Effect of volume for injection of the 15th to 16th thoracic interspinous space: An ex vivo study
Virginia Melly, Kara A. Brown, Julie Engiles, Holly L. Stewart, Darko Stefanovski, Kathryn W. Bills, Elizabeth DavidsonAbstract
Background
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.
Objectives
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.
Study Design
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.
Conclusion
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.