Page 532 - Adams and Stashak's Lameness in Horses, 7th Edition

498 Chapter 4

commonly results in a severe pain that may not block
out with local analgesia. Thus, if the PD nerve block
VetBooks.ir that the pain may be due to solar pain primarily and that
abolishes the lameness on the affected digit, it is likely
the lamellae are stable.
In the horse with suspected chronic laminitis in which
it is unknown whether the lameness is due to laminitis
or other musculoskeletal issues, it is important to per-
form distal limb flexion prior to nerve blocks; if the
horse is severely lame, the response to flexion can be
assessed at a walk. A response to flexion usually indicates
the presence of an arthropathy unrelated to laminitis,
the most common of which is arthropathy of the proxi-
mal interphalangeal joint.
To delineate lamellar pain from proximal inter-
phalangeal joint pain in the horse that does not respond
to a PD nerve block, the clinician can assess any remain-
ing pain emanating from the foot by performing a mod- Figure 4.72. Several measurements obtained from lateral
ified ring block immediately proximal to the coronary radiographs of the digit can be used to assess horses with distal
band to block the dorsal branches of the PD nerve. displacement of the distal phalanx. a = distance from the proximal
When attempting to rule out the other common cause extensor process to the proximal aspect of the hoof wall (immedi-
of bilateral forelimb lameness, namely, navicular syn- ately distal to coronary band), b = the distance from the dorsal
drome/palmar heel pain (cases with severe degeneration parietal surface of the distal phalanx to the dorsal surface of the
of the navicular bone can mimic the lameness of a mod- hoof capsule, b/c = the ratio of the distance from the dorsal parietal
erately severe case of chronic laminitis), the history, surface of the distal phalanx to the dorsal surface of the hoof
presentation, and radiographs are valuable in addition capsule (b) to the length of the palmar cortex of the distal phalanx
(c), and d = the distance from the dorsodistal tip of the distal
to the response to nerve blocks. Because PD nerve anes- phalanx to the ground surface of the sole.
thesia can block both palmar heel pain and solar pain
and likely a great deal of lamellar pain, radiographs
58
should be used to help differentiate between them. As distance between the dorsal surface of the hoof capsule
with any type of severe musculoskeletal pathology, the and the parietal surface of the distal phalanx is best
lameness may not block out entirely with local perineu- measured as the shortest distance between the two
ral anesthesia in some severely painful cases. immediately distal to the base of the extensor process (in
Figure 4.72b) to limit the effect of any rotation on the
measurement. This measurement is most valuable when
Radiography
pre‐disease radiographs are available and then it is a
Radiography is not only critical as a diagnostic tool to relatively reliable indicator of displacement in horses in
determine the presence of the disease but is also critical to which the distal phalanx has undergone distal displace-
monitor progress of the disease and guide treatment. ment. In normal horses (approximately 450 kg), the dis-
Radiographs should be taken at the first sign of acute lam- tance should be less than 18–20 mm (reported means of
initis to serve as a baseline for subsequent radiographic 14.6–16.3 mm). With digital radiographs it is possible to
comparisons and determine if radiographic changes sug- identify two layers of the hoof wall, the outer being
gestive of previous laminitis are present. The most impor- more radiodense than the inner layer. In horses with dis-
tant views are the lateral and the dorsopalmar/plantar tal displacement, the inner layer increases in thickness,
projections (Figures 4.67, 4.72, and 4.73). For both of while the outer layer is unaffected, at least initially.
these projections, it is important that the foot is placed on Therefore, an increase in the ratio of the inner layer to
a block and that the X‐ray beam is centered as close as the entire thickness of the wall should also increase with
possible to the solar margin of the distal phalanx (approx- distal displacement and is independent of the size of the
imately 1.5 cm proximal to the surface of the block). horse. The normal ratio is approximately 40%. 36
Early radiographic signs suggestive of distal displace- Additionally, an accurate measurement of the dorsal
ment of the distal phalanx include: 1) increased dorsal hoof wall thickness that takes into account the magnifi-
hoof wall thickness (e.g. widening of the distance between cation and the size of the foot is the ratio between this
the dorsal surface of the hoof wall and the parietal sur- dorsal measurement and the palmar cortical length of
face of the distal phalanx) and 2) increased vertical dis- the distal phalanx measured from the dorsodistal tip of
14
tance from the proximal aspect of the extensor process to the bone to its articulation with the navicular bone (in
the firm proximal border of the hoof wall located imme- Figure 4.72c). This ratio, which should be less than 28%
diately distal to coronary band (sometimes termed the in the normal horse, indicates possible distal displace-
founder surface of the hoof capsule). It is important to ment from 28% to 32% and indicates likely displace-
ensure that the lateral radiographic view is a true lateral; ment if greater than 32%. The vertical distance from
rotation of the axis of the foot by more than 10° causes the extensor process to the firm proximal border of
the degree of rotation to be underestimated. 40 the hoof wall is also well used and is –2 to 10 mm in
14
A radiopaque object or paste can be applied to the normal horses, depending on size (Figure 4.72a). Due
mid‐dorsal hoof wall and should end at the level of to the variability of this distance (–2 to 10 mm) in
the coronary band to help identify it (Figure 4.72). The normal horses, it is most valuable to compare this