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Original research
Anatomy of the thoracic paravertebral space: 3D micro-CT findings and their clinical implications for nerve blockade
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  1. Tae-Hyeon Cho1,
  2. Shin Hyung Kim2,
  3. Jehoon O1,
  4. Hyun-Jin Kwon1,
  5. Ki Wook Kim2 and
  6. Hun-Mu Yang1,3
  1. 1Department of Anatomy, Yonsei University College of Medicine, Seoul, Korea (the Republic of)
  2. 2Department of Anesthesiology and Pain Medicine, Yonsei University College of Medicine, Seoul, Korea (the Republic of)
  3. 3Surgical Anatomy Education Centre, Yonsei University College of Medicine, Seoul, Korea (the Republic of)
  1. Correspondence to Dr Hun-Mu Yang, Department of Anatomy, Yonsei University College of Medicine, Seodaemun-gu 03772, Korea (the Republic of); yanghm{at}yuhs.ac

Abstract

Background A precise anatomical understanding of the thoracic paravertebral space (TPVS) is essential to understanding how an injection outside this space can result in paravertebral spread. Therefore, we aimed to clarify the three-dimensional (3D) structures of the TPVS and adjacent tissues using micro-CT, and investigate the potential routes for nerve blockade in this area.

Methods Eleven embalmed cadavers were used in this study. Micro-CT images of the TPVS were acquired after phosphotungstic acid preparation at the mid-thoracic region. The TPVS was examined meticulously based on its 3D topography.

Results Micro-CT images clearly showed the serial topography of the TPVS and its adjacent spaces. First, the TPVS was a very narrow space with the posterior intercostal vessels very close to the pleura. Second, the superior costotransverse ligament (SCTL) incompletely formed the posterior wall of the TPVS between the internal intercostal membrane and vertebral body. Third, the retro-SCTL space broadly communicated with the TPVS via slits, costotransverse space, intervertebral foramen, and erector spinae compartment. Fourth, the costotransverse space was intersegmentally connected to the adjacent retro-SCTL space.

Conclusions A non-destructive, multi-sectional approach using 3D micro-CT more comprehensively demonstrated the real topography of the intricate TPVS than previous cadaver studies. The posterior boundary and connectivity of the TPVS provides an anatomical rationale for the notion that paravertebral spread can be achieved with an injection outside this space.

  • regional anesthesia
  • pain management
  • nerve block

Data availability statement

Data are available upon reasonable request. Deidentified data will be made available upon reasonable written request submitted to the corresponding author.

https://doi.org/10.1136/rapm-2021-102588

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    Introduction

    Thoracic paravertebral block is a well-established technique for regional anesthesia of the hemithorax in patients undergoing breast and thoracic surgeries.1 2 However, there remains a potential risk of pneumothorax or unintentional neuraxial injection.3 4 More superficial needle placement techniques with improved safety and feasibility, such as the erector spinae plane (ESP) block, have been introduced as potential alternatives to the conventional thoracic paravertebral block.5 6 Recently, several approaches with a deeper target point than that for the ESP block, but not piercing the superior costotransverse ligament (SCTL), have been reported, with anatomical evidence of paravertebral spread and clinical effectiveness.7–9

    The concept of the thoracic paravertebral space (TPVS) was shaped by clinical needs and has not been fully elucidated as a clearly delineated space from an anatomical perspective.7 Specifically, although understanding its anatomy in terms of the posterior boundary and connectivity are crucial to understanding the action mechanism of recently descripted block techniques, there is a lack of detailed information regarding these points.

    Manual dissection and histological observation are common methods in the investigation of human tissues. However, manual dissection can damage delicate structures during processing, and histological observation provides limited information through cross-sectional imaging. Micro-CT, as a non-destructive morphological method, is an effective tool for obtaining three-dimensional (3D) information with high resolution. Our previous studies using 3D micro-CT successfully demonstrated specific compartments within various complex structures of human tissue.10–12

    Therefore, we aimed to clarify the 3D structures of the TPVS, and adjacent tissues using micro-CT and investigate the potential routes for nerve blockade in this area.

    Methods

    Sample recruitment and dissection

    A total of 11 embalmed cadavers (mean age: 82.8 years, 6 men and 5 women) were used for conventional dissection and micro-CT imaging analysis in this study. All cadavers had no deformity, trauma, and operative procedures in the thoracic spine region. Two experienced anatomists (T-HC and H-JK) dissected the thoracic spine region and examined the TPVS and its adjacent structures. Five cadavers were used to extensively investigate the TPVS and its adjacent structures using manual dissection. For micro-CT imaging, the TPVS tissue blocks of the T4-5 level were harvested from the six remaining cadavers. Conventional macroscopic examination and a 3D topographic analysis of the micro-CT images were performed to acquire following information: (1) 3D topography of the TPVS, including its detailed boundaries at the mid-thoracic level at the 4th and 5th rib levels; (2) the neurovascular contents within the TPVS and adjacent spaces; and (3) the spatial connections allowing intersegmental spreading between adjacent TPVSs. For one cadaver, a Venue Go ultrasound unit (GE Healthcare, Chicago, Illinois, USA) was used, with a high-frequency linear probe (L4-12t-RS, 4.2–13.0 MHz).

    Micro-CT preparation with phosphotungstic acid and 3D visualization

    Our micro-CT preparation technique for enhancing soft tissue contrast was used.10–12 Briefly, to enhance the soft tissue contrast, TPVS tissue blocks were soaked in 1% phosphotungstic acid solution with 70% ethanol after serial dehydration (30%, 50%, and 70% for one night each). Micro-CT images were acquired using a micro-CT scanner (Skyscan 1173, Bruker, Kontich, Belgium), with a source voltage of 70 kV, source current of 114 μA, and image pixel of 20 μm2. For a 3D visualization and analysis, the acquired section images were reconstructed using NRecon, CTVox (Bruker, Kontich, Belgium) and Mimics Research V.20.0 software (Materialise NV, Leuven, Belgium). Two representative 3D images of the TPVS are provided in online supplemental videos 1 and 2.

    Supplementary video

    Supplementary video

    Results

    A total of 11 TPVS tissue blocks of the T4-5 level were finally analyzed using micro-CT imaging method in this study. The topography of the TPVS was very typical in all cases. Thus, the locations of the borders of the TPVS, connections among the spaces, and proceeding pattern of the spinal nerve branches can be described as follows.

    Topography of the TPVS

    There were several anatomical differences between the conventional TPVS concept (figure 1A) and the present study findings (figure 1B). The TPVS was vertically located at the intercostal levels. The posterior wall of the TPVS was mostly formed by the internal intercostal membrane and SCTL (figure 1B; online supplemental figure 5). The SCTL incompletely constituted a part of the posterior wall between the internal intercostal membrane and vertebral body (figure 2; online supplemental figure 1). In detail, the anterior layer of the SCTL ascended laterally and attached to the costal tubercle (figure 3; online supplemental figure 2). The SCTL was located very close to the pleura; the mean distance between the pleura and the attachment of the SCTL was 7.8 mm (SD: 0.7 mm). Two slits in the SCTL were observed, located medially to costovertebral joint and laterally to the internal intercostal membrane (figures 2B and 3B; online supplemental figures 1B,2B and 5; online supplemental video 1).

    Supplemental material

    Supplemental material

    Supplemental material

    Figure 1
    Figure 1

    Territories of the thoracic paravertebral space (TPVS) based on the previous concept (A) and the present findings (B). The present findings show that the posterior border of the TPVS largely comprises the internal intercostal membrane (IIM) and superior costotransverse ligament (SCTL). The retro-SCTL space was partially open to the intervertebral foramen and directly connected to the TPVS via the medial and lateral slits (M and L, respectively). The posterior rami of the spinal nerve (Po) passed through the retro-SCTL space, proceeding toward the erector spinae compartment. ICN, intercostal nerve; IICM, internal intercostal muscle; ITL, intertransverse ligament; SG, sympathetic ganglion; TP, transverse process of vertebra.

    Figure 2
    Figure 2

    Micro-CT images of the thoracic paravertebral space (TPVS) at the mid-thoracic levels (4th and 5th ribs) in a sagittal section (A). Cross sections are shown at the levels of the TPVS (B) and the costotransverse space (C). An illustration (D) indicating the individual spaces on a sagittal section. The blueish and green colored regions indicate the TPVS and retro-superior costotransverse ligament (SCTL) space, respectively. The SCTL and intertransverse ligament are emphasized by blue coloring. The asterisks (*) indicate the costotransverse space. Uncolored raw micro-CT images are presented in online supplemental figure 1. A, anterior; CT, costotransverse joint; CV, costovertebral joint; DRG, dorsal root ganglion; ICN, intercostal nerve; IIM, internal intercostal membrane; L, lateral slit; LC, levatores costarum; M, medial slit; S, superior; TP, transverse process of vertebra; VB, vertebral body.

    Figure 3
    Figure 3

    Posterior intercostal artery (PIA) and posterior intercostal vein (PIV), and intercostal nerve (ICN), within the thoracic paravertebral space (TPVS) are shown. The neurovascular bundle located beneath the pleura is shown (A). The medial and lateral slits (M and L, respectively) are clearly revealed after removal of the pleura and vessels (B). Uncolored raw micro-CT images are presented in online supplemental figure 2. IIM, internal intercostal membrane; L, lateral; RL, radiate ligament of the costovertebral joint; S, superior; SCTL, superior costotransverse ligament; SG, sympathetic ganglion.

    Neurovascular bundles of the TPVS

    The intervertebral foramina were located in a similar anteroposterior position, or anterior to the SCTL. Although the nomenclature, “para”vertebral, indicates that this space might mostly comprise the nervous component of the spinal nerve, the intervertebral foramen did not completely open to the TPVS. Instead, the medial base of the TPVS was mostly formed by the vertebral body. Only the anterior rami of the spinal nerve and sympathetic trunk emerged at the TPVS and the posterior rami of the spinal nerve directly proceeded into the space posterior to the SCTL and erector spinae compartment. Especially, the intercostal nerves entered the TPVS directly via the medial slit (figure 2B; online supplemental figure 1B) or indirectly after piercing the SCTL (figure 3B; online supplemental figure 2B). The sympathetic trunk was located within the TPVS, beneath the pleura (figure 3A; online supplemental figure 2A). Very tiny fibers of white and gray communicantes connected between the trunk and anterior rami within the TPVS.

    The vessels within the TPVS were observed as very close to the pleura (figures 3A and 4B; online supplemental figures 2A and 3; online supplemental video 2). The posterior intercostal artery entered the TPVS, and its branch proceeded through the lateral slit after a bifurcation on the SCTL. The posterior intercostal artery was accompanied by the posterior intercostal vein at its upper location. The posterior intercostal vessels continued laterally within the TPVS, beneath the pleura, midway between adjacent ribs (online supplemental video 2).

    Supplemental material

    Figure 4
    Figure 4

    Comparison between an ultrasound image (A) and the corresponding micro-CT image (B) of the thoracic paravertebral space (TPVS) and retro-superior costotransverse ligament (SCTL) space (RS). Arrowheads indicate the SCTL. The posterior intercostal artery and vein, and intercostal nerve (colored as red, blue, and yellow, respectively), are clearly observable only on the micro-CT image. The posterior rami of the spinal nerve passed through the retro-SCTL space, proceeding toward the erector spinae compartment (ES). Uncolored raw micro-CT images are presented in online supplemental figure 3. I, inferior; P, posterior; TP, transverse process of vertebra.

    Retro-SCTL space

    We observed a potential spreading space, which we termed “retro-SCTL space,” broadly connecting to the TPVS, intervertebral foramen, and erector spinae compartment (online supplemental video 1). Previously, the space posterior to the SCTL was considered a part of the retrolaminar space, preserving the erector spinae compartment (figure 1A). However, this retro-SCTL space was not situated at the retrolaminar (behind the lamina of the vertebra) position (figure 2B; online supplemental figure 1B). Although it was a continuum of the erector spinae compartment, there was only loose connective tissue in the retro-SCTL space (online supplemental figure 4). This space was partially open to the intervertebral foramen and directly connected to the TPVS via the medial and lateral slits of SCTL. The intercostal membrane was partially merged with the levatores costarum at the lateral slit position. The posterior rami of the spinal nerve passed through the retro-SCTL space, proceeding toward the erector spinae compartment. Moreover, the proximal part of the anterior rami was situated in this space.

    Supplemental material

    The ultrasound image of the posterior thoracic region corresponded to the micro-CT sagittal image (figure 4; online supplemental figure 3). On ultrasound, the SCTL demarcating the boundaries between the TPVS and retro-SCTL space could be distinguished, and there was an intensity difference between the retro-SCTL space and erector spinae compartment.

    Costotransverse space

    A topographic conduit for vertical spreading was clearly verified in the present study. It was previously known that there was ligamentous tissue (costotransverse ligament) between the rib and transverse process. However, in the present study, this costotransverse space was very clear, comprizing loose connective and adipose tissues, with a width of 3.1 mm (SD: 1.1 mm) (figure 2C; online supplemental figure 1C; online supplemental videos 1 and 2). The costotransverse space seemed to provide a distinct conduit between adjacent TPVS and retro-SCTL spaces.

    Some fibers of the SCTL (previously known as the posterior lamina or layer) crossed the retro-SCTL space and attached to the lower surface of the transverse process (figure 2A). Thus, the SCTL formed an incomplete ligamentous barrier between the retro-SCTL space or the TPVS and the costotransverse space.

    Interligamentary space between the SCTL and intertransverse ligament (ITL)

    At the retro-SCTL space, the ITL was located posterior to the SCTL (figure 5). The ITL was a narrow distinct ligament that attached to adjacent tubercles. The ITL was distinctly separated from the SCTL; thus, there was a distinct interligamentary space between them. Some twigs of the posterior rami of the spinal nerve passed through this space and proceeded laterally.

    Figure 5
    Figure 5

    Intertransverse ligament (ITL) and the posterior rami of the spinal nerve in dissection (A) and micro-CT (B) image, viewed from behind. Arrowheads indicate the twigs of the posterior rami of the spinal nerve. M, medial; S, superior; SCTL, superior costotransverse ligament; TP, transverse process of vertebra.

    Discussion

    The current study examined the 3D topography of the TPVS and revealed more specific information regarding its anatomy. The posterior border of the TPVS was incompletely formed by the SCTL and, in all cases, the SCTL had slits that were connected with the TPVS. The retro-SCTL space contained the posterior rami of the spinal nerve and broadly connected to the TPVS via the slits of the SCTL, costotransverse space, intervertebral foramen, and erector spinae compartment.

    The posterior boundary and connectivity of the TPVS have clinical significance for nerve blockade in this space. Our study confirms that anatomically the TPVS is not an entirely closed space, and its posterior boundary does not comprise a simple structure. In fact, this finding was already described in previously published studies, although conclusive anatomical evidence has not been sufficient. There have been many anatomical illustrations describing this space so far. Among them, a previous illustration of the TPVS by Boezaart et al1 2 included all of our current findings. The internal intercostal membrane largely constituted the posterior border of the TPVS, along with the rib and SCTL. This finding suggests permeability to the TPVS, as well as the volume of the TPVS itself, which seems to be considerably changeable in human subjects, with dynamic respiratory movement. The SCTL is the obvious barrier that must be passed to spread to the TPVS. The SCTL was a distinctly robust ligament, which may restrict injectate spread, but the slits at the medial and lateral ends can act as channels to the TPVS, as shown in the current study. The retro-SCTL space, between the SCTL and erector spinae compartment, is not an empty space, but is filled with muscles, adipose, and loose connective tissue complexes. This space was previously described in a valid study by Krediet et al13 along with histological findings. Our 3D micro-CT imaging demonstrated that this space contains the posterior rami of the spinal nerve, and the proximal part of the anterior rami was situated in this space. Additionally, this space was partially open to the intervertebral foramen and directly connected to TPVS via the medial and lateral slits of the SCTL. Moreover, the costotransverse space vertically provides a potential conduit between the retro-SCTL space and the TPVS at adjacent levels.

    The present study findings suggest the retro-SCTL space as a potential space for paravertebral spread. Indeed, this space has been described as a new target space in several recent studies.7–9 Using the midpoint transverse process to the pleura block has been shown to result in extensive paravertebral spread with sympathetic chain involvement, and has demonstrated successful postoperative analgesia in breast surgical procedures.7 Also, pleural displacement and ESP spread have been observed under real-time ultrasonography.7 The costotransverse block, comprizing an injection into the thoracic intertransverse tissue complex located posterior to the SCTL, has shown paravertebral spread, with anterior rami and sympathetic trunk involvement, comparable to that with conventional paravertebral block in cadavers.8 Recent reports have described a similar approach, comprizing a local anesthetic injection to the anterior region of the ITL, immediately posterior to the junction of the SCTL and cranial transverse process.9 Despite a limited sample size, this technique resulted in good analgesic efficacy in patients who underwent breast surgery or thoracotomy.9 14 Although the above mentioned reports used slightly different anatomical nomenclatures, block names, and needle trajectories, the final target space seem to be identical to the retro-SCTL space.

    The ESP block targets the fascial plane deep to the erector spinae muscle at the tip of the transverse process5; thus, there are no doubts about its safety when compared with the conventional paravertebral block. Additionally, clinical reports support the effectiveness of the ESP block, and even its analgesic effect on visceral pain.15–17 In contrast, human cadaver studies revealed limited, or even no, paravertebral spread in the ESP block, accompanied with more predominantly posterior back muscle or fascia spread.8 18 19 It is uncertain whether ESP block and blocks targeting the retro-SCTL space can replace the conventional paravertebral block. It may be reasonable to assume that these approaches differ in terms of their specific clinical indications and risk–benefit ratio. Regarding paravertebral spread, our results suggest that an injection into the retro-SCTL space, which has more potential pathways to the TPVS, may be anatomically more advantageous than an ESP injection. However, there is currently a lack of clinical information about the effectiveness of retro-SCTL space blocks. Thus, the significance of our results should be evaluated by clinical comparisons, especially in patient subgroups requiring anterior thoracic area coverage.

    Herein, thorough and reliable 3D information of the TPVS region was provided using high-resolution micro-CT images. However, the current study has inevitable limitations as an anatomical study using cadavers, in which dimensional changes in the space due to the respiration cycle and the real-time spreading pattern of the injectate could not be validated. In the current study, we intensively examined the mid-thoracic region; comparative topography at other vertebral levels might be elucidated in a subsequent cadaveric study. Also, further study is needed to determine if there is individual variation of the TPVS due to physical factors such as height and weight.

    In conclusion, the 3D topography of the posterior boundary and connectivity of the TPVS provides an anatomical rationale supporting the notion that paravertebral spread may be achieved with an injection outside the TPVS. In particular, our results suggest that controlled clinical studies are needed to verify the retro-SCTL space as a target space for neural blockade in the thoracic region.

    Supplemental material

    Data availability statement

    Data are available upon reasonable request. Deidentified data will be made available upon reasonable written request submitted to the corresponding author.

    Ethics statements

    Ethics approval

    The Institutional Review Board approved the study for exemption from formal review (No. 4-2020-0997). All cadavers were legally donated to the Surgical Anatomy Education Centre at Yonsei University College of Medicine (No. 21-001).

    Acknowledgments

    We are grateful to the people who very nobly donated their bodies to the Surgical Anatomy Education Centre at the Yonsei University College of Medicine. Additionally, the authors wish to thank Jun Ho Kim, Jong Ho Bang, and Tae-Jun Ha for their technical support (all are staff members of the Surgical Anatomy Education Centre at the Yonsei University College of Medicine). One of the authors (JO) produced the illustrations in all figures.

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    Supplementary materials

    Footnotes

    • T-HC and SHK are co-first authors.

    • Correction notice This article has been corrected since it published Online First. The corresponding author's ORCID ID has been corrected.

    • Contributors T-HC and SHK (these authors contributed equally to this work): dissection, experiment, validation, formal analysis, writing and editing of the original manuscript. JO: data curation, three-dimensional (3D) images analysis and figure illustrations. H-JK: dissection, experiment, and 3D images analysis. KWK: experiment, data curation, and formal analysis. H-MY: project administration, conceptualization, design, supervision, formal analysis, and critical revision of the manuscript for intellectual content. All authors read and gave final approval of the version to be published.

    • Funding This work was supported by the National Research Foundation of Korea grant funded by the Korea government (MSIT) (No. 2020R1F1A1058123). No other external funding or competing interests declared.

    • Competing interests None declared.

    • Provenance and peer review Not commissioned; externally peer reviewed.