[RC] FYI: A Theory About Equine Foot Physiology - Tamara Woodcock
From Robert Bowker, VMD, PhD, at Michigan State University College of
Veterinary Medicine: "A Theory About Equine Foot Physiology"
A New Theory About Equine Foot Physiology
A Michigan State University College of Veterinary Medicine researcher has
pieced together a new picture of equine foot physiology that suggests
vascular systems in horse hooves function in much the same way that air- or
gel-filled running shoes do. "Moving liquids are the best way to dissipate
energy," said Robert Bowker, a professor in the College of Veterinary
Medicine's Anatomy Department. "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. This 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, re-examined.
For example, Bowker's theory presents a wholly different view of how horse
feet respond to ground impact. 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 may 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 said.
This would 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 material." With digital cushions constructed of
more resilient tissues, less ground impact energy would be transmitted to
foot bones and ligaments, reducing internal foot problems, such as
navicular
disease.
Currently, equine foot physiology researchers subscribe to one of two
anatomical theories: pressure theory or depression theory. Both seem to be
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 cartilagineous
material instead of elastic tissue.
This is particularly true for horses in breeds that are said to have good
feet, such as Arabians. "It was also true for Quarter Horses in their
mid-20s with no history of foot problems," he said. Bowker believes
environmental factors also contribute to the formation of these kinds of
tissues. "We found more cartilagineous digital cushions consistently,
regardless of breed, in domestic horses from the Rocky Mountains, where
harder ground surfaces and higher altitudes may contribute to their
formation," he said. In regions of the country where ground surfaces are
softer, more horses have digital cushions made of elastic tissue. "We
believe these horses have a greater chance
of having internal foot problems."
Robert Bowker, VMD, PhD, (517) 353-4532
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