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. In a nut shell, if the balance around the centre of rotation (COR) is 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 also 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. The Inclination of the dorsal wall was shown to influence surface strain values in the hoof wall (Thomason et al., 1992, Thomason et al., 2002). The hoof angle was thought to influence stress/strain levels and distribution within the hoof capsule (Thomason et al., 2002, McClinchey et al., 2003). The hoof angle was thought to directly affect internal tissue strain levels in hoof and distal limb, and angulation of joints within the phalangeal axis (Balch et al., 1991). Thomason et al. (2001) stated that there was a deterioration in structural coherence of the foot in a low-angle hoof compared to a higher angle. The angle of the hoof wall is directly linked to the balance around COR.
Van Heel et al (2004) could be used to show that a lower angle hoof takes longer to reach full bearing meaning the caudal aspect maintains loading for longer and stated that full bearing support is beneficial in absorbing concussion so points at the more ideal hoof having a more efficient anti-concussive mechanism. Conversely 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.
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 and all of these studies outline why poor hoof morphology leads to poor hoof morphology, as the feet get caught in perpetual cycles.
Fig.1 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, compared to a foot with better balance around COR, 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, around COR, 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 protocols 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 externally 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. There will be an increased extending moment creating increased flexor strain and also create increased work for the deep digital flexor to initiate breakover.
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).
The shoe placement can then enhanced the trim, taking 2.5% from the toe and adding it to the heel length creating 50:50. This simple process helps to establish ideal biomechanics, however this is only on 2 axis’.
A video on shoeing around the COR can be watched at this link.
This image shows a hoof with the 50:50 split and balance around the COR achieved, but only in 2 dimensions meaning a BBHPA still presented.
When more ideal balance around the COR is achieved in 3 dimensions, the geometric proportions of the foot begin to be restored, this directly effects the conformation and posture and creates an environment for positive hoof morphology.
These X-rays show how poor COR balance presents radiographically. The long toe creates a leverage of the dorsal structures, the caudal structures are not supported, a broken back phalangeal alignment is present and the navicular area is strained. Toe to heel ratios, both as external COR splits on the solar view and toe to heel height ratios, play an integral role in the orientation of the internal structures around the COR of the DIPJ. Some feet would possibly come into three-dimensional balance around the COR with just a foot mapped trim and shoe placement, however when a BBHPA is still present, elevation of the heels can become necessary.
Having a straight phalangeal alignment, which corresponds (on the whole) with a straight HPA is part and parcel of creating balance around the COR.
The image expresses the long lever arm at the toe created by the imbalance, the long toe low heel conformation. Looking at this image we can see that although the basal surface split around the COR could be improved, the orientation of the structures around the COR would also need heel to toe height ratio improvement. Willeman et al (1999) discussed how toe:heel ratios can be positively affected by the application of a wedge, the elevation of the heels shortens the length of the toe, re-orientates P3 and helps to create correct internal geometry around the COR.
This image shows how trimming, shoe placement and a wedge application helped to create three-dimensional balance around the COR. This has created a substantial decrease in load on the navicular area and assuming the unintended consequences of elevation have been mitigated, also creates more ideal biomechanical forces and an environment for positive morphology. See my article on wedges
It is important to note that a BBHPA and phalangeal alignment predisposes to navicular syndrome, therefore creating optimal balance around the COR is vitally important in longevity of working life and reducing the risk of injury. A recent retrospective study by the author discussed the link between conformations with poor COR balance and this pathology. Further reading at https://www.theequinedocumentalist.com/post/navicular-a-retrospective-study
Bowker (2019) discussed how navicular was “A man made issue caused by incorrect trimming and shoeing methods.” And in the authors experience the majority of navicular cases present with poor three-dimensional balance around the COR, where poor morphology has been allowed to perpetuate by not creating the optimum balance discussed above. Bowker also described a hoof with navicular syndrome as having long toes and underrun heels, this correlates with Caldwell’s paper on Pre-Navicular syndrome (1987) that points toward poor COR balances discussed, as pre-cursers to the onset of the pathology. Bowker also stated that prevention and treatment start with “proper hoof trimming” and the author would add to this with “and creating balance around the COR in 3 dimensions.” Willeman et al (2010) expressed how wedges reduced the force on the DDFT over and above egg bars, the author believes this can be attributed to egg bars addressing COR balance in 2 dimensions but not its relationship with the other internal structures, specifically the phalangeal alignment, which we are calling the 3rd dimension. frog support is also vitally important as it helps to lock in the internal COR balance during locomotion. See articles on wedges and frog support for elaboration.
Holroyd et al (2013) established a link between poor balance around the COR and the development of foot lesions, DDFT and navicular bone lesions were associated with hoof conformations of the previous description and my recent article https://www.theequinedocumentalist.com/post/the-truth-about-hoof-pastern-axis outline many more papers showing links.
It is important to appreciate that often a foot labelled as “long toe, low heel.” Is in fact just low heeled, the angle of the hoof creates a long lever arm at the toe. Trigonometry dictates that when you increase the angle of the hoof the length of the dorsal wall decreases in relation to the base length, so these hooves often need elevation of the heels and not excessive rasping of the toe as this will lead to a weaker foot. In these cases, as part of COR balance restoration, it becomes important to create frog and solar support to de-load the compromised heels. More info on frog and solar support at https://www.theequinedocumentalist.com/post/frog-pressure-is-the-heart-bar-obsolete
This image expresses the ethos of the article, creating balance around the COR is not just about at least a 50:50 base split, which is good practice, but also about putting the structures around the COR in a position that creates optimum biomechanical efficiency and soft tissue load distribution. Note in the image (Courtesy of D. Madin) the breakover has been brought back, this is also an effective way of creating balance without over dressing the toe.
Many studies have linked hoof conformation to pathology, commonly, in relation to the parameters we have covered, navicular syndrome in the fore foot and higher pathologies in the hind. Poor COR balance in the form of long toes, low heels and a BBHPA is a constant re-occuring theme and is rife in the equestrian world. Shoeing around the COR is recognised as good practice. Due to hoof distortions and variation, COR is a more reliable and dependable location to balance the hoof around. Shoeing around COR should perhaps become a basic of farriery. The majority of my referred work consists of poor COR balance, establishing the COR and utilising it as a reference point to create balance around doesn’t seem to be part of everyday practice for a percentage of the population. Although studies such as Caldwell (2018) point at other measurements being used as a more reliable indicator, using the COR as a base point proves to be an efficient way of establishing and maintaining optimal geometrics and reducing the risk of injury. Other factors can play a role in COR balance not being achieved such as shoeing cycle and lost shoes forcing shorter shoeing, also financial restraints when farriery interventions using modern materials ect are not affordable. The client and the farrier must work together to create an environment for healthy functioning hooves. Recognition of imbalance is vital and objective assessment is a useful tool in monitoring hoof morphology. To create the ideal proportions of the foot, you first have to see the less than ideal, and then apply evidence-based practice to get the desired results.
M. A. WILLEMEN , H. H. C. M. SAVELBERG , A. BARNEVELD, 2010, The effect of orthopaedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses, https://doi.org/10.1111/j.2042-3306.1999.tb03787.x
M. Caldwell, 1987, Pre-Navicular Syndrome, Personal Correspondance
KateHolroyd, aJonathon J.Dixon, bTimMair, cNickBolas, dDavid M.Bolt, aFredericDavid, aRenateWellera, 2013, Variation in foot conformation in lame horses with different foot lesions, https://doi.org/10.1016/j.tvjl.2012.07.012
Kummer, M, et al (2007) ‘Comparison of the trimming procedure of six different farriers by quantitative evaluation of hoof radiographs’ The veterinary journal, volume 179, pp. 401-406
van HEEL, M, et al (2004) ‘Dynamic pressure measurements for the detailed study of hoof balance: the effect of trimming” Equine Veterinary Journal, volume 36, No.8, pp. 778-782
van HEEL, M, et al (2005) ‘Changes in location of centre of pressure and hoof-unrollment pattern in relation to an 8-week shoeing interval in the horse.’ Equine Veterinary journal, volume 37, No.6, pp. 536-540
J. Thomason, A. Biewener, J. Bertram
Surface strain on the equine hoof wall in vivo: implications for the material design and functional morphology of the wall
Journal of Experimental Biology, 166 (1992), pp. 145-165
O. Balch, K. White, D. Butler
How lameness is associated with selected aspects of hoof imbalance
Proceedings of the American Association of Equine Practitioners, 39 (1993), pp. 213-214
J. Thomason, H. McClinchley, J. Jofriet
Analysis of strain and stress in the equine hoof capsule using finite element method: comparison with principle strain recorded ‘in vivo’
Equine Veterinary Journal, 34 (2002), pp. 719-725
H. McClinchey, J. Thomason, J. Jofrriet
Isolating the effects of equine hoof shape measurements on capsule strain with finite element analysis
Journal of Veterinary Comparative Orthopaedics and Traumatology, 16 (2003), pp. 67-75