). Right side is superficial, and left side is deep dissection. Note the extensive dye in the multifidus muscle.
There was limited craniocaudal and medial-to-lateral spread of the injections, which is depicted in table 1.
Extent of dye spread in cadaver model
Cephalocaudal spread was observed from L3 to L5 in 100% of cases (nine specimens) with extension to L2 in 44% of the cases (four specimens). The lateral extent of the dye spread was most extensive at the L4 and L5 levels. The iliolumbar ligament limited caudal spread in all specimens, while the TLF confined the posterior and lateral spread of the dye to the deep back musculature. Dorsal primary rami were stained by the dye in all injections; however, the spread of the dye did not involve the paravertebral space, the dorsal root ganglion, nor ventral primary rami in any of the specimens (figure 3).
Dye spread to dorsal primary ramus. Note that ventral ramus and posterior abdominal wall musculature are not dyed.
Discussion
Anatomic investigations of the ESP block have been limited primarily to injections administered at various thoracic levels ranging from T4 to T11.20–23 These studies report extensive craniocaudal dye spread, with some reporting spread of up to 12 vertebral levels.20 Lateral spread of the dye with thoracic ESP injection was noted beyond the iliocostalis to the serratus anterior muscle20 22 and external intercostal muscle,23 potentially accounting for the involvement of thoracic dermatomes supplied by ventral rami. Despite a uniform volume of 20 mL injectate among these studies, high variability was observed in the amount of lateral spread. Similarly, although the dorsal rami were consistently stained with ESP blocks, these studies report inconsistent spread to the paravertebral space and ventral rami and limited spread anteriorly.26 27
The current investigation of the extent and mechanism of injectate spread in the lumbar region showed dorsal rami staining as was seen in thoracic ESP blocks. However, a notable difference is that the craniocaudal and lateral spread were not as extensive with lumbar ESP injection. Also, in our study, ventral rami were not affected by any of the potential pathways of dye spread: medially into the paravertebral space, laterally to the anterolateral abdominal wall muscles, or anteriorly onto the posterior abdominal wall muscles via the anterior layer of the TLF. A similar craniocaudal spread in a cadaveric model was also seen in a recently published study by De Lara González et al.28 Their cadaveric study of lumbar ESP injection found spread anterior to the transverse process in 67% of their specimens with spread to the corresponding spinal nerves in two cases (16%). In our study, we did not see any spread anterior to the transverse process. Of note, their injection site was more medial than in our study; they injected at the midpoint between lamina and the tip of the transverse process, while our injection was at the tip of the process.
Differences in the musculofascial anatomy in the lumbar versus thoracic regions of the back may explain the more localized spread of the lumbar ESP block compared with the thoracic ESP block. This includes differences in: (1) the complex, multilayered TLF, (2) the arrangement and thickness of lumbar musculature may account for the more limited spread observed in lumbar ESP blocks compared with thoracic ESP blocks (figure 4) and (3) for reduced caudad spread in the lumbar region, the iliolumbar ligament, which passes from the tip of the transverse process of the L5 vertebra to the iliac crest, forms the thickened lower border of two of the layers of the TLF and limits caudal spread.
Illustration demonstrating the anatomic relationships of the multifidus, erector spinae muscles, dorsal and ventral ramus, and thoracolumbar fascia.
In the lumbar region, the investing fascia and aponeuroses of the ES, latissimus dorsi and serratus posterior inferior muscles combine to form the TLF. The TLF is thicker and more complex in the lumbar region than in the thoracic region.29 In the thoracic region, the TLF is thin and difficult to see in this region of the back.28 Therefore, the TLF may not function to limit the spread of injectate in the thoracic region to the extent that it does in the lumbar region.
The deep back muscles are also more substantial in the lumbar region than in the thoracic region. In the lumbar spine, longissimus and multifidus are the most prominent muscles. In the thoracic spine, all three ES muscles are present but considerably thinner, with spinalis and longissimus comprising the majority of the muscle fibers and iliocostalis taking the form of a narrow fusiform muscle.30 The thinner muscle bellies in the thoracic region may permit more extensive cranial–caudal and lateral spread of the injectate compared with the lumbar region, where the injectate is concentrated within the thick musculature adjacent to the spine. Notably, Ivanusic et al hypothesize that the complex attachments of the deep back musculature to the transverse processes may explain the limited ventral dye spread in thoracic ESP injections,20 and Adhikary et al hypothesize that the intertransverse connective tissues and muscles may allow for anterior spread in some specimens.21
Our results suggest that the injectate was deposited within the deep back muscles and distributed along the planes of the TLF to neighboring muscles within the TLF complex. However, the injectate did not spread anterior to the transverse process, potentially because it was contained within the ESP by the middle layer of the TLF which attaches to the transverse processes and separates the abdominal wall musculature from the deep back musculature,31 or because the placement of the needle was medial to the attachment points of the posterior abdominal wall muscles on the lateral tips of the transverse processes. Elsharkawy et al assert that needle insertion site is more important than continuity of fascial planes, and results may vary considerably if the needle is inserted at different points along the same tissue plane.22 Aside from musculofascial anatomy, the lack of paravertebral spread could have resulted from needle placement relative to the transverse process, injectate volumes, as well as the relatively longer length of transverse processes in the lumbar vertebral levels as compared with thoracic (4–6 cm vs 2–3 cm, respectively).32 33
The extensive dye staining of the deep back muscles and dorsal ramus seen in this study provides evidence for the mechanism of motor and sensory blockade achievable with lumbar ESP block, though comparative clinical studies are needed for confirmation. The dorsal ramus divides into three nearly equal-sized branches that originate from a common point on the primary dorsal ramus: (1) a medial branch that innervates the facet joints, multifidus muscle, interspinous and supraspinatus ligaments, and skin near the midline of the back, (2) an intermediate branch that primarily innervates the longissimus muscle, and (3) a lateral branch that innervates iliocostalis and skin on the lateral back.34 As the lumbar ESP injection consistently spreads to the dorsal primary ramus, these structures would also be blocked with a lumbar ESP injection and may provide some muscular, bone and joint analgesia for lumbar spine surgery. However, our results do not support the anterior sensory analgesia reported with lumbar ESP for lumbar spine surgery, which may limit its efficacy in that surgical population.14 Furthermore, our results do not confirm spread to ventral rami, which is likely needed to provide adequate analgesia for hip surgery and lower abdominal surgery.
This anatomic study is not without limitations. The spread of dye in a cadaveric model may not replicate the spread seen in the living due to the lack of tissue tension in the cadaver. It has also been suggested that blood pooling within small vessels and tissues may prevent dye from spreading into the paravertebral space.35 Further, anterior or lateral spread may be affected by breathing and changes in intra-abdominal pressure. While the optimal volume for lumbar ESP block is unknown, several case studies have opted for larger volumes of injectate than was chosen for this study and/or catheter placement.36–39 It is possible that larger volumes of injectate may result in greater spread and this is an important area for future studies.32 In this current study, the subjects were positioned prone, but as the ESP block can be performed in the sitting position, the positioning of the patient and subsequent effect of gravity may have an effect on the spread of local anesthetic. Further, the spread of dye, rather than local anesthetic, may differ with clinical effect. While other cadaveric studies have used similar dye injections and concluded that the spread seen with dye is consistent with clinical effect,20 limited data from a patient case found extensive anterior spread from an ESP injection at L4.33
In conclusion, this cadaveric study documented limited craniocaudal spread and moderate medial-to-lateral spread with lumbar ESP injection. There was extensive dye deep and superficial to the ES musculature with dye spread to the dorsal rami in all specimens. However, there was no dye spread anteriorly into the dorsal root ganglion, ventral rami, or paravertebral space.
Acknowledgments
The authors would like to thank Catherine Raymond, Coordinator of the Mayo Clinic Arizona Center for Procedural Innovation, for her assistance with this study. They would also like to thank the individuals who donated their bodies for the advancement of science and medicine. Without their generous donation, this study would not have been possible.
References
Footnotes
Twitter @MonicaHarbellMD, @kraus_molly, @nrlangley
Contributors Study design/planning: MWH, DPS, NRL, VK, MBK. Data acquisition, analysis, interpretation, writing manuscript, revision and approval of final manuscript: all authors.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Patient consent for publication Not required.
Ethics approval This study was deemed exempt by the Mayo Clinic Institutional Review Board and was approved by the Mayo Clinic Biospecimens Subcommittee (IRB#19-008419, Bio00017871).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available upon reasonable request.