Morphological variations in equine hoof balance metrics and their influence on the mechanical behaviour of the foot and the probability of increased risk factors associated with lameness and pathology

The equine foot has a specific conformation (shape) that provides maximum biomechanical efficiency. Biomechanical efficiency allows the foot to withstand, accept, absorb, dissipate and transmit loading weight bearing forces in a manner that offers the greatest protection to the horse. This principle implies that there is some combination of foot size, foot shape, wall length and angles that make the foot an ideal shock absorbing, weight-bearing structure. It is the proper combination of these variables are said to constitute what has been described as the properly balanced foot. However, there are currently several conflicting hoof balance reference systems commonly utilised and what constitutes ideal balance has been the subject of great debate for many years. One goal of the research was to investigate the principle of equal geometric proportions and dependentcy on factors such as foot-type and environmental conditions. By utilising a standardised trimming protocol and a hoof mapping system to collect measurement data based on proportionality of the bearing border length the purpose of this study was, partly, to verify whether a commonly used theory of hoof balance, firstly described by Duckett, is achieved. Secondly to determine whether geometric proportions are equivalent following trimming, thereby achieving hoof balance.
Analysis suggested Currently accepted interpretations of static hoof balance including the achievement of an aligned phalangeal axis and a ground bearing border bisected by CoR are likely to be outmoded. This provides support to the hypothesis that feet should be managed on an individual basis rather than a “one-size fits-all” approach commonly applied and that implementing a prescriptive model may even be counter-productive to the functional integrity of the hoof.
Farriery technique have been shown to influence skeletal alignment within the foot. Standardised trimming and shoeing protocols were used to test the hypothesis that shoeing, over an extended period of time, would result in significant differences in static hoof balance proportions. Results showed that horses managed unshod had greater ability to manipulate bearing border length, re-align the heel angle and allow palmar heel migration than shod horses. Furthermore, proportional hoof balance measures were able to be altered in unshod feet and that equivalence of the proportional hoof measures were not present in either cohort (unshod/shod). The significant differences in hoof measures present in shod feet ie; flattening of the sole, heel contraction, reduction in dorsal hoof wall and heel angulation and dorsal migration of dorsal hoof wall and heel seemed likely to reflect the effect of the shoe over an extended period.
The application of a standard steel horseshoe appeared to influence hoof shape and is likely to both affect and be affected by mechanical forces acting on the foot. The affect of hoof shape and the mechanical forces experienced by the foot itself following the application of the standardised trimming protocol and the application of a shoe were investigated. Results highlighted significant post-shoeing statistical differences in all dynamic measurements between shod and unshod feet. Specifically post-shoeing reductions in peak pressure and the contact area resulting in differences in peak force and peak force time were noted. These results partially support the propersition of a difference in mechanical behavior of the foot under load and may reflect the differences witnessed in feet under different management regimes. Biomechanical analyses of this kind enable improved understanding of hoof function, and a rational, objective basis for comparing the efficacy of different therapeutic strategies designed to address hoof dysfunction and pathology.
There is considerable anecdotal information that poor foot conformation and balance are associated with an increased risk of foot-related lameness but foot imbalance may also result from lameness as an adaptation to chronic pain. For example, a long toe and a low, collapsed heel is still considered a risk factor in the development of foot-related lameness. Utilising MRI findings from a group of horses referred for lameness investigation bionominal logistic regression was used to test the hypothesis of risk of lameness associated with hoof measurement proportions. There is evidence to suggest a strong correlation between hoof conformation and the biomechanical inference on anatomical structures and foot-related pathologies. Results indicated significant relationships between hoof measurement proportions and individual disease status. variation in key hoof measurement proportions resulted in significant differences in risk factors of specific common foot pathologies ie; navicular disease and degenerative joint disease of the distal interphalangeal joint.
It has been argued that the form of the solar arch was indicative the pathologies. Results from the current study appear to support his hypothesis by linking hoof morphology to the incidence of disease. Whilst the author recognises that hoof shape is influenced by any number of other factors, proportional values along the solar axis may well prove to be a good model for biomechanical efficiency either by trimming alone or form the basis of a more biomechanically sympathetic standardised shoeing model.

Figure 1 Schematic views of the external reference points from lateral (A) and solar aspects (B). BBL = length in the sagittal plane between the heel buttresses and dorsal toe; COP = point 9.5mm palmar to the apex of the frog; COR = point formed by the intersection of the heel buttresses and opposite breakover point (dotted lines); DHWA = angle between the dorsal hoof wall and horizontal ground; DHWL = length in the sagittal plane from the coronary band to the dorsal toe; DT-COR = length in the sagittal plane from the dorsal toe to COR; HA = angle between the heel bulb and horizontal ground; HB-BO = length from the heel bulb to the point of breakover (BO); HB-COP = length from the heel bulb to COP; HB-COR = length from the heel bulb to COR; HB-FRA = length from the heel bulb to the apex of the frog; HBUT – COP = length in the sagittal plane from heel buttresses to a point 9.5mm palmar to the apex of the frog; HL = length from the coronary hair line to the bearing border of the heel; SL = sagittal length from the heel bulb to the dorsal toe

Figure 2 Photographs showing shoe fitting from solar (A), lateral (B) and palmar (C) aspects. The illustration highlights the main criteria of the shoeing protocol. Horses were trimmed and shod to a competition style as defined in the UK National Occupational Standards for Farriery.
Key characteristics are A) the shoe should not interfere with the natural functions of the foot. B) The shoe should be of the correct weight and size for the horse and the work the horse is engaged in. C) The shoe should be of adequate length so that there is no loss of bearing surface and fitting to the heel buttress with additional 5mm length in a palmar direction with symmetrical branches. D) The excess hoof growth should be removed ensuring that correct balance is maintained and according to the horses conformation. E). No daylight should show between the shoe and the foot, necrotic feet being the exception to this rule. F) The right number and size of nails should be used in relation to the foot, and the nails driven so as to fill the nail holes. G) The clenches should be in regular line and flush with the wall. H) Clips should be well formed low and broad, and flush with the wall.

Figure 3 Individual Kernel density distribution plots for the control group and samples from the lameness group with navicular lesions. Data displayed are key solar border measurements Heel (HB)/SL (A), COR/SL (B), COP/SL (C) and BO/SL (D). *A one unit increase in the COP/SL proportion (C) will increase the odds ratio of the foot having navicular pathology versus no disease by a factor of 1.284 (p value <0.0001, 95%CI: 1.132, 1.457), indicating that as this hoof measurement proportion gets larger, the odds of navicular disease increases.

Figure 4 Individual Kernel density distribution plots for the control group and samples from the lameness group with DIPJ lesions. Data displayed are key solar border measurements Heel (HB)/SL (A), COR/SL (B), COP/SL (C) and BO/SL (D). * A one unit increase in the COP/SL (C) proportion will increase the odds ratio of the foot having DIPJ disease versus control group C by a factor of 1.376 (p value <0.0001, 95%CI: 1.190, 1.591), indicating that as this hoof measurement proportion gets larger, the odds of DIP disease increases.