Diffusion of contrast medium after four different techniques for analgesia of the proximal metacarpal region: an in vivo and in vitro study.

This study investigates the patterns of diffusion os a local anesthetic solution after application through four different techniques on the proximal metacarpal region of horses. Discovering these patterns can potentially provide valuable information about how to best apply analgesic techniques, and the avoidance of inadvertent penetrations.

Objectives and Methodology

• The primary objective of this study was to document the potential distribution of local anesthetic solution following four techniques used for diagnostic analgesia of the proximal metacarpal region on horses.
• In this experiment, a radiodense contrast medium, which is visible on radiographs, was injected around the lateral palmar or medial and lateral palmar metacarpal nerves in eight mature horses, using four different techniques.
• During the experiment, radiographs were captured at the 0th, 10th, and 20th-minute mark post-injection, and were later analyzed subjectively.
• An identical procedure was carried out on four cadaver horse limbs, this time with a mixture of radiodense contrast medium and methylene blue, another dye used for tracking diffusion.
• The locations of the radiodense contrast medium and methylene blue in these cadaver limbs were determined through radiography and dissection.

Results

• The results indicated that after perineural injection of the palmar metacarpal nerves, the majority of the contrast medium was distributed in an elongated pattern axial to the second and fourth metacarpal bones.
• It was also found that the carpometacarpal joint was inadvertently penetrated in half of the live limbs after injections of the palmar metacarpal nerves from both the medial and lateral sides, and in only one of the limbs when both injections were performed from the lateral side exclusively.
• Following the perineural injection of the lateral palmar nerve using a lateral approach, the contrast medium was diffusely distributed in all but one limb, in which the carpal sheath was mistakenly penetrated.
• When using a medial approach to inject the lateral palmar nerve, the contrast medium diffused proximally to the distal third of the antebrachium in five out of eight limbs.

Conclusions

• The study concluded that inadvertent penetration of the carpometacarpal joint is a common occurrence after perineural injection of the palmar metacarpal nerves, but this risk is significantly reduced if both palmar metacarpal nerves are injected using a lateral approach.
• Also, the entire palmar aspect of the carpus may be desensitised after the injection of the lateral palmar nerve using a medial approach.