When working with horses, it is imperative for the farrier to mentally review aspects that affect the shape of the hoof. Some of these factors can be influenced by you as the farrier, while others are completely out of your control. All should be considered when trimming and shoeing the horse, as these factors will influence your decisions in maintaining its hooves.

“There are a lot of things that affect the hoof shape of the horse,” says Dr. Simon Curtis. “Hoof shape change, even in an old horse, can be quite dramatic.”

The Hall of Fame farrier and PhD from Newmarket, England, summarized many of the factors affecting the hoof capsule at the 2018 American Farrier’s Association Annual Convention in Reno, Nev. By showing information derived from his university research and observations derived from his years of farrier work, Curtis left conference attendees with fresh considerations for providing footcare for their clients’ horses back at home.

Horn Generation And Growth

Curtis began with a review of the anatomy of the hoof capsule. Much like the rest of the presentation, anatomy is an enormous subject, and only can be touched on in a lecture. As he reminds, the hoof isn’t a solid, rigid structure, but it is a flexible organ. For this presentation, Curtis focused on the hoof wall because it is the largest part of the hoof capsule and is subject to the most change.

“We also must remember that the horse is suspended in the hoof capsule,” he says. “That has ramifications for how the hoof acts and how it can deform.”

And while hoof anatomy is a substantial subject, Curtis did highlight horn tubules to dispel a common misconception that the hoof is made of horn tubules “glued” together. From his work, Curtis provided a diagram of the germative layer (Figure 1). “P” represents the papillae. The basement layer is where mother cells continually produce keratinocytes (horn cells).

“If you had a piece of rubber, you could stick your finger in the membrane and change the shape of the surface,” he says. “So we need to quit thinking of the papillae as if they are separate from the rest of the germative layer. All they are is the same germative layer being pushed down into the projections illustrated in this diagram.”

Curtis reminds us that this diagram is a two-dimensional representation of a three-dimensional body, limiting how we see these cells actually radiate out. When the cells are first created, they are the same as the rest of the body. Only after production do these cells start specializing — and do so rapidly. Keratin tonofiliaments are formed within the cells and these create a skeleton, giving the cell strength.

“The keratin tonofiliaments form part of the three-dimensional linkage across the whole stratum medium, adding more structural integrity,” he says. Tubular and intertubular horn cells are joined in the same manner. During this process of generation, maturation and dying, the cells flatten and are described as squamous (Figure 2).

The keratinocytes flatten in the direction of generation and keratin tonofiliaments are laid in the long axis of the cells. Effectively, this means that tubular and intertubular squamous cells lie at right angles to each other. They are joined by the desmosomes and as a mass, described as the stratum medium, travel proximodistally to the bearing border at the angle of the hoof wall.

“You are better off thinking of these as bricks layered in a different pattern,” he says. “We know that if you build a brick wall with bricks only in the same direction, it won’t be as strong as one in changing directions.”

Curtis stresses the difference distinguishing hoof generation (production of these cells) and growth (what farriers remove from the bottom of the hoof).

Curtis identifies several material properties of hoof horn:

Continual regeneration. The hoof begins to grow as soon as the foal is born. And while it may slow, the growth continues until the horse dies.

Heterogeneous. Horn tubules are not the same throughout the hoof — they change size and shape.

Isotropic and Anisotropic. “Isotropic is material that acts in the same way in all directions,” says Curtis. “As farriers, we understand that if we take a cube of iron, we don’t have to decide which way we will draw it out — it will act the same whichever way.”

The hoof is anisotropic, meaning it bends and compresses in differently in diverse directions.

Visoelastic. The hoof exhibits both viscous and elastic properties — recoils under load and in movement, but also can change in shape over time.

Resistant to wear. For the healthy horse in the correct environment, the hoof is good at resisting wear.

Durable. The hoof germinates and then grows down the hoof, but when it gets toward the bottom of the foot, it is pretty much as strong as it was.

“Hoof horn really is a remarkable material,” says Curtis. “Various industries have tried to work out how it gets such strong material properties.”

As Curtis noted earlier, hoof growth begins at birth. Prior to birth when the horse is in the early embryonic state, hooves haven’t yet formed. Hooves become apparent in images of the horse after it matures into a fetus, and single toe on each leg is recognizable. Interestingly, the single digital is one of the contributors to the remarkable athleticism of the horse, Curtis notes.

“It allows the horse to transfer energy very efficiently to the ground,” he says. “The horse isn’t the fastest animal, but it is very fast over short and long distances — few animals have that ability and none at the level of the horse.”

The hooves are more recognizable and the distal phalanx becomes noticeable as the horse matures. After 11 months of development, the hooves are ready to work, as shown by a newborn foal standing after birth.

Of course, while the hoof is ready to work, it still presents management issues, especially with angle changes. The hoof angle changes significantly over the initial months of life. In his research, Curtis noticed that on average, the foals in his study were at 59.5 degrees in the first few months of life.

“Textbooks will tell you that a club foot is 60 degrees or more,” he says. “My measurements at 59.5 degrees sound remarkably close to 60 degrees. Almost half of them were above 60 degrees.”

Overall hoof shape changes in the early development of the foal. Figures 3a, 3b and 3c illustrate the stark shape and size changes of the distal phalanx in a 10-month fetus, a newborn and a 4-month foal.

As the foal ages, the hooves rapidly distort due to conformation. Figures 4a and 4b show a 2-month-old foal with valgus deformity from the fetlock.

“Look at how the hoof is already distorted,” he says. “Few of you here likely deal with foals, so what chance do you have later when a foal like this already has a distorted hoof like this? One of my jobs is to get them to maturity and improve the hoof shape. All foals are born with formed symmetrical hooves. But how many farriers shoe horses with symmetrical front feet? So understanding how the hoof changes will help us understand the hooves of mature horses.”

Other Factors

There are other influencers beyond anatomy, conformation and development. By all means, this isn’t the exhaustive list of what can affect hoof shape, but what Curtis finds are major contributors.

Breed and environment. Farriers have long recognized how breeds can present different hoof characteristics. In Figure 5, Curtis shows a Thoroughbred he is working on.

“It is as wide as it is long,” he says. “Look at the shape of the paddock it is standing in. Compare that to the Andalusian (Figure 6), which has a steep dorsal wall and narrow, almost contracted foot. We shouldn’t try to make the Thoroughbred into an Andalusian or the reverse. We should understand the breeds and the differences. The environment also will affect the hoof shape.”

Loading and internal stresses. During his studies, Curtis performed measurements of loading using pressure mats to obtain data (Figures 7a and 7b). Curtis notes that difference of loading illustrated by the color. Areas of the hoof are loaded differently. Interestingly in this measurement, there are areas more significantly loaded that adjacent squares.

There are many conditions that affect loading and internal stresses, but Curtis highlighted upright and club feet, reminding that these two separate things, caused by different reasons.

“The only thing they have in common is the steep dorsal wall,” he says. “A horse will get an upright foot usually by two reasons. First is a limb length difference. So if the horse isn’t loading one leg as much, that limb’s hoof is desperately trying to growth toward the ground to get some even loading between the limbs. The other likely explanation is that it had some injury at some point in its life and if it hasn’t loaded that particular limb as much, the hoof will contract and become more upright.”

Laterality. While laterality causes different hoof angles, this is a more complex subject than the simple view of one-sidedness. As Curtis points out, the dominant leg and lead leg are not the same thing.

Wear. This Arabian stallion (Figures 8a and 8b) toes in a bit and is kept in sandy soil, creating the wear. “We have to trim horses as individuals and for the environment they are in,” he says.

Neglect. This is a major factor, especially when clients allow for appointments go too long between cycles. “The best shoeing job will look awful in 12 weeks,” Curtis says.

Injury and disease. Figures 9a and 9b show the foot before shoe removal and after debridement of a hoof affected by a wire cut. With the shoe off and trimmed, Curtis notes the lack of uniformity in the hoof wall. And diseases such as chronic founder can largely impact hoof shape.

Farriery. Of course, the farrier management of hooves positively or negatively affects the hoof shape. Curtis notes that trimming styles may differ dramatically, as those influenced by fads, or with more idiosyncratic differences between practitioners.

Research has shown that farriers do trim left- or right-handed. Curtis says that it shouldn’t be surprising, just recognize it as the practitioner and try to not let it influence your trim.

In closing, drawing a comparison to his body of work as a farrier, Curtis notes that everything with a foal is exaggerated.

“You will often learn things in principles through the exaggeration,” Curtis says. “But you need to look for things more subtly and with more finesses. Foals have taught me a lot because many of them have presented such distortion at times.”

July/August 2018 Issue Contents