NEW THEORY MAY HELP AVOID NAVICULAR
Michigan State University News Release March 1999
What does a horse's foot have in common with a Nike running
shoe? A lot,
according to Dr. Robert Bowker, an associate professor in the College of
Veterinary Medicine's Department of Anatomy.
Bowker has put together a new picture of equine foot physiology that suggests
vascular systems in the horse foot functions in much the same way that air-
or
gel-filled running shoes do.
"Moving liquids are the best way to dissipate energy," he said.
"That is why some of the major running shoe manufacturers market products
that contain liquids in their soles."
Bowker has theorized a "hemodynamic flow" process in which he proposes
that much of the blood in horse feet fulfills purposes other than providing
nutrients to
hoof tissues.
"It dissipates energy within feet that is created during the act of galloping,
trotting or walking," he said.
Currently, equine foot physiology researchers subscribe to one of two anatomical
theories--pressure theory or depression theory--which are, in many respects,
mirror images of each other.
"Pressure theory says that when the hoof hits the ground, the pressure
of the
impact hits the frog of the hoof, which causes the back part of the foot to
move
outward," Bowker said.
Depression theory suggests that when impact on the ground occurs, the pastern
descends and depresses the digital cushion inside the hoof.
"According to both theories, these actions push hoof cartilage to the
outside,
with the digital cushion absorbing the energy," he said. Both theories
state
that blood is pumped from the hoof at impact.
Yet both theories share a single problem. Researchers who attempt to duplicate
depression or pressure theory in the lab or on live horses are unable to do
so.
Problems arise when researchers attempt to account for how the energy of the
hoof's impact with the ground is dissipated.
"The digital cushion is made of soft, elastic tissue and acts like a
spring,"
said Bowker. "So for every action, we would expect a reaction of equal
force."
Yet when researchers put energy measurement devices into digital cushions,
that does not happen.
"When the hoof is in the air, it registers zero pressure," he said.
"But when it
hits the ground, instead of registering positive pressure, it is actually
negative."
Bowker's hemodynamic flow hypothesis suggests this negative pressure is actually
created by the outward movement of the hoof cartilage. This movement creates
a vacuum action that sucks blood from beneath the coffin into the rear portion
of the hoof.
"As the blood moves to the rear of the hoof through microvessels in the
lateral
hoof cartilage, it dissipates the energy caused by its impact on the ground,
much
like fluid-filled running shoes do," he said.
In developing this new theory, Bowker observed that horses with good feet
have more blood vessels in the lateral cartilage of their hooves than those
that had histories of foot problems.
Additionally, blood vessels in healthier animals were located inside the lateral
cartilage of the hoof, and the digital cushion on these animals tended to
be made
of cartilaginous material instead of elastic tissue.
Bowker's theory not only proposes a new physiology for horse feet, it also
suggests some of the more widely held views in the equine industry should
be
revised or, at least, reexamined.
For example, he presents a wholly different view of the horse foot that sees
it
as very responsive and adaptable to ground impact. Stimulation of the internal
foot structures will result in the development of more efficient ways to
dissipate energy and support the horse's weight, he believes.
It also suggests horses with navicular disease may not need to be put down
and
that hoof trimming techniques might need to be reviewed.
"We need to be trimming hooves so that more of the back part of the
foot--including the frog--bears the initial ground impact forces and weight,"
Bowker stated.
He has been working with Gene Ovnicek, a farrier from Montana, and Dr. Barbara
Page, a veterinarian in Colorado, who are putting some of these principles
into practice. They are trimming the feet so that the breakover is much shorter
and the frog and back part of the feet support a lot of the weight of the
horse.
These farrier techniques seem to encourage development of tissues that dissipate
more energy when hooves hit the ground.
"If hooves are trimmed so that the frog rests on the ground," Bowker
said, "it
stimulates the back part of the hoof to grow more fibrous and cartilaginous
digital cushions, which appear to be 'protective' of the more chronic foot
problems."
Conversely, with digital cushions constructed of only elastic and fatty tissues
with little or no fibrocartilage, the ground impact energy is transmitted
to foot
bones and ligaments, resulting in internal foot problems, such as navicular
disease.
Across most of the breeds examined, environmental (i.e. nongenetic) factors
seem to be a major contribution to the development of the internal tissues
of the back part of the foot, Bowker said.
Some of these factors include the frog resting on the ground, husbandry of
the
horse, trimming and shoeing methods, and so forth.
"Horse feet with good environmental stimulation of the back part of the
foot, the
digital cushion, and the lateral cartilages will respond by becoming more
robust
structures to dissipate the initial impact energies and provide better support
for the horse when the horse is standing," he said.
In those horses that do not have good environmental stimulation of the back
part of the foot (for example, a trimmed or recessed frog, rather than one
resting on the ground), regardless of the breed, the internal tissues of the
back part of the foot will be underdeveloped, which will lead to internal
foot problems such as navicular syndrome, according to Bowker.
"We can develop trimming and shoeing methods that can stimulate the back
part of the foot--much like physical therapy to the back part of the foot,
which will stimulate the internal tissues of the foot--to provide a more efficient
system of energy dissipation and provide better support for the weight of
the horse," he said.
Bowker firmly believes that as we begin to understand internal foot biology,
we
will be able to decrease the incidence of navicular-type and other foot diseases.
