Courtesy of America’s Horse Daily
Ever wondered, “What is a horse’s hoof made up of?” We take you on the journey of understanding the structure of the healthy equine hoof.
The equine foot is a mechanical marvel of joints, cartilage, ligaments, tendons, blood vessels and bone encased within a horny capsule that is vital to the horse.
Dr. Stephen E. O’Grady of Keswick, Virginia has devoted his professional life to its study, both as a veterinarian and farrier. He’s an acknowledged worldwide expert on equine podiatry who enjoys few things more than educating horsemen, farriers and veterinarians about it.
“For more than 2,000 years, humanity has recognized the importance of healthy and proportional feet if the horse was to be maintained as a useful beast of burden,” Dr. O’Grady says. “The ancient Greek horseman Xenophon said, ‘If he has not good feet, there is no profit in him as a war horse.’ ”
Hence the old adage: “No hoof, no horse.” And, as with everything related to horse anatomy, the hoof’s form, “the structure of its parts and the way they are arranged,” greatly affects its function.
If you look at the foot externally from the side (laterally), the front portion is largely static, whereas the back part of the foot is dynamic or deformable. The foot’s structures are all contained within the hoof capsule.
“The hoof wall is an ‘elastomeric’ structure,” Dr. O’Grady says, meaning that it has the ability to both flex and retain its shape with load-bearing. “Its design allows weight-bearing functions, as well as shock dissipation.”
He continues, “The soft tissue at the palmar (or rear) aspect of the hoof further acts as an anti-concussive mechanism to dissipate energy.”
When it’s functioning correctly, the hoof provides protection and support for the horse’s “digit,” the bones and ligaments that make up the lower limb. It is also weight-bearing and absorbs concussion. All of those functions make for a tall order on those structures when you consider a horse’s size and the force of its foot hitting the ground.
“The horse’s foot is unique as it is a biological entity that follows the laws of physics,” Dr. O’Grady says. “When you change the hoof (through farriery), you change the orientation of the structures within the hoof.”
Understanding the fundamental anatomy and function of the horse’s foot gives any horseman the necessary tools for recognizing when the hoof is working correctly, and when it’s not. Dr. O’Grady walks us through the basics here.
The horse’s digit is its farthest extremity in the lower limb, from the fetlock down to the ground. Its primary structures include the proximal phalanx (P1 or long pastern bone), middle phalanx (P2 or short pastern bone), distal phalanx (P3 or coffin bone), navicular bone, the pastern and coffin joints, suspensory ligament of the navicular bone, deep digital flexor tendon, etc.
The tendons and ligaments act as support structures for the bones, as there are no muscles in the digit.
“The navicular bone allows the deep digital flexor tendon to maintain a constant angle where it inserts on the distal phalanx,” Dr. O’Grady points out. “The navicular bone coupled with the deep digital flexor tendon makes the joint elastic and yielding, thus counteracting shock.”
He continues, “The distal phalanx (coffin bone) is unique. Like the scapula (shoulder), it only articulates with another bone on one end. And the majority of the bone is covered with soft tissue.”
The coffin bone has three surfaces: the articular (at the joint), the parietal and the solar (bottom).
“The parietal surface forms a large surface area that is porous and roughened – ideally adapted for attachment of the lamina (or lamellae),” he explains, “remembering that the lamina attach the bone to the hoof wall.
“The solar surface is very hard and smooth with no vascular foramina (openings). The articular surface lies between the coffin bone and the middle phalanx, which makes up the lower part of the pastern joint.”
He adds that on either side of the coffin bone lie the lateral cartilages (or sidebones), which also function to decrease concussion in the foot. They also appear to allow for the lateral expansion of the hoof capsule while equalizing pressure within the hoof.
Palmar to or behind the coffin and navicular bones lies the digital cushion.
“The digital cushion is composed of fibrous trabeculae (microscopic rods), elastic fibers and fat cells,” Dr. O’Grady says. “It shapes the palmar (rear) section of the foot, therefore dictating poor or good hoof conformation, and plays a major role in absorbing concussion. It is not meant to bear weight; rather, it has the ability to share weight.”
When a horse has low heels or underrun heels, generally speaking, part of the problem is that the structure of the hoof wall has been altered so that weight bearing has been shifted to the digital cushion.
“The hoof capsule is a protective shell, and its internal dynamic factors rarely change but they are susceptible to damage,” Dr. O’Grady says. “The hoof is the integument (or skin) of the foot. Unlike the skin, which is relatively uniform all over the body, the hoof integument can be divided into five distinct regions – coronary band, hoof wall, sole, frog and heel bulbs.”
The coronary band is specialized to produce the hoof wall and adjacent dermal structures.
The hoof wall’s primary function is weight bearing. It’s made up of three layers that work to do that: a thin outer layer (stratum externum), the primary horny wall (stratum medium) and an inner wall (stratum internum), which is also known as the epidermal lamina or lamellae that help suspend the coffin bone within the hoof capsule.
The stratum medium is made up of vertical tubules of varying density, with the highest density toward the external part of the wall. The varying density appears to help distribute concussive forces and also “acts as a crack-stopping mechanism.”
“At the heels, the hoof wall is inflected forward and inward at an acute angle to form the bars,” Dr. O’Grady says. “The bars are weight-bearing structures for the palmar (rear) aspect of the hoof. They lend additional strength to the heel and allow for its lateral expansion.”
He points out that the “external influences on the hoof’s form and function include the environment, the horse’s use and, above all, farriery.”
The hoof capsule “has the ability to deform under load and then return to its original shape when the weight is removed.
“When subjected to a sudden high stress, it deforms elastically and reforms. In contrast, when subjected to a constant stress (such as an ill-fitted shoe or poor conformation), it deforms slowly in a viscous manner, which will often reverse when the stress is removed.”
Dr. O’Grady adds, “In fact, the hoof wall is so resistant to sudden high stresses that it is considered more fracture resistant than bone.”
The sole is the bottom or ground surface of the foot.
“It should be concave,” Dr. O’Grady explains, mimicking the plane of the coffin bone above it. “That concavity appears to be ample evidence that the general surface of the sole is not meant to bear weight.
“Its purpose is protection, and it plays a role in concussion and weight distribution. The sole cannot really be too thick.”
The frog is made up of elastic horn and aids in the foot’s expansion. The frog and digital cushion above it work together to allow expansion at the heels and to dampen concussive vibrations as the foot meets the ground.
“The frog has more moisture than any other part of the foot, aided by sweat glands in the digital cushion,” Dr. O’Grady says. “It also plays a role in concussion and aids in traction.”
He explains, “The hoof is also a ‘hemodynamic’ system. The veins of the hoof collect into a complex of interconnecting blood vessels, or ‘plexuses’ (found at the coronary band, either side of the lateral cartilage and underneath the sole). These plexuses are also designed to reduce concussive forces on the bone.”
Look for Balance
Again, just as with evaluating other aspects of equine conformation, train your eye to look for balance or conformation in the horse’s foot.
“We don’t really know what a normal foot is,” Dr. O’Grady says, because that qualification is individual to each horse. Instead, Dr. O’Grady looks for a “good” or “functional” foot, from the side, front, rear and bottom.
- From the side, or lateral aspect, Dr. O’Grady envisions a triangle. The first line runs from the fetlock down the front of the pastern, across the coronary band and down the front of the hoof: “This forms the hoof-to-pastern axis, which should form a straight line.” The second line begins in the middle of the lower cannon bone above the fetlock and drops vertically to the ground. This line “should intersect with the ground at the most palmar aspect (rear) of the weight-bearing surface of the hoof wall at the heels.” The third line connects the previous two lines, along the bottom of the foot. “This triangular relationship can be used to evaluate a balanced, functional foot regardless of the size of the horse,” Dr. O’Grady says.
- Viewed from the front, or dorsal aspect, a vertical line that bisects the cannon bone and the digit should separate the foot into two equal parts. Any line drawn horizontally across the hoof at any point from the coronary band on down, should be parallel to the ground.
- Viewed from the bottom, or solar aspect, “The properly trimmed foot should be as wide as it is long.”