The hoof/ horse connection

To 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 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 increased dorsiflexion were found to be mechanically predisposed to navicular and that DDFT lesions corresponded with areas of increased load. It is quite clearly outlined by the majority of the existing literature that “long toe, low heels” predispose to navicular syndrome. The predispositions of this hoof conformation aren’t isolated to navicular syndrome. Clayton 1990 looked at the kinematic of the stride between a more upright foot and a more acute angled hoof where the hoof was allowed to grow into a broken phalangeal alignment. It found longer time to full solar surface bearing, and longer time to breakover, which has a biomechanical effect on the heels of the hoof and the navicular, but one very significant finding was that of toe first landings, something that has been linked to navicular, but also can cause other arthritic changes. As well as increased strain on structures such as the suspensory of the navicular and the whole podotrochlea apparatus, the concussive forces of toe first landings also predispose to articular ring bone, both high and low.

In the front feet most of the implications, although we will outline others later, of poor dorso-palmar balance are isolated within the digit. Mainly affecting the navicular region and distal joints. Medio-lateral imbalances have been shown to affect the horse further into the musculoskeletal system along the front limb myofascial lines.

At this point it is important to outline that outside of poor farriery, medio-lateral imbalances are a factor of poor conformation such as angular limb deformities or laterally offset hooves, as these create increased off axis loading leading to morphological deformations. Kilmartin (2014) outlined an example of the implications along the front limb myofascial lines. He stated that even a small amount of imbalance can cause a change in muscle development and tension in the upper body, “In cases of medio-lateral imbalance in one of the forelimbs the medial wall of the hoof is more vertical, and the lateral wall is flaring out. Looking at the sole of the hoof the medial wall is higher than the lateral wall. In these cases, the Transverse, Ascending, and Descending Pectoral muscles are working along with the Subscapularis and Brachiocephalic to keep the fore limb under the body. These horses again consistently show pain or reactivity over the cartilage of the scapula.” This statement clearly shows the compensations the horse has to make as a consequence of poor hoof balance.

As well as imbalance in the individual foot, imbalances between feet create compensatory patterns within the horse. These imbalances between pairs of feet are great examples of full body compensations that arise and have been compared to subtle, sub-clinical lameness. Hobbs et al (2018) found that the lower hoof in the pair had increased breaking forces as found in Wiggers et al (2015) and an increased vertical force, mirroring the pattern of Weishaupt (2008) for lameness. Links between lameness, which has been shown to be linked with poor hoof balance, and compensations in the spine, were outlined by Landman et al (2004) and Gomez-Alvarez et al. (2007).

With asymmetric propulsive forces the animal has to stiffen its spine and has an adapted hind limb locomotion in order to maintain straight line propulsion. Dr Ridgeway (2016) discussed her experience in some detail on the physiological effects on the musculoskeletal system and highlighted the benefits of interventions that increase symmetry and balance, expressing the “functional limb length disparity” of high-low hooves and described how the difference in joint angles affecting the muscular development. Ridgway (2016) also talked about vertebral function touched on by Hobbs et al (2018) who speculated that vertebral stiffening may be required to apply the locomotive adaptations required. Ridgway described the animal’s response to the imbalanced propulsive forces, “The horse has to twist his head and neck to keep the eyes level. Horses, therefore, often exhibit muscle pain, stiffness and spasm at the base of the neck. Moreover, because of dural torque (the tube in which the spinal cord is suspended and anchored), vertebral dysfunction and fixation occurs at the base of the neck. This, then, accounts for the muscle tension and pain around the sixth and seventh cervical vertebrae. It also causes dural torque (twisting) at the level of the poll and at the lumbo-sacral connection. Chiropractic issues are therefore common at all three levels as a result of the High Heel/Low Heel” Ridgway (2016).

Dyson (2011) stated that poor hoof balance was directly linked to lameness, lameness creates full body compensations and therefore, hoof balance of the front feet can potentially create issues anywhere along the extensive myofascial system. The implications of poor hind hoof balance have been shown to be different from the front limbs, suspected to be due to different ground reaction forces experienced as a result of a more propulsive job. Studies have also suggested the links to musculoskeletal issues being more profound in poor hind hoof balance as a result of the hind limb being connected to the trunk via a joint. When looking at the hoof horse connection in the hinds, posture becomes a very important factor. Every horse will have biological variation, different length bones, different angles to their joints, but Something that is emerging as an ideal is the idea of vertical metatarsals at rest.

DR Shoemaker, DR Gellman, and DR Rombach also express in their work and writings the ideal of vertical metacarpals and metatarsals. When they are in this orientation, they are counteracting gravity efficiently and bearing load in pure compression. When they aren’t vertical, like in a camped under posture, horses need to use immobiliser muscles as stabilising muscles due to a reduced base of support and brace themselves before initiating locomotion and in general over stress the musculoskeletal system.

Importantly having the ideal digit plays a vital role in creating this ideal of vertical metacarpals and tarsals. Poor hoof balance creates the necessity for compensatory adaptions and also distorts the neurological in put being received from the feet. this will directly affect limb orientation. In my research I have found this posture to be ubiquitous in the domestic horse population. The causes are only just starting to be uncovered but come very much down to domestication. Influences such as confinement, eating from hay nets and modern riding techniques, and of course hoof balance. Therefore, there is very much a fluid relationship of the hoof-horse connection emphasised in the hind end.

While the implications for poor hoof balance in the front limbs are somewhat isolated for the main to the distal limb, in the hind pathology is referred to higher structures. Dyson et al. (2007) eluded to poor hind hoof balance being a predisposing factor in suspensory lesions, Mansmann et al. (2010) linked “long toe, low heel” conformation with gluteal pain, Pezzanite et al. (2019) concluded that horses with hind limb lameness localised to the distal tarsal and proximal metatarsal regions were likely to present with negative plantar angles and most recently Clements et al. (2019) correlated negative plantar angles with stifle pathology amongst others, with all of these studies highlighting the benefits of farriery intervention. Mannsman and Clements discussed the importance of the dorsal myofascial line and suggested further implications into the trunk of the horse. Pezzanite, Mannsman and Clements all discuss the camped under posture associated with poor hind hoof balance but focus on kinematic implications being responsible for the link to higher pathologies. Pezzanite alluded to the morphological implications on posture on the hind hoof, suggesting an increased load on the heels, anecdotally, experiential opinion would agree although there have been no studies to quantify this. This is where the two-way fluid relationship becomes apparent. Either poor hoof balance creates an adapted posture, increasing strain on the dorsal myofascial line, predisposing to the associated pathologies. Or, the influences of domestication and possible higher pathologies create an adapted posture, increasing heel loading and creating a negative morphology. The cycle then becomes self-perpetuating whichever way it starts. My personal research looked further into this cyclic relationship. I correlated the Prescence of negative plantar angles with a camped under posture, measuring metatarsal angle. Then using thermography, I correlated this with the presence of increased surface emissivity suggesting areas of increased inflammation along the dorsal myofascial line. Then I did farriery intervention and measured the changes in posture. Farriery intervention to improve hoof pastern axis directly affected limb orientation. There was no significant difference in hock angle pre and post intervention, despite a significant negative correlation between how broken back the hoof pastern axis was and metatarsal angle. This also suggests, due to the reciprocal apparatus, that the stifle angle was not significantly changed. This pointed toward the change in limb orientation coming from higher structures, likely the pelvis, or lumbosacral area. 91% of the cases had caudal thoracic inflammation, 91% had Sacro-iliac, 58% had gluteal, 58% had hamstring, 58% sciatic, 91% hock, 16% stifle and 25% had proximal metatarsal inflammation.

The study strongly suggests a clear relationship between hoof balance, posture and musculoskeletal pathology. This study shows the importance of a multi-profession approach to managing both hoof balance and higher pathologies. Postural assessment should become part of farriery protocol and incorporated into intervention decisions. Veterinarians and practitioners should consider hoof balance could be a product of the physiological state of the horse as well as a contributing factor. Also acknowledging the importance of its correction in the treatment of the higher issues.

While research into the effects of medio-lateral imbalance in the hinds is limited, there is plenty of anecdotal and experiential evidence of its implications. Medio-lateral imbalance in the hinds can create Rope walking/rope standing where the limbs are moved toward the midline, this creates uneven loading and wear of the hoof. This then exacerbates the necessity for the posture as the horse instinctively wants to stand on a level surface. Conversely, improving the hoof balance commonly results in improvement of the static and dynamic posture.

Again, we see a cause-and-effect cyclic relationship. Irrespective of the causation, this relationship creates Imbalanced joint load all the way up the limb, notedly in the hock and problems in the stifle affecting collateral ligaments all the way up the limb. This can create compensatory contraction of the abductor, a contracted iliopsoas and torque in the pelvis, creating issues with the Sciatic nerve, SI and lumbar. This medio lateral imbalance behind creates issues along the dorsal and ventral myofascial lines.

The fluid relationship is apparent, the poor hoof balance that may or may not be created by confirmation or pathology predisposes to pathology and postural adaptation. This brings us back to the recognition of the perpetuating cycles that occur between hoof morphology and higher pathology. As we further research the myofascial lines, and most relevantly the ones that extend into the hoof we will uncover further kinetic chain relationships between the hoof and the rest of the body. The more we look at the horse as a bio-tensegrity, the more we will appreciate that any anatomical point within its systems, can and will affect every other