Page 180 - Anatomy and Physiology of Farm Animals, 8th Edition

The Equine Foot and Passive Stay Apparatus / 165

tendons of insertion, including the lacer- the distal joints without muscular effort
because they both feature accessory liga-
tus fibrosus, a substantial band that con-
VetBooks.ir tinues into the deep fascia (epimysium) of ments, ligamentous connections between
more proximal bones and the insertions of
the m. extensor carpi radialis. This feature
of the m. biceps brachii creates a continu- the tendons.
ous ligamentous connection from the The superficial digital flexor muscle
scapula through the length of the bra- possesses a ligamentous head that
chium, across the elbow, and to the proxi- attaches to the caudal aspect of the distal
mal radius via the biceps’ tendon of radius and joins the muscle’s tendon near
insertion, and to the proximal metacarpus the carpus. This accessory ligament of
via its connection with the tendon of the the superficial digital flexor muscle is
m. extensor carpi radialis. It is this band of commonly known as the radial or proxi-
connective tissue that will, without muscu- mal check ligament, and its presence
lar effort, resist flexion of the shoulder creates a continuous, ligamentous band
when the limb is bearing weight. This same from the radius to the insertion of the
continuous band, through its connection tendon on the proximal and middle pha-
with the m. extensor carpi radialis, langes. This provides additional support
contributes further to the stability of the to fetlock and pastern.
carpus in extension. The deep digital flexor muscle also
The elbow is surprisingly stable when features an accessory ligament, this one
fully extended and loaded. The m. triceps attaching to the caudal part of the carpal
brachii, the primary extensor of the elbow, joint capsule and blending with the mus-
contributes to this stability by maintaining cle’s tendon just distal to the carpus. This
a slight amount of tone, even when the accessory ligament of the deep digital
horse is sleeping on his feet. flexor muscle is more commonly called
The rest of the stay apparatus is primar- the carpal or distal check ligament. It
ily concerned with holding the fetlock and creates a continuous ligamentous band
to a lesser extent pastern and coffin joints that extends from the carpus to the distal
in a physiologic position. Without liga- phalanx, supporting all of the joints in
mentous support on the palmar aspect of the digit.
these joints, the fetlock would drop to the Injury to the flexor tendons produces
ground and the toe would point upward, some dropping of the fetlock as some of
with the sole off the ground when the limb the resistance to hyperextension is lost.
was loaded. If injury includes the deep digital flexor
The primary support of the fetlock and tendon, the toe is likely to come off the
pastern joints comprises the suspensory ground, as this tendon is the only one
ligament, the proximal sesamoid bones, resisting hyperextension of the coffin
and the ligaments of the proximal sesamoid joint. The most devastating injuries,
bones. Recall that these structures form however, involve the suspensory liga-
a continuous ligamentous connection ment and/or the proximal sesamoid
between the palmar aspect of the carpus bones. Fractures of the proximal sesa-
and proximal metacarpus distal to the moids are the most common of all frac-
proximal and middle phalanges (Fig. 8‐10). tures in the forelimb. If the fractures
The tendons of both digital flexor mus- are complete (transverse fractures of
cles offer additional support to the fetlock both sesamoids), or if the suspensory
and pastern joints, and the tendon of the ligament is transected, the ligament
deep digital flexor muscle resists hyperex- loses its connections to the phalanges
tension of the coffin joint. Remember, and the fetlock will drop to the ground.
though, that muscle tends to stretch easily Such an injury is often irreparable and
and would tire quickly without ligamentous may necessitate euthanasia of the horse
modifications. The digital flexors can support (Fig. 8‐13).