I'd like to know exactly what causes hoof wall separations. I can delineate the obvious, the same factors sometimes cause cracks in the hoof wall: poor farriery, genetics, imbalance, mechanical pressure, failure to maintain moisture equilibrium, trauma, and pathology. But, what triggers the phenomenon?
Why is it that two horses standing side-by-side, full siblings and indistinguishable in most all respects, often have feet so different from one another that they might as well belong to horses from different planets? ¿Quién sabe?
We do know a little about how hoof wall separations are formed after whatever triggering mechanism has occurred.
The horny covering of the horse's hoof capsule we call "hoof wall" is anatomically analogous to a human's middle fingernail. The hoof wall consists of three main layers, the stratum externum, the stratum medium and the stratum internum. Technically, the hoof wall consists of parallel strands of keratinized epithelial cells that arise on the coronary corium of the coronary band. The hoof wall has no nerves or blood vessels. Put another way, the hoof wall is a modified hair.
The stratum externum is the outermost layer and serves mainly to help maintain moisture equilibrium within the inner layers of the wall by retaining moisture. Most of this layer is routinely rasped off in show horses, worn off in horses at liberty, seemingly without ill effect.
The middle layer of hoof wall, the stratum medium, forms the bulk of the wall. It plays an important role in the way the horse disperses shock and in the horse's blood circulation.
The innermost layer of wall is called the stratum internum. This structure provides the means, laminae, by which the coffin bone is attached to the hoof capsule.
For whatever of the reasons mentioned, and goodness knows how many others, the rows of parallel epithelial cells in the stratum medium sometime part company. This phenomenon is called a "wall separation." Wall separations can have far- reaching consequences and, if not addressed on a timely basis, can compromise the hoof's ability to function normally. Wall separations can and do occur in both shod and unshod horses, but are more often found in barefoot horses. The most insidious part about wall separations is that they very seldom get better without mechanical treatment because, once begun, their growth is accelerated by mechanical means. Once started, the separation, however tiny, fills with dirt, rocks, ****** matter, etc. Whatever the material filling the crack, when the foot is weighted ("loaded"), the material exerts mechanical pressure on the strands of keratinized epithelial cells forming the wall, which causes more tearing, allowing more dirt, rocks, ****** matter, etc., to enter the crack, causing more mechanical pressure, ad nauseam.
In addition to becoming somewhat of a self-perpetuating phenomenon, wall separations create the anaerobic conditions conducive to the growth and proliferation of the pathogens responsible for the pathology sometimes called "White Line Disease." Actually, this pathology does not affect the white line, it affects only the stratum medium of the hoof wall, and is much more accurately described as "Hoof Wall Disease." HWD is epidemic on the Texas Gulf.
Wall separations are most often successfully treated mechanically by unloading the affected area. "Unloading", translated from farrierspeak, means that the affected portion of the hoof wall is not allowed to bear weight ("load") when the wall bears weight (is "loaded").
How is this accomplished?
One unloads a portion of the hoof wall by cutting it away until that portion of the hoof wall affected is no longer an integral portion of the perimeter of the ground surface of the hoof. In other words, hoof separations are treated by stopping their tendency to self-perpetuation by negating the mechanical forces which separate the strands of epithelial cells and push them outward.
The extent of this cutting away, or debridement, is governed by the following axiom: "New growth follows old." Unless all the aberrant growth is removed, all new growth will follow the old; therefore, the wall must be removed to a point above the separation or the treatment will be unsuccessful. If known, the original cause of the separation is not addressed until after mechanical treatment is initiated. Most often, the affected hoof is shod in order to better distribute the stresses normally taken by the portion of the wall debrided and, in the case of extensive debridement, to maintain the structural integrity of the hoof capsule. In the case of small separations, especially at the quarters, the foot is sometimes left bare after debridement.
At times, the separation is so extensive that the affected area of the wall cannot be debrided without compromising the function of the hoof capsule and the procedure must be done in stages, with the most affected portion of the wall usually debrided first.
Tom Stovall is an American Farriers Association Certified Journeyman Farrier since 1983, a Member of the Texas Professional Farriers Association, and a Member of the Artists-Blacksmiths Association of North America. Thanks to him for his permission to post this article.