Page 642 - Adams and Stashak's Lameness in Horses, 7th Edition

608 Chapter 5

that this level of congruity can change with fatigue, when subchondral bone damage alone occurs, synovitis
increased speed, poor racing surface, poor trimming, leading to joint effusion may not be appreciated, and the
VetBooks.ir racing surfaces, perhaps leading to fracture. Horses with small osteochondral carpal fragments are
horse may or may not be positive to carpal flexion.
possibly uncoordinated movement, and variability in
often only subtly lame with mild amounts of effusion
Subtle geometric abnormalities that may be within
these joints also could predispose these horses to frac- and soft tissue swelling.
ture. Considering the incidence of ALD of the carpus in The common characteristic of their movement is
young horses, it is likely that these subtle geometric abduction of the forelimbs, which some have hypothesized
abnormalities occur, causing problems at speed. This is a is an attempt to minimize carpal flexion. Consequently,
known occurrence in the distal third metacarpus in the this decreases hoof height during flight. Horses with
fetlock joint. Pathologic studies that have been per- osteochondral damage are often positive to carpal flex-
formed in an effort to determine the cause of these frac- ion. Horses that have complete fracture such as slab
tures have led to the observation that many are chronic fractures may have intense pain even with passive flex-
in nature in racehorses. 111 ion. Horses with fracture or fragmentation of the pal-
It is known that increased axial loading leads to mar aspect of the joints are often significantly responsive
increased dorsal compression between opposing carpal to flexion, which reflects the extensive soft tissue dam-
bones. Therefore, chronic high loads and the instantane- age that can occur in this area. In more chronic stages of
94
ous maximum loads likely lead to disease. Although osteochondral fracture, there may be physical limita-
these horses typically present with acute lameness and tions to the amount of flexion in the carpus, with or
swelling, histopathologic observations have demon- without pain.
strated a chronic disease process that not only leads to Horses with comminuted fractures of the carpus are
bone microdamage but attempts at bone remodeling often axially unstable, and effusion may be associated
and healing. 34,35,92,95,111,131 It has been shown repeatedly with subcutaneous swelling. If the horse can tolerate it,
that intense bone modeling and remodeling occur at the it is a good practice to palpate the dorsal aspect of the
sites at which osteochondral fracture occurs. It is carpal bones while flexed, because horses with osteo-
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likely, then, that some of these fractures may in fact chondral damage often demonstrate pain on palpation
occur during the peak bone remodeling phase, which of certain bones. Often, fibrous thickening of the joint
includes intense osteoclastic bone resorption. Not unlike capsule can be appreciated if a significant amount of
dorsal metacarpal disease, some of these horses may be joint capsule is involved in the fragmentation. However,
predisposed to fracture during these intense remodeling in these cases, the presence of a hygroma or extensor
periods. Therefore, it is likely that chronic repetitive sheath swelling must be ruled out.
stress can lead to a chronic pathologic process that ulti- If there are obvious signs of osteochondral fracture, it
mately leads to bone failure and demonstration of clini- is often a good idea to perform a radiographic examina-
cal signs. tion prior to performing intrasynovial analgesia because
Soft tissue fatigue that can occur within the joints with loss of pain, the horse no longer protects the limb,
includes a decrease in soft tissue support of the joint possibly leading to worsened damage. If a subtle lame-
capsule and the surrounding tendons and ligaments. ness is present or radiographs are inconclusive, then
This is compounded by the hypothesis that the hoof intra‐articular analgesia is often needed to confirm the
often impacts the ground before maximum extension site of pain. For intra‐articular analgesia of the carpal
occurs, thereby causing the joint to snap shut and lead- joints, 5–7 mL of anesthetic is injected into the joint and
ing to the assumption that these intensive, pinpoint the horse checked after 10 minutes and again at 30 min-
3,5
stresses can occur at these sites and may lead to dam- utes. Horses that have incomplete slab fractures that
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age. In addition, it has been observed that fatigue of have not broken into the joint may not respond signifi-
the flexor tendons and muscles in the caudal aspect of the cantly to intra‐articular anesthesia. The same may be
leg can lead to hyperextension, as has been observed in true of horses that suffer from subchondral bone disease
many horses at the end of the race. Because of the of the middle carpal joint. However, in these horses,
prevalence of damage that occurs dorsally, several there may be subtle preexisting lameness that prompts
investigators have hypothesized that a back‐at‐the‐knee further diagnostic imaging.
conformation can lead to increased incidence of dis-
ease. 3,103 However, Barr et al. showed that there was no Diagnostic Imaging
correlation to the incidence of carpal damage when this
conformation was evaluated. Others have observed A minimum of six radiographic images are needed to
9
that most horses in Europe are “over at the knee” and fully characterize the carpal joints, especially those with
therefore likely have a decreased incidence of this prob- small fragmentation (Figure 5.11). Most of the frag-
lem, although this has not be substantiated. 3 ments are on the very dorsal aspect of the bones; there-
fore, some practitioners recommend that oblique views
be taken more parallel with the lateromedial view to
Clinical Signs
fully characterize the damage and detect subtle lesions.
Horses with fracture in the carpus can appear with Images should be evaluated for osteochondral damage
various degrees of lameness, synovial effusion, soft tis- that can be surgically addressed, as well as osteoarthritic
sue swelling, and carpal flexion during standing. If oste- changes such as osteophyte, enthesophyte, and joint
ochondral damage is complete and enters the joint, space narrowing that can affect the prognosis for any
synovitis typically results, which leads to clinical signs of surgical outcome. Not all radiographic abnormalities
joint effusion and pain. However, on rare occasions can be clinically significant. Simon and Dyson showed