There is a long list of factors that influence the amount of length a farrier shoes a horse with, many are environmental; type of work, type of turnout and shoeing cycle, to mention a few. Even without these variables the ideal amount of length of shoe remains highly subjective, research into the different biomechanical effects of different lengths needs to be done to work toward a standardisation of the industry, however we can use some physical principles to help establish some ideals to work towards, understanding that these may still have to be compromised according to the environmental factors.
The hoof is a structure that responds to the biomechanical forces that act upon it following the laws of physics, therefore we can use maths and physical laws to help establish the ideals in shoeing that structure. When applying a shoe we are looking to create centralised load and biomechanical efficiency to preserve all biological structures, so shoeing around the centre of rotation (COR), in the authors opinion, has got to be the first step in establishing the correct length of shoe. We also have to appreciate that the front and hind hoof have different conformations and should be assessed differently for their appropriate shoe lengths.
This picture from Progressive equine ( https://www.facebook.com/hoofscanandhoofcarecentre/ ) raised this question online, suggesting that establishing heel length to correspond with the dorsal wall angle is possibly a viable protocol. The author suggests this is a method that deserves investigation.
The following principles are the authors protocols for establishing length, as stated previously the industry requires evidence-based research to validate shoeing length.
Shoeing around the COR goes some way to establishing the authors chosen shoe length.
Foot mapping helps to establish the COR with the absence of radiographs.
Trimming to the highest widest part of the frog helps to point at the appropriate length of shoe. This point is expressed by O’Grady (2019) as the point at which to ideally trim and could be our first potential for shoe length fit, a common protocol for many farriers would then be to shoe to the widest part of the frog, being slightly further back, so this is our second potential shoe length. The length can be worked out as a 50/50 split around the COR instead of to an external anatomical point, although some would argue that 50/50 is not practical in some circumstances it does remain, in the authors opinion, an ideal and is often achievable by shoe fit.
The length of shoe should help to establish the balance around the centre of rotation. Had this foot been in a toe clip with the same fit it would have remained at 45/55 which may be acceptable to some, or 50/50 could still be achieved with even more shoe length at the heels, essentially, in this context, shoe length depends on toe length and toe length depends on the length of lever arm from the COR. The author most often choses to come under the toe and add length at the heels in appropriate proportions to establish the 50/50. For further reading on shoeing around the COR https://www.theequinedocumentalist.com/post/hoof-balance-shoeing-around-the-cor-in-3-dimensions
Radiographic appearance of 50/50 split around the COR. The length of shoe (assuming appropriate toe length) establishes this dorso-palmer balance.
This is a fundamental basic of the authors establishment of shoe length but there are other factors and reinforcing principles. When establishing balance around the COR a centralised load is established because both halves of the foot have the same surface area and therefore pressure.
We can then look at this from a lateral view.
From a lateral view we can see where this length equates to in terms of the digit, before and after applying a larger shoe around the COR, the author would suggest that this is the basic minimum length of shoe fit (with environmental factors removed) with a foot with minimal distortion, one could argue this foot still looks like it could have more length, so how do we establish maximum length? When does length turn from support to leverage? Other factors begin to also come into play, what about when the bulbs are prolapsed? What about when the heels end forward of the highest widest part of the frog? Often these morphologies come hand in hand, we can use geometry to help re-establish reference points.
An important point to drive home is that trimming plays a huge role in creating optimal physical influences, the shoe should enhance this, trimming down the heels to the highest widest part of the foot is a good start. Often farriers are reluctant to lower heels too much, however often the heels that require lowering are of such a low angle that the gain in basal length far outweighs the drop in hoof angle, trigonometry can help to explain this.
This image shows how underrun heels can be taken down with a smaller effect on hoof angle compared with more upright heels this is relevant because “length” of shoe applied depends on how close to the heel length required the heels already are with the trim.
In physics the triangle is the strongest shape and O’Grady (2013) expressed how the ideal conformation of the digit should present with a triangle, Balchin (2017) also discussed the applications of Pythagoras in assessing hoof conformation in his assessment of the golden ratios in the hoof.
Balchin (2017) right angle triangle created in 59% of feet, feet outside of this range were thought to have been pathological in some way. This finding could point at another suggestion for shoe length, the golden ratio, the length of the shoe is divided by 5 to have the same figure as the dorsal wall divided by 3 and the hairline divided by 4 and creating a right angle triangle with the dorsal wall and hairline. However this could still be questioned when we have pathological feet not following the golden rule.
We can continue the use of triangles to create further perhaps more practical possibilities for ideal shoe length.
A plum line taken from the centre of the distal condyle of the 3rd metatarsal, which often follows the middle of the leg mass close to the back of the bone, creates a right angle triangle with a straight hoof pastern axis, where this lands at the ground could be an indicator of appropriate shoe length. Radiograph courtesy of Progressive Equine.
The author uses this geometrical indicator and has found that anecdotally it commonly correlates to shoe placement around the COR, which also commonly correlates with the widest part and base of the frog. However, again this is not always practical especially if the horse has particularly sloping pasterns but could point toward a possible ideal.
With prolapsed heels/bulbs often this point can still appear too short, the heels will be run forward of the widest part of the frog often with parts of the bulbs almost becoming weight bearing in extreme cases, the author will then often use a line dropped from the back of the bulb as an appropriate shoe length. This correlates with O’Grady (2018) which discussed the appropriate length of hind shoe.
Illustration of O’Grady (2018) and a common authors reference point for the fitting of hind shoes. Hind feet often have a shorter distance from the COR to the base of frog/end of heel, in order to still maintain COR balance they may require more apparent length then fronts. When there is substantial forward migration in a fore foot the same proportions can present, therefore they may require the same increase in length. This point often corresponds with the same line dropped from the distal condyle as in the fronts.
Fore limb plum line discussed applied to the hind shoe length. Note this may be more length after the heel than it would be in the fronts.
Severely prolapsed hoof. The heels are way forward of the base of the frog, the shoe length was placed to the bulbs of the heel, with elevation this helped to create the right-angle triangle, the shoe was almost 2 inches longer at each heel than previously fitted. Appropriate shoe length helps to create ideal geometrics, note that before intervention using a line dropped from the condyle of the metatarsal would have been inappropriate and impractical, however this point does correlate after the other points were brought into check. This shoe fit went beyond 50/50 around the centre of rotation from external reference points, however we know that foot mapping is less accurate with the increase of distortion so the other shoe length reference points were used in conjunction. Note that trim, shoe length, shoe placement and elevation all worked together to work towards establishing the ideal and therefore all have an influence on appropriate shoe length. Posture also points toward appropriate shoe length and shoe length affects posture, anecdotally the author has noticed that commonly the fore and hind feet move backwards with more length (and sometimes elevation), however the shoe length markers usually correspond with fore mentioned parameters, if the rest of the conformation doesn’t look ideal then hoof geometrics need to be questioned, shoe length being one of them.
Appropriate length of shoe helps to create ideal posture/conformation. Hind feet are often shod too short allowing prolapse of unsupported heel bulbs. When you create the ideal many external markers correspond.
Another method suggested for optimum shoe length, as advocated by Mark Caldwell uses proportional lengths from certain reference points.
Caldwell suggests, following from Duckett (1990), that the dorsal wall length should correspond to the toe to the COR and Heel to COP and therefore is a measure of optimum shoe length. The author is yet to test and apply this principle in practice and it may prove to correlate with other reference points.
In conclusion appropriate shoe length remains highly subjective mainly due to environmental factors, however when shoeing for the ideal and removing these factors, certain principles can help to establish correct shoe length. Shoeing around the COR should be a base principle reinforced by other external reference points. Creating triangles and geometric proportions is proving to be sound practice in an emerging era of mathematical shoeing and these often correlate with anatomical reference points. Creating an optimal base of support is vitally important in supporting healthy hoof morphology and therefore appropriate shoe length remains an important subject for further research and daily practice. The author hypothesises that the widest part of the frog would be the minimum length of shoe appropriate outside of very short shoeing cycles, the maximum shoe length still remains elusive but the author proposes that lengths beyond the geometry created by the skeletal system i.e. the right angle triangle formed with the plum lines dropped from the metacarpal/tarsal could be the point at which support becomes leverage, or when the shoe extends beyond the extremities of the hoof in prolapsed heels, of course these all need quantifying by research. I leave you with a graphic showing the potential shoe lengths, including the dorsal wall angle suggestion by Progressive Equine, some will correspond with each other on some feet and others will not. This subject remains subjective, this article outlines the authors assessment of appropriate shoe length which is sometimes compromised due to the usual factors.
Caldwell, 2020, Personal Correspondance