Page 194 - Adams and Stashak's Lameness in Horses, 7th Edition

160 Chapter 2

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B
A2
A1

Figure 2.150. Needle location to perform an abaxial sesamoid
nerve block in the forelimb.

Figure 2.149. Injection sites for local anesthesia. (A1 and A2) success of the block. While it is common to find that
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Sites for the pastern ring block. (B) Site for the digital nerve block at skin sensation still exists over the dorsal surface of the
the base of the sesamoid bones (basisesamoid block).
phalanges, this does not mean that the phalanges and all
the deep structures are not desensitized.
Basisesamoid or High PD Block
Low Palmar Block (Low 4‐Point Block)
An alternative to the pastern ring block is the basis­
esamoid block. This block is performed similarly to the The low palmar block is often referred to as the low 4‐
PD block, only it is more proximal on the limb at the point block because both palmar and palmar metacarpal
base of the proximal sesamoid bones (often referred to nerves are anesthetized at the distal aspect of the metacarpus/
as a high PD block). The PD nerves can be palpated at metatarsus (Figure 2.151; Video 2.11). The lateral and
this location, and 1.5–2 mL of anesthetic is deposited medial palmar nerves lie between the suspensory ligament
directly over the nerves using a 25‐gauge, 5/8‐inch (1.5‐ and the DDFT. Since they assume a vein–artery–nerve rela­
cm) needle (Figure 2.149; Video 2.8). The basisesamoid tionship, these nerves are located closer to the DDFT and lie
block will desensitize the dorsal branch and the PD nerve on its dorsal edge. These nerves are relatively deep but
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at a more proximal location in the pastern. This block can be reached in most cases with a 5/8‐inch (1.5‐cm),
desensitizes the palmar/plantar soft tissue structures of 25‐gauge needle (a 1‐inch, 22‐gauge needle may also be
the pastern, PIP joint, and all structures of the foot. used), after which 2–3 mL of local anesthetic is deposited. It
Because it is performed at the base of the sesamoid is best to perform these blocks 1 cm proximal to the distal
bones, it is unlikely to desensitize any of the fetlock joint. ends of the splint bones to avoid injection into the digital
flexor tendon sheath. 34,48,68
Blocking the palmar nerves alone will not completely
Abaxial Sesamoid Block
desensitize the fetlock joint. Two additional nerves, the
The neurovascular bundle is easily palpable along the medial and lateral palmar metacarpal nerves, innervate
abaxial surface of the proximal sesamoid bone. With the the deep structures of the fetlock. 56,75 These nerves
limb elevated and holding the fetlock in the palm of the course parallel and axial to the second and fourth meta­
hand, isolate the palmar nerve by rolling it away from carpal bones. A 5/8‐inch (1.5‐cm), 25‐gauge needle or a
the artery and vein with the thumb or forefinger. A 5/8‐ 1‐inch (2.5‐cm), 22‐gauge needle is used to inject 2–3 mL
inch (1.5‐cm), 25‐gauge needle is used to inject 1.5–2 mL of anesthetic around these nerves as they emerge from
of anesthetic perineurally (Figure 2.150; Video 2.9). the distal edges of the second and fourth metacarpal
Alternatively, the block can be performed with the horse bones (Figure 2.151b). However, because the palmar
standing (Video 2.10). It is best to use a small volume of pouches of the fetlock joint can be inadvertently entered
anesthetic and direct the needle distally to avoid partial at this location, these nerves can also be anesthetized
desensitization of the fetlock joint. The biaxial block more proximally. 48,68
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desensitizes the foot, middle phalanx, PIP joint, disto­ Both the palmar and palmar metacarpal nerve blocks
palmar aspects of proximal phalanx, distal portions of can be performed while the horse is bearing full weight
the SDFT and DDFT, distal sesamoidean ligaments, and or the limb can be held with the opposite hand. It is usu­
digital annular ligament. 48,66 Loss of skin sensation at the ally easier to perform in the standing position. Anesthesia
coronary band in the toe region together with loss of skin of these four nerves effectively desensitizes the deep
sensation on the palmar pastern is used to determine the structures of the fetlock region and all structures