These x-rays show the relationship of the coffin bone to the breakover point – an absolutely perfectly shod horse. It also shows the great depth of sole beneath the coffin bone.
The horse’s foot is very complex and yet quite simple once we understand the physiology and anatomy. It is complex because, as the saying goes, without feet we have no horse.
When it comes to performance, the foot is one of the most important structures to consider and proper trimming/shoeing is paramount. While most of us horse owners are not farriers, if we have a clear understanding of the anatomy and physiology of the horse’s foot, we can make more concise decisions when considering a farrier in addition to understanding what your farrier of choice may or may not be doing.
Without properly trimmed/shod feet, the owner is left in a query when a problem may arise as secondary problems commonly occur will ill-trimmed/shod feet, essentially hiding the primary cause. If we do what we can to prevent issues in the horse’s foot, we have already essentially ruled out those areas as the initiator and we are that much further ahead and closer to “fixing” what may go array.
Whether you choose to go the barefoot route or you decide to use shoes, the function of the horse’s foot remains the same. Please refer to the diagram below as well as x-rays for a clear picture.
There are two main portions to the hoof, the sensitive lamina and the insensitive lamina.
The outer portion of the hoof is the “horny hoof” called the insensitive lamina. This is like your fingernail. The inner lamina is the sensitive portion of the hoof attached to the insensitive lamina (horny hoof). The coffin bone is shaped like the hoof and is a hard, spongy bone with perforations for vessels and nerves to pass through the bone and supply the sensitive structures around it. There are grooves on each side of the coffin bone for attachment of the lateral cartilages, the thin plates that go up and end above the coronary band. You can feel these beneath the skin at the heel, and these help shape the bulbs of the feet.
The sensitive lamina is called the corium, which has five parts as shown to the right in figure 1. The corium is highly vascular and attaches to the coffin bone and the lower part of the lateral cartilages and produces the insensitive lamina (horny hoof wall and sole). The structures of the corium are responsible for producing the hoof wall and supplying its nutrition, lining the inside hoof wall from the coronary band to the sole, producing the horn of the sole, the horn of the frog, and producing a waxy waterproof covering that cuts down on evaporation of moisture from the hoof (this waxy material is called periople).
The elastic tissues of the hoof are the digital cushion, hoof wall, sole, frog and bulbs of the heel. These structures have the ability to change shape in response to foot impact. The digital cushion is the main shock absorber and the back of it forms the bulbs of the heel.
The hoof wall has three layers. The horn wall is the outer layer that gives the hoof its gloss and protection. The coronary band is located at the hairline along the top of the hoof wall, which is the hoof’s primary source of nutrition. The middle layer is called the stratum medium. The thickest portion of this layer is at the toe and then gradually becomes thinner toward the heel. It is also this layer that gives the hoof its color. The inner layer of the hoof wall fuses to the corium, which attaches to the coffin bone.
It is important to note that the weight of the horse is transferred to the bearing edge of the hoof wall and not to the sole of the foot.
The bottom of the foot is made of up the sole, frog, bulbs and bearing edge of the hoof wall. There are four quarters called the toe, the heel and the two side quarters. The sole covers most of the ground and the thickness of the sole is a key component in the protection of the coffin bone from sharp/hard objects such as rocks.
There are two grooves inside the bars called the lateral sulci (the area you pick out with a hoof pick). When weight comes in contact with the frog, these lateral sulcii spread slightly. There is a depression along the center of the frog called the central sulcus. This depression is a ridge on the inside of the hoof, called the frog-stay, which presses into the digital cushion.
The coronary plexus is the blood supply to the hoof, a network of veins that takes the blood back to the heart. The coronary plexus allows blood to flow out when the foot bears weight and then back into the foot when the foot contracts.
The hoof acts as a shock absorber, dissipating concussion laterally against the hoof wall and a number of mechanisms aid in this process. When the foot hits the ground, the pressure flattens the sole’s surface, distributing weight laterally against the hoof wall and expanding; thus, the reason for the bars. The foot could not expand if it were a solid ring. The heel pressure compresses the digital cushion, which, if you remember from above, is the main shock absorber of the hoof. It is the back of this cushion that forms the bulbs of the heel. When the horny frog compresses, the digital cushion flattens and pushes the lateral cartilages in opposite directions, pushing out venous blood from the coronary plexus to make room. As the foot contracts, removing the weight of impact, all these structures go back to their original position.
The navicular bone is also a shock absorbing structure of the hoof, supported by the deep digital flexor tendon and ligaments. With concussion, the navicular bone moves the load from the digital cushion to the short pastern bone, bypassing the coffin bone and relieving some of the load.
As you can see, it is important that the bearing surface of the hoof wall is level with the frog to distribute weight evenly, allowing this intricate process to work properly. The breakover point is identified by the placement of the coffin bone. Being able to identify this breakover point is absolutely paramount when shoeing/trimming a horse. If a horse is not properly balanced, like in a long-toe, for instance (the toe hitting the ground first), the concussion is coming through the toe of the hoof wall and not at the heel, not compressing the frog properly and not dissipating the concussion through the digital cushion and the navicular bone. Instead, in this case, the brunt of the force is going to be felt through the coffin bone/joint. The coffin bone/joint is already the first weightbearing surface and takes a lot of concussion so we don’t want to add more impact there than necessary.
I believe too long of a toe to be one of the worst detriments to a horse’s movement and seems to be a common error seen among shod horses. Problems arising from a long toe include tripping, forging, incorrect coffin joint alignment, navicular issues, etc. Obviously, these are problems that we would like to avoid.
Improperly shod horses don’t always show signs of a problem right away. In fact, many times it is a gradual imbalance and effects do not arise for some time and oftentimes may arise in other parts of the body, making it even more difficult to ascertain what is going on. With that said, I do believe the feet to be one of the most important “preventative” medicines we can provide our equine athletes.
I hope this article gives you better understanding of the anatomy and physiology of the hoof and allows you to be more comprehensive in communicating with your farrier and evaluating your horse.