Farriery has, up till recently, been largely based on anecdotal evidence and experiential opinion. We are entering a new era of evidence-based research into farriery interventions, what is the research telling us? How do we extrapolate practices for our daily work?
Firstly, it’s important to highlight that when it comes to research, single studies often only show a truth for a certain population at a certain time and according to the protocols that study used to gather its information. Very importantly studies are subject to the way the author analysed the data collected and what conclusions they came to, this always needs to be critically appraised. One needs to use ones own critical thinking to dissect the studies and decide if what it said the data shows is actually logical. What is more powerful, is when you start to get groups of studies that point toward the same set of logical principles. That’s when you can create logical implications for daily practice.
When you start doing that, you still need to apply those principles correctly and gain practical experience as to what theories stand up in practice. The reality is that with podiatry any theory is only as good as the application of that theory. As much as knowledge can point toward certain practice, practical skill will always be a limiting factor in its efficacy.
Take wedges for example, something widely debated in the industry. Many people and some studies have suggested that they crush heels. Weller (2020) discussed this very recently, we know that the point of force is moved toward the heels with wedges (Wilson et al.) and the length of time that the heels are loaded through the stance phase is prolonged. So, people logically extrapolate that the heels will suffer as a result, but, Weller also discussed how moving backwards of the point of force, reduces the extensor moment and therefore reduces the strain on the flexor structures.
Fig.1 Schematic illustration of the movement of the point of force (PoF)or centre of pressure (CoP) with elevation of the heels. A reduction in extensor moment arm = a reduction in flexor structure strain.
If we provide adequate frog support, length and get the wedge fitted correctly it has obvious benefits for certain horses, fitted wrong without consideration for the consequences, it does perpetuate the conformation that leads to their need in the first place. In the authors experience, if they are fitted with certain important factors in place, that risk is mitigated and they become a very useful therapeutic and transitional tool as you begin to see improvement in the heels.
Fig. 2 Schematic diagram of a well fitted wedge vs a poorly fitted wedge. With every application the efficacy is often subject to correct application.
The point is that we need to critically appraise both theoretical sides and balance them with real world experience and logic. For instance, using the same subject, one of the biggest problems with studies that have suggested an increased intra articular pressure and increased load on the other flexor structures with wedges, is that they didn’t established an ideal. They are telling you that the numbers change, but without an objective quantification of what the intra articular pressure should be, how can one establish whether to attribute a positive or a negative to the change in pressure. When you put these findings against the studies that show the predispositions of broken alignment, you then have a balancing act to work through to decide your own practice. For me the catastrophic injuries associated with poor alignment outweigh the risks of heels being crushed, especially when those risks are mitigated and In practice i have actually found the opposite to be true.
So that’s the process that one has to go through with the research, take the studies that may even be opposing and work through them. Try to establish what can actually be gleaned from them and what makes logical sense. You can’t take a single study and especially the authors interpretation of its data and just use that one finding to dictate your daily practice.
Another really good example, in my opinion, for this process of critical appraisal is a recent study, Craig (2020). It took thousands and thousands of x-rays and found that perfect alignment was very rare. Something that the author would agree with from real life experience and something that other studies have confirmed (Dyson 2011, Clements et al. 2019) .
The question is what do these studies tell us? Is common correct?
looking at Craig (2020), I would come up with a different analysis of the same study, and this is the problem with science, in inverted commas, because the data is the data but the interpretation of the data will always be somewhat subjective. Some people use that study to imply that aligned is not correct and perhaps we shouldn’t aim for it. But I question, is it saying that, or is it saying we have an issue that needs addressing? considering we also know that caudal hoof failure is found in as much as 75% of the shod population and is directly linked to lameness (Dyson 2011). These findings also raise other questions, where do we draw the line between acceptable natural variation and something that predisposes to pathology, especially in the light of all the other studies that clearly show a relationship between poor alignment and navicular for example.
Weighing up individual studies against the wider literature helps to guide practice. For the author this has led me to choose a practice that looks to establish as straight a hoof pastern axis and near straight phalangeal alignment as is sensibly possible, or at least work towards it. If we aim for aligned we will probably miss it by a working tolerance, if we use such studies to disregard alignment as a result of bio diversity without assessing the individual cause, then for me that becomes dangerous. Podiatrists don’t have the luxury of radiographs before and after every job, however, a straight hoof pastern axis is easily recognisable. A straight hoof pastern axis externally will usually still be a few degrees out at each joint of the digit. If we don’t aim for a straight HPA then how much more would those angles be out. The point though of all that is that we need to use our own critical thinking and logic to firstly question the reliability of and then correctly apply a studies data before we just accept the authors interpretation.
Podiatrists should have groups of studies for different aspects of podiatry that compliment each other and point toward certain logical principles.
There are different aspects of podiatry that need to be considered and a library of studies utilised to build practical principles from.
The following are my principles for daily practice.
Hoof Function, Boney Alignment and Biomechanical considerations.
Taking the studies into the barefoot and its function can act as a blueprint for what would be ideal functionality. Nature and evolution has had thousands of years to create something that works, so we should try and replicate that.
lets start with Hoof wall biomechanics. How does the hoof capsule work and move/flex and distort? The studies of Thomason et al., specifically the study dating back to 1992 showed us how the hoof naturally distorts.
The hoof wall is an obliquely truncated cone that opens posteriorly between the heels. The wall has to withstand two types of loading: high-velocity impacts with the ground and transmission of forces between the ground and the skeleton. The deformation of the hoof is therefore very important in absorbing concussive forces. The distortions that were documented were,
- Inward Movement of dorsal wall
- Expansion of heels
- Depression of coronary band
- Sinking of heels
- Flattening of sole
- Biaxial Compression of Dorsal Wall
Fig. 3 The natural viscoelastic deformation of the hoof.
If we look at the old studies of snow and birdsall (1990) suggesting A change in hoof shape may be the result of a shoe restricting normal capsular deformation, which was recently cited by Dyson (2011), which was looking at essentially caudal hoof collapse, we can see a logical link between this natural viscoelastic deformation and hoof proportions and health. This was also suggested by Gunkelman and Hammer (2017) who stated the ability to efficiently dissipate the forces of locomotion directly affects hoof morphology.
We can put these studies together with the studies of Bowker and Poss on haemodynamics. Bowker has discussed 3 theories for how the haemodynamic system works, to briefly outline them there is the depression theory, compression theory and the negative pressure theory. One of those theories state, displacement of the digital cushion presses against the lateral cartilages and subsequently compresses the vascular structures. Another theory suggests the descension of the middle phalanx induces an outward displacement of the lateral cartilages, this theory is somewhat backed up Taylor et al (2005) which indicated that the function of the digital cushion was mainly to counteract this displacement of the middle phalanx and not to provide a pressure force.
Bowker suggests another theory, “When the foot hits the ground, the bars of the heels and pillars of the hoof wall force a small “shelf” of the cartilage outward, creating negative pressure in the digital cushion. Impact is thus transmitted to a complex venous network inside the cartilage, creating more negative energy, which draws blood up from the solar area of the hoof.”
Fig. 4 The traditional haemodynamic theories
Which ever theory one accepts and perhaps all are true, what we can take from this is that the caudal structures of the hoof are important in dampening the forces of impact AND responsible for aiding the natural deformation, which in return is important for the correct morphology of the hoof.
So what does this have to do with shoeing and how do I address this in daily practice. Well if you look at those studies and combine them with the Roepstorff (2001) study which showed that the expansion in a shod foot was restricted compared to a barefoot but frog support padding returned functionality to the foot closer to that of the barefoot. Then you add the studies showing the improved morphology of feet taken out of shoes by Clayton, Malone and Davies and Proske it all starts to point toward caudal hoof support being an important factor in a functionality and therefore a healthy hoof. Going back to Bowker who also spoke about the difference between a strong and weak foot this tells me that in daily practice, weaker feet need this support even more so.
Fig.5 A strong vs weak haemodynamic system
So to conclude that section, by looking at the functionality of the bare hoof and the impact of shoeing and non-frog contact, when a horse cant go barefoot, which is a debate for another time, then I want to provide the same functionality as close as I can by providing frog support padding.
If you reverse engineer the positive morphology of going barefoot, and consider that weak caudal structures will fail, we then see how the correlation of the health of the caudal structures, namely the heels, directly affects the hoof proportions, toe to heel ratios and therefore alignment. Which brings me onto my next aspect of practice.
Going back to the study I mentioned suggesting alignment is not normal or ideal and other teachings that talk about the ideal of a ground parallel pedal bone, this has created a lot of confusion about phalangeal alignment and hoof pastern axis. The amount of peer reviewed evidence-based studies to agree with these suggestions are limited, however there are a substantial amount of agreeing papers that outline alignment as an ideal and discuss the predispositions of a broken alignment. Even breaking alignment from hoof growth was outlined as having negative implications.
Moleman et al (2006) showed that hoof growth broke the HPA and increased the moment arm around the distal interphalangeal joint (DIPJ) surmising that the effects of a Broken back HPA (BBHPA) translate into increased load onto the DDFT, distal phalanx and DIPJ structures. Earlier studies also showed similar effects of hoof growth in breaking the HPA and the corresponding negative correlation biomechanically. Van Heel et al (2004,2005) and Moleman et al (2006) findings highlighted the effects of hoof growth on both biomechanics and soft tissue load.
Clayton (1990a, 1990b) showed the increase in breakover time of the fronts and hinds with a BBHPA and an increase in toe first landings in the forefeet.
A straight HPA has been described as important for optimum biomechanical functionality (O’Grady 2018, Brown 2020) but more importantly a BBHPA has been linked to pathology and inefficient biomechanics by repeatable peer reviewed studies.
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.
Logie (2017) which stated “In the negative HPA more force is placed on the flexor tendons, which is transmitted as pressure into the navicular region; the horse may try to alleviate this pressure by changing its stance, so that its feet are in front of the perpendicular. The hoof capsule is overloaded in the posterior portion and may crush as a result, this and the change in stance can create a vicious pain cycle creating collapsed feet which are slow to recover, if at all.”
Turner (2020) “there is no reason not to shoe for a correct hoof axis and a broken hoof axis can predispose to lameness problems and it has been associated with a greater risk of breakdown in racehorses.”
Witte (2014) “In order for the horse to perform optimally, it is important that the foot is in balance. A balanced foot requires medio-lateral and dorso-palmar balance, with a straight foot-pastern axis. A balanced foot allows for the correct distribution of force within the foot and limb and reduces the likelihood of injury.”
Brown (2020), “HPA determines the alignment of the bony column and therefore, the digit's ability to perform mechanically to its potential.”
Zani et al (2015), “A reduced palmar angle and increased angle between the middle and distal phalanx were observed in horses with alterations of collateral ligaments of the distal interphalangeal joint and navicular bone spongiosa, respectively.”
And all of these studies are only talking about the front feet. The implications for the hind feet extend all the way into the trunk of the horse, according to Mannsman et a. (2010), Pezzanite et al. (2018), Clements et al. (2019) and Walmsley et al. (2019).
So when you put all of these studies together it becomes very clear, at least to me, that alignment is something that we should work towards in our daily practice. In my experience hoof functionality and improvement in hoof proportions and therefore alignment go very much hand in hand, we also know from some of the studies mentioned and from basic physics, that the distance from the centre of rotation to the point of breakover increases with broken alignment and also the point of force application moves toward the toe.
Eliashar 2004 told us that for every 1 degree reduction of solar angle we get an increase in deep digital flexor strain of 4% and we know that the extensor moment, which is the collapsing force acting on the limb is calculated by the ground reaction force multiplied by the moment arm from the point of force to the centre of rotation of the joints. And this is counteracted by the flexor structures at the back of the leg.
Fig. 6 The increase in tendon strain with the reduction of palmar angle. Possibly causing micro damage with every cycle.
So the factors follow on from each other and influence each other, these biomechanical considerations are affected by alignment and toe length and that is affected by caudal hoof health which is affected by function. But also this cycle works backwards, biomechanics directly affect hoof morphology. So this brings us then onto the next factor, biomechanical considerations.
Biomechanical considerations mainly, or rather simplistically come down to shoe placement and lever arms and balance around the centre of rotation on every axis. That of course starts with the trim, which shoeing should compliment. Trimming for alignment and proportions around the centre of rotation is just as important as the shoe you put on. We just mentioned toe lever arms and how they increase flexor strain, so biomechanics is about creating efficiency of movement and correct load share of the internal structures of the hoof. The lever arms are measured from the centres of rotation, the centre of rotation of the distal interphalangeal joint is the point that the hoof rotates around, so our balance around that point dictates the work load of the flexor structures in counteraction to the extensor moment and the initiation of breakover.
The centre of rotation has been established as an important factor for balance around by studies such as Mark Caldwell’s PhD, which stated using the COR as a base point proves to be an efficient way of establishing and maintaining optimal geometrics, and also the work of Dave Ducket, Gene Ovenick and Jim Ferrie also all discuss this point as a datum for hoof balance. All of these studies also suggest a way of finding this point externally. Caldwell advocates a hoof mapping system where you take a line from the base of the buttress of the heel to the inside of the white line of the toe and then cross from end to end to create a crossover point where the COR would be. Duckets bridge is across the widest part of the foot and this will usually correlate with the termination of the bars. Jim Ferrie takes a lateral of the hoof and divides the hairline into thirds. The intersection between the first and second thirds is the COR.
Fig. 7 The COR by different mapping systems.
Personally I use Caldwells mapping system or ferries lateral third but however you do it this point becomes where optimum biomechanics can be created around. Now we have that anatomical reference point we want to create balance around it on every axis as I mentioned earlier. For me I use a mixture of these studies to establish my shoe position for balance. The simplest way to describe it is that I am looking for a 50/50 split of the base around the centre of rotation, from the widest part of the frog, which is my personal minimum length of shoe to the point of breakover.
This is where the alignment comes in, it becomes increasingly harder to achieve 50/50 around COR the more broken back the HPA is, because the distal phalanx and therefore the toe is rotated dorsally, everything has a more acute angle and the toe lever arm gets further away from the COR. Its actually basic trigonometry, if you have a reduction in the angle of the hypotenuse then the base has to become longer. So this is where the balance on every axis comes in and alignment becomes an important factor in biomechanics.
Something that is important to understand about breakover is that just because reducing breakover has positive effects, like reducing the tension in the flexor structures, doesn’t mean that an even smaller distance to breakover gives even more benefit. We have to appreciate that the limb is like a spring and the tension in the flexor structures acts to provide propulsion with reduced muscular effort, if we reduce the breakover too much then the horse has to start using more muscular effort to initiate breakover.
There is an optimum position, Ovnicek first suggested it and this has been developed since by Myers and kornherr and the ELPO.
Fig. 8 ELPO Mapping system
In the average foot, the tip of P3 is 1” forward of the apex of the trimmed frog, the optimal breakover is then ¼” forward of this. Ducket and more redently Caldwell advocates for the distance from the heels to the centre of pressure to be the same as the toe to the centre of rotation.
Fig. 9 Ideal proportions of the hoof as advocated by Ducket and Caldwell.
When we address these considerations we create an environment for good hoof morphology. This is when all of the 3 factors come together to tell us what we want to create and what the ideal is that we want to work towards. The truth is we are so used to seeing poor hooves that we forget what it should look like and what I call unacceptable norms. Assessing the both the horse and its hooves has got to be a integral part of daily practice.
The process of assessment.
Firstly, I look at the posture of the horse way before I get to the feet, if horses are perpetually camping under for instance, in my opinion this is a red flag and I know I am likely to see morphological changes in the feet, and the feet could well be causing the posture, in which case I need to do something about it. I want to see the horse standing, for the most part with vertical metacarpals and tarsals.
Fig. 10 The ideal digit and the ideal posture.
Drop a line from the horses point of buttock and it should run down the back of the metatarsal, in front a line dropped from the point of shoulder should bisect the limb and fall just behind the heels.
The reason I start with posture is because the next thing to look at is hoof pastern axis, and posture directly affects hoof pastern axis, if the horse is camping under behind then this will disguise a broken HPA and this is why this is very often missed behind. To assess hoof pastern axis you simply take a line through the middle of the pastern and this should be the same angle or very close, to the dorsal hoof wall. The best way to do this is to take a photo of the digit from ground level, so with the phone actually on the ground sideways and at 90 degrees to the hoof. From this view we can assess a few things that give us a lot of info on how close to ideal we are. Many studies tell us that the pastern and hoof angles should match.
If they don’t then we start to look at the difference between the heels and the toe angles and heights. As we said before these ratios directly affect alignment. Dyson 2011 suggests that an angle difference of greater then 5 degrees could suggest heel collapse. Another way to see this collapse from the same view is the look at the hairline, if it curls downwards toward the heel bulbs then this can be a sign that that caudal hoof is failing. Then we can take a line from the front of the hairline to the back and divide it into thirds, ideally we should have up to but not more then 60% of the foot in front of that line, or rather the toe.
To summarise, from the lateral ground level view we should have a straight hoof pastern axis, the heel and toe angles should be within 5 degrees of each other or there abouts. The amount of hoof in front and behind the centre or rotation should be no more then 60% to 40%.
Fig. 11,11b The ideals from a lateral view. These can be easily, quickly and accurately assessed in the field using HoofmApp.
Now we can start to look at the underneath of the foot. In reality this is the first thing most people do, but you have missed so much information about the balance of the foot. The first thing I want to talk about here is the frog, for me frog health is one of the most important factors in a healthy hoof and yet we see the majority of frogs being thrushy, withered, contracted and the likes. Taylor recently described what the frog should look like, its width should be between 50% and 60% of it height and it should have a small shallow central sulcus. Something that I look for is that the heels, and by that I mean heels of the hoof and not shoe, should be at least the same height as the frog, if it is less then I would suspect they are beginning to fail or have been trimmed too low. The bars should be straight and strong as they play an important role in the haemodynamic system.
After look at the frog we are looking for general symmetry, the foot is never perfectly symmetrical, but gross imbalances suggest something is wrong. Again 50/50 – 60/40 around the centre of rotation and a nicely concaved sole.
Fig. 12. Healthy caudal hoof structures.
One of the biggest influences on my practice is looking at the barefoot. Going back to what we spoke about regarding function, the Roepstorff study and then seeing the studies that have shown improved morphology barefoot, to me its completely logical that functionality of the hoof and importantly frog and caudal hoof function is vital for hoof health. In my opinion and I have done a videocast on the subject, for the hooves that fail shod, the lack of frog and caudal hoof support is a huge contributing factor. I think that is the biggest thing that I take from the barefoot, the fact that it works as a singularity, it has many different structures that work together to provide specific functions. The frog, bars and digital cushion, as we saw from the Bowker studies, play a huge role in a functional haemodynamic system, concussion absorption and therefore structural integrity. Shoes negate this unified function to a certain extent creating different levels of consequence for differently conformed and managed hooves. But, Shoeing is necessary for different reasons. The barefoot vs shod argument, in my opinion, is counter productive and not in the best interests of the horse. Many horses are shod unnecessarily, but some horses, very much depending on what they are used for as well as conformation, need shoes. I have recently written an article on the future of horse shoes where I explore the idea that looking back at the barefoot is the way to innovate technologies for its own protection. But for now this is why I push for frog and caudal hoof support in horses that need to be shod and actually I now question more readily if the horses under my care actually need shoes. If people use composite shoes and glues, then yeah that makes sense to me considering Back et al showed the shock forces to be more similar to that of the barefoot, but again they have their own set of limitations. We and that’s the collective we, barefoot, farrier or otherwise need to keep an open, scientific mind. Having said that, if we are going to shoe horses then we have to mitigate the consequences, for certain feet, as best we can. That is why I use everything we have spoken about to direct my practice, and the same principles apply to barefoot. We need to do the best we can for the time we are in, and that means using the latest research, while understanding that at any point new science could prove everything we are doing to be wrong. We can’t be afraid and resistant to that, the whole point of everything we have discussed is that science, critically and logically appraised should be our plumb line and dictate our daily practice.