Sole Pressure
If your horse wears shoes, you may have heard about something called "sole pressure." This supposedly can cause a horse to come up lame...that is, to have pain in the hoof and limp.
What is sole pressure? Well, let's think about the hoof. (Please refer to the video Understanding the Hoof--From the Horse's Perspective for an explanation of the parts of the hoof and their functions.) The sole is seen on the underside of the hoof. It's basically the area between the frog and the lamina, or white line. The sole is there to protect the coffin bone, and on a horse with natural hoof function, the sole will bear much of the weight of the horse and will contact the ground when the hoof is fully loaded. The forward part of the sole, in the toe area, is a special area of sole called the "sole callus." It is approximately the area of sole that is colored red in the picture. The term "callus" is a little misleading, because it implies that this is just regular sole that becomes callused if allowed to contact the ground. But the sole callus is actually of a different composition than the rest of the sole. It's tougher and more resilient because its purpose is to protect the fragile tip of the coffin bone.
What causes sole pressure? When a horse is trimmed, the hoof is often trimmed too close from the bottom at the toe. You may have seen someone trim, and near the toe, they open the nippers wide and cut not only the hoof wall but also into the sole area. Also, it's common practice to take a hoof knife and carve out some of the sole. You know this has been done if you see little flakes of sole lying around on the ground after your horse has been trimmed. When this is done, the sole callus is cut into. The horse's body interprets that as an injury. And as is so often the case with an injury, the area swells up. Then when the shoe is nailed on, there is an area of swelling up underneath the shoe. This creates pressure; it's sort of like when you mash your finger and it throbs. This is what is known as "sole pressure," and your horse may limp in response. To understand other, more serious complications that may result from cutting into the sole callus, please refer to the video, Understanding the Hoof--From the Horse's Perspective.
To avoid sole pressure, after a farrier has cut into the sole with the nippers, he may take a hoof knife and carve out even more sole at the toe, lowering that area in relation to the hoof wall. If he lowers it enough, that will prevent sole pressure, but it will cause other, more serious problems, as explained in the video Understanding the Hoof--From the Horse's Perspective. Some farriers use hot fitting to avoid sole pressure, as the hot shoe will burn off any sole that sticks up above wall level. However, some farriers will use both methods...they will use hot fitting, and then take the hoof knife and lower the sole even more.
Just remember that sole pressure is caused by trimming into the sole callus, and that there is absolutely no reason to do that.
