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Effect of volume for injection of the 15th to 16th thoracic interspinous space: An ex vivo study.
Abstract: Impinging (overriding) spinous processes (ISPs) are commonly identified in horses with back pain. Diagnostic analgesia is widely used as the clinical reference standard for confirming association with clinical signs of back pain, but the specificity of dorsal spinous region diagnostic analgesia remains poorly understood. Objective: To evaluate the specificity of a midline thoracic interspinous space injection in cadaveric horses using three injectate volumes, assessed by computed tomography (CT) and anatomical dissection. Methods: Ex vivo experimental study. Methods: The T15-T16 interspinous space of isolated thoracolumbar segments from 10 cadaver horses was injected using a dorsal midline approach with 5, 10, or 20 mL of a 3:1 iohexol:methylene blue solution under radiographic guidance. CT was performed post-injection to assess contrast distribution, and specimens were serially sectioned to evaluate methylene blue distribution. Bayesian linear regression was used to assess the relationship between injectate volume and spread distance. Results: Injectate was identified in the interspinous space in six specimens. Contrast injectate distribution to ≥1 thoracic articular process joint occurred in nine specimens and ≥1 intervertebral foramen in all specimens. Injectate entered the epidural space in two specimens (10 and 20 mL injectate volumes). Larger injectate volume was associated with greater craniocaudal spread (Bayesian linear regression: +4.094 mm/mL, 95% credible interval [CrI] 0.038-8.399), but not dorsoventral spread. Conclusions: Dorsal midline injection at T15-T16 is a technique with poor specificity for targeting the interspinous ligament and frequently results in periarticular and intervertebral foramen distribution, placing injectate in proximity to adjacent articular and neurovascular structures. This may lead to misinterpretation of clinical results after injection and potential complications. Craniocaudal injectate spread increased with larger injection volume.
© 2026 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
Publication Date: 2026-09-30 PubMed ID: 42813955DOI: 10.1002/evj.70341Google Scholar: Lookup The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
Summary
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Overview
- This study investigated how different volumes of an injected solution spread when injected into the space between the 15th and 16th thoracic vertebrae in horse cadavers.
- The goal was to determine the specificity and potential risks of this injection technique commonly used in diagnosing back pain in horses.
Background
- Horses with back pain often show impinging or overriding spinous processes (ISPs), which are spinal bony projections that press against each other.
- Diagnostic analgesia—injecting anesthetic agents into specific spine areas—is used to confirm whether these ISPs cause pain.
- The dorsal spinous region’s diagnostic injections are clinically common but have unclear specificity, meaning injected substances may affect more than just the intended tissues.
Objective
- The study aimed to evaluate how specifically injections targeted the interspinous ligament at the T15-T16 thoracic vertebral level.
- Researchers analyzed the distribution of different volumes of injected solution using CT imaging and tissue dissection in horse cadaver spines.
Methods
- Ex vivo study was conducted on thoracolumbar spine segments from 10 horse cadavers.
- Researchers performed dorsal midline injections into the space between the 15th and 16th thoracic vertebrae (T15-T16) using volumes of 5, 10, or 20 mL.
- The injectate was a 3:1 mixture of iohexol (a radiocontrast agent) and methylene blue dye to visualize spread.
- Radiographic guidance ensured accurate injection placement.
- Post-injection, CT scans assessed contrast agent spread in three dimensions.
- The injected specimens were serially sectioned anatomically to observe methylene blue distribution within tissues.
- Bayesian linear regression modeling evaluated the relationship between injectate volume and how far the solution spread along and across the spine.
Key Findings
- The injectate stayed within the interspinous space in only 6 of 10 specimens, indicating imperfect targeting.
- In 9 specimens, the contrast agent reached at least one thoracic articular process joint, areas adjacent to the interspinous space involved in joint movement.
- All specimens showed injectate spreading into at least one intervertebral foramen, openings where spinal nerves exit the spinal canal.
- In two specimens (injected with 10 and 20 mL), injectate entered the epidural space, which surrounds the spinal cord and nerves.
- Increasing the injection volume was statistically associated with a greater craniocaudal (head-to-tail) spread of injectate by approximately 4 mm per milliliter.
- No significant relationship was found between volume and dorsoventral (back-to-front) spread.
Interpretations and Implications
- The dorsal midline injection at T15-T16 has poor specificity for targeting only the interspinous ligament.
- Injected substances often spread beyond the intended site to adjacent joint areas and nerve exit foramina, which may affect nearby nerves and blood vessels.
- This lack of specificity can cause diagnostic confusion if pain relief is observed, as the injected solution might anesthetize other structures, not just the interspinous ligament.
- There is also potential for complications, such as unintended epidural injection, especially with larger volumes.
- Clinicians should be cautious with injectate volume, balancing diagnostic sensitivity with the risk of off-target spread.
Conclusions
- The study highlights limitations of the dorsal midline interspinous injection technique in horses for diagnosing back pain related to impinging spinous processes.
- Using smaller volumes may reduce undesirable spread but targeting remains inherently imprecise.
- Further refinement of injection techniques or imaging guidance may improve diagnostic accuracy and safety.
Cite This Article
APA
Melly V, Brown KA, Engiles J, Stewart HL, Stefanovski D, Bills KW, Davidson E.
(2026).
Effect of volume for injection of the 15th to 16th thoracic interspinous space: An ex vivo study.
Equine Vet J.
https://doi.org/10.1002/evj.70341 Publication
Researcher Affiliations
- Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan, USA.
- Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
- Department of Pathobiology, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
- Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
- Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
- Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
- Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, Pennsylvania, USA.
Grant Funding
- 580-5805-1-400666-XXXX-2000-5318 / Firestone/Tamworth/Raker-Tulleners Grant from the Department of Clinical Studies, New Bolton Center, University of Pennsylvania
- 400319 / Allam Center for Equine Sports Endowment Fund from the Department of Clinical Studies New Bolton Center, University of Pennsylvania
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