Updated: Mar 26, 2019
“No Foot, No Horse!” what does that mean? Originally it came from the cavalry, you needed the hoof of your deceased horse in order to receive a new one, but today the expression has come to show the importance of caring for our horses hooves, It has been widely documented that shoeing has a direct effect on the musculoskeletal system of the horse (Kilmartin 2014), farriery until recently has been based on tradition, anecdotal evidence and personal experience, but with the introduction of objective locomotion assessment and importantly pressure plate systems, a new revolution of evidence - based farriery research is taking place (Oosterlinck et al 2019). Scientific study of the relationship between hoof balance and the wider musculoskeletal system is still in its infancy, however, with the expression we began this article with its easy to see that this relationship is recognised as having a huge factor in achieving peak performance from our horses, keeping them sound and treating them after injury.
The hoof is the horses point of contact with the ground, the biomechanics of this interaction dictate the physiological effects of movement on the given animal, however this doesn’t mean it’s the farriers fault if this relationship isn’t ideal, we must understand that the form of the hooves follows the forces that act upon it (Curtis 2002, Caldwell 2016), conformation and hoof growth have direct effects on hoof morphology, from the second the farrier puts down a finished hoof, these components are having a negative effect on physiology, a farrier can not change a horses conformation after the point at which its growth plates have closed, what he can do is facilitate a more balanced interaction with the ground by assessing the individual and shoeing it accordingly.
Even with ideal conformation, just normal hoof growth has negative effects on the internal structures of the hoof (Moleman et al 2006, Van Heel et al 2004,2005) but coupled with conformational defects that every horse will have, you have a complexed array of physiological effects. Curtis (2002) discussed how the forces acting upon the hoof are infinitely different in each horse, however you can predict the predispositions of certain conformations and how they will morph the hoof. For the most part (assuming correct farriery) hoof distortion is caused by poor conformation due to the uneven loading from above, not the other way round, however it is the farriers job to recognise the imbalances and look to re-establish balance.
Fig 1 Normal hoof growth causes an increased moment around the distal interphalangeal joint, increases load on the deep digital flexor tendon and the navicular area, this means that just leaving your horse too long between shoeing’s already predisposes it to serious issues.
As stated in the opening statement Dr Kilmartin (2014) discussed the relationship between hoof balance and the wider musculoskeletal system, he stated that even a small amount of imbalance can cause a change in muscle development and tension in the upper body, the imbalances and their effects are too many to count (Curtis 2002). A recent review (Larson 2019) echoed these findings concluding that hoof anatomy and biomechanics were closely interlinked and therefore farriery interventions can have a significant effect on equine locomotion. These points help us to establish the first and possibly most important key to evidence-based farriery, optimizing hoof balance!
Hoof balance is widely debated and remains subjective, Caldwell (2016) asked questions of the long established “Duckets dot” theorem and other widely used balancing protocols and concluded that trimming protocol should be individual to the subject, new studies (Johnson 2018) have again asked questions of what true balance means, adding a new measurement to the equation, impulse forces, these are the cumulative load on the hoof through all of the dynamic phases, raising new questions on hoof balance, but certain principles hold true regardless, establishing as close to ideal as possible, medio-lateral and dorso-palmer balance is critical before any shoe is applied! (Oosternick 2019). Balance, put very simplistically, ensures the limb is evenly loaded, each joint, bone, tendon, cartilage and ligament shares load as its designed to do and therefore has a much lower risk of injury (Oosternick 2019). Wilson et al (1998) highlighted the effects of imbalance showing that the point of force moved toward high points of the hoof causing uneven loading of the internal structures of not just the hoof but the entire limb. Establishing hoof balance is the foundation of any shoeing job, remedial or otherwise, but what science do we know of the different shoes and materials we can apply to that trimmed foot?
To assess the farriery applications administered, which are too numerous to assess individually, we can categorise them according to the biomechanical forces they address through each phase of stride; Initial impact: Secondary impact phase: support phase/mid stance and breakover.
Ideally the fore hoof should land flat across its solar surface to the naked eye, however studies have shown that the majority of horses have a preferred landing pattern of lateral heel first (Fig.2) (van heel et al 2004).
Fig.2 Van heel et al (2004) showing the trace of the centre of pressure from initial impact through to breakover. Courtesy of J.Mather.
Different conformations can result in different initial impacts so each horse should be assessed individually, however, with the unfolding of more research into Johnson (2018)’s findings, trimming to optimise balance will possibly depend on more than simple initial impact. Trimming can be complimented by shoeing to encourage level footfall, sometimes an “unlevel” shoe can aid in a “level” footfall, and in the case of certain pathologies, uneven footfall can mean increased comfort. The hoof by design dissipates force on impact, shoeing and physiology can affect the efficiency of this mechanism, a steel shoe and an upright conformation decreases the dampening effect for example (Oosternick 2019), pads (Fig 3-4) are widely used to counteract this, there is very limited evidence based science on the effects of pads, anecdotally they reduce vibration, spread load across the solar plane and make horses more comfortable, as well as offering protection to thinner soled animals. A recent study (Young 2018) found that horses were happier to load a padded foot over a heart bar shod foot, suggesting more comfort associated with the softer interaction with the ground.
Slide is an important and perhaps neglected factor in secondary impact, it further dissipates the decelerative longitudinal forces and this differs on every surface, the shoe and its characteristics will have a direct effect on this interaction (Clayton and Hobbs 2019), if the hoof decelerates too quickly an increase in force will be transmitted up the limb and through its structures, however if the hoof slips too much there is an inherent risk of falling (Oosternick et al 2019), horses also need grip to generate propulsion (Parkes and Witte 2015). The difficulty arises in the fact that the modern horse commonly works on multiple surfaces, the most appropriate shoe must be selected on an individual basis, shoes that offer grip on slippery surfaces and shoes that offer slip on harder, high friction surfaces. The horse is designed for high-speed locomotion, a reduced number of bones and proximally located muscle mass are just 2 examples of their adaptations but even barefoot they are not as well adapted to changing surfaces as other mammals (Parkes and Witte 2015). As impact peaks are dissipated by the time they reach the fetlock, it is during the secondary impact and subsequent load bearing phase that injury to more proximal structures is most likely, so getting the foot-ground interaction right at these points is crucial.
Support phase/mid stance
This is the point at which the hoof is bearing maximal weight (Parkes and Witte 2015), variations in hoof anatomy play a role in the stress distribution under this load, although again, surface plays a vital role in the biomechanics of this phase (Hobbs et al 2014), farriery can have huge effects on the centre of pressure (COP), the orientation of the hoof, and the hoof-ground interaction, although much of the evidence is anecdotal. Farriery can move the COP by creating a high point, either medio-laterally or Dorso=palmerly (Wilson et al 1998), heel wedges for example, often fitted to reduce strain on the Deep Digital Flexor Tendon (DDFT), move the COP toward the heels, this movement of the COP can be used beneficially in the case of medio/lateral pathologies such as collateral ligament damage. Changing the width of the shoe can reduce sinkage, as this has shown to have detrimental effects on the internal structures as they are strained (Hobbs et al 2014) wide webbed shoes help to keep the foot above the surface and work optimally, in the case of certain pathologies altering the width of certain parts of the shoe can play a part in prevention and treatment, for example, a wide webbed toe stops the toe sinking into the surface, beneficial when treating suspensory injury, a wider branch of the shoe stops that side sinking, beneficial in treating collateral ligament damage (Oosternick et al 2019), a wider heel reduces heel sinkage which has shown to strain the DDFT and can predispose to navicular (Hobbs et al 2014), conversely, there may be times when you wish to encourage sinkage, for example bone/cartilage damage can be relieved of tension.
The lack of evidence-based research into farriery techniques causes many of them to be subjective, the use of lateral extensions is a good example, what constitutes an extension to begin with!? In the authors opinion a true lateral extension extends beyond what would be the proportions of the hoof, establishing solar symmetry by shoeing bold of a contracted wall is just good practice. Anecdotally lateral extensions (Fig 5) make a horse suffering with bone spavin more comfortable by changing the weight distribution, however with further understanding of the pathology of bone spavin the benefits of this technique are questionable (Conroy 2019), a lateral extension is also designed to encourage quicker ossification of the distal tarsal joints, however with the close proximity of the highly mobile proximal tarsal joint, which if affected has a much poorer prognosis, this technique could prove risky. The key factor in the load bearing phase is optimal pressure distribution (Oosternick 2019) many of the mechanisms for achieving this are still anecdotal, as further research is carried out we will better understand the farriery role in all the stages of equine locomotion.