In a normal foot, the angle of the toe and heel approximate each other. The hoof pastern axis forms a straight line down to the ground.
Any change in normal conformation can be detrimental to the horse.
Dr. Tracy Turner, an equine veterinarian at the University of Minnesota, defines an underrun heel as a foot with a heel angle that is 5 or more degrees lower than the angle of the toe.
Nobody has stated whether a broken-back pastern axis should be part of this definition. But as the heel moves forward in this conformation, the pastern is also moved forward, creating the broken axis.
The lengthened lever arm caused by the long toe places excess strain on the foot’s dorsal laminae and prolongs breakover. This increases deep digital flexor tendon tension. As the toe and heel grow forward, the foot is compromised, leading to heel problems such as the inability to support weight, bruising, separations, cracks, etc.
O’Grady: While Dr. Turner defines this syndrome as the angle of the heel being 5 degrees lower than the angle of the toe, this doesn’t always account for those horses that are born with or develop a low heel. When we define the long-toe and underrun heel, should the broken-back pastern axis be part of the definition?
Hood: When this syndrome is severe, a broken foot axis is present. In the syndrome’s early phase, an in-between state may exist where the broken axis is mild or hasn’t yet developed. Making a broken…