Hoof pastern axis (HPA) is a term for the relationship between the hoof and the pastern. There are three categories for HPA; broken back, straight and broken forward. For the most part we will focus on broken back.
Fig.1A How to assess HPA. A line through the middle of the pastern is compared to the angle of the dorsal wall. This HPA is on the broken back side of aligned but is pretty good. Straight would have the same angle between the pastern and hoof.
Fig.1B Broken back is when the hoof is at a more acute angle then the pastern and broken forward is the opposite.
HPA correlates to some extent with bone alignment of the three phalanges but shouldn’t be confused with phalangeal alignment as they are not the same thing. For the purposes of this discussion they will be discussed together, as re-alignment of the HPA will correspond with improvement of the phalangeal alignment.
Fig.2 This shows the difference between HPA and phalangeal alignment. HPA is an external reference marker and phalangeal alignment can only be assessed radiographically. Note how the two are different from each other.
Something else to clarify before we continue is that palmar angle and phalangeal alignment or HPA are not the same thing, often people make the mistake on assessing “angles” purely according to the orientation of the distal phalanx. Horses will have varying degrees of palmar angles and therefore hoof angles, what is important is the relationship between the phalanges and not the absolute angles of the hoof. The discussion is not about creating more upright hooves per say, it is about creating correlation between hoof angle and pastern angle. For instance, a thoroughbred will have a more acute angle then a warmblood, the angle required for a good HPA will be different in the two.
Fig.3 This image shows a range of good HPA’s. They all have different angles to them, created by the palmar angle and bone angle of the distal phalanx, but the HPA is good regardless because what determins a good HPA is not the absolute angle of the pedal bone/hoof but its relationship with the phalanges/pastern. Re-imagined with permission of EPC solutions.
The question in the industry is, should we improve HPA, should every horse be perfectly aligned and how much intervention is appropriate. These questions were discussed in a previous article “The truth about HPA” and a videocast “The Importance of Phalangeal Alignment.” These outlined the predispositions of a broken alignment, so in this article we will discuss the parameters that possibly create the need for an improved HPA.
HPA, to a large extent, especially in broken back HPA, is dictated by hoof proportions, outside of other influences such as muscle unit contraction. Studies have shown that certain measurements and ratios directly affect the orientation of the hoof. Moleman et al (2006) and the van Heel studies showed that the majority of the effect of hoof proportions is taken up in the distal interphalangeal joint (DIPJ), this equates to hoof angle as the distal phalanx rotates around the DIPJ, in turn this creates a broken HPA. Page and Hagan (2002) discussed the distance between the distal phalanx and the toe and how it directly affected HPA, Redden (2003) took this measurement and expressed how it created toe leverage when it was too great, causing toe flares. Snow and Birdsall (1990) and Dyson et al (2011) discussed how a difference in angle between the hoof wall and heel greater than 5 degrees suggested a collapsed heel, affecting HPA. Turner (1992) and Dyson et al (2011) mention the established ideal of there being a 3:1 toe:heel height ratio and that this ratio affected the angle of the hoof, inevitably this will affect HPA. The Moleman and van Heel studies as well as Clayton (1990) were on the effects of hoof growth in the shod foot, which will be expanded on later in our discussion.
Fig. 4 The measurements and ratios that affect HPA.
Looking at fig.3 we can see the same foot that had its proportions changed, this created a change in HPA. Although the toe/heel length ratio and the length of P:3 to breakover have an effect on HPA, this is only up to a certain point and is minimal, what really has an impact is the toe:heel height ratio. In the top pic the ratio is 10 heel heights to 1 toe height, in the bottom, 4.5 heel heights to 1 toe height. Note if this ratio became closer to 3:1 the phalangeal alignment would be almost perfect.
Fig. 5 There are 3 axis to consider when looking at the influences on HPA. Toe length, heel length and the bone alignment which is directly affected by the toe:heel height ratio. You can shorten the length of the toe and it will have a minimal effect on the orientation of the phalanges, however change the toe:heel height ratio and you hugely affect the phalanges. As you can see from this radiograph the heel height is almost non-existent which correlates with a broken back HPA and a negative plantar angle.
The question is whether the HPA should be artificially improved. What we need to ask it what created the poor toe:heel ratio? Is it natural and therefore to be left alone? Or is it a product of external factors that should be mitigated?
We are asking this question of feet that cannot reach ideal with the trim. The trim plays a large role in orienting these axis’ as the farrier can directly affect the toe:heel ratio. There are different teachings on trimming, some protocols state trimming the heels down to the widest point of the frog, some state trimming to the live sole plane and some aim to create a ground parallel distal phalanx, just to mention a few. Caldwell (2018) states trimming should be decided on an individual basis and provides a trimming protocol based on mapping internal reference points on the external foot. In my opinion trimming to the internal structures and to create biomechanical efficiency is vital and HPA has to be considered along with the health of the caudal structures and strength of the heels. Trimming for a ground parallel distal phalanx, for instance, will invariably create a broken HPA and for the reasons explained in the articles and videocast mentioned earlier this could prove to be detrimental to the musculoskeletal system of the horse.
Fig.6 A foot with strong heels trimmed to the highest widest part of the frog to maintain a good HPA.
You can see from Fig.5 if the heels were trimmed to the red lines how it would break the HPA. The barefoot often presents the same, left to its own devices the frog and heels will gain height on the same plain. Having said that heels left too long still run forward and collapsed run forward heels need taking down to something strong and under the haemodynamic structures, which may then require elevating back up. There is always a balance to have and create, again trimming should be individual to the horse, with certain goals in mind to create balance on the 3 axis’.
Malone and Davies (2019), Clayton (2011) and Proske et al (2017), all found improved hoof morphology in the barefoot out of shoes. Relevant to this study they all found increased hoof angles and palmar angles from improved toe:heel height ratios, which would invariably mean improved HPA, this is assuming the schools of trimming that encourage taking the heels down as mentioned previously haven’t been involved.
What does this tell us? Potentially two things, 1. A proportion of shod hooves should ideally be at a higher angle, or rather more aligned then they are and would be, in time, were they to go barefoot. Meaning they could benefit from elevation now. 2. Traditional shoeing creates a reduction in heel height compared to toe height in a proportion of horses. Let’s explore these.
Fig.7 The difference between what caudal hoof structures should look like (top left) and what is commonly seen from a solar view.
Horse owners and farriers alike are so used to seeing contracted, collapsed and prolapsed caudal hoof structures that it has become an (unacceptable) norm. Relevant to this discussion these morphologies will have a direct effect on the proportions of the hoof, including the toe:heel height ratio. What we need to understand is that if the horse presents with these morphologies it does not have natural hoof ratios, or rather, hoof ratios that are ideal for that particular horse.
Fig.8 The possible caudal collapse of the foot due to non-contact of the frog. Bottom right image courtesy of Progressive Equine Services.
Looking at the same collapse of the foot from a caudal view, perhaps we should question the traditional rim shoes possible effects on the role of the frog and the haemodynamic system. Lifting the frog off the floor means the ground reaction force cannot transfer through it in opposition to the weight of the horse, in weaker feet this leads to downward migration of the caudal structures over time and collapse of the heels. Although the studies of Bowker found other mechanisms of the haemodynamic system, like the depression theory and negative pressure theory, previous articles have questioned whether frog support pads should become default practice in farriery to go some way to mitigating this widespread collapse. Conversely this explains the positive morphology that occurs if and when a horse goes barefoot, these caudal structures work more optimally and the hoof goes through the changes stated previously.
What does this mean for our discussion? 1. Feet presenting with these negative morphologies will not be at the angle they would be if all factors were optimal. 2. These negative morphologies create a poor Toe:heel ratio and will again have a direct effect on HPA 3. The majority of horses would ideally want a hoof with a higher angle and straighter HPA then it presents with as these negative morphologies are ubiquitous in the shod population.
Coming back to hoof growth, we need to consider that the moment a hoof is put down after shoeing, hoof growth is having a negative effect on the angles of the hoof. The toe and heel angles are becoming more acute and the heels are running forward under the wings of the distal phalanx at which point they suffer from being situated under and in between hard structures, rather than the soft structures of the haemodynamic system, this can lead to weaker heels over time affecting the toe:heel ratio. This is of course the same in a barefoot, unless however hoof wear and hoof growth is balanced. In the authors experience bare feet tend to wear the toe more heavily then the heels, possibly due to the heels experiencing impact where as the toe experiences friction, this would need quantifying through research and in reality is probably an oversimplified hypothesis. What we do see however in the barefoot is the toe:heel height ratio is better maintained and biomechanical forces acting on the hoof remain more optimal which directly affect its morphology.
To summarise, HPA is a product of heel:toe ratio. Considering a high percentage of caudal hoof structures are distorted and collapsed to varying degrees suggests that most horses would be more upright and therefore more aligned then they are, so therefore a high percentage of horses require elevation of the heels proportionate to the amount of caudal hoof distortion and/or collapse. Perhaps if farriery sought to proactively mitigate the unintended consequences of shoeing with the use of modern materials to more closely emulate the bare foot we would see less broken HPA’s as a result of healthier caudal structures.
In the absence of other issues that would perpetuate caudal hoof collapse, if these same horses went barefoot they would likely create a straighter HPA with improved hoof morphology. The fact that the barefoot increases in angle again suggests that a proportion of shod horses present with a lower hoof angle then they would naturally have, thus likely to have a broken back HPA and likely to require some form of elevation.
How much elevation? And where should the HPA be? Some people would suggest a healthy range for HPA and bone alignment, well If you could somehow calculate the cumulative effect of the distortions and collapses that are present into an amount of reduced heel angle and height, there may be your answer. I would suggest it would be close or at least closer to aligned. Until we can calculate that, perhaps we should aim for as close to ideal as possible. We cannot say that a broken back HPA is normal and acceptable if and when there are obvious distortions of the caudal structures, due to shoeing or otherwise. Show me a foot that has no caudal distortion and a therefore a good toe:heel ratio and for the most part ill show you a foot that has an HPA within an acceptable range. Not only that but poor morphology leads to poor morphology and the poor caudal structures tend to perpetuate without farriery intervention that improves the biomechanical forces acting on that hoof.
In our discussion of ideal HPA and whether farriery intervention to improve it is appropriate, we should consider the health of the hoof, its proportions and whether it would be a more upright hoof in the absence of these distortions.
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