The Hoof Horse Connection – ACPAT Conference 2021 Yogi Sharp DipWCF BSc (Hons)
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o understand the hoof – horse connection there are certain principles of hoof biomechanics we must first appreciate. How the hoof functions and what affects its functionality, how the hoof morphs according to loads it experiences from conformational and pathological influences and finally how its shape then affects posture and predisposes to injury. These different factors create and perpetuate positive or negative morphology-pathology cycles. Hoof capsule biomechanics were outlined by the studies of Thomason et al. (1998), the truncated oblique cone with thinner walls at the heels, affords the hoof certain normal deformations it uses to absorb concussion. The hoof is able to deform and return to shape, as long as it stays within its elastic capacity. Due to the massive concussive forces the hoof experiences, it has another mechanism to disperse these shock vibrations. Three main haemodynamic mechanisms of the hoof were outlined by Bowker, as well as the differences in strong versus weak systems. Differently conformed hooves will utilise different haemodynamic mechanisms with the ideal hoof perhaps utilising all three. This becomes important in the discussion of hoof morphology as the elastic capacity and therefore shape of the hoof is directly affected by its ability to disperse shock. The hoof is a Hookean material, basically the amount of strain the hoof experiences, which is the amount of deformation in response to a stress, is directly proportional to the amount of stress applied. The size of the increments of strain depends on the stiffness of the object, its inherent composition. This composition also plays a direct role in the hoof’s elastic capacity. A weaker hoof will fail under the strains of a dysfunctional musculoskeletal system and inability to disperse shock, before a strong one does. Douglas et al. (1996) outlined the elastic modulus of the different areas of the hoof, clearly showing that the heels had a lower elastic modulus to match their function of expansion, however this lower modulus means they will fail before the dorsal wall, changing the proportions of the hoof, creating negative cycles. Hooke’s law and young’s modulus therefore become factors in dynamic morphological implications for the hoof. Via its visco-elastic property, the hoof is
good at dispersing rapid shock when it is working efficiently, within that elastic limit. It deforms and returns to shape. If we go outside that elastic limit, even slightly, we can get cumulative plastic deformations of that area, leading to measurable morphological changes over time. This is often insidious, leading to a lack of recognition until there are obvious hoof imbalances. As well as morphological changes from dynamic forces, the hoof is also subject to time dependant forces. When we look at longer loading times, or even accumulative short loading times we start to see the effects of a phenomenon called creep. There is little research into creep in the hoof, but it applies to all viscoelastic materials. Creep it is the tendency of a solid material to move or migrate slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress even if they are below the yield strength, or elastic capacity of the material. The rate of deformation will be a function of the hoofs individual properties, exposure time, and the applied structural load. This becomes important in the hoof connection discussion because if we have a centralised load, we can assume uniform strain within the hoof, and it will deform symmetrically. If we have off axis loading, then we will have increased load on one side and we can assume increased strain and deformation. This off axis loading can come of course from conformation or postural defaults in the horse as well as poor farriery. Through these phenomena the hoof becomes subject to the forces arising from the physiological state of the animal, the forces coming from above and the ground. The hoof is a deformable structure, subject to the weight of the horse and its interaction with the ground. The morphology of the hoof is subject to the magnitude and direction of forces it experiences, and these create cycles. Hoof shape is a factor of its mechanical function, its mechanical function will create morphology, affecting its mechanical function and so on. When these become inappropriate, negative pathological cycles are created. These cycles then extend beyond the hoof and spread through the whole body. Ideal physiology, creates ideal hoof loading, leading to ideal morphology, leading to
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ideal load on the musculoskeletal system. Conversely, we can get negative influences from biological or environmental variations and a negative cycle ensues. Another important factor in the link between the hoof and the horse, is the hoofs role as a neuro-sensory organ. The hoof is the primary way for the horse to gain information about the physical features of its surroundings. Its feedback comes from the distortion the hoof undergoes as well as the physical touch of the ground etc. This feedback instructs the horses posture and way of going and importantly its adopted postural stance. The increased strain in the deep digital flexor tendon as a result of “long toe, low heel” conformations coupled with this change in proprioceptive input from the hoof are hypothesised to be responsible for changes in limb orientation. The ideal hoof gives ideal proprioceptive feedback, creates the ideal digit conformation, which enables correct limb orientation and this positive effect transfers through the entire musculoskeletal system. The implications of poor hoof balance have shown to be different in the front and hind limbs. To understand the predispositions in the front limb we need to firstly outline some biomechanics. Weller (2020) outlined that the extensor moment acting on the limb is calculated by the ground reaction force acting through the COP times the distance of the COP from the centres of rotation. The extensor moment is a rotational and collapsing force acting on the limb. In order for the limb to not collapse the counteracting force, being the tension in the flexor structures times their moment arms, has to increase. As the flexor moment arms are stationary, the only way to counteract the increased collapsing force is to increase strain in the flexor structures, predisposing them and the fulcrums they pass over, markedly the navicular, to injury. Waguespack and Hanson (2010, 2011, 2014) outlined the biomechanical considerations and stated that the primary source of pressure on the navicular bone (NB) is compression from the deep digital flexor tendon (DDFT) also stating that creating a straight HPA was an effective treatment for navicular. Ruff et al (2016) expanded on this, expressing the increased compressive force on the NB from the DDFT in conformations exhibiting increased dorsiflexion. This was echoed by Uhl et al (2018) which stated conformations with
