I had enough interest in my remark about rim notching, that I asked for help in explaining it from my hoof care provider, teacher and friend, Tara Wilson. First, I'll show you the photo I gave her of Beamer's foot with the hairline arches that I have been trying unsuccessfully, for the last year, to remedy by relieving the quarters. The remarks below the photo and the drawings are hers.
The rest of this post is provided by Tara, and I thank her for taking the time to put this together for us. Feel free to ask questions in the comments box.
Rim Notching is simply removing the ground pressure on tubules that are being bent. Now that we understand what tubules are and we can see them with our own eyes let's think of them as little individual strings within the wall's structure. Now lets think of a 6 inch long string laying on the table. It's 6 inches long right? Now put a few bends in it, it's no longer 6 inches on the ruler, its 4. So we trim our horses foot, everything looks good, everything is how it should be BUT our strings (tubules) still have bends in them so in reality they are still longer then they should be. (This is one of the things that cause arches in our hairlines at the quarters.) Now we think, "Our bends are above our arch relief so they are not pressured," but they are. As the foot lands the arch relief expands and makes ground contact keeping these tubules from relaxing and hanging straight, and allowing the hairline to travel in a relaxed 30 degree angle towards the heel.
Tubules are not glued together within the wall, they just grow in a very tight knit community creating strength in numbers! They are so individual, we can manipulate a very small number of them and they will move within the wall or "community," up or down, forward or backward.
Rim Notching comes into use when our quarter hairlines won't relax, when we have a contracted foot, when there is FFS (Forward Foot Syndrome) or any time there is a capsule (wall) deformity.