• theequinedocumentalist

Horse Shoes - Back to the Future

Updated: Mar 1

For thousands of years horses roamed the grasslands, surviving, thriving and all without the need for something applied to the bottom of their feet to provide extra protection, or traction, or to enhance their performance.

An optive word in that sentence is roam, when humans domesticated the horse their work load and intensity of work increased exponentially. Add to this the disregard for the concept of survival of the fittest and you can suddenly see the necessity of protecting the feet of beasts of burden. Horses who couldn’t cope with escaping predators with bare feet were easy prey, but humans do not have survival of the fittest in the forefront of their mind, economic reason far outweighs Darwinian principle. So…the horse shoe.

One of the first devices for protecting hooves in the roman empire was the hipposandal. It was a removable metal slipper that was tied on. Originally made of rawhide or woven plants and then developed to have metal soles, it was born from the roman and ancient Greek recognition of the need to protect their horses feet as they traversed the roads of a vast empire. Exactly who nailed the first shoe on remains a mystery, whether it was the Celts, Gaul’s or Romans the application has remained all but the same for thousands of years.

The reality is these ancient civilisations did not have the knowledge, technology or materials to use anything else.

Now the question is, do we? And is steel still the best material to protect our horses’ feet?

Lets first look at the hoof, a miraculous evolutionary phenomenon that provides both protection and impact absorption while acting as a blood pump and sensory organ. Looking back at the hoof itself may give us a glimpse as to the future of our applications to protect it.

The hoof is a viscoelastic, anisotropic, heterogenous structure, its complexed makeup means it has evolutionary mechanisms that make up its functionality. Lets explore these words.

Fig. 1 Force Dispersion Mechanism of the Hoof. Viscoelastic, anisotropic, heterogenous structure.

The hoofs composition, being complexed and changing (Heterogenous), is such that it can bare different forces in different directions (anisotropic), Kasapi and Gosline (1999) studied the hoof on a microscopic level explaining how the hoof was a series of (crystalline) tubules within an (amorphous) matrix (Fig.2).

Fig.2 The cells within the hoof proliferate at different orientations creating horn tubules filled with intra tubular horn and surrounded by inter tubular horn. The different orientations of the cells play a role in its ability to with stand both compressive and tensile forces.

These properties of the keratin composite vary through the wall thickness and the different layers offer different support mechanisms. As we can see in fig.1 and 2 the tubules in the wall are arranged and spaced so that they can take different forces, the outer hoof wall is harder, it contains more tubular horn which is arranged in a direction to withstand compressive forces. As you get deeper the hoof wall changes to have more inter tubular horn which is arranged more horizontally to withstand the tensile forces that are created as the compressive forces are redirected toward the laminal attachment. This transference of energy absorbs some of the concussive forces of ground impact. There are other relationships within the hoof capsule that allow movement and therefore energy dissipation.

Fig. 3 The natural deformation of the hoof as an energy dissipation mechanism.

The viscoelastic nature of the hoof means that it can deform and return to shape with out damage as long as it remains within its elastic capacity. Fig.3 shows the natural deformations that occur, again this dissipates the concussion of locomotion. The relatively thinner wall at the heels means they are more flexible then the toe (Douglas et al. 1996).

Huang et al. (2019) discussed in depth the material property of the hoof, describing it, as we have seen here, as an efficient energy absorption layer that protects the skeletal elements from impact when galloping. The study also outlined its heterogenous structure calling it an efficient energy absorbent natural polymer composite.

The hoof is made of keratin which is a very common building block in nature for structures that need to be used for protection or impact absorption, take rams horns and our own finger nails for examples. As in the hoof, the different arrangement of these keratin cells provides different functionalities. As the hoof is subject to impact after impact it must have material properties that are high in absorption capability or else it would simply fail.

Some other interesting facts about keratin is that it comes in different hardness’ according to how much sulphur it contains and the difference in crystalline versus amorphous volumes affects the young’s modulus of the structure (Mckittrick et al. 2012). Mckittrick et al. (2012) compared the hoof to vulcanized rubber and found the mechanical properties of keratin, like most biological materials, to be extremely sensitive to hydration, with stiffness and strength decreasing accompanied by an increase in toughness with increasing hydration. It’s unclear as to whether this is beneficial or a disadvantage of the material, but it could prove significant in the application of steel and nails to the hoof in different states of hydration.

The hoof wall, from its microscopic level to its macroscopic composition all plays a role in its ability to serve the horse (Huang et al. 2019), the hoofs changing material and structural design is what makes it energy absorbent and impact resistant. Interestingly Huang et al. (2019) express how the hoof wall serves as a potential inspiration for designing new lightweight, energy absorbent and impact resistant synthetic structures and materials. Ironic considering the hoof is still, on the whole, protected by steel, perhaps it should serve as a design for a new generation of its own protection systems.

Youngs modulus of elasticity, as mentioned above is a way of measuring the stiffness of a material, The basic principle is that a material undergoes elastic deformation when it is compressed or extended, returning to its original shape when the load is removed. Hooke’s law states that the amount of deformation or strain of an object is directly correlated to the amount of stress placed upon that object, within it elastic capacity, young’s modulus measures the rate of stress to strain.

Fig.4 The Young’s modulus of the different areas of the hoof.

Douglas et al. (1996) showed that the different areas of the hoof had different elastic modulus under both compression and tension. Mild steel has an elastic modulus of ~210GPa that is 210000MPa so you can see just how stiff steel is in comparison to what it is applied to. The higher stiffness and hardness of steel inevitably creates increased shock through the hoof. Higher landing velocities, higher peak forces and higher and longer impact vibrations in traditionally shod feet were found by Willemen et al 1997, Roepstorff et al 1999 and Parkes and Witte 2015. Back et al. (2006) also discussed the possible deleterious implications of steel shoes, with the same findings of increased shock, interestingly this study described how these shock vibrations were diffused by the time they reached the proximal phalanx in both the shod and unshod foot showing how the structures of the hoof are primarily responsible for absorbing concussion.

Interestingly in Douglas et al. (1996) the heel quarters had significantly less elastic strength then the dorsal wall and Hinterhofer et al. (2001) found the quarters were softer on a shore rating. Considering its increased flexibility and actual thinning of the wall at the caudal aspect of the hoof this all shows how it is designed to move and flex allowing expansion and contraction. This also goes someway to explaining why this area of the hoof fails much more readily then the toe and collapsed heels are prevalent in the shod population (Dyson et al 2011). Roepstorff et al. (2001) showed that the expansion and contraction of the hoof was restricted by the application of a steel open heel shoe, but the application of frog support padding helped to return this closer to the barefoot in terms of expansion but not contraction, how relevant contraction of the foot is to its health needs to be researched, but we can extrapolate from this study that the unified mechanism of the hoof wall and haemodynamic system plays a role in its dynamic function and ultimately its health. Previous of my articles and Videocasts have discussed this in more depth, explaining how frog function is possibly one of the main reasons why, in general, a horse taken out of shoes (barefoot vs shod) will experience positive morphology of the hoof.

The question of this article is, if we could provide protection of the hoof in a way that more closely replicated the hoof itself, not just in material properties but in it's dynamics, could this fact become a thing of the past? Clearly steel does not have the properties we have explored above, however adding frog support padding is an evolutionary step in the right direction.

Is something available now or in the future that has or will have more similar properties?

A recent article (O’Grady 2016) discussed how barefoot methodologies could be applied to traditional farriery to improve the health of the shod hoof. O’Grady expressed the same results as the author when taking horses out of shoes seeing an improvement in the palmar structures, resulting in improved hoof proportions, something discussed in a recent VideoCast on the barefoot racehorse.

In relation to this article an interesting statement made by O’Grady (2016)

“. In actuality, the horseshoe is not an extension of the horse’s foot. Placing a horseshoe with different properties than the hoof capsule between the hoof and the ground replaces a single interface with 2 interfaces.”

Very much expresses the sentiment of this article. The article also highlighted the difference in wear of the shod foot versus the barefoot, in the shod foot the expansion and contraction of the heels against the shoe wears them down while the less dynamic toe continues to grow, inevitably leading to a change in hoof proportions as found by van Heel et al (2004,2005) and Moleman et al. (2006).

So with the understanding of the steel shoe creating an additional interface due to its vastly different properties to the hoof, what is available now and what could be the future of horseshoes?

Firstly lets summarise the possible important factors in choosing a material to protect our horses feet with. The mechanisms of the hoof are such that it is a very efficient shock absorbing structure, so shock absorbing characteristics would seem to be a primary concern. As reduced constriction of the normal deformations of the hoof as possible and engagement of its haemodynamic mechanisms would also logically be implied. These factors would be met by a flexible material and applications that re-unified the entire solar surface as a load sharing entity, while still allowing the different parts to move as different rates.

Composite shoes are becoming more widely used and a study Back et al. (2006) studied the use of polyurethane (PU) shoes, although the effects were individual to the horse the main findings were that