Page 575 - Adams and Stashak's Lameness in Horses, 7th Edition

Lameness of the Distal Limb 541

THE FETLOCK

VetBooks.ir Matt BRokkEn and alicia BERtonE

OSTEOCHONDRAL FRACTURES AND cannon bone) is commonly found. Horses in race train-
FRAGMENTATION OF THE PROXIMAL PHALANX ing often present with a history of lameness, which
increases after exercise, and a workout or a race may
Osteochondral fractures of the proximal aspect of the cause the horse to be markedly lame. After prolonged
first phalanx (P1) can occur in any horse used for per- rest, the horse may seem to be sound, only to go lame
formance but are particularly common in horses that again when returned to training. Horses with fractures
exercise at speed. Most fractures of this type involve the of the palmar/plantar eminence of proximal P1 will typ-
dorsal articular surface of proximal P1 in the forelimb, ically have an easily observable lameness (normally lame
medial or lateral to sagittal midline. The left forelimb at the walk) along with varying degrees of swelling/
and medial aspect of the joint are affected more often. edema associated with the fetlock region.
Chip fractures from the dorsodistal aspect of the third Some horses, particularly with chronic chip fractures,
metacarpal/metatarsal (MC/MT) bone also occur but have only a small amount of swelling or lameness to
are less common. indicate that there is a chip fracture. There may be
Other less frequently occurring fractures of P1 include fibrous enlargement on the dorsal surface of the fetlock
fractures of the lateral and medial eminences of the proxi- joint that is easily palpated. It is difficult to produce pain
mopalmar (or proximoplantar) surfaces and avulsion frac- in the affected region by digital pressure, but some heat
tures of the midproximal palmar articular margins just may be detected over the dorsal surface of the joint.
below the sesamoid bone. In addition, there are fragments Flexion of the affected fetlock often elicits pain, and a
associated with the palmar or plantar proximal aspect of fetlock flexion test usually exacerbates the lameness. If
P1 that may be traumatic or developmental in origin. the examiner is unsure of this response, it should be
compared with the opposite fetlock.
Etiology In most cases it is not necessary to use local anesthe-
sia to identify chip fractures within the fetlock. If the
Dorsoproximal P1 “chip” fractures are caused by con- examiner is suspicious that the fetlock is involved,
cussion and overextension of the fetlock joint. Typically, radiographs should be taken. However, if confusion
similar to osteochondral fractures of the dorsal aspect of exists regarding the contribution of the fetlock to the
the carpus, these fractures occur toward the end of the lameness, either intrasynovial anesthesia of the fetlock
race in racehorses. From the appearance of the fractures, it (preferred) or a low four‐point nerve block (proximal
seems that excessive overextension of the fetlock joint is to the fetlock) can be performed. Diagnostic analgesia
probably involved. Overextension places stress on the is recommended for some fragments of the palmar/
dorsoproximal aspect of P1 as it is pressed against the dis- plantar aspect of proximal P1 to determine their clini-
tal third metacarpal bone. Limb fatigue is a factor in over- cal significance.
extension of the fetlock joint, noted at the end of races
when the back of the fetlock may contact the ground (run-
ning down). The fracture most frequently occurs medial to Diagnosis
midline most likely due to the fact that the medial aspect
of proximal P1 is more prominent and extends slightly A definitive diagnosis is best made with radiographic
more proximal than its lateral counterpart. examination. A minimum of four views (five views
Fractures of the proximopalmar (or proximoplantar) including the flexed lateral) should be obtained. Oblique
eminence are also a result of trauma (most likely tor- radiographs should be taken to determine whether the
sional) and may be associated with complete or partial chip is on the medial or lateral side of the midline
tearing of the collateral ligament of the fetlock joint (Figure 4.121). For palmar/plantar fragments, oblique
with possible subluxation/luxation. 43 radiographs raised ~20° from horizontal can be helpful
The cause of proximal palmar/plantar P1 fragments is in limiting the superimposition of the sesamoids on
debatable. In most horses, the belief is that they are devel- proximal P1 (Figure 4.122). However, it is important to
opmental. Axial fragments are classified as type I and are note that the downward oblique projections may make
36
articular. Type I fragments can be associated with lame- detection of dorsal P1 fractures more difficult. It is
ness at speed with clinical signs similar to dorsoproximal important to radiograph the contralateral fetlock,
P1 fractures. Type II fragments are located abaxially and because bilateral fractures are not uncommon, and clini-
typically have minimal articular component; however, cal signs may not appear until the horse is back in train-
they can be a source of performance‐ limiting lameness. ing. 1,24,28,49,108 Ultrasonography also may be used to
diagnose chip fractures of the dorsal aspect of P1 and
Clinical Signs concurrent proliferative synovitis of the fetlock synovial
pad if present. Acute proximal eminence fractures can
Synovitis of the fetlock joint indicated by distension be difficult to appreciate on the lateral and dorsopalmar/
of the palmar/plantar pouch (between the suspensory plantar radiographic views and are best detected on
ligament [SL] and the palmar or plantar surface of the oblique projections (Figure 4.123).