Part 3 of the Brainy Hoof Blog by Christa Lesté-Lasserre
The smooth outer surface of the hoof is the hoof wall. It is the part of the hoof that is visible when the horse’s foot is flat on the ground, and it’s aptly named. It’s a fairly robust, essentially solid shell covering a multitude of complex tissues inside the foot. Built with its own level of complex construction, this wall serves as a barrier between the intricate inner structures and the outside environment.
Cross-sections of the hoof wall reveal three distinct zones: an outer, a middle, and an inner zone. They’re primarily made up of what scientists call “wall tubules”, these are long, thin cylinders about 0.2 mm wide. The outer, drier zone has greater tubule density, and the inner zone, which is moister, has less density. This makes for an interesting design that promotes optimum crack resistance and energy transfer, according to Christopher Pollitt, BVSc, PhD. All these tubules and the inter-tubular horn are cemented together with hard keratin, which is the basic substance of horn, hair, nails, and claws. The disulphide bonds in hard keratin gives it great physical strength, he says.
The inside surface of the inner wall is called the stratum lamellatum (which means, “layer of leaves”). It's name originated from its 550-600 epidermal lamellae projecting from the inner surface, in parallel rows, like the pages of a book, Pollitt explains. At the ground surface of the hoof wall, the tips of the stratum lamellatum transform into the white line.
Movement and impact absorption
A healthy hoof wall is hard, shiny, and resistant, but that doesn’t mean it doesn’t move. On the contrary, like the rest of the hoof, it has a role in absorbing forces from the impact of when the horse's foot hits the ground; bringing in forces up to 2.5 times his body weight on a single hoof at full gallop.
When a barefoot, natural hoof comes down, it spreads out a little, cushioning the landing forces. A healthy wall is elastic enough to follow that temporary change in foot shape, without cracking. It slightly stretches out and then contracts back into its usual shape once the forces are removed.
Unhealthy hoof walls can be brittle or cracked, can separate, or can flare too much on its sides. Poor hoof wall health can come from a variety of problems, such as insufficient nutrients, bad weather conditions, suboptimal footing, unfavourable genes, and trimming mistakes. In addition, hoof walls can get diseased from various environmental conditions.
Hoof walls can also be damaged from the use of shoes and nails. A fixed shoe (whether it’s made of steel or plastic, nailed in or glued on) by its very nature, restricts normal expansion of the hoof wall during impact. The hoof wall tries to expand anyway, causing friction against the shoe and putting pressure on the nails, which can lead to cracking. Meanwhile, the entire hoof is prevented from carrying out its natural shock absorption abilities. When the hoof wall can’t expand, the hoof is unable to cushion the impact as well as it should, meaning the landing forces are instead moved higher up into the horse’s limbs, where they may be absorbed by the tendons, ligaments, bones, and cartilage (which can lead to pathologies).
Effective wall trimming
Barefoot horses need specific barefoot trimming of their hoof walls in order to keep their hooves healthy. Farrier and veterinarian Stephen O’Grady of Northern Virginia Equine in the United States, prefers to call it 'shaping' rather than trimming. He recommends 'beveling' the weight-bearing surface of the hoof wall by giving it a 45-degree angle up from the white line (the sole wall junction), all around the perimeter of the foot. The sharp edges from the beveling then need to be rounded off, making a round edge that is at least half the thickness of the wall. Importantly, excess hoof wall flare must be removed from the outer hoof wall, O’Grady says. Then, those areas can be rasped and blended into the rest of the rounded perimeter of the hoof wall. Since there’s no shoe to restrain the hoof walls from spreading out, shaping the barefoot hoof correctly is essential to prevent excess spread. Many trimmers leave too much hoof wall flare, creating an angle that doesn’t resist those impact forces very well, ending up with a splayed look. Hoof walls need to be trimmed straight in barefoot horses, with a 45-degree, rounded beveled angle close to the white line. The ground bearing surface of the hoof wall should never be trimmed flat as this encourages flare through peripheral loading.
Boots: Letting the wall expand naturally
Removable hoof boots, such as Scoot Boots, fit snugly over the hoof wall without being attached, allowing the wall to expand as it normally would. Because of this, the hoof can continue its natural job of expanding during landing and absorbing shock. Since hoof boots are only worn occasionally, the hoof wall still gets plenty of natural exposure with the ground and air.
Owners should ensure they properly follow manufacturer's instructions on boot fit in order to avoid friction between the boot and hoof wall.