An investigation of injection techniques for local analgesia of the equine distal tarsus and proximal metatarsus.

A study was conducted to investigate certain injection techniques for local pain relief (anesthesia) in the lower ankle and upper foot (distal tarsus and proximal metatarsus) of horses. The research found that the injections were successful, but the results varied with different injection techniques.

Research Procedure

• The researchers began by injecting a positive radiographic contrast agent into the tarsometatarsal (TMT) joints of both hind limbs of 10 horses. Radiographic images were then taken at intervals of 5, 15, and 30 minutes after the injection.
• A staggering 95% success rate was observed as injections were successful in 19 out of 20 limbs. The research found that communication between two specific joints, centrodistal (CD) and TMT, was identified in 7 limbs, making up 35% of the total.
• The contrast agent extended around the tendons of tibialis cranialis (TC) and fibularis tertius (FT) in 18 limbs, and in 7 limbs, some contrast entered the tarsal sheath.
• In 13 limbs, a slight to moderate plantar (related to the sole or underside of the foot) and/or distal (away from the center of the body) extension of contrast agent was also identified.
• On a later note, the contrast agent was injected subtarsally using one of two techniques and the process of acquiring radiography was repeated.

Main Findings

• The radiographic contrast was primarily distributed on the plantar aspect of the 3rd metatarsal bone, the plantar aspect of the suspensory ligament, and between the superficial and deep digital flexor tendons.
• Indication of the extension of contrast agent into the TMT joint was found in only 1 limb, but in 8 limbs contrast agent extended into the tarsal sheath.
• The practical implications of these results suggest a possibility that when a local anesthetic solution is injected into the TMT joint, it could potentially alleviate pain from the CD joint, the insertions of TC and FT or the tarsal sheath.
• Furthermore, the solution may result in perineural analgesia of dorsal metatarsal nerves or the plantar metatarsal nerves. In some cases, subtarsal injection of local anesthetic solution might result in alleviation of pain from the tarsal sheath.
• However, there’s a potential risk of false negative results for subtarsal analgesia which could be achieved by inadvertent injection into the tarsal sheath or into a blood or lymphatic vessel.