Frog Pressure - Historic vs Modern Applications
Updated: Feb 15
Frog function is a vitally important factor in hoof health. A cliché has emerged amongst some farriery circles “use it or lose it.” when referring to the frogs relationship with the digital cushion, as the digital cushion has a poor blood supply and therefore limited regeneration properties.
When we shoe the horse we reduce the functionality of the frog as we lift it further from the ground. Studies have shown that both the expansion and contraction of the hoof is reduced with the application of a shoe compared to the barefoot (Roepstorff 2001), historically, heart bar shoes are used to utilise the frog in the treatment of hoof pathology and weak dysfunctional hooves.
The heart bar shoe has been used as a remedial shoe for many pathologies, laminitis being one. Eustace and Caldwell (1989) expressed its efficacy in the treatment of solar prolapse as a result of laminitis and many studies before and since have advocated its use in treating the pathology. Brown (2015) outlined the mechanisms for its use, stabilisation and load sharing, explaining that applying “positive pressure” to the frog de-loads the laminal bed where damage is occurring. O’Grady and Parks (2008) also expressed the same use but stated that it was unknown if the heart bar provided any further benefits to other treatment methods, in the same paper it explained how this focal pressure can cause adverse effects and utilising a larger surface area can be more beneficial.
In recent years there has been research into other effective farriery treatments such as the clog, which Reynolds (2018) found to produced the least amount of surface tension of the hoof when compared to the heart bar and other techniques including the use of modern materials to provide this vital frog support. See my article on Laminitis for further reading.
Other uses for the heart bar has been flat/weak heeled feet and cracks, again in an effort to utilise frog support to unload the compromised structures, the shoe is advertised as mimicking the unshod foot restoring the natural pattern of weight bearing and supporting the coffin bone, yet Podol (2006) stated that incorrectly applied the heart bar can become a “Damaging piece of equipment” and Castelijns (2002) listed other disadvantages including their lack of efficiency in deeper frogs and their tendency to induce frog atrophy.
It can clearly be seen that the foremost reasoning behind the application of the heart bar is (re-) utilisation of the frog after application of a rim shoe that lifts it off the ground, but the frog is deformable and soft, while the heart bar is rigid and hard. Let’s take a min to outline the functions of the frog.
Young (2018) outlined these important functions, the dissipation of forces from locomotion, weight transference of the bone column and blood circulation, it also discussed how these functions are made possible by its makeup and harmonious relationship with the other hoof structures. My own articles and webinars have a re-occurring rhetoric, expressing the importance of the haemodynamic mechanism.
Fig.1 Diagram showing how the frog transfers and dissipates the ground reaction force by its relationship with the other internal structures, this displacement of these structures is also the mechanism of blood circulation. The frog provides support and cushioning for the descending body weight.
What’s important to appreciate is that once the foot is unloaded so too is the frog and everything returns to its unloaded position, this is also part of the hoofs blood circulation mechanism.
Now lets consider the heart bar, as stated by Young (2018) it is a fixed metal plate which can be fitted with positive or negative pressure to either provide constant support or loaded support respectively, however it can not provide any further advantages or enhancements as agreed by O’Grady and Parks (2008) and can actually either be useless or harmful depending on the fit, which is also influenced by hoof growth as the heart bar only sits where its intended until the hoof growth moves it. In the case of positive pressure the internal structures do not get any relief or return to origin, this must, in the authors opinion, influence the pumping mechanism and also the expansion and contraction mechanism as suggested by Watson (2016).
So the question is what can provide the benefits of the heart bar while more accurately mimicking the unshod foot and add other advantages?
Young (2018) perhaps points us in the right direction, although it was only a pilot study, it used pressure plates to assess the comfort of horses with caudal hoof packing versus a heart bar and found that the horse was happier to bear more weight on the hoof packing than the heart bar. Casserly (2018) also expressed improvement in palmer angle and sole depth in horses with frog support padding over and above the use of heart bars.
Fig. 2 One of the added benefits of padding over a heart bar is the increased surface area, shown in this image. Allowing load sharing not of only the frog but of all the solar structures, more closely mimics the bare foot. The increased surface area leads to reduced pressure on the heels, which is actually increased with the application of a bar shoe. The greater the restriction of the hoofs natural deformation, the greater peak load is experienced by the heels, shown by the colour changes in the pressure mat readings. While increasing surface area to include the frog, the increased restriction from bar shoes increases the pressure on the heels. This findings means that applying bar shoes for weak heels is contraindicated.
Roepstorff (2001) showed that shoes restricted the expansion and contraction of the hoof while adding hoof padding restored the hoof function closer to that of the bare foot. This can go some way to explaining the success of applying hoof padding to weak feet as experienced by both the author and Casserly (2018).
Fig.3 The authors preferred intervention to address collapsed palmar/plantar hoof structures. Mustad hoofcare comfort mix.
These findings, alongside practicality and economic reasons is why frog support padding,in different forms, has become the main stay of the authors theraputic intervention.
The studies of Dr Hagen have shown that bar shoes restrict even further the natural deformation of the hoof. With the statement of Gunkelman and Hammer (2017) and the experiential opinion of the author, suggesting negative hoof morphology is a product of reduced hoof capsule functionality, treating low weak heels with bar shoes becomes illogical. Instead increasing hoof function and natural deformation is indicated.
Another further technology that could be utilised in place of the heart bar is Formahoof, although studies like Young (2018) and Roepstorff (20