Hoof Balance - Shoeing Around the COR in 3 Dimensions
Updated: Dec 6, 2020
A question that is asked very often whenever people are looking for advice on farriery related issues is “what shoe would you put on for this?” and the true answer is that it’s irrelevant, if the trim, the shoeing period and the shoe placement are also not correct.
This accounts for why two farriers can use the same shoe and get very different results, this has been shown by Kummer et al (2007) that found differences in 14 of 15 trimming parameters between 6 different farriers and stated that the individual trimming procedure had a direct effect on hoof conformation and geometry.
One of the parameters that was found to be different was the distance from the distal phalanx (P3) to breakover (BO), this is an important finding as the ratios of the basal surface around the centre of rotation (COR) play a distinct role in the efficiency of locomotion and loading of both the hoofs internal and external structures. Page and Hagan (2002) expressed how both the hoof pastern axis (HPA) and the position of the navicular bone were both affected by this measurement, finding that a reduction in this distance unloaded the navicular apparatus, the paper also discussed the prevalence of a broken back HPA (BBHPA) which corresponds with the authors experiences of the general population of horses and certainly in those referred.
Redden (2003) expressed the increased leverage on the toe in longer P3 to BO lengths causing “dishing” or toe flares and stated that morphology was never isolated and in the authors experience toe flare commonly accompanies low, run forward heels.
Other studies have outlined the negative effects of a decreased hoof wall angle, although through the mechanism of hoof growth (which expresses the importance of shoeing interval) the same principles apply.
Van Heel et al (2004) could be used to express that a hoof with more ideal angles’ landing duration was reduced, leading to the hoof having complete bearing support sooner and the centre of pressure (COP) moving centrally quicker, which in theory positively effects the load distribution on the internal structures. Conversely this study could be used to show that a lower angle hoof takes longer to reach full bearing meaning the caudal aspect maintains loading for longer. Van Heel et al (2004) adds that full bearing support is beneficial in absorbing concussion so points at the more ideal hoof having a more efficient anti-concussive mechanism.
Van Heel et al (2005) could be used to express the movement of the COP towards the heels in longer dorsal wall lengths predisposing to crushed heels and Moleman et al (2006) could be used to express an increase in moment force around the navicular in increased P3 to BO distances and the forward migration of the heels. These findings express the importance of correct geometric proportions of the hoof. Trimming and then fitting to the COR helps to establish these proportions and therefore evenly distribute load on all structures.
Fig.1 Expresses Van Heel et al (2004,2005). A foot with disproportionate toe to heel ratios (in favour of the toe) around the COR would have increased landing time and a more caudal COP at mid stance, creating an environment for negative hoof morphology.
Clayton (1990) and Weller (2020) suggest an opposite movement of the COP due to hoof growth or a long toe/low heel conformation. However, Clayton agreed with van Heel regarding the changes in kinematics of the stance phase.
Fig.2 Clayton 1990 showed the effects of a broken alignment and poor balance around the COR. Also expressing why poor balance around the COR may predispose to stumbling as it increases the frequency of toe first landings, which also predisposes to arthritic changes.
Weller (2020) discussed the movement of the COP with poor balance around the COR.
Fig.3 The effects of the movement of the COP. The extensor moment acting on the limb is calculated by the ground reaction force acting through the COP times the distance of the COP from the centres of rotation. The extensor moment is a rotational and collapsing force acting on the limb. In order for the limb to not collapse the counteracting force being the tension in the flexor structures times their moment arms, has to increase. As the flexor moment arms are stationary, the only way to counteract the increased collapsing force is to increase strain in the flexor structures, predisposing them and the fulcrums they pass over, markedly the navicular, to injury.
Whether the COP moves back toward the heels or forward toward the toe needs to be clarified by further research, however either way appropriate balance around the COR is validated.
Caldwell (2018) discussed in depth the different balance theories, stating the widely accepted understanding that there is a link between poor hoof geometry and pathology and that static balance reduced the risk of injury. This study questioned the accuracy of commonly adopted trimming protocols stating that the influences of hoof morphology and pathology affected its efficacy, however it also stated that creating proportional dimensions was a good model for creating biomechanical efficiency. Interestingly this study also brought into question the relationship between the macro conformations of long toe, low heel and lameness and highlighted the importance of the individuals micro conformation in the form of viscoelasticity.
See my article on conformation for further reading on micro conformation https://www.theequinedocumentalist.com/post/farriery-related-conformation-macro-micro-dynamic
Caldwell (2018) tested the foot mapping protocol and found that the external reference points did correlate with internal structures and that the COR mapped on the external hoof was a reliable indicator of the centre of rotation of the DIPJ. Taking these findings and the previous studies mentioned we can see that trimming and shoeing to establish balanced ratios around the COR is good practice in creating efficient biomechanics, sympathetic forces and an environment for positive or maintained morphology.
Shoeing around the COR should be a three dimensional concept, creating balanced proportions on every axis, not only Dorso-palmer and mediolaterally but in terms of the COR’s relationship with the other internal structures.
Here we have a foot, run forward with poor dorso-palmer balance. This will have a COP more caudal toward the heels perpetuating the running forward of the heels and putting extra load on the navicular region.
Foot mapping creates an external reference point for the internal COR, this helps establish the interventions for dorso-palmer and medio-lateral interventions, remembering that we need to establish balance on every axis. The trim here has started the process to creating a 50:50 split around the COR. Shoe placement can then enhance this. A line is drawn at the origin of the heel buttress from heel to heel, then a straight line is drawn from the buttress to the toe at the white line, then lines are drawn diagonally from the point of buttress to the white line intersection with the line from the buttress. Where these diagonal lines intersect is the location of the internal COR (deep in the hoof).