Nature has an inherent ability to improve, to adapt, to survive and to overcome. The miracle of the evolutionary complexities that it creates, within animal systems, to be able to respond to external stimuli, change and therefore withstand, is nothing short of incredible. But sometimes these changes are not conducive with long term health, sometimes these changes can be detrimental.
The hoof is a viscoelastic structure, it is designed to deform and return to shape under load to absorb the concussive forces of locomotion.
Fig.1 The hoof wall is designed to deform under load, its specialised composition deforms enough to dissipate some of the forces of impact and load but not too much that the internal structures become damaged. This illustration shows some of the normal deformations that take place.
This elastic nature of the hoof is limited by the individual elastic modulus, once this limit is reached the hoof suffers plastic deformation. The hoof is not wholly elastic but also has viscous materials in it which can suffer from viscous creep. Loads and environmental stimulation are important in the continued development of the hoof, but accumulative, unbalanced loads on the hoof can cause plastic deformation and cause the hoof to negatively morph.
Thomason et al. (2005) discussed how both the structures of the hoof and the musculoskeletal system respond to changes in loading from external influence, but the question is, what is adaptation for the better and what changes are having negative effects?
We can see the changes in the skeletal systems as outlined by Wolf’s law.
Fig.2 This schematic illustration shows how the bone remodels itself to better withstand the forces being placed on it. These changes happen over time and the internal structure of the trabecular bone also changes.
Wolf’s law states that bone adapts to the loads under which it is placed. If loading on a bone increases, the bone will remodel itself over time to become stronger to resist the loading. The internal architecture of the trabecula bone adapts and follows the lines of stress and the external cortical portion of the bone can become thicker.
Fig.3 This drawing by Wolf shows the architecture of the trabecular bone and how the lines follow the stress trajectories.
Fig.4 This close up image of the navicular shows how the trabecular bone is orientated in the direction of the force from the deep digital flexor tendon. Waguespack et al. (2010) stated that Trabecular Bone orientation in the navicular bone suggest primary force is compression from the DDFT (Wolf’s Law).
Mullender and Huiskes (1995) used a computer‐simulation model to investigate the remodelling of trabecular bone. Their findings supported the hypothesis that osteocytes located within the bone sense mechanical signals and that these cells mediate osteoclasts and osteoblasts in their vicinity to adapt bone mass. They found that when they changed the external forces the architecture of the trabecular bone followed suit and the orientation of the trabecular followed the lines of stress. Considering this mechanism one could hypothesise that abnormal load over a long period of time could lead to negative bone reformation.
Many structures in the horse are dynamic, just like the bones and the hoof is no different, also containing many microscopic structures that sense and respond, not unlike the osteocytes, to the stimulation of the hoofs structures.
Are the changes positive or negative? Are they adapting or being affected?
knowing the ideal can help us to recognise changes in shape (morphology) for the positive or negative, toward the ideal, maintaining the ideal or away from the ideal. Once we assess this, can we change the stimuli/influential factors to encourage adaptation?
The ideal hoof
Fig. 5 Solar view of a good hoof. Photo from Barefoot South.
Many studies have discussed the parameters of the ideal hoof. Caldwell (2016) outlined that proportions and symmetry around the foot mapped centre of rotation, here pictured as the crossing point of the dark red lines, were optimal for biomechanical efficiency. Proportions between 40/60 to 60/40 have been outlined as acceptable for the barefoot and are largely dependant on hoof slope. More upright feet will present with a larger back half while feet with lower angles to the pastern axis present with a larger front half.
Taylor (2020) discussed the proportions of the frog and caudal structures, it stated that Frog width should be 50-60% of its length with a shallow wide central sulcus and at a height that incorporates it in the bearing surface of the foot. It stated that the Digital cushion should be 2” thick, 3-4 fingers wide, it Should be quite dense like a tennis ball hard to deform and Fill the lateral cartilages. The Bars should be straight and not folded to mid frog. The sole should be concaved as no to little concavity means the bone is close to ground.
Fig. 6 Some established ideals from a lateral view.
The ideal from a lateral view is assessed with proportions and geometry. Although this foot fits many of the parameters for ideal if we look closer we can see slight prolapse of the caudal structures, seen in the downward curve of the coronet band and horn tubules at the back of the foot, the foot is adapting to the forces its under, this can also be seen by the fact that the heel has a greater then 5 degree difference to the toe angle. This is a morphology and caused by influential loads on the hoof. Could we create an influence that creates adaptation, straightens the coronet band and creates a higher angle to the heel?
Its important to understand that the developmental stage of the horse is vital in creating ideals for adulthood, creating ideal and stimulating loads on the musculoskeletal system and the hoof sets them up for longevity by creating robust structures.
Bowker (2003a) quantified how the hooves of the young horse adapted to environmental influences and stimulation. The internal architecture of the primary (PEL) and secondary epidermal laminae changed, becoming more dense increasing in number and changed in shape, the study hypothesized that these morphological changes of the relative number and shapes of the PEL along the perimeter of the hoof wall represented adaptive responses by these tissues to the many and varied stresses being imposed on the foot during its interaction with the environment. This influence continues into adult hood, Thomason et al (2005) found differences in the PEL in adult horses and hypothesised that regionally variable mechanical behaviour in the laminar junction, induces corresponding variations in morphology of the primary epidermal laminae, again showing how the architecture of the hoof responds to forces in a similar mechanism to Wolf’s law. This also tells us that potentially, we can still influence the morphology of the foot beyond the point of its developmental stages.
Both these studies suggest that the foot as it is at any point of time is a product of the environmental influences up until that point, if we have had positive influences we will likely have a good hoof, however that hoof is still an adaptive structure. If we create more optimal influences on a poorly morphed hoof we will create improvement, conversely if we do not optimise forces on that foot it will negatively morph or remain in a poorly morphed state. In the words of Einstein, you cannot keep doing the same thing and expect a different outcome.
The caudal structures of the hoof play a vital role in the proportions and geometry of the hoof (Dyson 2011) and Bowker (1998,2003) showed that the internal architecture of the haemodynamic system was different in a strong versus weak foot. The studies of Bowker and Poss have also expressed the importance of the developmental stages in the formation of these structures. Bowker (2016) showed the extensive network of nerves within the hoof, Taylor (2020) suggest these play a role in the development of the caudal structures. Bowker and Poss hypothesise that the lack of stimulation in young horses shod from very early on stunts the development of these caudal structures, this applies to race horses especially, as they are shod from a young age and have some of the weakest feet of the breeds (Labuschagne et al 2017). Anecdotal evidence and some studies have shown that thoroughbred feet do well training barefoot and the findings are attributed to the stimulation and adaption of the hoof to more optimal stresses..
Taylor (2020) suggests that many of the negative morphologies seen commonly in the adult horses frog and digital cushion is in part due to the lack of stimulation of these structures (fig.7). This study and the authors experience show improved adaptation of these structures when they are re-stimulated, the hoof adapts in response to stimulus of the nervous system and positively remodels in response to new loads. The study also highlighted the importance of different substrates to provide different stimuli and resultant adaptive changes. Even when we go barefoot it is still important that the environment is one that would encourage positive adaption.
Fig.7 The adaption of a hoof taken out of shoes. The new and different stimulations and loads on the hoof have created adaptations. Much of this morphology can be attributed to increased stimulation of the caudal hoof structures. The question is, can we replicate this more closely and what loads do we need to address to do so?
Taylor cited Bowker et al (1998, 2003) and described how an active frog in weight bearing changed its composition from mainly fatty and elastic to fibrocartilaginous. This supports previous articles on the barefoot and haemodynamic system by the author discussing the findings of studies such as Clayton (2011) and Taylor (2014) which showed positive morphology of the hoof when going barefoot as it adapts to the new loads. Taylor (2020) also found increased concavity of the sole after a period of time on pea shingle, again expressing the adaption of the hoof to external stimuli. This also supports the authors findings and Casserley (2018) that offering stimulation of these same structures with frog support padding (while shod) showed similar adaptive response (fig.10).
Taylor (2020) did however support that we are restricted by the developmental stage, suggesting that horses allowed to develop strong structures when young will have inherently bigger distal phalanges and digital cushions, this hypothesis was extrapolated from a study into cows. The finding of Thomason et al. (2001) that there was a deterioration in structural coherence of the foot in a low-angle hoof compared to a higher angle, again shows that what you start with has a direct effect on what you get in cycles of morphology.
Fig. 8 Morphological feedback loop
Thomason et al. (2005) described a feedback mechanism where external biomechanical influences have cumulative effect on the mechanical behaviour of the hoof. Fig. 8 is the authors representation of this feedback loop where outside influences can break a positive or maintained morphological cycle and quickly create a chaotic downward spiral when forces acting on the hoof from the ground and/or the body become less than ideal. This same cycle is seen in the adaption or negative effect on hoof morphology in relation to this article and the stimulation or lack of, of the structures of the hoof.
The hoof and the musculoskeletel system are inextricably linked, posture is emerging as a significant connection between the two, creating influential factors on both. Postural adaptation directly affects the loads on the hoof and the dimensions of the hoof directly effect the posture of the horse, in kinetic chains and myofascial trains. Posture is therefore another cause of hoof adaption and can have abstract causations, again this influence can be positive, maintained or negative.
Fig.9 This image shows the adaption of the hoof in response to improved posture, created by improved hoof proportions and the treatment of higher pathologies that played a part in the causation of the posture. The accumulative load on the heels from a camped under posture, considering the horse stands for the majority of the day, plays a huge role in the morphology of the hoof. To break the cycle we have to change the loads. The posture of the horse will also affect the loads on the skeletal system and therefore have possible influence on bone remodelling and has been linked to osseous pathologies up the hind limb and into the pelvis and spine.
The hoof we have is a product of its history and the horses conformation, yes, but also the influence we had on it in the last cycle, whether that is farriery, management or the horses neuro-musculoskeletal state (which will affect its posture). How we stimulated it and how we loaded it created what we see. What we do to it now will have an effect on what it looks like at the end of this cycle, back to Einstein, if you haven’t changed the stimulation, load or environmental input, how can you expect to see a positive adaption. The fact that the barefoot positively morphs shows us that the hoof is still capable of adaption, so what can we implement to provide the necessary stimulation?
Thomason et al. (2010) stated changes in external capsule shape modifies mechanical behaviour, which in turn modifies internal structure. So do we need to manipulate the shape of what we have? Dyson (2011) discussed and cited papers outlining the effects of hoof proportions on hoof morphology. Do we need to change the proportions and geometry of the foot to create more optimal loads on the hoof so it has a chance to positively adapt to those stimuli? Studies have shown us that the caudal structures improve in depth, width and height once stimulated more naturally (Clayton 2011, Malone and Davies 2019, Proske et al 2017). Do we need to create more natural functionality by loading the caudal structures (Roepstorff 2001) and allow them to positively adapt in response to the stimulation?
In my experience, creating an environment that allows positive adaption comes down to a few certain important factors.
1. Balanced shoeing around the centre of rotation on every axis to create optimal leverage and biomechanical forces on the hoofs structures.
2. Focusing on the structures that need positive adaption and maintaining integrity of other areas. This is most applicable to leaving toe integrity when addressing “long toe, low heel” conformations. Often a long toe is compromised in order to shorten it when in fact it isn’t long but at a low angle due to low heels.
3. Frog, bar and sole support. Stimulating and supporting the caudal hoof structures to make them load sharing structures and optimising the use of the haemodynamic system is proving vital in positive adaption.
4. Shoeing cycle. It is vital that shoeing cycle is appropriate to the individual to ensure the effects of hoof growth don’t create excessive negative forces.
Fig.10 This image shows the results of the 4 factors being addressed. Changing the loads and providing support and stimulation to the frog, bars and sole has caused positive adaptation similar to the results from going barefoot. Going barefoot would, for the majority, be an ideal for the best possible loads on the hoof, but when this is not practical we can still provide those same stimulations by modern interventions.
In conclusion, the statement “… just has bad feet” becomes questionable in the light of this article. Yes the hoof is a product of its developmental stage and some will respond to new stimuli and new loads better than others, but non the less, how the foot is at the next shoeing cycle will be a product of what loads you create this cycle. Taylor (2020) used the term “smart structure” to describe the hoof, a term used to describe structures that adapt to external influence.
We asked at the beginning of this article, can we change the stimuli/influential factors to encourage positive adaptation? Well that depends if you are giving it smart input. Documenting the morphology of the hoof and watching which way its changing, toward ideal or away from it, can point toward what we may have to change to allow the hoof to positively adapt.
References
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