The Influences on Shoeing Cycle
Shoeing cycle length is a vitally important factor in hoof health and controlling the morphological changes associated with hoof growth and the biomechanical forces acting on the hoof. However, it is still a factor that is often dictated by economical and logistical factors, rather than a need dictated by the hoof itself. There are often misconceptions about “allowing the horse to grow more hoof” to create more positive farriery intervention, when in fact the opposite is often true. The effects of hoof growth have been quantified and these parameters should be the deciding factor in shoeing cycle length, as well as the understanding of the structure and properties of hoof itself.
Firstly, not every hoof is the same, not only in size but in microscopic structure (Bowker 2003), this structural difference will have a direct effect on the shoeing cycle. Bowker (2003) showed the vast differences there can be in micro conformational markers, these differences equate to ability to withstand cyclic load and force dissipation. The fibro-cartilage content of the digital cushion, for example is stated as being important in force dissipation due to its proteoglycans. A foot that has poorer micro conformation will inherently need more regular intervention due to its tendency for collapse. This comes down to the elastic modulus of the hoof, Hooke’s law and Youngs modulus of elasticity helps us understand how these structures fail when they exceed their elastic modulus by taking unintended (or intended) load.
Fig.1 This is a representation of these laws applied to two different hooves with different elastic modulus’. Note the different deformation rates and fracture points.
Hooke’s law, F=kx, and Youngs modulus of elasticity, state that the deformation of an object is directly proportional to the force applied to it, up until the point at which the elastic modulus is reached when the material begins to fail and eventually fracture. Different materials will have a different stress-strain relationship depending on the stiffness of its structure, just as two different hooves will have the same difference in their stress-strain graph. This tells us the makeup of the individuals hoof structures will react differently to forces applied on them, some feet inherently can cope with force more efficiently than others and therefore return to form after force application more easily and at higher forces.
So we can see that the micro conformation of the horse plays a large role in its optimum shoeing cycle, as it is important to intervene before the elastic modulus is exceeded by cyclic loading. This point will differ for every horse and those horses that “Need more time to grow” are often horses with a weak elastic modulus and therefore the “growth” will be negated by its own collapse. Keratin structures also suffer from elastic creep (Fung 1993, Wei 2018) where their modulus is constantly tested by pressure over time, so even just standing will have a different effect on different hooves, so the other saying “we haven’t really done anything this cycle, so we can go a bit longer” also becomes a null hypothesis as the wear on the shoe is a less important deciding factor in shoeing cycle.
A factor often overlooked is also the gender and seasonal growth rates of the hoof, Frackowiak & Komosa (2006) found that “The dynamics of hoof horn growth turned out to be the highest in the group of young mares, followed by the group of stallions and the group of adult mares. In the winter months the lowest growth increment of the hoof horn was observed in all the analysed groups. In the period of the elongating solar day, i.e. from May to July, the growth was rapid and reached the highest values. Starting from August the growth of the hoof horn decreased.” This finding shows again how shoeing cycle must be individual and changeable according to the environmental factors in play.
The biomechanical effects of hoof growth further cement the necessity for individual cycle assessment. Van Heel et al (2004,2005) and Moleman et al (2006) findings highlighted the effects of hoof growth on the structures of the foot.
Fig.2 Van heel et al (2004) discovered a common trace of the centre of pressure (COP). The study found that hoof growth affected this trace, the COP moved caudally with hoof growth creating higher loads on the heel and the landing time increased meaning longer load times of the heels. This was exacerbated in a lower angled hoof showing that weaker heeled horses will suffer greater deformation from the same length of shoeing cycle.
Fig.3 Hoof growth affects the proportions of the foot and therefore the forces acting upon it, the distance from the centre of rotation (COR) to breakover increases, creating a larger lever arm for the deep digital flexor (DDFT) tendon to overcome and the COP moves back towards the heels predisposing them to exceeding their elastic modulus and exposing them to greater creep forces.
Fig.4 Adapted from van Heel et al (2005), this shows the changes in hoof wall length and angle over a shoeing cycle with the subsequent backward movement of the COP. Although the horse has compensatory mechanisms which counteracted some of the effects of growth the increased load on the DDFT and navicular is pronounced. This study also outlined that the changes in angle of the hoof was more distinct in a lower angle hoof, again showing that shoeing cycle should be dictated by the individual conformation with low angled hooves needing more regular shoeing.
Fig.5 van Heel used trigonometry to calculate the change in COP. This image the authors use of trigonometry in expressing how the more acute the heel angle is the greater the forward migration of the hoof will be, echoing the findings of van heel et al (2005) in highlighting that lower heel feet need re-balancing more regularly as the forward migration directly affects the forces acting on the DDFT and navicular.
Fig.6 Shows the radiographic effects of hoof growth, it directly influences DDFT strain by breaking the hoof pastern axis and creating a larger moment arm. Moleman et al (2006) showed that hoof growth increased the moment arm around the distal interphalangeal joint (DIPJ) meaning that the effects of hoof growth are transferred onto the DDFT and DIPJ structures. Considering that long toe low heel feet are already strongly predisposed to navicular, one can see how important their regular shoeing interval is. Fig.6 also shows the findings of Lesniak et al (2017) which found “trimming the dorsal wall, weight bearing and coronary band lengths resulted in an increased vertical orientation of the hoof. The increased dorsal hoof wall angle, heel angle, and heel height illustrate this further, improving dorsopalmar alignment.” Lesniak et al (2017) demonstrated that a four to six week interval, even shorter then the 8 weeks of van Heel and Moleman, “is sufficient for a palmer shift in the centre of pressure, increasing the loading on acutely inclined heels, altering DIP angulation, and increasing the load on susceptible structures (e.g., DDFT).”
These images and studies relate to dorso-palmer balance, however hoof growth is 3 dimensional and affects the medio-lateral aspect as well. Some horses, due to their conformation grow imbalanced.
Fig.7 Wilson et al (1999) found that the COP moved toward the high point of the hoof creating higher stress forces on that side of the hoof as well as creating unbalanced forces transferred all the way up the limb.
Wilson et al (1999) showed how horses with medio-lateral imbalances also require more regular intervention, specifically if they are high medially, the study discussed that horses with the outside of the foot higher could compensate by widening their stance, however horses that were high on the inside of their foot showed greater discomfort as they were unable to compensate for the imbalance.
Clayton 1990 found that acute angulation was associated with fewer heel‐first impacts and a greater number of toe‐first impacts than the normal angulation and Breakover time was prolonged with the acute angulation. This suggests that at the end of a shoeing cycle, with the angulation of the hoof being more acute, a horse may be more likely to have a toe first landing. A longer breakover found by Clayton and a longer landing phase by van Heel et al (2004) are both seen as having negative effects, a longer landing phase suggests more time before the hoof has full support and a longer breakover suggests increased loads on the DDFT (van Heel et al 2005). Page et al (2002) also discussed breakover, stating the hoof-pastern axis and the position of the navicular bone will be affected by the distance from the apex of the frog to breakover, a parameter directly affected by hoof growth. It also expressed the resulting decrease in strain (by a regular shoeing cycle) of the deep digital flexor tendon while standing and during movement will decrease inflammation and disease in the equine digit.
Remembering that the hoof is the beginning and end of a kinetic chain the effects of hoof growth will extend into the body of the horse, starting just above the hoof van Heel et al (2006) found that the pastern and fetlock compensated for the change in hoof angle. Kane et al (1999) linked catastrophic injury in race horses to poor dorso-palmer balance, considering race horses hoof conformation, this emphasises the importance of very regular shoeing of these types of feet. Faramazi et al (2018) stated that “Stance-phase duration, swing-phase duration and consequently, gait-cycle duration decreased significantly (by approximately 12%) after trimming” also expressing that “proper contact with the surface may improve foot vascular perfusion and that changes in hoof conformation after trimming improved the force and pressure distribution. The improved vascular perfusion could be attributed to more optimal frog contact as hoof growth can create a bigger distance for the frog to travel before making ground contact.
Fig.8 Authors illustration of the role of the frog in force dissipation and subsequent role in haemodynamics of the hoof. This displacement of the digital cushion and lateral cartilages creates a pumping action of the blood.
Excess hoof growth can render the frog underutilised or even obsolete in more upright conformations, this can affect the functions of the frog, generally leading to reduced hoof health, increased concussive forces and create contraction of the heels. While the flatter foot may need shoeing more often to negate its forward migration, a steeper angled hoof may need more regular shoeing to maintain frog function.
When a horse has one of each, mismatched feet, this can also create the need for more regular intervention.