Joint injection and regional anesthesia (Proceedings)
Palmar Digital Block
The palmar digital nerves are blocked by injecting up to 2 ml of anesthetic over the nerves, along the edge of the DDFT. Much discussion has taken place regarding the proximal to distal level that the injection should occur. The PDN can be blocked anywhere from the proximal margin of the collateral cartilage to the mid pastern region. Some believe that it is important to anesthetize the nerves near the proximal margin of the collateral cartilage because blocking the PDN as far distally as possible will decrease the likelihood of anesthetizing the dorsal branches of the palmar digital nerve. The relevance may be limited because studies have shown the dorsal branches are unlikely to contribute much more than sensory innervation to the dorsal aspect of the coronary band and dorsal lamina of the foot.Performing a PDN at the mid to upper portion of the pastern may result in anesthesia of the pastern joint. Because of this, the preferred location is to block the nerves as distal as possible at the level of the collateral cartilages. The old concept that a PDN blocks the caudal ⅓ of the foot is not accurate. It has been shown that anesthesia of the palmar digital nerves just proximal to the bulbs of the heel alleviated lameness caused by endotoxin-induced pain in the DIP joint, indicating that the palmar digital nerves innervate most, if not all of the DIP joint. This report corroborated an anatomical study that demonstrated that the dorsal branches of the palmar digital nerves do not innervate the DIP joint.
Pastern Ring Block
Some clinicians prefer to do a pastern ring block. This will include the dorsal branches so it will desensitize the entire foot. It is often done in place of a PDN because a ring block after a negative response to a palmar digital nerve block is unlikely to result in a positive response. This is likely because the dorsal branches of the palmar digital nerves contribute little to sensation within the foot. The palmar digital nerve block will already have anesthetized the majority of the foot, with the exception of the dorsal portion of the coronary band and the dorsal lamina of the foot.
This block is performed by doing a midpastern PDN and placing approximately 1 ml of local anesthetic on each side of the extensor tendon.
Abaxial Sesamoid Block
An abaxial sesamoid nerve block desensitizes the foot, pastern, middle phalanx and associated soft tissues, the distal and palmar aspects of the proximal phalanx, and possibly, the palmar portion of the metacarpophalangeal joint including the sesamoid bones. Performing the nerve block at the base of the proximal sesamoid bones decreases the likelihood of partially desensitizing the metacarpophalangeal joint. Using a small volume of local anesthetic solution (i.e., < 2 ml) and directing the needle distally, rather than proximally, also decreases the likelihood of partial analgesia of the metacarpophalangeal joint. Some clinicians prefer to use a basi-sesamoid block to decrease the likelihood of blocking the fetlock joint, however the volume and needle placement are probably as important.
A low4-point blocks the medial and lateral palmar nerves and the medial and lateral palmar metacarpal nerves blocked proximal to the button of the splint. All 4 nerves must be blocked to completely desensitize the fetlock region.
The block can be done with the horse standing or by holding the leg. My preference is to do it holding the leg between my knees so there is no tension on the flexor tendons and skin. A one inch needle can be inserted dorsal to the DDFT across the limb to the subcutaneous space medially. You can inject 2 ml at that location, withdraw to the subcutaneous space laterally and inject another 2 ml. At that point, leaving the needle in the skin, but withdrawing and tenting the skin, you can direct the needle to the axial aspect of the distal lateral splint bone. The needle is then removed and inserted palmar to the medial splint bone just above the level of the button and angled between the splint and suspensory to block the medial metacarpal nerve.
It is possible to enter the palmar pouches of the fetlock if the block is done too distally. The injections should be maintained above the buttons of the splint bones. This block will completely desensitize the fetlock.