Investigation of the extent of injectate after single-level thoracic erector spinae plane injection: A cadaveric study

INTRODUCTION The thoracic erector spinae plane (ESP) block, first described in 2016, has rapidly gained acceptance as a method for providing analgesia in thoracic, breast, cardiac, and upper abdominal surgeries.[1,2] However, cadaveric evidence on its mechanism of action and dye distribution has shown inconsistent findings.[1,3-6] A major reason for this variability is methodological heterogeneity. Studies differ in embalming technique (fresh frozen, formalin, or Thiel), injectate (methylene blue, India ink, gelatin, or latex), volume, and dissection strategy (surface vs cross-sectional). Formalin cadavers are rigid, desiccated, and poorly suited for studying fascial plane spread, whereas fresh cadavers degrade rapidly and pose biohazard risks. The Thiel embalming method preserves soft tissues in near-life conditions, maintaining elasticity, natural colour, and identifiable fascial compartments.[7] Similarly, latex injection offers distinct advantages over aqueous dyes by retaining a three-dimensional imprint of neural and fascial spread.[8] This study aimed to characterise the spread pattern of thoracic ESP injections using Thiel-embalmed cadavers and latex dye, with detailed cross-sectional analysis. The objective was to delineate the pathways of cranio-caudal, paravertebral, intercostal, and epidural extension and to highlight the methodological advantages of this model. METHODS Six Thiel-embalmed cadavers (four male, two females; age 72–81 years) from the Medical University of Graz donation program were studied under institutional and Styrian legal guidelines. Therefore, no further approval was necessary for the conduct of this study. Injection technique: Cadavers were placed prone and acclimatised to 22–23°C for 4 h. Using ultrasound (Edge 2, Fuji-Sonosite, 6–13 MHz probe), T5 was identified via rib counting and visualisation of the costotransverse junction. After parasagittal alignment, structures visualised included trapezius, rhomboid, erector spinae, costotransverse junction, inter-transverse ligament, and pleura. A 22-G Stimuplex D needle was advanced in-plane (caudal to cephalic) to contact the T5 transverse process. Twenty millilitres of latex were injected deep into the erector spinae muscle: Blue (left) and green (right). The injection lifted the erector spinae anteriorly, confirming spread. Injection sites were identified/marked for subsequent cross-sections at T3, T5, and T7. Cadaver Sectioning: Cadavers were frozen at −20°C for 2 weeks to solidify latex. Horizontal sections were created with an electric band saw at T3, T5, and T7. Sections were labelled per cadaver and level, stored in trays, and examined macroscopically for dye distribution. The primary outcome was paravertebral spread (T3, T5, T7). Secondary outcomes included spread to the ESP, retrolaminar, intercostal, and epidural planes. RESULTS In all 12 injections, the needle tip was visualised, and the injectate spread lifted the erector spinae. Injectable distribution was sonographically assessed. At the marked T3, 5, and 7, the cross-sections resulted in clear, well-preserved specimens without disruption of the latex. The cross-sectional analysis resulted in the dye consistently occupying the ESP (100%). Latex in the paravertebral space was observed in 2/12 specimens, both at T5, occasionally enveloping the thoracic nerve root. No paravertebral staining occurred at T3 or T7. The lateral spread was visualised dorsal to the costotransverse junction in 83% and the rib (100%). There was no diffusion in the intercostal and epidural spaces [Table 1]. A panoramic view of the cross-section at the level of T5 illustrates the latex on each side after injection in ESP [Figure 1]. The other representative cross-sections are depicted in Figure 2a-f.Figure 1: Cross-section at the level of the 5th transverse process (T5) demonstrating green and blue latex in the left and right erector spinae plane. Bilaterally, the dye seems to travel dorsal to the erector spinae plane. SP = spinous process; ESM = erector spinae plane; SC = spinal cord; ICS = intercostal space; PVS = paravertebral space; CTJ = costotransverse junction; ESP = erector spinae plane)Figure 2: (a) Cross-section at the level of the 3rd transverse process (T3) demonstrating blue latex in the right erector spinae plane. The dye is dorsal to the costotransverse junction. Some extension in the retrolaminar area. The paravertebral area is delineated in light blue. (b) Cross-section at the level of 5th transverse process (T5) depicts the extension of blue dye in the paravertebral space, courses caudal and lateral along the erector spinae plane, and laterally dorsal to the rib. (c) Cross-section at the level of 7th transverse process (T7) illustrates dye in the paravertebral space, surrounding the ventral rami. It courses caudally and laterally in the erector spinae plane and further laterally dorsal to the rib. (d) Cross-section at the level of 3rd transverse process (T3) demonstrating green latex in the left erector spinae plane. The dye is dorsal to the costotransverse junction. Extension to the erector spinae muscle is visible. The paravertebral area is delineated in light green. (e) Cross-section at the level of the 5th transverse process (T5) depicts the green dye dorsal to the costotransverse junction with far extension dorsal to the rib. Dye traverses in the planes between the erector spinae muscle. The paravertebral space is delineated in light green. (f) Cross-section at the level of the 7th transverse process (T7) illustrates very minimal green dye dorsal to the rib. (ESM = erector spinae muscle; SC = spinal cord; ICS = intercostal space; PVS = paravertebral space; CTJ = costotransverse junction; ESP = erector spinae plane; ICNP = intercostal nerve pathway; ICN/V = intercostal nerve and vessel; CVJ = costovertebral junction)Table 1: Summary of dye spread to various areas at T3, T5, and T7 levels (left and right)DISCUSSION Thiel embalming provided life-like elasticity, minimal tissue shrinkage, and distinct fascial compartments.[6,7] Unlike formalin, which stiffens tissues and obscures fascial layers, or fresh frozen cadavers, which degrade rapidly, Thiel cadavers permit repeated dissection with near-physiological tissue compliance.[8,9] This fidelity is particularly important for fascial plane blocks like ESP, where spread depends on preserved tissue planes. The selection of injectate influences the accuracy of spread assessment. The latex dye offers durable, three-dimensional fidelity in injectate distribution, presenting a distinct advantage compared to methylene blue, India ink, or gelatin/resin, which are prone to excessive diffusion, artefacts, or fragility. Latex dye forms a three-dimensional cast within fascial and neural pathways, resisting post-injection diffusion. This allows accurate visualisation of the true extent of the spread. In contrast, methylene blue and India ink diffuse across tissues, exaggerating spread, while gelatin or resin injections are less stable and lack contrast.[10,11] Latex also permits photographic documentation and durable teaching models. Serial cross-sections were critical in confirming anterior spread, which may be underestimated by surface dissection alone. This method provided panoramic correlation of ESP injectate migration from posterior fascial compartments into the paravertebral and epidural regions, explaining the clinical efficacy of ESP blocks in providing both somatic and visceral analgesia. Cross-sections are preferred over dissections if quantitative analysis and precise mapping of injectate within specific layers are required.[11] Previous ESP cadaveric studies using formalin cadavers and aqueous dyes reported variable anterior spread.[1-6] Our findings suggest that methodological differences, particularly embalming and injectable, are major determinants of observed spread. Thoracic ESP blocks in Thiel-embalmed cadavers with latex dye demonstrate a consistent ESP diffusion and unreliable paravertebral and intercostal spread. The combination of Thiel preservation and latex injection provides superior fidelity compared with formalin cadavers and aqueous dyes, making it an optimal model for studying fascial plane blocks. However, the absence of paravertebral spread in this cadaveric model cannot be directly extrapolated to the behaviour of local anaesthetics in vivo. Local anaesthetic solutions differ from latex in terms of viscosity, diffusion dynamics, and tissue interaction, and clinical studies and imaging evidence suggest that anterior spread into the paravertebral and even epidural space can occur. Additionally, the volume of injectate plays a critical role in spread. Cadaveric and imaging studies suggest that larger volumes (e.g., 30 mL) are associated with increased cranio-caudal distribution and a higher likelihood of anterior/paravertebral extension compared with 20 mL injections.[12-14] Therefore, the results of our 20 mL latex injections should be interpreted with caution when translated to clinical practice. Our cadaver injection findings are consistent with a previous cadaver research study, indicating that the ESP block predominantly influences the dorsal rami, with limited involvement of the ventral rami.[15] CONCLUSION Understanding the precise mechanism of action of the erector spinae plane block is crucial for optimising its clinical application in regional anaesthesia. Study data availability De-identified data may be requested with reasonable justification from the authors (email to the corresponding author) and shall be shared after approval as per the authors’ institution’s policy. Disclosure of use of artificial intelligence (AI)-assistive or generative tools The AI tools or language models (LLM) have not been utilised in the manuscript, except that software has been used for grammar corrections and references. Declaration of use of permitted tools The scales, scores, figures, and tables are freely available and not copyrighted. Authors contributions SD was involved in concepts, Design, Definition of intellectual content, literature search, conduct of cases, data acquisition, data analysis, and manuscript preparation; GF was involved in Concepts, Design, Definition of intellectual content, literature search, conduct of cases, data acquisition, data analysis, and manuscript preparation; ASN was involved in Data analysis, manuscript editing, review, and approval. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.