Anima Books - Trimming (and shoeing) for comfort, performance, and injury prevention



			Dr. Christine King Licensed Veterinarian Germanton, NC Ph: (336) 608.8552 e-mail: king(at)animavet.com

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			"Trimming (and shoeing) for comfort, performance, and injury prevention" Most of the problems I see with both trimming and shoeing originate in a single, fundamental misunderstanding about how the horse’s hoof is designed to be loaded. ... [Full-text article by Dr. Christine King] Please note that this article is copyrighted, so if you want to reuse it in any sort of publication (web site, printed media, etc.), then please contact me for permission and guidelines for reuse. Enjoy! ******************************************

			Trimming (and shoeing) for comfort, performance, and injury prevention Dr. Christine King Whole books have been written about the structure, function, and care of the horse’s hoof. Perhaps one day I’ll add to that library. But for now I want to focus on some common problems I see with trimming (and shoeing) that adversely affect the horse’s comfort and function. I say “trimming (and shoeing)” because this article applies both to barefoot horses and to those who are shod. The principles are the same because they are based on the structure and function of the equine foot—on how the foot is fundamentally designed to work. The choice and placement of a shoe certainly are important, as they can help or hurt. But the success or failure of any shoeing strategy begins with the trim: in what is done with the rasp, not with the shoe. A good shoe cannot make up for a bad trim. The hoof is designed for heel loading... Most of the problems I see with both trimming and shoeing originate in a single, fundamental misunderstanding about how the horse’s hoof is designed to be loaded. Here’s what we know from examining the anatomy of the equine foot and studying how it functions: The horse’s foot is designed to bear the bulk of its load in the back two-thirds—basically, from the point of the frog back to the heel bulbs. Take a look at this image and you’ll understand why.

	Here it is again, with some reference points. The toe is on the left and the heel is on the right. The foot has been sectioned along its midline, revealing the center of the second phalanx (P2, or the short/middle pastern bone), coffin joint (CJ), third phalanx (P3, or coffin bone), deep digital flexor tendon (DDFT), navicular bone (NB), digital cushion (DC), wall, sole, and frog.

There are several things I want to discuss about this image. The first is this: notice the mass
of soft tissue in the back two-thirds of the foot.

The predominant tissue back there is the frog and the digital cushion (DC) directly above it.
Both of those tissues are bulky and strong yet elastic. The digital cushion is composed primarily of collagen and elastin, so it is fibroelastic. The frog likewise is quite elastic. These two tissues fill the area between the heels and are designed to withstand repeated cycles of compression.

The primary role of the frog and digital cushion is considered to be as a shock absorber, to absorb and dissipate the forces of ground impact and loading. (The frog is also assumed to act
as a brake and an anti-slip device.) These tissues are like the high-tech materials used in the soles of running shoes—only, these are living tissues. Unlike the stuff in your running shoes, they have the capacity to adapt and regenerate.

...so, move the loading surface back

Whether leaving the horse barefoot or preparing the foot for a shoe, the hoof should be trimmed so that it is loaded as far back as possible—ideally, all the way back to the very base of the frog—and the frog and digital cushion are fully engaged when the foot is loaded. The goal is to have the hoof loading all the way back to where the frog and the hoof wall meet at the back of the foot. Doing so makes the best use of those fibroelastic tissues that are designed to be loaded.

Another way to put it is that, when you’re looking at the foot from side-on, you want to move
the loading surface of the hoof further back toward the heel bulbs. That is particularly important in feet with underslung heels (such as the one shown below), where the heel bulbs are hanging out there in space, unsupported from below—and underutilized. In this image, the blue arrow indicates where loading ends in this hoof. A long way forward of where it could, and should!

It is not enough to make sure the shoe extends back there, though. The hoof itself must be fully loaded back to the base of the frog. It is a common misconception that extending the heels of the shoe accomplishes the same thing; it does not.

It is also not enough for there to be some rasping done back there. The loading surface must be level all the way from the point of the frog back to the base of the frog in order for heel loading to be optimal. A common fault I see is that, although some rasp marks go all the way back, for all practical purposes the loading surface doesn’t go any further back than the angle of the bars.

Here is an example. Functionally speaking, the loading surface does not extend behind the green stars, even though the rasp marks go further back. The depth and shape of the grooves either side of the frog give it away, as does the side-on view of this foot:

The goal also is to have the loading surface of the hoof wall no higher than the surface of the frog. In feet with very low or very high heels and/or very shrunken frogs, that may not be possible in one go; but it is the direction to head in as quickly as possible. What I usually say
to trimmers and farriers is this: go as far as you safely can with that foot on that day, and then gently but relentlessly keep at it until the hoof is maintaining a healthy shape on its own.

But note this: change too little, and too little changes. A common problem I see is that the trimmer or farrier just doesn’t do enough in one session to make much of a difference. It is essential to set up the foot for optimal biomechanics, which also optimizes blood flow and hoof growth, in order to make any headway; and that needs to happen sooner rather than later.

Now, it is true that in the process of moving the loading surface back, you may sacrifice a little heel height. However, what is gained in moving the loading surface back, and making full use
of those elastic structures in the frog and digital cushion, more than makes up for any small reduction in wall height. Furthermore, over time the horse grows stronger, more robust heels, thereby replacing any weakened, defective, or ineffective wall that may have been trimmed
in the process of moving the loading surface back.

Look at the frog

One simple way of knowing if you have optimal heel loading is to look at the frog. In this respect, frogs are like little kids: they just blurt out the truth, whether you want to hear it or not.

If the frog is plump and juicy, occupying all of its available space between the bars of the heels, and it has a broad, shallow central groove, then you likely have optimal heel loading. But if the frog is small for the size of the foot and the size of the horse, if it has broad and deep grooves either side, and/or a deep, narrow central groove that traps muck and is prone to thrush, then you do not have optimal heel loading. Simple as that.

Remember that the frog and digital cushion are living, dynamic tissues. Just like any other living tissue, they respond and adapt to the loads placed on them. Load them too little, and they will shrink (atrophy, in medical parlance). They don’t like to be overloaded, either, and can certainly be damaged by excessive load. But by far the more common problem I see is under-utilization, leading to a frog and digital cushion that need to be re-engaged and nursed back to robust health.

(To gauge the approximate depth and health of the frog and digital cushion, place your thumb
in the center of the frog and your middle finger on the skin between the heel bulbs at the back
of the pastern; or the other way around if it’s easier. Your thumb and finger should be 2–3 inches apart in the average sized horse with a healthy frog and digital cushion. Also, the tissues between your thumb and finger should feel springy, like there’s some elasticity and resilience to them.)

The good news is that the frog and digital cushion are living, dynamic tissues that respond and adapt to the loads placed on them. Once the heels are loaded properly through appropriate trimming, and the frog is being engaged as it was designed to be, it quickly starts to regenerate, as does an atrophied digital cushion. After just one good trimming, the frog begins to plump up. It may take some months for these tissues to fully regenerate, but positive change begins to happen right away.

Next, move breakover back at the toe

Take another look at the sectional image of the foot. Notice the path and destination of the deep digital flexor tendon (DDFT). It runs down the back of the leg, over the back and underside of the navicular bone (NB), and ends at a strong attachment on the underside of the coffin bone (the * on the lower margin of P3).

The role of the deep digital flexor tendon is to flex the coffin joint (CJ), as well as the other joints in the digit (the pastern and fetlock). It does that by pulling on the coffin bone from underneath as the flexor muscle further up the limb contracts.

As you may imagine, the longer the toe, the more effort it takes for the deep flexor to rotate P3 around the lower end of P2, the more strain is placed on the tight bond between the inner hoof wall and P3 at the toe, and the more pressure is placed on the sensitive tissue of the sole under the tip of P3.

The toe on the foot shown in this image is too long. The wall is too tall from coronet to ground, relative to the height of the wall at the heels (not shown); and it is too far forward, relative to the tip of P3. Here is how this hoof ought to be trimmed (green line) if being left barefoot or where the breakover or pivot point of the shoe ought to be placed if the horse is shod:

In the bare foot it is done by rasping a “half round” or “bull nose” contour at the toe, between about 10 and 2 (the center of the toe being 12 o’clock if you imagine the underside of the foot
as a clock face). As a general guide, the tip of P3 in the average sized foot is located about the width of a man’s thumb (approximately 1 inch) forward of the tip of the frog. Add a generous
¼ inch to ½ inch more and round from there forward.

Here’s what the foot at the bottom of page 3 should look like when trimmed (green line). Note that the total surface area of the hoof has not been reduced, because we’ve moved the loading surface back at the heels as well as at the toe.

It may look extreme, but consider this: all of the material outside the green line on both this image and the sectional image is insensitive horn; removing it is like trimming overgrown fingernails. The horse even feels the better for it, and moves better, too, if it’s done correctly.

That’s because all of the material forward of this green line at the toe, and to the left of the green line in the sectional image below, is not only unnecessary, it’s interfering. It’s making more work for the deep flexor tendon to rotate P3 around P2 at the coffin joint. And it’s putting excessive strain on the bond between hoof wall and P3 at the toe and excessive pressure on the sole under the tip of P3—with every stride.

As the blood supply to the germinal layer of the sole lies between the insensitive horn of the sole and the underside of P3, extra pressure = slowed sole growth = perpetually thin, tender soles. It is particularly important to optimize the biomechanics of the foot in horses with thin soles.

Note that with this strategy, no sole is removed from under the tip of P3. That is very important; few horses can afford to lose sole depth there. Also note that this trim leaves more depth of protective horn around the tip of P3 than there is directly below P3 (compare the lengths of the two arrows below). Nothing is removed from under P3, and plenty is left around the tip of P3.

	Also note how this trim (or breakover position if using a shoe) moves the breakover/pivot point back closer to the tip of P3 (white line, below). That makes it much easier for P3 to rotate around P2 at the coffin joint. Generally speaking, the breakover point should be no more than about 1½ inches forward of the tip of of the frog in the average sized foot. (The tip of P3 is about 1 inch forward of the tip of the frog; and add a good ¼ to ½ inch forward of there as a buffer.)

One final note is that this trim also removes the bulge of sole this hoof (as well as the one shown in the solar views) has immediately behind the white line that is often referred to as the “sole callus” or the “toe callus.” This bulge of sole forms when the toe is allowed to get and stay too long. Contrary to popular opinion, this bulge is not protective to P3. It is too far forward to provide any protection to P3. In fact, it indicates improper loading and growth that does not serve P3.

Here is the foot again, with the bulge highlighted on the left by a pink line running along the underside of the toe. By trimming as shown with the green line on the right, the plane of the loading or solar surface of the hoof is changed a small but significant amount.

The orange line shows the plane of the hoof pre-trim and the white line shows where the plane would be if this foot was properly trimmed. When a sole callus is present, the heels will be overloaded because the bulge in the sole at the toe tips the foot back onto heels that are ill prepared for the load. But when the foot is trimmed to restore a healthier plane (white line),
we recreate healthier biomechanics, blood flow, and thus hoof growth.

This article is long enough, so I’ll stop here for now. In the next one I’ll discuss the sole plane in more detail and also trimming for symmetrical loading of the hoof (lateral-medial balance).

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