Mepivicaine is typically used for local blocks. It acts quickly and lasts ~2 hours. Lidocaine starts to act quickly but also wears off quickly (sometimes within the hour). If it wears off before the lameness is fully localized, things get very confusing. For example, if the horse improved 50% with a palmar digital block but you are still trying to get to 100%, you need that block to stay working. Bupivicaine is very long lasting (3-6 hours) and more tissue toxic. It is usually reserved for pain relief with laminitis or fractures, rather than for lameness evaluation.
Joint injections are prepped as for surgery and a fresh (unopened) bottle of local anesthetic used. Infection can occur with injections through skin that is not well prepped. All sites anywhere near joints or tendon sheaths are scrubbed as for a joint injection as sometimes the joint or tendon sheath is inadvertently pricked by the needle. Larger needles are used for joint blocks to prevent the needle breaking off in the joint when the horse bends its limb.
Perineural blocks are assessed by checking the related skin sensation with a pen or pen cap. Eg a palmar digital nerve block should numb the skin of the heel bulbs but not the skin of the dorsal coronary band. Joint blocks don’t have related skin sensation but joint fluid may be seen in the needle hub (this isn’t always true) and the injection should flow smoothly with minimal pressure. Both are checked at 10 minutes (perineural blocks can be checked at 5 minutes); joint blocks may be reassessed up to 30 minutes or longer as the local anesthetic diffuses through and out of the joint.
Horses are typically blocked without sedation as most drugs alter the gait. Twitches and good restraint are used along with careful positioning and attention to horse limbs.
Palmar digital nerves
The neurovascular bundle is a group of structures that you can “strum” in the midpastern region. Usually blocked as low in the region as possible. A 25ga needle is used to inject local anesthetic subcutaneously around the nerve. Usually the order of the structures is vein -artery – nerve, starting dorsally (nerve is most palmar). To desensitize heel region and sole, block both medial and lateral branches.
This block numbs the foot except for the dorsal laminae and dorsal coronary band. This includes the navicular bone, coffin joint, bursa, caudal heel region and sole. The caudal heel includes the impar ligament, navicular suspensory ligament, collateral cartilages, frog, digital cushion and part of the deep digital flexor tendon. The PDN block does not always fully block the collateral ligaments of the coffin joint.
Coffin (DIP) joint
- Thumbs’ width above the coronet and thumbs’ width off dorsal midline to either side : find a depression adjacent to the extensor tendon. Insert 20 ga needle either medial or laterally, angling to opposite corner of foot (eg if lateral stick, aim toward 2:00). The needle should sink into a “hole” and it should be easy to inject fluid
- On the lateral or medial aspect of the foot, one thumbs’ width above the coronet (dorsal to the collateral cartilages, aim toward the center of the foot (again, should sink into a hole and be easy to inject).
This block numbs the coffin joint, parts of the coffin bone, the coffin joint collateral ligaments, the impar ligament and the navicular bone. Over time it diffuses out to block the navicular bursa and the caudal portion of the foot (coffin bone, soft tissue structures in the heel).
Navicular bursa block
A spinal needle is inserted between the heel bulbs, parallel to the sole of the foot. Location is checked radiographically before and after injection of local anesthetic combined with contrast dye.
This block numbs the navicular bursa, the navicular bone and the caudal portions of the heel.
Pastern (PIP) joint
Very similar to coffin joint but just a little higher – find widest part of long pastern bone and go just below this area. Insert 20 ga needle just under extensor tendon, superficially into joint pouch (don’t need to get between bones). Can also inject with limb flexed : find “V” shaped notch at palmar distolateral aspect : aim needle just distal and dorsal.
This block numbs the pastern joint. With time it can diffuse out and also act as a PDN block.
Basisesamoid nerves (abaxial nerve block)
The same neurovascular bundle is readily palpable as it courses over the sesamoid bones. Nerve remains most axial. 25ga needle used for subcutaneous injection in area of nerve. Both medial and lateral branches blocked to desensitize foot and pastern.
This block numbs the foot and the palmar/plantar aspects of the pastern.
- Insert 20ga needle in lowest area of rectangle formed by splint bone proximally, cannon bone dorsally, suspensory ligament palmarly (plantarly) and sesamoid bones distally. Needle should be parallel to ground if horse is weight-bearing. Injects easily.
- Insert 20 ga needle under extensor tendon dorsally (shallow injection into joint pouch).
- Insert 20 ga needle between cannon bone and sesamoid bones when joint flexed
This block numbs the fetlock. Over time it can also act like an abaxial sesamoid block.
Fetlock joint injection – video
Low 4 point/ low volar nerve block
4 nerves to block : deep nerves course between the splint bones and the cannon bone on the axial surface of the splint bones-block just distal to the end of the splint bone using 25ga needle
Superficial nerves course between the suspensory ligament and the deep flexor tendon but can be on either side of a fascial reflection: need to fan needle (move it around) with sufficient local anesthetic to cover moderate sized area.
This block numbs the fetlock and distal cannon bone (palmar or plantar aspect).
High 4 point/ high volar nerve block
very similar to low volar block but performed above communicating branch ; can also inject all but deep medial branch by injection between distal aspect of accessory carpal bone and cannon bone.
This block numbs the fetlock and more of the cannon bone and below. It can also numb the suspensory area and distal carpus or hock joints.
Flex the carpus. Note the 4 indentations. Inject above of proximal row to block radiocarpal joint; inject above the distal row to block the intercarpal (midcarpal) joint which communicates with the more distal carpometacarpal joint.
The indentations lie on either side of the wide tendon of the extensor carpi radialis m. that runs vertically over the dorsal aspect of the carpal joint. When removing carpal chips the surgeon must work on either side of this tendon.
On the lateral aspect of leg, follow splint bone up until you reach a flat, thumb sized area (slight depression). This is the region above the caudolateral extension of the splint bone. This flat area is above the tarsometatarsal joint (find a skeleton). Insert a 1″ 22ga needle slightly distally and slightly dorsally until it sinks its full length into a hole.
This joint can also be injected medially in a very small depression on the distal aspect of the cunean tendon.
Distal intertarsal block
On medial aspect of hock, find small depression on proximal aspect of cunean tendon; insert 22ga 1″needle (see upper needle on image above).
Inject on either side of saphenous vein on dorsal aspect of leg using 20 ga needle parallel to ground
Tibial/peroneal nerve block
Tibial nerve : about 4″ above point of hock, insert 22ga needle from lateral side until point can be felt just under skin medially.
Peroneal (fibular) nerve : at same level, inject on lateral aspect of leg between muscle bellies; fan local anesthetic.
This block can numb the hock and the plantar surface of the distal limb.
Local anaesthetics for regional and intra-articular analgesia in the horse, Equine vet. Educ. (2021) 33 (3) 159-168. Review article
Injecting the stifle video