An intricate and complicated host of tissues, blood vessels, tendons, ligaments and bones comprise the [horse’s] foot, and the whole package is sealed within a hard, horny shell – the hoof wall. – Lester Sellnow, 2001
Image: Anatomy of a Healthy Hoof, Animal Health Trust
Structure of the Hoof
The hoof has evolved to serve a number of important functions for the horse, including supporting an incredible amount of weight, absorbing shock, providing traction and conducting moisture. The hoof wall is essentially a self-replenishing keratinized horn. More specifically, it is a series of keratinized cells arranged in parallel tubules that run perpendicular from the coronary band at the top to the bottom surface of the wall. The outermost layer that forms the hoof wall begins at the coronet, the band separating the skin of the lower leg from the hoof, and is composed of the stratum externum (periople) and the stratum tectorium (Hood & Larson, 2013; Sellnow 2001).
The periople is similar to the cuticle of the human fingernail. It protects the junction between the horn and the skin, and extends less than ¾ of an inch down from the coronary band around the entire hoof, and to the bulbs of the heels at the back. The bulbs are the most flexible part of the hoof, allowing the foot to change shape in response to weight-bearing forces upon ground contact. The stratum tectorium is a thin shiny layer similar to the human fingernail. It gives the hoof a glossy appearance and helps retain moisture (Equine Research, 2004; Hood & Larson, 2013; Sellnow, 2001).
The stratum medium, the middle layer of the hoof wall, is the densest portion of the hoof, and as such, bears most of the weight of the horse. This layer consists of two zones. The outer zone is quite rigid and serves as a barrier between the foot and the external environment, while the inner zone is quite flexible and elastic, contributing to the hoof capsule’s ability to change shape in response to force. The innermost part of the hoof wall is the stratum internum (insensitive laminar layer). It consists of hundreds of alternating epidermal and dermal folds called laminae. Approximately 600 insensitive primary laminae project from the inside surface of the hoof wall, each one branching off into secondary and tertiary laminae. The laminae secure the coffin bone and attach it to the hoof wall (Hood & Larson, 2013).
The solar surface of the hoof is directly exposed to the ground. Its main function is to support the internal weight of the foot and dissipate concussive forces in the leg. In most domestic horses the outer wall is the primary weight-bearing structure, while the sole takes on more of this role on softer footing. Other sections of the solar surface that come into contact with the ground are the heels, frog, and bars, with the heels being the first part of the foot to make contact when the horse is in motion. The size and hardness of the frog changes in response to water content and ground contact, and plays an important role in biomechanical function and shock absorption with its ability to widen and dissipate concussive force when the heel strikes the ground. The bars are thought to act as a spring, adding to the biomechanical forces that simultaneously cause the hoof to flatten and the heels to expand (Equine Research, 2004; Hood & Larson, 2013).
The Feral Hoof
The hooves of feral and domestic horses are not the same. They differ vastly in shape, look and feel. Horse’s hooves have developed over thousands of years to adapt to a range of ground conditions, and the hooves of feral horses are shaped by their environment. Even though some feral horses tend to have a ‘conventional’ looking hoof shape that has been used as a model for domestic hoof care, the way in which the hoof capsule functions and interacts with the ground is in sharp contrast to the way the hooves of domestic horses are typically managed, and should not be readily adopted.
In contrast to the domestic horse, feral horses have thicker, stronger hoof walls that sometimes become rounded and have little to no ground contact. In many cases, particularly for horses living in dry conditions with hard footing, the sole has more contact with the ground than does the hoof wall. This ground contact of the sole changes the shape and function of the frog, which becomes much larger. In the words of Gene Ovnicek, the soles of feral horses are “calloused and the frog like a patch of leather” (Jurga, 2001). Robert Bowker has discovered similar findings, indicating that for the most part feral horses bear “their weight on their soles and frogs, which [are] enormous compared to the frogs of domestic horses” (Jurga, 2001).
At the opposite end of the spectrum, the hooves of horses living in sandy environments tend to develop spike-like hooks on their heels, which are present to a lesser degree on horses living in prairie, sod or gravel environments. In any of these softer conditions, horses are prone to hoof growth rates that exceed natural wear, resulting in a longer, flared hoof capsule (Jurga, 2001; Ovnicek, 2003).
Brian Hampson and Chris Pollitt (2011) elaborate that the balance between hoof capsule wear and growth rate in horses living in environments with hard footing results in a short hoof capsule that is worn to the level of the peripheral sole, leading to the sole bearing a significant portion of the horse’s weight. It is this hoof shape that has been promoted as being an ideal model for the domestic horse. However, Hampson and Pollitt believe this is actually detrimental to foot health, as indicated by the high incidence of laminitis and other foot pathologies in the horses they have studied.
The majority of the feral hooves studied by Hampson and Pollitt (this included 14 separate studies with horses ranging in number from 12 to 100 per group) showed extremely high incidences of pathology, ranging from less severe abnormalities like hoof wall cracks, flares, imbalances, narrow and uneven heels, to more serious abnormalities in the horses living in hard, rocky or desert environments, like concussive laminitis, rotated coffin bones, excessive hoof wall thickness, and ungual cartilage calcification (sidebone).
Basically, although the hooves of feral horses look robust and have been naturally modified to withstand locomotion in their respective environments, there are many negative pathologies, which, somehow, the horses are able to live with and continue to thrive under natural and sometimes harsh conditions (Hampson & Pollitt, 2011).
The “feral” or “natural” hoof, although touted by some based on the strength and durability of the hooves of feral horses living in hard, rocky environments, is not suitable for domestic horses because a) they do not wear their hooves enough naturally to maintain a healthy hoof length, and b) while the hooves of feral horses are “self-trimmed” because of distances travelled for food and water, they do have a high incidence of foot pathology. In the case of domestic horses, whether the hoof is shod or bare, the skills and knowledge of a good farrier are essential to maintaining healthy, balanced feet.
Animal Health Trust. “Learn About Laminitis.” Retrieved from http://www.aht.org.uk/cms-display/cal_laminitis.html
Hampson, B.A. & C.C. Pollitt. (2011). Improving the Foot Health of the Domestic Horse: The relevance of the feral horse foot model. Australian Government, Rural Industries Research and Development Corporation: RIRDC Publication No. 11/140, RIRDC Project No. PRJ-002510. November 2011
Equine Research. (2004). Horse Conformation: Structure, Soundness, and Performance. Guilford, Connecticut: The Lyons Press.
Hood, D.M. & C.K. Larson. (2013). Building the Equine Hoof. Eden Prairie, Minnesota: Zinpro Corporation.
Jurga, F. (2001). “The Natural Hoof – A Sign of the Times.” The Horse Oct. 10, 2001. Retrieved from http://www.thehorse.com/articles/10671/the-natural-hoof-a-sign-of-the-times
Ovnicek, G. et al. (2003). “Natural balance trimming and shoeing: its theory and application.” Vet Clin Equine 19 (2003) 353-377
Sellnow, L. (2001). “The Equine Foot – Form and Function.” The Horse Oct 15, 2001. Retrieved from http://www.thehorse.com/articles/10753/the-equine-foot-form-and-function