One Foot For All Seasons? (9-8-06) Pete Ramey
Note: The prospect of supporting P3 through the sole often seems impossible and generally foreign to many farriers. They know from experience that if you shoe with sole pressure, the corium will bruise and abscess. But understand the difference between constant pressure and pressure and release. P3 can only be truly supported through the sole when the pressure to the sole's corium is being released during flight. This is one of the primary reasons I favor hoof boots with padded insoles, rather than shoeing systems with impression material for laminitic horses.
When many people first study the deep solar concavity of most desert feral hooves and healthy domestic hooves, they get the idea that the sole and frog are passive – lifted off the ground by the bowl-shaped bottom of the foot. This is not true.
The problem is, many farriers and trimmers were incorrectly taught to evaluate and think about hooves while the horse is standing square on concrete, with less thought given to the hoof and limb mechanics in motion or the terrain the horse lives and works on. In fact, when a horse is on the varied terrain – deep, soft footing and/or rocky footing that forges these deeply concaved soles, everything that casts a shadow bears weight. In the front half of the foot, given the larger surface area, even a deeply concaved sole bears more weight than the much smaller surface area of the hoof wall around the periphery. This is very safe, natural and comfortable for horses if adequate sole thickness and callusing are present in the hoof.
Study of healthy, sound horses and recent research from Robert Bowker VMD, PhD suggests that the horse was never intended to “hang” from the laminae as most of us were originally taught. Direct support of the coffin bone through the sole is essential to preventing unnatural, excess stress on the laminae that can contribute to lamellar failure. The deeply concaved soles provide this support when the horse is working on rocks, soft dirt and grass as the foot sinks into the varied terrain.
But what if the same horse routinely lives and works on perfectly flat surfaces? Then our natural P3 support is missing and that same deeply concaved foot will cause peripheral loading. The horse is right back to hanging all of his weight unnaturally from the laminae. Healthy laminae are strong enough to withstand this occasionally, but the repeated insult will cause damage or failure of the hoof wall/coffin bone attachment. It is my opinion that the swelling caused by concussion on a hard surface that is commonly blamed for road founder (mechanical damage to the laminae) is not the culprit at all; instead it is the peripheral loading that usually results from a hard, flat surface. The horse simply needs sole pressure/support.
The expansion of the hoof capsule on hard impact is essential to the horse for both energy dissipation (much like the flexion of your car tires) and for healthy circulation [Bowker ‘99]. The primary force that causes this necessary expansion is the weight of the horse descending onto the concaved sole, drawing it out flat, thus pushing the walls apart on impact. To my knowledge, no one has ever actually studied how much hoof expansion is necessary or natural, but my personal observations leave me suspecting most hooves should expand about ¼- to ½-inch on hard impact. To achieve this, the arc of the sole should be able to flatten about ¼-inch vertically on hard impact as well.
On gravelly surfaces or soft terrain that the outer perimeter of the hoof can sink into, the "naturally" deep (1/2-inch to 3/4-inch) vault of solar concavity is necessary for this ¼-inch of sole flattening to occur; causing expansion of the hoof capsule. However on a hard, flat surface ¼-inch of total solar concavity may be all that is needed and having more solar concavity can actually deprive the P3 of necessary support.
In the real world (if you have grown well-attached hoof walls), achieving the correct amount of solar concavity is usually pretty easy if the horse is moving on the same type of terrain 24 hours a day. The foot will do it for you if you allow it. If the horse is always on a hard flat surface, the inner part of the sole (closer to the frog) will fill in, and the ¼-inch of total solar concavity will usually wear into the foot, provided the walls are not allowed to overgrow and the horse has freedom to move.
If the horse is, instead, always moving on a yielding or rocky surface, the soles will tend to callus into a deeply concaved shape with uniform sole thickness over the coffin bones and lateral cartilages, mirroring the individual's coffin bone shape. I think either way, in each terrain type, the sole is seeking its ability to provide the ¼-inch of flattening (at impact) that is so critical to hoof expansion, while also seeking a critical need for this flattening to “bottom out” so that the coffin bone is supported and excess shear stress does not damage the laminae.
Our problems begin when we suddenly switch the horse from one terrain to the other. For instance, in many areas the correct foot for winter is all wrong for summer. If your pasture or paddocks are yielding in the summer, but frozen solid in the winter, it is healthy to allow your horse to wear completely different feet during the “hard” times in winter. Let the center of the soles “fill in” to a flatter state. If you try to force your hooves to be deeply concaved, you will probably cause the damage, stress and distal descent of P3 associated with peripheral loading.
When springtime rolls around and the ground softens, it is usually best to help the horse adapt to a more concaved summer foot by taking out the retained sole adjacent to the frog, paralleling the inner structures and respecting natural sole thickness. The horse will usually start to try to 'let go' of this excess, indicating it is time to help it come out with your tools. Look carefully for and respect the true healthy sole plane that will usually be lying underneath the exfoliating material. Generally speaking, you only have to do it one time, then natural wear and callusing will take over for you. Please be careful, here. It is much better to do too little or nothing to the soles than it is to do too much. Again, the previous article, http://www.hoofrehab.com/HorsesSole.html will help you do this correctly. After you do this “cleaning out,” recognize that the healthy sole underneath is a natural thickness, but is probably not well callused. There is a big difference between a 5/8-inch-thick sole and a 5/8-inch-thick callused sole! Use hoof boots for rocky terrain or when you add the weight of a rider until densely callused sole is achieved.
It is very important not to overdo it. When in doubt about the sole or frog; leave them alone. I strongly believe that the most common cause of hoof tenderness is constant micromanagement of the sole and frog, so I'm not suggesting you run out and change the feet every time the weather changes. Instead you should accept, understand and respect the need for these different adaptations to terrain throughout the year. If you understand the need, when the terrain changes you will be able to notice the hoof trying to change on its own; you are just there to help it along, because domestic horses don't typically move enough to do a complete job in a reasonable time.
In other regions it works in the exact opposite way. Soft mud and wet pastures in winter turn to dry, baked ground in summer. Other people deal with soft snow in winter and hard ground in summer…… The list goes on, but the point is, wherever we live we need to listen to the hooves and think about the mechanics in the living/working terrain rather than only in the wash rack on trim-day.
Another question that often comes up is, “What if the riding terrain is wildly different from the living terrain?” For the very healthiest answer, I’ll quote Dr. Bowker again, “Bed your horse in the terrain you wish to ride.” If you can’t provide that, the next best option is usually to trim for comfort and function in the living terrain and boot for the riding terrain. If the soles are packed into a deeply concaved shape, I like to use dense foam insoles in the boots so the foot isn’t peripherally loaded by the boot floor.
If the horse is worked enough, the foot will adapt to the work conditions instead of the living conditions, even if the horse actually spending more time loafing around in the pasture. It is a question of miles, not time. For example: hard-working barefoot carriage horses and mounted patrol horses tend to “want” the flatter, thicker “concrete hooves” that mirror their working conditions. I allow it, even encourage it. I did worry in the past that these hooves might experience problems because they were too flat and too hard to function properly in the pasture, but I’ve yet to see any real-world problems occur that suggested there was inadequate circulation or hoof function.Unfortunately none of this can be put in a box. I can’t tell you exactly how much sole to leave or take from the center of the foot. I can tell you it is wrong at least 99% of the time to do anything at all to the outer inch of sole adjacent to the white line on a barefoot horse. The retained sole and bar that sometimes builds between this outer area and the frog, though, is a very complex variable – extremely important to optimum function and performance. No answer is always right or wrong. The best thing I can do for you here is give you something else to think about – another reason to constantly question everything you "know."
Since the collateral grooves tend to be 7/16” (9mm) away from the corium, if you concave the sole all the way to the bottom of the collateral groove around the apex of the frog, you will have cut the sole too thin (right side of drawing). In order for the entire sole to be of adequate (5/8”-15mm) thickness (much needed for safety and soundness), there needs to be a 1/4-3/8” vertical dropoff into the collateral grooves adjacent to the frog (left side of drawing). Note that the collateral groove heights-off-the-ground are the same on both sides, but the left side has a much thicker sole.
The same reasoning is important when considering the sole ridges that tend to pop up along the sides of the frog. On the right side of the photo, the sole ridge is simply the horse growing adequate sole thickness in that spot – leave it alone, and hope that adequate thickness spreads to the rest of the sole. Look for that 1/4” dropoff into the collateral groove. On the left side of the photo, there is a 1/2” dropoff into the collateral groove – this ridge of sole has formed on top of an adequately thick sole. It might be appropriate to trim this ridge off so that the dropoff is only 1/4” deep, though do consider that it might also be a terrain adaption as discussed above.