What is hoof concavity?Apr 18, 2022
This blog post tells you why concavity is an important factor when assessing hoof health.
Concavity can be described as the cupped appearance of the horse's sole - like an upside-down bowl. Imagine you're holding the horse's hoof up and looking at the sole; the frog should be nestled at the bottom of the bowl shape and the sole should rise up to the periphery of the foot and the hoof walls.
Concavity is a sign of a healthy sole with enough depth to keep adequate distance between the coffin bone and the ground. It allows the coffin bone to “sink” with each step and spring back up to its normal position again via the hoof mechanism. It means there is enough space between the coffin bone and the sole to allow for healthy circulation – blood is the food of the hoof. The blood supply is called "corium", and it is like a red sock that envelops the coffin bone. Without a healthy corium, tissues including bone begin to deteriorate.
Below is a photo of a hoof with good concavity. We've marked up the photo to show you the shape of the sole better.
The amount of concavity differs from horse to horse since each coffin bone is unique. In large horses with large hooves, concavity is spread over a larger area so the sole may appear more flat even when it’s healthy, such as in a Clydesdale hoof, whereas small ponies can have a lot of concavity. Hind hooves usually have more concavity than front hooves due to the different shape of the pedal bone.
Below is a draught horse X with less concavity but good sole depth and healthy hooves, front hoof pictured. The next 2 photos show a gypsy cob X with nice concavity in a hind hoof. The laminae could be a little tighter (newly purchased horse who had long trim cycles before).
Why does a horse need concave hooves?
The pedal bone is covered in corium – vital blood supply that feeds the bone and soft tissues inside the hoof. The corium has a physical thickness, and adequate sole depth together with the laminae keeps the coffin bone high enough inside the hoof capsule to facilitate enough space for the corium to function optimally. If there isn’t enough blood entering the tissues, then those tissues deteriorate or become necrotic.
Remember that the shape and health of the hoof capsule, along with appropriate nutrition and movement with a purpose are all part of healthy circulation- if any of those are lacking, then circulation is compromised.
Movement with a purpose: Horse following his herd mates to water. Moving one foot every 20 seconds for the next bite of grass does not count as movement!
A healthy concave sole allows a horse to travel over varying terrain comfortably (note that we’re not talking about a horse under saddle here, that is a different matter). It allows the horse, along with proper development of the caudal structures (soft tissues in the back of the foot) to avoid the ever so damaging toe first landing.
Horses with poor soles are more susceptible to bruising- they are more easily hurt as their bones are literally too close to the ground- and will not travel well on hard or stony ground. Not that we gauge a horse’s soundness by how well they go over rocks - we definitely don’t do that - but it can give us valuable information about their hoof health. Horses with thin, flat soles should NOT be made to walk over stones in the name of “conditioning” the feet. Enduring pain and discomfort should never be on the path to better health - and in this case will never lead to better health.
A horse with a sole this thin should never be forced to go over rocks. This is permanent damage due to inappropriate diet, lifestyle and years in shoes.
We have hoof boots and pads for these situations, and some horses have suffered such extensive damage to the internal hoof structures that they will always need boots and pads for exercise. Some horses require protection in their living environment, at certain times of the year.
This horse lives in hoof boots during summer when the ground is hard and the rocks bother his thin soles, the hoof above belongs to him.
It’s also a bad idea to let the hooves overgrow and see a false sense of comfort over harsh terrain; consider it the same as shoeing the horse. That is a topic for another blog post so we won't go into it here, but I thought it worth mentioning as something that seems quite a common practice. To help a horse with poor soles move comfortably on hard or stony ground, use hoof boots!
This overgrown hoof, despite the damaged coffin bone and thin sole, walks over rocks pretty well. It's a false sense of comfort since the high walls are acting like a horseshoe and placing excessive strain on the laminae.
The typical neglected hoof which walked over the sharpest large gravel with no problems. The absence of lameness does not equal soundness. Simply lifting the sole off the ground is not the answer.
What if my horse has flat soles?
The opposite of a healthy concave sole is a flat sole, or even a convex sole. Flat soles are unfortunately seen everywhere, and the convex type is seen in founder cases, which should be considered an emergency!
Here are photos of two hooves with a very small amount of concavity.
There is a tell tale "ridge" of sole at the toe - when you see this, assume the horse's sole depth in the concave area is lacking.
Unfortunately, so many horses we see have flat soles. Flat soles are caused by distal descent. We can picture this as either the coffin bone sinking down inside the hoof capsule, or, the hoof capsule being “pushed” up the bony column. The latter is truer but “sinking” is a widely used term which most people recognise. Distal descent is caused by weakening of the laminae. For many of our domestic horses, this very unfortunately starts to happen soon after birth due to lack of, or poor trimming, inappropriate living environment, lack of movement and a poor (by which we really mean a RICH) diet. I have seen newly weaned foals with laminitis, foundering yearlings and so forth. 2 year olds with the most terrible sinking - these are horses that later in life get labelled with "born with bad feet" or "they were always like that".
Below is a photo of a yearling, future TB racehorse, and we will later say he "always had poor feet". He received no hoof care for the first year of life, lived on a diet of rye grass pasture, supplemented by large amounts of alfalfa (lucerne) and "breeder pellets". This hoof is in the middle of a severe laminitis event, and in 6 months time when he's 18 months old, will have a shoe nailed to it.
This photo below shows a hoof with a thin sole and distal descent. The red lines are drawn in to help you see the shape of the sole.
On the catastrophic end of the scale, we have the foundered horse whose pedal bones penetrate the soles, and on the less severe end - if we can call it that - the horse with thin flat soles. They’re both a symptom of the same problem!
If you have a well-marked lateral x-ray (x-ray taken from the side of the hoof at ground level), it should indicate what is commonly termed “founder distance”, or CE (coronary extensor distance). Simply put, it is a measure of vertical distance between the top of the coronary band and the coffin joint. They should be aligned horizontally. The greater the vertical difference, meaning the coronary band is sitting HIGHER than the coffin joint, the more the pedal bone has “sunk”. This is so common it’s often disregarded entirely, and the damage- whilst largely able to be improved- is rarely completely reversible. This is a great example of "common does not equal normal"- most X-rays are not properly marked, and the CE distance is entirely ignored. The horse is said to have thin soles with no explanation of WHY this is the case and how to improve it without causing further damage. The remedies usually involve a band aid of applying a shoe, since this appears to give the horse a great deal of instant comfort. If only we remembered that most of these horses have already been shod earlier, and the thin soles have a lot to do with the shoes. Anything that places undue stress on the laminae- sugar rich diet, long trim cycles, trim technique- contribute to "sinking".
Severely foundered horse, the soft sole would peel away with fingers to expose the coffin bone.
How do we get concave soles?
The answer is simply by looking after the laminae and circulation.
- We look after the laminae and circulation by providing frequent trims so the laminae don’t endure excessive strain, so the walls do not flare out and the toes do not get long.
- Frequent trims keep the hoof capsule in its anatomically correct position and hugging the internal structures tightly.
- By avoiding shoes which add excessive stresses to the laminae and reduce circulation by getting in the way of hoof mechanism.
- By lots of movement. Optimal circulation demands physical movement.
- An appropriate low sugar diet which keeps laminae-destroying inflammation at bay.
- Using hoof boots and pads, they are the best physical aid to stimulate good growth and healthy movement patterns when those are lacking.
It’s really pretty simple, and something that we fail rather miserably at. We need to do a lot more work on education, learning to ask the important questions and being curious enough to question what we're seeing, and always asking "but why" like a little kid!
This next photo shows the difference between a 6-week trim cycle (two top images) and 1-week trim cycle (bottom images). Notice how much had to be trimmed when the hoof was left for six weeks, compared to the minimal trimming required when the hoof is trimmed weekly. The hoof is flaring in the 6 week trim pictures, the bars are laying over the sole. The toes are long and wearing unevenly.
And another comparison photo of a front hoof, showing the sole and oblique views before and after a weekly trim. (The frog is exfoliating and looking ugly as a result.)
Often, when the hoof care, diet and environment is changed to be species appropriate, a flat sole can over time become concave. Sometimes this happens just by doing more frequent trims with a better technique, and no other changes are necessary. This results in the new hoof growth bringing in a tight laminar connection between the coffin bone and the hoof wall, and the coffin bone "rises up" in the hoof capsule, allowing space for the corium to build a healthier sole, and the sole to callous into its correct form.
What we trim from the bottom, affects what grows up the top so we need to have a more long-term view and be mindful to watch the changes – are they positive or negative, or is nothing changing?
Above is a 6 year old horse who came out of shoes 4 weeks ago when he moved to a new home. He is now on an appropriate diet and moving lots. The healthy growth is already visible under the hairline. It will take 9 months or so for the laminitic hoof wall to grow out.
Often a small change can have a big effect. This pony was moved from a paddock with a small amount of grass, to a paddock with no grass (large paddock not a yard). The inflammation events stopped and the wall is growing down more smoothly. When the laminae tightens up, concavity can be seen to "appear overnight" when the hoof has grown about half to 2/3 of the way down, provided the internal damage isn't extensive.
What if the soles stay flat or my horse has other problems relating to concavity?
When the coffin bone sinks down, the corium under the bone becomes compressed, and can't feed the bone. We have a flat sole. Over time, the edges of the bone "erode" away, which means the bone itself is less concave. If this state goes on for long enough, too much damage has occurred to the blood supply and the bone to grow a healthy sole, and we have a permanently thin, flat soled horse. These horses are not condemned to “need shoes'' or be put to sleep. They need excellent management, weekly trimming and padded hoof-wear for riding, and at times in their living environment depending on the conditions. They must have a forage based/low sugar diet to remove those laminae inflaming events, watch out for the growth rings- they are indicators of inflammation! They must have lots of movement which creates blood flow, to protect and nourish what’s left of the internal structures.
The cure is the prevention and vice versa - just simply how horses should live. The term pedal osteitis evokes terror in owners, but in truth, I’d say most horses I see suffer this condition to a degree. It’s more common than you’d think, if you know what to look for.
Can concavity be wrong?
There is one type of sole that appears concave, but is actually very thin. This kind of sole is sometimes called a “retracted sole”. (Although, a sole cannot retract, there is no empty space between the sole and the bone). This kind of sole usually has a distinct ridge around the periphery of the hoof, and then it drops abruptly into deep concavity. If you press on the concave parts, it may give with finger pressure - something anyone can do and definitely ascertains the sole is possibly only mere millimetres in thickness. This is a sign of a very unhealthy hoof.
Below : "retracted sole" - they are always a thin sole. Picture taken the day of the shoe pull, owner had been told the horse has brilliant feet.
Sometimes, there can be excessive concavity at the toe which in effect tips the coffin bone in the wrong orientation (the back of the bone sits lower than the front). This is called a negative palmar angle for front feet, and negative plantar angle if we’re discussing hind feet. If an x-ray shows that there is too much sole under the tip of the bone, a knowledgeable trimmer or farrier can correct this. This particular scenario is more often seen in shod horses where the digital cushion has “failed”, but it can be seen in barefoot horses too, and the back of the foot isn’t always unhealthy. It’s all about the ability to read the hoof and apply the trim according to the horse’s anatomy. We have discussed this issue in another blog post here: https://www.flexhoofboots.com/post/npa-what-is-it-and-how-to-fix-it
And finally, on occasion, there IS such a thing as too much concavity. Some hooves grow taller and develop deeper and deeper concavity (often seen in those mini ponies or Shetland ponies that are irregularly trimmed or desert hooves which fail to exfoliate on their own). This is accompanied by collateral grooves that plunge to dark depths and sometimes a huge amount of frog growth. Reading the hoof correctly is the key again to resolving the issue.
Below is a strong hoof with too much concavity. An easy trimming fix but not a DIY job - any sole trimming should be done by a skilled professional, and preferably under veterinary guidance.
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