Page 193 - Adams and Stashak's Lameness in Horses, 7th Edition

Examination for Lameness 159

accurate interpretation of complete nerve desensitiza­
tion is important. Therefore, it is recommended that
VetBooks.ir perineural anesthesia was or was not successful.
multiple approaches be used before deciding whether

The Forelimb Medial Lateral
Palmar Digital (PD) Block
The medial and lateral PD nerves are located just pal­
mar to their respective artery and vein and lie along the
dorsal border of the SDFT proximal to the pastern joint B
and along the DDFT distal to the pastern joint. The
injection is done with the foot elevated in most cases.
Some prefer to stand with their backs toward the ani­
mal’s rear end while holding the hoof between their A
knees. Others prefer holding the pastern with one hand
while injecting with the other and assume either a lateral
or frontal position in relation to the limb (Figure 2.147;
Video 2.7). The PD nerves should be anesthetized just
distal to or at the proximal border of the collateral car­
tilages (Figures 2.147 and 2.148). Blocking the nerves at Figure 2.148. Injection sites for a PD nerve block. In (A), the
this location reduces the risk of anesthetizing the dorsal needle is inserted parallel to the lateral PD nerve, while in (B), the
branches of the PD nerve. 64,66 If the PD block is per­ needle enters just off midline and is inserted in the subcutaneous
formed 2–3 cm above the collateral cartilages, the prox­ tissues to approximate the medial PD nerve.
imal interphalangeal (PIP) joint is often desensitized in
65
addition to the foot. The PD nerve and neurovascular Loss of skin sensation at the coronary band in the
bundle are easily palpable at the level of the collateral heel region and loss of deep sensation between the heel
cartilage just behind the DDFT. A 25‐gauge, 5/8‐inch bulbs after 5–10 minutes are a reliable indication that
(1.5‐cm) needle is inserted into the subcutaneous tissue the block was successful. Previous hoof tester pain
48
in a proximal to distal direction over the nerve, and should also be eliminated following the block. Lameness
1–1.5 mL of local anesthetic solution is injected peri­ assessment of the horse following the block should be
neurally. 66,75 The needle is retracted slightly and redi­ performed in a similar manner as was done prior to the
rected if excessive pressure is needed to inject. Since block. In cases of many foot‐related lameness condi­
there are several tissue planes, it is advisable to inject a tions, the lameness will often shift to the opposite fore­
small amount of local anesthetic as the needle is being limb. Structures that are desensitized with a biaxial PD
withdrawn. Each nerve can be injected individually nerve block include the entire sole, the navicular appa­
(preferred by the author) or the needle can be directed ratus and soft tissues of the heel, the entire DIP joint of
across midline under the skin to inject the contralateral the forelimb, the distal portion of the DDFT, and some
nerve (Figure 2.148). of the distal sesamoidean ligaments. 21,48,63,66 Partial
desensitization of the PIP joint is also thought to occur
in some horses. 75

Pastern Ring or Semi‐ring Block
A pastern ring block is performed just above the col­
lateral cartilages of the distal phalanx at the same site as
the PD block (Figure 2.149A). A 20‐ to 22‐gauge needle
is used to inject 3–4 mL of anesthetic subcutaneously
laterally and medially from the site of the respective PD
block. The needle is directed dorsally perpendicular to
the long axis of the pastern to about the level of the
medial and lateral collateral ligaments. This will anes­
thetize the dorsal branches of the PD nerve and will
desensitize all the deep structures distal to the block.
However, because the dorsal branches of the digital
nerves are thought to contribute little to sensation
within the foot, the pastern ring block is unlikely to
improve lameness that has not been eliminated with a
PD nerve block. 21,48 The pastern ring block can be per­
Figure 2.147. This image illustrates the positioning to perform a formed with the limb in a full weight‐bearing position
PD nerve block when facing the back of the horse and holding the but is easier to perform with the foot off the ground.
limb with one hand. The needle is directed toward the hoof and is Horses tend to resent this block more than the PD or
inserted at or below the level of the collateral cartilages. basisesamoid nerve blocks.