Hoof Form & Function | HELP 4 HOOVES

This page is an overview of some of the whys & wherefores, to give reasoning behind my trimming and management philosophy. You can read more, if you wish, in the Members section.

Attention donkey and other equine owners! What you here below absolutely applies to other equines too! All the same principles - and practice - applies, except for hoof balance for donkey hooves. Regarding the small differences noted in donkeys(and mules), please visit my Donkeys page.

**You can magnify most images by clicking them.

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Anatomy and Function of the Hooves

Below is a brief summary on anatomy and function, including some points to consider when analysing alternative viewpoints. You can find my far more detailed paper discussing reasoning of different theories, complete with references,

'Contrasting Morphology of 'Good' and 'Bad' Feet' in the Member's section.

Considering other Ungulates

To gain an objective, broader view, it is worth first considering other ungulates (hoofed animals) and their similarities, to better understand the horse's hoof anatomy and function. A horse's foot is built similarly to most other ungulates, except that only equidae(horses, donkeys, zebra) have one single toe, whereas others, such as cattle, sheep, deer, camels & giraffe have two toes, and others, such as rhinos & tapiers have 3 or more.

Horses(even wild ones) also have smaller pads(we call the frog in horses) than other ungulates, some, such as rhino & camels being mostly 'foot' with smaller horned toes, more like fingernails or claws.

All ungulates, including horses living optimally, have strong, well developed pads and caudal foot(the structures above the 'skin') which acts as the primary 'landing gear' and weight bearing structure. No other animal, including other species of equidae, or even wild or feral Equus Cabellus supports itself wholly or even mostly on it’s hoof walls or toe nails.

**In my research, I have found NO evidence or logical reason that equids, domestic or otherwise, would have evolved to support themselves on their hoof walls. It seems illogical that Mother Nature would have made the bottom of the horse's foot to be raised from the ground, supported only peripherally by the walls.

The Equine Foot

The bones of the lower limb

The horse's carpus (or in hinds, tarsus) bones ~ 'knee' or 'hock' - is equivalent to our wrist or ankle, made up of lots of little bones. The metacarpal(or in hinds, metatarsal) bones - which are our hand/foot bones, extend below. The 3rd metacarpal forms the main 'cannon bone' while remnant 2nd and 4th metacarpals form the 'splint bones' which with maturity, fuse to the 'cannon'. The 'fetlock' is actually equivalent to our knuckles, but at the back there are two 'proximal sesamoid' bones, which aid smooth movement of the flexor tendons.

Below the fetlock joint is where we are most focused on here. There are 3 phalanges - equivalent to our finger bones. The first phalanx is the longest, just below the fetlock. The second phalanx should sit just above the hoof capsule. The third phalanx, also known as the Distal Phalanx or pedal or coffin bone, is the bone inside the hoof capsule. The Distal Sesamoid, commonly known as the Navicular Bone is a small bone which is attached by the impar(main) & other ligaments, to the back of P3 and the distal end of P2.

**As to the make up of the 3rd Phalanx, I find it curious that it is described in text books as the only bone in the horse's body (indeed it would make it the only bone of any species) that does not have a periosteum(outer membrane) and that it is considered in anatomy books to be naturally 'porous' - such as can be seen in the above pedal bone picture. Any other bones in an animal found to be in this state are classified as osteoporotic. Coincidence?? Have vets been too used to seeing damaged feet, that it, like 'sidebone' has been mistaken as the natural state??

The bones and joints function best when they are in alignment. Looking at the foot laterally, such as the above pictures, the phalangeal bones should form a straight line. Additionally, when considering balance, laterally or dorsally, when assessing medio-lateral balance, joint spacings should be equal, not compressed or stretched from one side to the other.

The hoof capsule

Very simply, the horse's hoof capsule is the thickened 'nail' and 'skin' that covers the internal structures, providing protection to the foot from external 'assault'.

Unfortunately I took no photos of capsules I had when they were fresh, and they contract greatly like this when they dry out, but you should get the idea.

It's a complete 'boot' for the horse. The lines that look like mushroom gills on the inside of the wall are the epidermal laminae.

The external hoof wall grows down from the coronary border or hairline. It is formed from densly packed tubules, which form a dry and largely impervious layer. The bulk of wall material grows out from the laminae - the tissue joining the hoof capsule to the corium (meat) of the foot. Close in to the laminae the material is soft and moist, while it compacts and dries out the further out towards the external wall it moves. Technically, we can include the bars as hoof wall.

The sole is a keratinised 'skin' layer which grows on the ground surface. Dr Bowker's research at cellular level suggests that much of this forms from the bar laminae and 'spreads' forward and out. A thick, healthy sole provides great protection and support for the underside of the foot. It is compressed into a strong 'toe callous' around the front periphery of the sole, under the distal edge of P3. Unfortunately SOOO many horses have far too thin soles to provide adequate protection under P3. The first, undeveloped foot pictured below is an illustration of extremely thin sole under the edge of P3. If this is not rectified, damage to the solar corium and eventual distortion(bone remodelling) to P3 will occur.

The frog is also keratinised 'skin', of a more flexible, elastic nature than the sole. It grows from the frog corium and joins sole material at the 'collateral grooves'. It protects and allows stimulation to the caudal foot.

**Examination at a cellular level shows intertubular material is far stronger than the tubules of a hoof wall, and that hoof walls are vastly stronger across the width of the wall rather than vertically, (Pollitt 2008, p.7) as would be likely if the material had evolved to support the weight of the animal.

The Caudal Foot

The last to be discussed but arguably the most important part of the foot!

Through effective use, the caudal section of the hoof matures to become an effective shock absorber, to support the impact of the horse and protect the muscoskeletal system from undue wear and tear.

The biochemistry and composition of the tissue is different between young, mature & aged animals. When a horse nears maturity – from around four to five years of age – with good hoof function, this tissue begins to become more fibrous.

a) Undeveloped caudal foot. DC comprising soft adipose tissue (Lavender 2011)

b) Relatively well developed, fibrocartilaginous healthy digital cushion (Lavender 2005)

c) Slice of hoof showing well developed lateral cartilages and fibrocartilaginous digital cushions(Bowker 2011)

A lot of blood passes through the foot. Far more than is required for feeding the cells. It has been hypothesised that the caudal hoof acts as a pump for circulation. That the digital cushion fills with blood when the hoof is off the ground and the blood is pushed up the legs when it's loaded and the DC's are squashed. Or conversely, depending which theory you read, that when the foot is loaded, it expands and creates a vacuum sucking in blood, to be pushed up the leg when the hoof is off the ground and 'contracted'.

**It has been found, that in an undeveloped hoof, blood flows more freely, while Bowker has shown, such as in the above picture, that in a well developed foot, there are 'bazillions' of veno veno anastimoses (very tiny blood vessels) that the blood is forced through, acting as effective shock dissipation and slowing down blood pressure & flow up the legs.

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Hows & Whys of Hoof Balance

Below is a brief summary and discussion of some alternative ideas. Read the full article, complete with references, in the Member's section, titled Hoof Balance and The Live Sole Plane.

Scientific studies of 1000's of horses have shown that the hoof functions optimally when the centre of articulation - the middle of the Distal Inter Phalangeal joint - is central to the load bearing area of the foot. The third phalanx is level to the ground surface at the distal border medial laterally and at a slightly positive palmer/plantar angle(ELPO 2009)

The big question is how does a farrier know exactly where the internal structures are located within the hoof capsule in order to correctly trim for good balance?

Different methods of balancing

Guidelines for balancing hooves have been varied. Many people have used the hoof walls as the benchmark. While a healthy, already well balanced hoof wall can give a good idea of the position of internal structures, the walls are prone to deformation, losing their relationship to P3.

Many people have tried to use standard angles and lengths as guidelines for trimming 'balance'. However, angles and lengths all vary within a species, even within 'normal' conformation, so at best there can only be an 'average' range.

A popular reference for the centre of articulation is 'Duckett's Dot'. It is indeed a helpful 'landmark'. However, recent studies of x-rays have shown many irregularities that make this point not quite as accurate as was believed.

In 2008 the Equine Lameness Prevention Organisation put together a scientific study to look at other reference points of the external hoof and how static their relationship to the DIP Joint, to determine a more accurate method of determining 'balance'. Their studies, backed up now by 1000's of radiographs indicate that...

The widest part of the foot is not always directly below the centre of articulation of the DIP Joint, but it is a reliable distance from the tip of the distal border of the distal phalanx(P3/pedal bone).

In feet size 00-3, measuring back 1” from the apex of the frog was a more reliable estimate for finding the centre of the DIP joint than 3/4”(ELPO 2009)

Using the ELPO guidelines to 'map' the foot, using both the apex of the frog and the widest part of the foot gives a very accurate – within 1/8” on hooves size 00-3 – guideline for finding the tip of P3 and therefore correct 'breakover' for the horse.

Mike Savoldi's studies, spanning thousands of dissections have shown that the live sole plane is of a uniform thickness at the lamellar line. In his paper “Uniform Sole Thickness” he states “The functional sole or the true sole where it attaches to the zona alba (white line) is very unique in that it will only vary within 1 to 2 mm in its vertical depth.

Trimming To The Sole Plane

In Savoldi's paper The Arch of The Sole, he explains about thin soles being due to pressure from P3 and the palmer processes pushing on parts of the sole. Despite this, the external rim of the sole, proximal to the walls is still of uniform thickness.

Duncan McLaughlin's recent thermography study of endurance horses also shows reduction in circulation and 'distal descent' in horses who's soles aren't supported. These studies illustrate the importance of not leaving excess wall length in relation to the live sole, as well as not thinning live sole.

Pete Ramey states that where live sole is trimmed, the internal structures tend to drop lower in the hoof capsule, but the live sole “adjust to perfect uniformity quickly after you stop trimming into it.”(Ramey 2005) Ramey also talks about using the depth of the collateral grooves as an accurate assessment of how much sole a horse may have in any region of the foot. Collateral grooves should also be the same medially as laterally(Ramey 2005).

There are a variety of reasons why we may not want to exfoliate to live sole. These include horses fresh out of shoes or unused to rough ground, as well as perhaps club footed horses, if Ramey's theory of extra sole growth doesn't hold true, who may need the extra thickness of dead sole for support and protection. Subsolar abscessing, where dead 'false' or 'retained' sole is protecting live sole which is too thin to do without it is another reason for leaving extra sole material.

Studies on sole depth of wild and domestic horses have shown also that sole depth and wall height vary with environment. On harder footing, the soles are thick and the walls are worn to level with the peripheral rim of the sole(Hampson, Connelley, de Laat, Mills & Pollitt). This appears to indicate that thicker sole is needed for providing protection on rough ground, while walls being short allow for full support of P3 from underneath.

Conclusion

Using the live sole plane as a reference for how far to trim the walls has been shown as an accurate way of determining anterior/posterior balance and well as medial/lateral balance of P3.

Using the widest part of the foot and the true apex of the frog is an accurate way to determine where the centre of rotation of the DIP Joint and the tip of P3 is located within the hoof.

Trimming into live sole can cause further issues of imbalance along with sensitivity. When trimmed or unsupported, the live sole, being too thin, can allow the distal phalanx and palmer processes to sink to a lower position within the hoof capsule. This also indicates that peripheral loading via shoes or long walls on hard surfaces would lead to the same effects.

I have found no good reason at all why live sole should ever be pared from a horse, although according to Ramey this may not be the case for donkeys and minis.

Exfoliating to the live sole can be important, especially in determining correct hoof balance that may be hidden by dead sole.

It is important to leave the retained sole in many circumstances. While understanding what and how much is dead is important, and trimming the walls accordingly, if the sole doesn't exfoliate easily or there is any doubt about what should be removed, it is probably best left in place.

It appears that trimming to balance the wall height around the entire sole, without invading it or leaving very much length at all above it – depending on environment – is an accurate ways of balancing a hoof, taking into consideration the postural and conformational needs of the horse at any given time.

It also appears that balancing the collateral grooves and ensuring they are at least 5/8” deep may be another good 'landmark' to help ascertain balance and adequate sole depth.

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What is a High Performance Barefoot Horse?

These animals are MADE, not bred! However, it takes more than just good hoofcare to create them. It is about their whole diet, management and environment.

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Genetically weak feet?

Genetics do indeed play a part, but it's a small one. For the vast majority, it's 'deed over breed'. To reiterate from the last article, These animals are MADE, not bred! However, it takes more than just good hoofcare to create them. It is about their whole diet, management and environment.

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But my horse Needs Shoes!

'Ideals' are all very well, but they don't work for all. When it may be best to consider shoes and how to minimise or avoid negative effects.

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