If you’re being technically correct, you would say “palmar angle” when discussing the angles of the coffin bones of the front feet, and “plantar angle” when discussing the hinds. However, in common parlance, most people use “palmar angle” to talk about both, so we will do that here.
There are two schools of thought on what the “correct” palmar angle of the coffin bone should be. Some argue that the bottom edge of the bone should be parallel to the ground, while others believe it should have a positive angle, which just means it should be slightly higher in the back.
The ground parallel camp believes that lining up the bone with the ground when the horse is standing still (stance phase) allows the best distribution of load forces around the entire bone when the horse is in motion. They say that if the bone is higher in the back, there will be unnatural stresses on various parts of the bone and soft tissues during loading. In particular, they are concerned that a positive palmar angle puts excessive pressure on the front bottom margin of the bone, which would also place the solar corium beneath that margin under stress from compression. It is certainly true that a coffin bone rotated too far forward (as often happens in cases of laminitis) can lead to both bone and soft tissue damage.
However, the positive palmar angle folks argue that at peak loading, when the distribution of forces is most critical, the heels expand and effectively “squish down,” allowing the back of the coffin bone to drop. This results in a ground parallel position when it is actually needed, whereas if you start out with a bone that is ground parallel during the stance phase, it will actually end up having a negative palmar angle during peak loading. A negative palmar angle, as you might expect, puts excessive pressure and strain on the back of the foot, which may cause or contribute to a whole host of problems ranging from deep digital flexor tendinitis to damage in and around the navicular bone. When you add the fact that sound horses typically land heel first when traveling at any pace above an average walk, the argument for the slightly positive palmar angle becomes even more logical. This is why most experts today agree that a slightly positive palmar angle of approximately three to five degrees is correct.
As for how you can tell, the most reliable way to gauge the angle of the coffin bone is with a lateral (from the side) x-ray taken when the horse is standing square. You can then “eyeball” the angle in the radiograph to get an idea of what is going on, and you can also do measurements on the x-ray if you want to get the precise angle of the coffin bone. Even without x-rays, you can often get a fair idea of the angle of the coffin bone by just looking at the hoof itself. For example, a foot with a broken forward hoof-pastern angle and a tall, upright heel is likely to have the coffin bone raised up too much in the back. Correspondingly, a foot with a broken back pastern angle and low, underrun heels may very well have a negative palmar angle.
Feet with a “bullnosed” profile, meaning there is a degree of bulge in the dorsal wall, are also likely to have coffin bones with a negative palmar angle. The bulge is caused by pressure from the leading edge of the coffin bone pressing up and out on the inside of the dorsal wall. A bullnosed profile should not be confused with a “dubbed toe,” which occurs when the outer layer of the toe wall has been removed, either naturally, as can happen when a horse drags its toes, or intentionally, when a farrier rasps away that portion of the wall to make the toe fit a shoe or appear shorter. In a dubbed foot, only the bottom portion of the hoof wall looks rounded, while a bullnosed hoof will be convex from top to bottom or may have a distinct bulge in the middle.
Healthy palmar angles are extremely important, not only for hoof health, but for overall biomechanics, so if you have any concerns about the angles of your horse’s feet and how that might reflect what is happening with the coffin
bones, speak to a qualified hoof care professional or your veterinarian.