Wedges - A Necessary Evil?
Updated: Jul 20
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.
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).