We receive at least three to four calls a week concerning thin soled horses. The stories have common threads running all through them. My horse has been tender footed for as long as I can remember. I have been trying to toughen his sole up for the past two years. Every spring my horse has bruised soles.
What it all comes down to is poor sole health, but why? We first need to look at the foundation for the sole. What is the structure that host the corium (sensitive structures, where circulation is present)? When observing the "sole" most people refer to the area of the sole where the coffin bone is the foundation, the area of the sole around the apex (tip) of the frog. The horn that is produced by the corium covering the coffin bone is called "primary tubular horn." This horn is very hard, unlike the horn produced at the outer edge of the coffin bone. The horn produced at the outer edge of the bone and where cartilage is the foundation is called "terminal tubular horn", this horn is tough and flexible. As a reference; tool steel (chisels, drill bits) is hard and unyielding, where iron is tough and flexible (pry bar, hinges). A benefit of being able to recognize and distinguish the difference between the two types of tubule is gaining the ability to identify the location of the coffin bone within the capsule. Each type of horn serves a purpose. Can you recognize the difference in the photograph below left?
The Primary tubular horn below the coffin bone helps in preventing torque / twisting of the bone and aids in establishing correct hemodynamic response (circulation), whereas the Terminal tubular horn helps protect against concussion and allows for proper distortion, this helping to protect the underlying sensitive structures and allowing for correct foot function.
When I hear someone say my horse has a thin sole they are most often referring to the Primary Sole located below (distal) the coffin bone and not the sole at its outer most edge (terminal sole).
Why does this primary sole have such difficulty in growing healthy? There are several reasons, but the most common are a lack of proper circulation and excessive pressures on the corium resulting from a compromised suspension. (I am not simply referring to a lack of circulation, but rather to proper circulation and its timing). There are a number of causes for improper circulation to the solar plexus (vascular bed). One is improper conformation of the hoof capsule itself. You see, the foot is subject to the laws governing fluid dynamics and one law states that fluids always seek low pressure. The foot is subject to these laws simply because systolic pressure is superseded by the forces created by the stride, therefore blood flows from high pressure to low pressure throughout the stride. Having proper conformation of the hoof capsule becomes very important. To short of a toe for instance can reduce blood flow to the circumflex artery that partially supplies blood to the solar plexus. At the time of break over it is important that the toe be engaged, producing pressure on the outer wall creating high pressure in the lamellae plexus, thus causing the blood to seek the lower pressure presented in the primary solar plexus. If the pressure on the hoof wall of the toe and its corium (lamellae plexus) is not higher than that of the solar plexus then proper circulation may not occur. This is only one small part of how circulation (hemodynamics) occurs in the foot.
Overall circulation in the solar aspect (bottom of the hoof) can also be affected by a lack of suspension of the Internal Arch Apparatus (Internal foot). Lack of suspension results in chronic mild compression of the solar corium, which can cause bruising, abscessing, and poor horn growth. Where does this lack of suspension come from? Most often it is the result of unhealthy Ungular Cartilages. The ungular cartilages and coronary band are partly responsible for suspension of the foot within the capsule. When ungular cartilages becomes unhealthy they suffer a loses elastic potential (the ability to resist distortion). When the suspensory structures suffers a loss of elastic potential the sole is the first place we see the symptoms. This is primarily because the internal foot and the relationship it holds to the hoof capsule are under the greatest amount of stress. How does this occur? Loss of Ungular cartilage health (poor development of extracellular matrix) is often the result of a metabolic disturbance of some sort. Following a harsh winter many horses can suffer metabolic imbalances due to seasonal changes in environment, dietary changes, and hydration changes. Spring can be very stressful for our horses.
Hoof wall integrity also plays a roll in suspension of the internal foot. If hoof wall integrity is compromised by poor wall matrix, excessive moisture, or excessive trimming, distal decent of the internal foot can become excessive resulting in abnormal pressures on the solar corium.
I suggest you take a close look at how environmental changes could be a cause of stress in your horse.
I treat thin soles by first addressing the metabolism of the horse, making sure the horse is well hydrated, trimmed properly, free of infection, and exercised to promote the health of the Internal Arch. Take care of the inside, to gain health on the outside.