Wedges are widely debated in the farriery world, do they help to create better geometric hoof proportions in a broken back hoof pastern axis’, de-loading affected structures? Or do they simply put a plaster on a symptom, perpetuating the adverse biomechanical forces that created the morphology in the first place? The answer can be both, depending on the application!
Firstly lets evaluate why wedges are applied in the first place. Studies have shown that a decrease in hoof angle directly effects the moment force around the distal interphalangeal joint (DIPJ) (Moleman et al 2004) meaning a flatter hoof has increased load on the navicular region, this excess load is exacerbated in a broken back hoof pastern axis (BBHPA) and many studies have linked this conformation to the pathogenesis of Navicular syndrome (Fig.1).
Fig.1 Showing research directly corelating phalangeal alignment or hoof pastern axis with navicular syndrome.
Uhl et al (2018) discussed the mechanical predispositions of navicular, stating that areas of increased DDFT load directly correlated to areas of DDFT core lesions, confirming the theory that conformations which increase dorsiflexion predispose to navicular pathology. Knowing that a BBHPA predisposes to navicular many farriers will choose to apply a wedge to get more ideal phalangeal alignment (Fig.2) as studies have shown that the change in hoof angle mainly effects the DIPJ and pedal bone alignment.
Fig.2 The application of a wedge immediately corrects the phalangeal alignment and de-loads the navicular area, also helping to establish better solar ratios either side of the centre of rotation. Studies have shown 1 degree difference equates to 4% decrease in load. Willemen et al (2010) showed maximal force on the navicular was reduced by 24% with the application of wedges.
The importance of hoof pastern axis goes beyond reducing the strain on the deep digital flexor and affects every phase of the stance.
So knowing that wedges can create such an immediate positive effect to the internal structures of the hoof why is it still widely debated as the common place intervention for these conformations?
Every action has a consequence, and often it’s the long term consequences of wedges that brings the use of them into debate. Wilson et al (1998) showed how the centre of pressure (COP) moved toward the high point of a wedged intervention and in the case of heel wedges the heels bore load for a longer duration of the stride resulting in the heels bearing more load and for longer, not ideal when they are already weak as is common in this conformation. Degueurce et al (2010) expressed the effects of elevation on an earlier and increased flexion of the proximal interphalangeal joint, Crevier-Denoix et al (2010) found an induced elbow flexion and Lawson et al (2007) highlighted the transference of load onto the superficial digital flexor and the suspensory ligament, so the unintended consequences of addressing the pedal angle must be taken into consideration. However, when returning the HPA to ideal, one would assume the load sharing of the soft tissue structures would return to more ideal, but this is in need of more research to be an evidence based statement. The most common reservation for applying wedges seems to be the possibility of prolapse and heel crushing. Possibly an important factor to remember in the subject is the frog.
Fig.3 Diagram expressing the function of the frog in providing a ground reaction force in opposition to the descending body weight, once the frog is loaded the body weight is transferred to the digital cushion and the bone column which now share the ground reaction force (GRF) with the heels, essentially load-sharing with them. The further the frog has to descend the more the base structures have to displace or bear load, once these structures reach their elastic capacity they begin to fail.
Fig.4 With the application of a wedge the BBHPA is restored but the COP is moved toward the heels. The distance the frog has to descend (yellow lines) to provide a GRF to the descending body weight is much larger and possibly never reached leading to possible prolonged excess loading of the heels and prolapse, when added to the already prolonged heel load time and movement of the COP caudally this goes someway to explain why the heels are crushed with this application.
The movement of the COP caudally, is actually not a bad thing. From where and to where it moves, is the question. When the heels are elevated, the COP moves from toward the toe to more centrally within the capsule (Fig.5).
Fig.5 The movement of the COP caudally with wedges reduces the lever arm at the toe the extending moment and collapsing force on the limb. This reduces the strain in the flexor structures.
A recent presentation by Haydn Price at the BEVA congress on his studies at the RVC outlined the positive impact packing can have on the internal structures of the hoof, essentially creating immediate frog support on impact which “locked” the internal structures of the hoof into place, this correlates well with the experiences of the author and when applied to the application of wedges can go a long way in mitigating the negative effects of wedge application mentioned above.
Fig.6 Packing is applied with the wedge, now frog support is immediate, the frog has no distance to descend before being loaded possibly mitigating the heel crushing, other studies have found that heel expansion with packing was returned closer to that of a barefoot then that of a normally shod foot, again showing that packing goes some way toward re-establishing normal hoof function which is affected by shoeing and exacerbated by wedges.
The effects of wedges will also be subject to other factors, touched on by Hagen et al (2017) who discussed that individual conformation and loading patterns will create different responses to the farriery interventions as well as the application techniques of the individual farrier creating different outcomes. The elastic modulus of each individual foot must also be taken into consideration,
Fig.7 Hooke’s Law and Young’s modulus of elasticity dictates that When a strain is applied to a material it deforms elastically proportional to the force applied, the material will return with the release of the strain, this law applies to the hoof, it will deform under load and return to its original shape until its elastic modulus is reached, as the forces get larger the return to shape gets slower but remains elastic until a certain point at which the distortion becomes permanent, the rate of distortion will be proportionate to the strength. This graph could represent two imaginary hooves and their heel strength, the weaker hooves heels will deform faster under less strain and reach their capacity quicker, a foot that is strong enough may accept a wedge and the extra forces may remain within its elastic capacity and not need extra support in the form of packing, where the wedge would tip the weaker foot over the elastic edge.
Hooke’s law expresses the need for pro-active early addressing of hoof morphology, being aware of any changes from shoeing to shoeing, understanding that the hoof is subject to the forces applied to it and doing what is needed to keep the hoof within its elastic modulus.
Often wedges are being fitted to heels that are already weak and underrun, a common co-morbid and predisposing factor of a BBHPA, the extra load applied to these feet, with a wedge, without mitigation, can cause even more crushing, but in the authors experience sufficient frog support can mitigate and possibly reverse the morphology.
Fig. 8-10 Examples of genuine hoof capsule proportion improvements, resulting in transitioning back out of wedges into flat shoes.
The efficacy of wedges comes down to appropriate trimming, frog support which addresses the cause in many cases, and creating ideal proportions.
Correct trim is paramount and has a whole article to itself. Just as important is frog support as discussed, but also correctly fitting wedges that create reduced leverage and ideal proportions around COR while supporting the prolapsed structures. (Fig.11)
Fig.11 The importance of correctly fitting wedges with proportions around COR.
Very often wedges are fitted too short, sometimes the prolapsed bulbs of the heel become part of the bearing surface in severely prolapsed feet, these should be supported, if not, they often prolapse further over the back of the application. But most importantly the application should centralise the centre of pressure by creating as close to 50/50 proportions of the base around COR. This normalises the strains within the capsule, both on the capsule itself and the soft tissues, this then facilitates improved morphology.
These findings are currently anecdotal evidence, more research is needed to quantify the differing effects of wedges with and without frog support, however, although wedging remains a subject for debate in the authors opinion it becomes a necessary evil when confronted with the pathologies associated with a BBHPA or negative palmer and plantar angle. These conformations pre-dispose to catastrophic and unfortunately common injuries. Protection and treatment against these should perhaps take precedence over the possible crushing of heels, but there are ways of providing some mitigation toward the consequences of providing elevation and appropriate frog support and creating ideal proportions are certainly of utmost importance.
With hooves that need an improvement in angle but are not yet pathological, there could be some argument that methods could be firstly adopted to encourage positive hoof morphology over time rather than an immediate fix which addresses the symptom but not the cause. But, what should be taken into consideration is how strongly the conformation predisposes to injury and the current work load. In the authors opinion the risk of further crushing to the heels, which is possibly reduced by providing adequate frog support, ranks lower than the risk of life threatening injury. With horses that already have injury or are post navicular surgery for example, research shows that appropriate wedges or other forms of heel elevation can have immediate positive effects and aid in a better long term prognosis.
In conclusion, wedging is beneficial in the prevention and treatment of certain pathologies but should be done in conjunction with providing frog support and correct proportions and fit, with whichever modern material is deemed appropriate, to help maintain foot function and shared load.
References
S.E. Lawson, H. Chateau, P. Pourcelot, J.M. Denoix, N. Crevier-DenoixEffect of toe and heel elevation on calculated tendon strains in the horse and the influence of the proximal interphalangeal joint
J Anat, 210 (2007), pp. 583-591
C. Degueurce, H. Chateau, H. Jerbi, N. Crevier-Denoix, P. Pourcelot, F. Audigié, et al.Three-dimensional kinematics of the proximal interphalangeal joint: effects of raising the heels or the toe
Equine Vet J Suppl, 33 (2001), pp. 79-83
N. Crevier-Denoix, C. Roosen, C. Dardillat, P. Pourcelot, H. Jerbi, M. Sanaa, et al.Effects of heel and toe elevation upon the digital joint angles in the standing horse
Equine Vet J, 33 (2001), pp. 74-78
Moleman, M, et al (2006) ‘Hoof growth between two shoeing sessions leads to a substantial increase in the moment about the distal, but not the proximal, interphalangeal joint.’ Equine Veterinary journal, volume 38, No. 2, pp. 170-174
Uhl. E.W, Blas-Machado. U, Kirejczyk. S.G.M, Osborn. M.L, 2018, Correlating Increased Mechanical Forces with Tissue Lesions in Equine Navicular Disease
(Accessed 03/03/2019)
Wilson, A, et at (1998) ‘The effect of foot imbalance on point of force application in the horse.’ Equine Veterinary journal, volume.30, No.6, pp. 540-545
Willemen MA, Savelberg HHCM, Barneveld A (1999) The effect of orthopaedic shoeing on the force exerted by the deep digital flexor tendon on the navicular bone in horses. Equine Vet J 31, 25–30.