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 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 theses horses have a greater chance of having internal foot problems.”