Anatomy of the Horse Hoof

Let's start with those parts of the hoof that you can see from the outside. First, there is the hoof wall: this gives the hoof its outer shape and stability. Ideally, the hoof wall should grow at a consistent angle from the coronary band to the ground (i.e. have a straight wall) and run evenly all the way around. This means the horn capsule should not appear asymmetrical or look crooked/distorted. It should also not have any bulges ("flares"), cracks or "beaks".

The hoof shape should resemble a truncated cone: at the coronary band, the circumference should be smaller than at the ground.

The hoof wall consists of different layers: the outermost layer is the glaze layer, followed by the pigmented wall and finally the unpigmented wall. The unpigmented wall layer is usually almost snow-white and is therefore often confused with the "white line" (see below).

Photo: Anatomy of the hoof wall

These layers exist in every horse's hoof, regardless of whether it is dark or light on the outside. The pigmented and unpigmented hoof wall are firmly fused together, but have different hardness. If the hoof's nutrient supply is not optimal, these two layers can also split, which means that small stones or similar foreign bodies can easily become lodged.

The "inside" of the hoof wall is finally formed by the connecting layer, which connects the hoof capsule to the internal structures of the hoof. This layer consists of fibrous laminar horn (lamellae) and is also called the "coffin bone suspension" because it forms the suspension for the coffin bone (see below). You can imagine this laminar layer like interlocking fingers: one set of fingers extends from the inside of the wall, the others from the corium, which encloses the coffin bone and the ungual cartilages. These "fingers" interlock and form a stable yet elastic connection. However, this connection can also tear open in diseases of the hoof, e.g. in the dreaded laminitis. In such a case, this is referred to as "separation of connection". More on this in the following section on the white line.

Photo: Laminar layer

However, the hoof wall does not enclose the entire hoof: in the rear area (the heels), the hoof wall essentially folds inwards and forms the bars. So the bars are nothing more than wall horn running inwards. This construction means that the hoof is very stable but still flexible to a certain degree. The hoof capsule can widen in the heel area and even adapt to uneven ground. This ability is important, among other things, for the hoof mechanism.

Experts from the different hoof schools argue about exactly what healthy bars look like — how they run, what length they should be, and how they should be trimmed. However, all trimming approaches agree on one thing: bars should run more or less straight (i.e. not form a curve) and should not lay down on the sole.

The transition from the hoof wall to the rest of the horse's leg is called the "coronary band". The coronary band also includes the "perioplic band", which in turn forms the glaze layer of the horn wall. The perioplic band is also often referred to as the "periople" and the horn formed from it is therefore also called "perioplic horn". Depending on the breed and weather, this horn layer can be more or less visible: often it is completely worn off on compact hooves and in dry weather, whereas it is often clearly visible on cold-blooded horses and in wet weather. So the next time you find a frayed layer at the top of the hoof in wet weather, you know: it's just the perioplic horn and no cause for concern.

Photo: Anatomy sole view

The underside of the hoof is finally formed by the sole. The sole horn is slightly more elastic than the wall horn, which allows the sole to adapt to the deformation of the hoof capsule with each step. A healthy hoof has an even sole arch (that is, the shape of the sole from the hoof wall to the frog resembles a shallow bowl) and is sufficiently thick.

Experts in the horse world are once again divided on the exact thickness and arching of the hoof sole. A sole thickness of approx. 1.5cm is often given as a good reference, likewise as an average for the sole arch (measured from the tip of the frog to the ground). However, these values must always be viewed individually and are also dependent, for example, on the size of the horse. However, the arching of the sole in particular does not scale 1:1 with the horse's body size. This means that a large horse normally has proportionally less sole arch than a pony.

By the way, the sole arch can be either too much or too little. With excessive arching, sole pressure is often found (too much sole material presses inwards into the hoof), whereas too little arching very often leads to sensitivity because the hoof under the tip of the coffin bone no longer has enough distance to the ground and therefore even the smallest stones can cause pain. The same applies to soles that are too thin: here too, the protective layer to the coffin bone is too thin and unevenness in the ground therefore quickly presses uncomfortably on the bone in the hoof.

Horse owners are often concerned when whole pieces of the sole (or the bar) break away when picking out the hooves. In the vast majority of cases, however, this is not a problem, but merely a sign that the hoof is helping itself and wants to shed excess sole material. You can observe this phenomenon particularly often during rainfall after a longer period of drought.

The white line (also called "Zona Alba") is the transition between the hoof wall and the sole. Strictly speaking, the white line is nothing more than the laminar layer between the hoof wall and the inner structures, which grows down here and is therefore visible from below. The name is misleading, however, because the white line often appears rather yellowish and not white (whereas the unpigmented part of the hoof wall is usually almost snow-white).

Due to its tuft-like structure, the white line is significantly softer than the hoof wall or the sole. Nevertheless, the white line should be closed — the individual lamellae should not be visible in dry weather and certainly should not look frayed.

Due to this softer structure, however, the white line is susceptible to intruders: both foreign bodies (small stones) and bacteria and fungi quickly become lodged here when the cohesion is no longer optimal (usually due to nutrition) or the husbandry conditions are not appropriate.

When the white line is infested with germs, this is referred to as "White Line Disease" (WLD). The white line is eaten away by bacteria and/or fungi, which manifests itself when cleaning the hooves in white crumbly material in the white line. However, the germs are often not the cause of this problem, but much more frequently imbalances in the hoof: for example, if the hoof wall is levered forward with every step by a toe that is too long, mechanical tension arises on the white line and the connection can be strained. Fungi or bacteria quickly become established this way and can migrate upwards in the laminar layer. If this is not counteracted quickly, it can eventually lead to the problem of the hollow wall: the germs have essentially eaten a hole in the connecting layer and a cavity has formed where the hoof wall has no connection whatsoever to the coffin bone. As a result, this part of the wall can quickly break away, causing the hoof to lose stability.

So you see: it is also important for the white line to ensure a balanced horn capsule and regular hoof trimming.

Also clearly visible from the outside is the frog. The English term for it ("frog") already indicates that this is a softer structure than the rest of the horn material. Nevertheless, the frog should have sufficient firmness to provide enough protection for the structures underneath. You can recognise a healthy frog by rather "waxy" material and a smooth surface. So when picking out, you should not find any pockets or grooves. Exceptions are of course the central and lateral frog grooves — these should be clearly visible, but not excessively deep. In particular, the central frog groove should be wide and dry, i.e. no dirt should be able to collect here. Overall, the frog should extend almost the entire width of the heels and take up approximately ⅔ of the hoof length.

The frog essentially connects the two ends of the horn capsule and is essential for a healthy hoof: due to its V-shaped structure and its rubber-like horn substance, it forms an important cushion in the otherwise rather brittle hoof. The importance of this cushioning function becomes particularly apparent when you visualise the horse's footfall: a healthy horse shows a heel-first landing, that is, with each step it first sets down the heel area of the hoof on the ground. At this moment, very large forces act on the hoof, the horn capsule widens in the rear area and the frog becomes wider, but is compressed in height.

If we now look inside the hoof, we can see that the so-called "digital cushion" sits above the frog. This has a high proportion of collagen fibres, interspersed with cartilaginous and fatty tissue. That is why the digital cushion is very elastic and can adapt very well to deformation when pressure is applied to the frog. It thus optimally protects the sensitive area located above the digital cushion: the navicular complex (see below).

Ground contact of the frog

With its digital cushion, however, the frog not only provides damping of the forces occurring with each step, but also indirectly presses the bulb area of the hoof apart. Therefore, the frog should have ground contact in the rear third, whereas the front two thirds of the frog should not have direct contact with the ground in the unloaded state. If, on the other hand, the frog also has ground contact towards the toe tip, it can press on the coffin bone and cause pain. Frequently, discolouration or bleeding is then seen in this area when the frog is trimmed.

If, on the other hand, the frog has too little ground contact (or even hangs in the air, i.e. does not touch the ground at all), it can no longer press the bulb area apart. The result: contracted heels. In this case, the hoof wall "collapses" at the bulb, so that when viewed from behind, the heel wall is no longer perpendicular to the ground but is significantly narrower at the bulb than at the ground. This condition in turn promotes thrush, which leads to the already atrophied frog additionally being eaten away by bacteria. A vicious circle arises!

One of the most important factors for a healthy hoof is therefore sufficient frog support, which should be given good consideration in every hoof trimming, every hoof protection and also in the husbandry conditions.

You can easily check for yourself where the frog has ground contact when picking out the hooves.