The Bearing Surface- Creating optimal basal support
The digit is on the end of the limb, the limb is attached to the body and all this weight is transferred through the hoof, creating a bearing surface that allows even loading of the limbs helps to protect the musculoskeletal structures of the entire horse. The weight of the horse is transferred through straight gravitational lines down the limbs, putting the heels of the hoof, the toe and everything in between, in the right place and putting a shoe on that compliments the horses’ conformation provides the limb with a base that can best support that load, statically and dynamically. Uneven or excessive loading will predispose the horse to injury and degeneration of structures throughout its entire musculoskeletal system. The horse has different stages of locomotion (see my blog on evidence based farriery) in this article we will mainly be focusing on the benefits of achieving static and mid-stance basal support. Studies have shown (Caldwell 2016) that every horse should be assessed on an individual basis and shod according to its unique physiology to achieve the best balance. Here in lies the secret to a good shoeing job, not only looking at the external structures of the bottom of the foot, but the entire horse, its age, work and musculoskeletal system, as workload, conformation and physiology directly affect the hoof (Curtis 2002) and the hoof directly effects the entire anatomy (Kilmartin 2014)(Further reading see my blog on evidence-based farriery). Balchin (2017) outlined the fact that maths should be the basis to any shoeing job, one of the fundamentals talked about was the importance of trimming the heels back to the highest, widest part of the frog to establish the correct ratios of the solar plane.
Fig1-2. When a horse is shod, the bearing surface of the foot becomes the shoe. In the before pictures the heels are way forward of the widest part of the frog and the shoe followed, this caused the bearing surface of the foot to be forward, the centre of pressure (COP) moves caudally and the internal structures are under increased strain, simple hoof growth over a shoeing period will create this effect. Re-establishing the dorso-palmer bearing surface is as important in everyday shoeing as in remedial shoeing as many studies have shown that hoof morphology has a direct effect on the internal structures of the hoof (van heel et al 2004,2005), these studies showed that the forward migration of the hoof puts strain on the navicular structures, the re-establishment of the bearing surface through trimming to the correct protocols and shoe placement that compliments this trim, giving full caudal support to the hoof, will put the COP back to a more ideal position.
As well as static load the bearing surface has dynamic implications too, Van Heel et al (2004) observed a trace followed by the majority of horses (Fig.3).
Fig.3 The trace of the COP during locomotion. Courtesy of J. Mather Bsc(hons)
Van Heel et al (2004) Discussed how post trim there wasn’t a substantial change in how the hoof landed but it did affect other aspects of the stride, most markedly how long a hoof took to land was reduced leading to the hoof having complete bearing support sooner, subsequently the CoP moved centrally quicker which in theory positively effects the load distribution on the internal structures. Conversely this study shows that hoof growth has a negative effect, the hoof takes longer to reach full bearing meaning the caudal aspect maintains loading for longer and at full bearing the load would be more medio-laterally imbalanced again negatively effecting the surrounding internal structures, Van Heel et al (2004) added that full bearing support is also beneficial in absorbing concussion.
Moleman et al (2006) added to the findings of Van Heel et al (2004) showing that in a shoeing interval of 8 weeks the hoof wall angle decreased significantly, this caused the CoP to move toward the joint away from the toe, this in turn increased the moment force significantly around the distal interphalangeal joint (DIPJ) however the moment around the proximal inter phalangeal (PIPJ) joint did not change significantly, this showed a compensation for the change in hoof conformation happened mainly in the DIPJ, the extension of this joint created an increased load on the deep digital flexor tendon (DDFT) and the navicular area, predisposing these soft tissue structures to injury. This showed that although the horse can compensate to an extent for hoof growth this compensation is detrimental even with minimal hoof growth and farriery intervention is important in protecting the internal structures of the hoof.
Moleman et al (2006) showed that there was no significant difference in the change of the CoP in a high angle foot (HHA) to a low angle foot (LHA), but the increase in moment was 2.4% larger in the LHA around the DIPJ, this indicated that horses with high low conformations should be shod according to the needs of the LHA and horses with LHA in general require re-establishment of their bearing surface sooner, in another words, the flatter your horses feet, the shorter shoeing interval they need. The ground reaction force (GRF) did not significantly change over the shoeing period in Moleman et al (2006), this indicates that the horse did not change its centre of gravity, however the distance between the GRF and the centre of rotation did change, indicating that the change in the lever arms was directly due to the change in the angles of the joints (See my YouTube vlog on navicular for an explanation on lever arms) even with a shorter shoeing interval the changes in hoof angle have a detrimental effect on the underlying structures.
Fig 4-5. These pictures show the re-establishment of dorso-palmer balance bring the base of the horse back under the limb, reducing the loading of the DIPJ and bringing the centre of pressure back to a more central position. Fig 4. Expresses Moleman et al (2006) the pre trim hoof has migrated forward and the centre of pressure (Approx. in red) is in the back third of the hoof, post trim it falls more centrally.
The hooves in fig1-2 are reasonably balanced, symmetrical and square to the limb, so only the dorso-palmer balance needed to be addressed outside of normal medio-lateral balancing, when this is not the case creating as much symmetry and basal limb support as is safely possible becomes good practice. Medio-lateral imbalances can cause the base to de-centralise Fig 6.
Fig.6 hoof growth exacerbates conformational defects, uneven hoof growth has caused an exaggerated “toe-out” conformation de-centralising the basal support, re-establishment of medio-lateral balance can bring the hoof back into a more central position under the limb.
Fig 7-10 The lateral heel of this horse has been crushed and has contracted due to conformational defects (Base Narrow), the shoe has been fitted to re-establish a symmetrical bearing border and support the limb. The red line shows the path of the contracted heel.
In an ideal conformation the bearing surface is centralised under the load from above (Fig.11).
Fig.11 An ideal conformation showing the bearing surface of the hoof is bisected by the centre of load of the limb above it.
When the load does not bisect the hoof evenly morphology occurs, and predispositions present. Commonly if the animals line of load was to fall laterally or medially of the centre of the hoof the wall closest to the load can suffer compression and contraction, different load transferences will produce different hoof distortions. Angular limb deformities are a perinatal and/or developmental disorder, associated with imbalanced loading of the growth plates in young bones, at this point establishing a centralised load is most important in producing a straight limb into maturity. There are conservative (Fig.12) and surgical (Fig.13) treatment options, Watson (2016) outlined the appropriate treatments at different stages. Many limb deviations, especially if untreated before the closure of the growth plates, continue into maturity and become conformational defects (Fig.14).
Fig. 12. Remedial trimming and foal extensions are a conservative way of correcting angular limb deformities in the perinatal and development stages, the principles are similar to shoeing methods for establishing central loading in the mature horse with the same deviation. Picture courtesy of Formahoof.com.
Fig.13. Depending of the severity and time line of the deformity surgical intervention may become necessary, this picture shows screws and wires have been inserted to slow growth on that side. Radiograph taken from Watson (2016).
Fig.14 Common fore limb deviations and their load transferences, the weight of the horse follows straight gravitational lines dropped from the point of shoulder in the fore limb and point of buttock in the hind.
As discussed earlier, conformation has a direct effect on hoof morphology, uneven loading of the hoof can cause deformation (Curtis 2006) and hoof asymmetry within each hoof and between pairs of hooves can cause movement asymmetry (Wilson et al 2016). Curtis (2006) outlined the flares created by varus and valgus conformations, flares are again caused by biomechanical forces acting on the hoof from above, dressing them out, re-establishing balance and providing a more centralised load can help to address them, the effects of the excessive force on one side of the hoof can have different outcomes, the wall can flare, underrun, get compressed or have a reduced growth rate, in fig7 and fig14 the hoof wall has become compressed and underrun.
Curtis (2006) described shoeing for the deviations and addressing flare and creating a bearing surface that supports the load of the limb (Fig15), put simply the shoe placement should look to provide a base central to the line of load. Taking one of the conformations we see in fig.14 and in the hind in fig.7-10, base narrow, we can see that to bring the base under the gravitational line the shoe bearing surface would extend outside the lateral hoof wall, widening the stance and creating a bearing surface under the central load line, it is debated whether this would be called a lateral extension, in some circumstances it would be a re-establishment of bearing border and in some cases extending beyond symmetrical would be beneficial although this can possibly cause crushing/tensile forces, more research is needed to quantify the effects of true “lateral extensions”.
Fig. 15 picture taken from Curtis (2006) and edited showing re-centralisation of bearing surface by dressing flare (dark red) and fitting full of contraction (Light red line), note that nail position would have to change to correctly fit the white line.
Fig.16 Extreme fetlock varus showing the gravitational line of the limb above. The “lateral extension” has sought to re-establish centralisation of load, in this case getting the line completely central would be impractical and unsafe, the medial wall was also dressed of as much flare as was safe for nailing. This is an extreme example of farriery intervention looking to re-establish central loading but on a more daily basis applying a symmetrical shoe that may fit bold at certain points is enough to create a centralised bearing surface (Fig.17).
Fig.17 An example of contracted lateral walls, the shoes have been fitted to re-establish a symmetrical centralised base but not with a lateral extension, the right fore had suffered perpetual horizontal cracks at the lateral heel due to the biomechanical forces, after being shod in bar shoes fitted for centralisation the cracks desisted.
This article only scratches the surface of the range of conformations that present in the equine, assessment of individual conformation aids the farrier in his engineering of as close to an ideal conformation as possible, an ideal horse stands on 4 legs, on four corners, that are directly under the gravitational lines dropped from the point of shoulder and point of buttock, this is very rarely found and a horses conformation has a direct effect on how its load it transferred through its hoof, both statically and dynamically. Hoof growth has a direct effect on stance and the basal bearing border and is also directly affected by the biomechanical forces from above, the farriers job is to create as centralised a bearing surface as possible (often by simply trimming to the correct solar ratios) to help create as near to central loading as possible, but will always be fighting the natural physical forces of nature acting upon the hoof, having a shoeing interval that is bespoke to a horses conformation is vital in keeping those gravitational lines supported, often shoeing intervals are stretched but if the shoes stay on “what’s the problem?”, the internal structures are being overloaded from top to bottom as the basal surface moves away from the central lines of support. Keeping your horse standing on four feet that are in the right place is a joint venture between the owner and the farrier, the owner must understand that their horse has unique farriery needs from shoes to shoeing interval and the farrier must assess those needs and create a shoeing plan to suit.
Fig. 18 Courtesy of N.Grooby, Horse presented at camped under, post shoeing the hoof is now centralised under the gravitational line, great example of the principles explained in the article.
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