Page 202 - Adams and Stashak's Lameness in Horses, 7th Edition

168 Chapter 2

restraint is recommended by the authors for all intra­ Distal Interphalangeal (DIP) Joint
synovial injections unless it is not tolerated by the The DIP joint can be entered using three dorsal
VetBooks.ir are often routinely tranquilized, but this is not usually approaches (perpendicular, parallel, or dorsolateral) and
horse. Horses being treated by intrasynovial injections
one lateral approach. These approaches are usually best
possible for injections used for diagnostic purposes.
Local anesthetic (1–2 mL) at the site of injection also performed in the standing patient, and a maximum of
66
may aid in the injection process, especially for synovial 4–6 mL of anesthesia is recommended. The dorsolat­
cavities in the proximal aspect of the limbs. In addi­ eral and dorsal parallel approaches are used most com­
tion, the smallest possible gauge needle (usually 20 monly by the author. The site of injection for the
gauge or smaller) should be used to minimize objection dorsolateral approach is 0.5 inch (1 cm) above the coro­
by the horse. nary band and 0.75–1 inch (2–3 cm) lateral (or medial)
Sterile gloves are recommended to permit careful pal­ to midline. A 1.5‐inch (3.8‐cm), 20‐gauge needle is
pation of the anatomic landmarks and to be able to inserted from a vertical position and directed distally
handle the shaft of the needle without contamination. and medially toward the center of the foot at approxi­
The injection should be done carefully but also as rap­ mately a 45° angle (Figure 2.160; Video 2.14). The nee­
idly as possible. Once the needle has penetrated the dle should enter the DIP joint capsule at the edge of the
synovial space, synovial fluid may be observed draining extensor process. If entry into the joint is uncertain, the
from the needle hub. The synovial fluid is allowed to needle can be directed at a more acute angle (more hori­
run freely until its ejection pressure is reduced to a slow zontal) to the skin and inserted until the needle contacts
26,79
drip. The syringe is then inserted on the finger‐stabilized the distal end of P2. It then is “walked” distally until
needle hub, and the anesthetic is injected as rapidly as the joint is penetrated.
possible. If synovial fluid is not observed, a small syringe Some prefer to enter the joint on the dorsal midline using
can be attached to the needle to withdraw fluid. the proximal outpouching of the DIP joint above the exten­
48,63,66
However, lack of synovial fluid does not necessarily sor process. The injection site is just above the coro­
mean that the needle is not within the synovial cavity. nary band 0.25–0.5 inches (8–12 mm) above the edge of the
To confirm correct needle placement, one can inject a hoof wall on the dorsal midline of the foot. With the dorsal
small amount of sterile solution; if there is little or no perpendicular approach, the needle is directed down­
plunger pressure, it is reasonable to assume that the nee­ ward perpendicular to the bearing surface of the foot
48
dle is within the synovial space. However, the only (Figure 2.161; Video 2.15). With the dorsal parallel
definitive method to confirm that the needle is in the approach, the needle is directed parallel or slightly down­
correct location is to obtain synovial fluid. Mepivacaine ward (hub of the needle is moved proximally) to the ground
is the anesthetic of choice for intrasynovial anesthesia to a depth of approximately 0.5 inches (12–15 mm)
because there is some evidence that it is less irritating (Figure 2.162; Video 2.16). The dorsal parallel approach is
than lidocaine after intra‐articular injection. 8,48 Guidelines usually easier to perform and is recommended by many
48,63,66
for performing diagnostic intrasynovial anesthesia, clinicians.
including suggested volumes of anesthetic to use, are The site for injection for the lateral approach is
given in Table 2.6. bounded distally by a depression along the proximal
Assessment of intrasynovial blocks is usually per­ border of the collateral cartilage approximately midway
formed 5–30 minutes after completion of the injection. between the dorsal and palmar/plantar border of P2
If there is no improvement after 30 minutes, it is unlikely (Figure 2.163; Video 2.17). A 1‐inch (2.5‐cm), 20‐gauge
that waiting longer will change the response. In one needle is directed downward at a 45° angle toward the
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study, lameness was not apparent after 5 minutes, and medial weight‐bearing hoof surface. Most horses
the improvement lasted for 55 minutes. In addition, appear to tolerate this technique very well. However, the
3
improvement in lameness within 5–8 minutes of injec­ specificity of the lateral approach is thought to be less
tion is often seen after intra‐articular injection of the than the dorsolateral approach. In one study using
DIP joint in horses with navicular disease or experimen­ cadavers and live horses, contrast material entered the
tally induced navicular bursal pain. 18,52,63 Evaluation of DIP joint in 100% of the cases injected using the dorso­
the effectiveness of the block should include repeating lateral approach and 85% of the cases in which the pal­
81
the exercise that resulted in the most significant signs of mar/plantar lateral approach was used. Importantly,
lameness and possibly re‐performing the manipulative/ with the lateral approach, only 65% of the limbs had
flexion test that made the examiner suspicious that this contrast exclusively in the DIP joint, 20% had contrast
region was involved. It is important to remember that in the digital sheath, and 5% had contrast in the subcu­
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structures superficial to the synovial cavity may retain taneous tissues. Because of this and the ease of per­
75
their sensitivity. In addition, false‐negative intrasyno­ forming any of the dorsal approaches to the DIP joint,
vial blocks have been reported but tend to be uncom­ the lateral approach is rarely used.
mon. One report identified a failure of intra‐articular Several studies have documented that injection of the
anesthesia of the radiocarpal joint to abolish lameness DIP joint with a local anesthetic is not selective for the
71
associated with chip fracture of the distal radius. joint and it will cause analgesia of the podotrochlear
7,8,17,39,
Diffusion of anesthetic to local structures, inadvertent apparatus, navicular bone, and navicular bursa.
52,61,64
anesthesia of peripheral nerves closely associated with In addition, injection of the DIP joint may cause
the synovial cavity outpouchings, and the possibility partial, and often complete, analgesia of the sole, toe,
61,62
that the injection was not in the synovial cavity should and heel regions of the foot. The analgesic effect
all be considered when assessing the response to intra­ increased with time, and 10 mL of anesthetic was more
62
synovial anesthesia. effective than 6 mL in alleviating pain in the foot.