In vivo diffusion characteristics following perineural injection of the deep branch of the lateral plantar nerve with mepivacaine or iohexol in horses.

This study involves investigating the diffusion of contrast medium and mepivacaine – a local anesthetic – after perineural injections in the deep branch of the lateral plantar nerve (DBLPN), located in the hind limb of horses. The research aimed to understand whether these substances inadvertently affect the tarsal sheath and tarsometatarsal (TMT) joint, adding to the knowledge of pain management in horses.

Investigation Method

• The researchers conducted an in vivo experiment involving the perineural injection of a horse’s DBLPN.
• 16 limbs were injected with 3 ml of either a mepivacaine solution or a contrast medium which could be observed under X-ray.
• Certain limbs which were injected with the contrast medium were X-rayed 5, 15, and 30 minutes post-injection and their diffusion characteristics were recorded.
• In those limbs injected with mepivacaine, synovial fluid was extracted from the TMT joint 10 and 20 minutes post-injection, and the concentrations of mepivacaine were analyzed.

Results of the Study

• At 5, 15, and 30 minutes after injection, the average extent of contrast medium dispersion was 19.6, 20.6, and 21.0 mm above the injection site and 38.0, 43.5, and 51.9 mm below, respectively.
• Contrast medium was found in the tarsal sheath of 3 of 8 limbs (37.5%).
• In 25% of the limbs, mepivacaine concentrations sufficient to provide analgesia (>300 mg/l) were found in the TMT joint 10 minutes after injection.

Conclusions

• The contrast medium dispersed further below the injection site than above.
• The study found that analgesia of the DBLPN can unintentionally involve the tarsal sheath and/or the TMT joint. This points to the need for more specific techniques to avoid unwanted side effects when administering pain relief injections for horses.