Edited April 2020 to add Dr Robert Bowker talks to Wendy Murdoch of Sure Foot Equine Stability Program April 2020 (parts 1 and 2).
The digital cushion is a natural shock-absorption mechanism, which, when well developed, consists of strong
ligaments that have adapted into fibrocartilage.
Myxoid tissue is a primitive connective tissue (similar to stem cells) that can adapt and respond to whatever is needed if it is stimulated correctly. It is important for the formation of fibrocartilage from fibrous tissue, especially for the frog.
Digital cartilage consists of fat cells, myxoid cells and elastic tissue with a fair number of blood vessels between the ligaments. As the digital cushion develops, ligaments start to thicken and the myxoid tissue contributes to fibrocartilage.
In a well developed foot, these ligaments run under the DDFT connecting the lateral cartilages, like a trampoline.
Fibrocartilage is tissue that is halfway between cartilage and fibrous tissue; it develops or adapts in response to stimulation of the foot under compression/strain. Under compression or strain the fibrous tissue tries to protect itself by producing fibrocartilage – this is a normal adaptive response, not a pathology.
In 1941 the Germans had discovered the ligaments in the foot, but not the blood vessels. People have claimed there are scant numbers of blood vessels between these ligaments but there are in fact a fair number of blood vessels.
When the foot is on the ground, the ligaments become taut and the volume of the frog increases (causing negative pressure) – RB believes that this causes much of the blood in the foot to be directed into the vessels in between the ligaments, causing it to start to swell (like erectile tissue) and thereby supporting the navicular bone and the DDFT. In a good foot the navicular bone sits on a cushion of thickened ligaments and blood vessels.
Dissections of horses with navicular syndrome (NS) show a lack of ligaments and fibrocartilage or damage to ligaments in the digital cushion, damage to blood vessels and pathologic DDFT tissue. They have less of a cushion for the navicular bone.
In comparison, a “good” foot has robust ligaments and a developed digital cushion beneath the navicular bone - “like a training shoe”.
When a foot with NS is loaded:
1. it can't support the navicular bone/weight because it doesn't have strong enough ligaments and vessels (due to damage), and
2. the vibrations can't escape the foot, setting up a continuous vicious cycle.
In a “good” foot, the lateral cartilages added together usually make up 25-37% of the total width of the foot, and the blood vessels are inside of the cartilage itself. When the vessels are inside the cartilage, when the foot hits the ground, the vibration shock wave goes up through the heels and the vibration is transmitted through the lateral cartilage and can get into the blood vessels to be transported out of the foot.
In a foot with NS the total thickness of the lateral cartilages is less than 10% of the width of the foot, and the blood vessels are on the inside of the lateral cartilage.
When there’s no support for the navicular bone:
the navicular bone starts to move on P3 and starts to sink towards the ground, and
the blood vessels are damaged – the vessels not only provide support for the navicular bone but they also nourish the DDFT and other ligaments. Pathology of navicular syndrome shows that it occurs outside of the navicular bone itself & beyond the pressure of the DDFT against the navicular bone.
Navicular is an entire foot problem - it doesn't just affect the navicular bone.
The frog should not be trimmed (between the collateral grooves and the tip of the frog apex), it should be swollen, large and on the ground/weight bearing.
When the foot is trimmed to remove the bars so that they are no longer weight bearing, peripheral loading of the walls is increased.
The hoof should support the thickest part of P3 through the collateral groove/bar area.
When shod, the load of the foot is distributed around the shoe. The rim of P3 is the thinnest part of the bone. When the foot is loaded peripherally, the rim of P3 becomes susceptible to problems like pedal osteitis and frequent fracturing of the edge of P3.
The centre of P3 is the thickest part – the hoof should support this area, directly under the roof of the collateral groove. By removing frog and bar material, there is no longer support under the centre of P3 so the foot has to be supported another way.
When the foot is loaded peripherally – on the hoof wall – all of the horse’s weight is on the hoof wall. P3 is being suspended from the laminae/hoof wall. RB believes that the hoof wall should support only 5-20% of the horse’s weight, the other 80-95% should be supported by the solar structures, i.e. the frog, sole, bars and dirt plugging the collateral grooves and sole which will support the centre of P3. “The coffin bone really shouldn’t be suspended from the hoof wall by 100% of the weight.”
People think the horse’s weight should be suspended from the hoof wall because when a horse develops laminitis, the laminae can no longer support the horse and it appears that P3 comes through the bottom of the horse’s sole - but actually the hoof is moving up the leg.
There is a large surface area of laminae, and if the foot is trimmed so that the hoof wall is below P3 (the sole), P3 IS suspended by the hoof wall. But if the foot is trimmed so that there’s minimal weight load on the hoof wall, P3 is not suspended by the laminae/hoof wall, and most of the weight will be on the solar surface of the foot.
RB has seen a lot of pathology in horses’ feet where the hoof wall is the loading structure and P3 is suspended from the hoof wall – the foot adapts to try to minimise stress on the connective tissue.
In other mammalian species, e.g. ruminants, camelids, the nail (hoof wall) is not the primary loading structure – they walk on some sort of solar pad. The horse has a more keratinised solar pad and a very large digital cushion which contains a lot of ligaments and blood vessels – when the foot is loaded, the blood vessels become engorged and remove the energy/vibration of impact.
You should be able to pass a ruler under the hoof wall all the way around – the weight should be on the solar part of the foot.
PR made foot imprints as horses left an arena, cleaned the dirt plug out, and the area where the foot was loaded on the arena was inside of the hoof wall.
It is thought that P3 doesn’t have a periosteum. This depends on how the foot is loaded. If loaded from the hoof wall the connective tissue overlying P3 (the periosteum) changes orientation so that it is no longer parallel to P3. When the solar surface is loaded, the periosteum is present.
RB believes that P3 wants the majority of its weight to be loaded (supported) from the solar ground surface rather than the hoof wall.
Using Dopler ultrasound, able to show that perfusion of the foot changes with the surface the horse is standing on.
On a hard surface the same amount of blood is going to the foot, but it stays in the larger vessels.
On a conforming surface, small blood vessels become perfused which leads to greater blood flow through the entire dermis of the foot and the internal parts of the foot. Softer conforming surfaces increase perfusion of the foot – studies in humans have found conforming surfaces “more comfortable” and “relaxing”.
RB believes the horse may relax psychologically as well as physiologically when standing on a conforming surface.
The footing should be firm, e.g. pea gravel or larger gravel. Pasture probably isn’t sufficient for stimulation. In other species on harder (gravel) surfaces bone density increases in the foot and the digital cushion becomes more fibrocartilagenous on pea gravel/gravel rather than pasture.
There is no exact distance a horse has to travel – one herd in Canada averaged only 0.5 km/day.
Optimal hoof health = the hoof wall bearing only 5 – 20% of the horse’s weight, 80 – 95% of the horse’s weight being born by the solar structures (frog, bars, sole) – this will set up a decent foot. It’s not about distance travelled, but ground contact, suface, degree of hardness are all components of the whole formula
The digital cushion contains tiny blood vessels which make up a kind of hydraulic shock absorption mechanism.
General recipe for trimming:
Take a marker pen and mark outside the white line. Trim the hoof wall at 45’ to this line. You should be left with the pen line, the white line, sole, frog and bars.
Over 100 years ago it was advocated not to touch the sole, frog, bars under any condition.
Feet are very adaptable, the foot will adapt to the environment.
Keep the toe short, bevel walls, lower heels to the level of the frog.
Foot should be 2/3 frog, 1/3 in front of frog.
Farrier shouldn’t touch the bottom part of the foot at all.
Why do horses become sore when shoes are removed?
Shoes make the foot less sensitive to the ground. When the shoe is removed, the foot is much more sensitive to the ground surface – this sensitivity isn’t necessarily pain. It should take a few days, no more than a week, to accommodate.
When the shoe is removed, don’t trim the foot apart from rasping the peripheral parts, and see what happens. If sore for more than 4 – 7 days, look for bacterial/fungal infections, examine the foot for pathology.
Use boots to protect the foot, keep the horse on a conforming surface e.g. sand, pea gravel, deep dirt. Avoid hard surfaces, allow the horse to adapt and see what happens. (NB this may assume a foot with no known pathologies before shoe removal).
Horses shouldn’t be sore for months after coming out of shoes.
They could be sore because of thin soles, so protect the foot with boots and pads or keep the horse on a conformable surface. If the sole moves when finger pressure is applied in front of the apex of the frog, the sole is thin.
There are natural fluctuations in the morphology (shape) of a healthy foot over time, e.g. the thickness of the sole changes particularly with humidity – in dry conditions the sole becomes flatter and firmer. Don’t trim this – this is the hoof protecting itself.
In a normal hoof, regularly check the temperature of the dorsal hoof wall. Heat could indicate an abscess.
Take the digital pulse – a soft pulse indicates good perfusion of the foot.
A bounding pulse suggests more resistance inside the foot and is not good.
When you trim feet, the digital pulse should become softer.
RB said owners can rasp their horses’ feet in between professional trims.
A pretty foot does not equal a healthy foot.
A healthy foot has a very dense P3, with lots of robust ligaments – fibre ligaments have changes to fibre- cartilage. There should be lots of blood vessels through the frog and back part of the foot to help remove energy.
These tissues develop as a result of stimulating the solar part of the foot. The front part of the frog is crucial – by trimming the frog you decrease the ability of the frog to develop a good internal foot.
RB sees many P3s with varying degrees of osteoporisis (OP) – bone loss.
Toe clips apply pressure and lead to reduced bone density. As do nails.
A crena at the front of the foot is normal.
Holes in P3 are not normal but are common.
Larger holes are vascular channels and normal.
The calcium in P3 is replaced every 5 years so the structure can change over a period of time. So an osteoporotic P3 can increase in bone density by loading the foot from the solar surface and with movement. The back part of P3 will also lay down more bone and become more dense.
20 – 30 years ago research showed that when a horse is shod and moving on a firm surface, the vibration energy is very high – 2,000 – 3,000 MHz – this is much less if the horse is barefoot. Vibration energy does not attenuate from the distal to the proximal hoof, and it has a deleterious effect on tissues.
If vibration energy is greater than 300 – 500 MHz, this causes blood vessels to constrict - 15 seconds of exposure to this level of vibration caused blood vessels to constrict for 3 days - this destroys the tissues inside the foot, e.g. a shod horse trotting on a hard surface e.g. a road. Pads between shoe and foot attenuate vibration to varying degrees. A shoe loads the hoof wall 100%, RB believes most of the horse’s weight should be on the solar surface – a question of biomechanics.
The problem with shoes isn’t that they prevent the hoof expanding and contracting, the problem is the vibration. Research in Denmark concluded that shod horses shouldn’t move above a walk on a hard surface.
Don’t pick out the feet before riding – the dirt plug increases the weight bearing area and acts as a cushion and support for the solar surface of the foot, removing the dirt plug increases peripheral loading.
Won’t horses get problems with e.g. thrush if the feet aren’t cleaned out? Thrush is caused by an opportunistic bacteria – if the hoof is functioning correctly and the back part of the foot is stimulated, thrush is less of a problem.
Dr Robert Bowker talks to Wendy Murdoch of Sure Foot Equine Stability Program April 2020 - parts 1 and 2.