*Please note: I am NOT a hoof care professional!*
Underrun heels are unfortunately a very common occurrence in domesticated horses, and one that a lot of horse owners don't recognize (including me until about two years ago!). So, what exactly is an underrun heel? It's a heel that has been pulled too far forward, and it is often accompanied by an overly long toe. It's interesting that a lot of people will look at an underrun heel and think that the horse "has no heel," while in fact it has a lot of heel but it has just been pulled so far forward.
To illustrate the point, I've borrowed a picture of Laz's hoof from Kristen's blog (not picking on them, just using sweet Lazaroo as an example!). I've drawn a line from Laz's heel straight up through his pastern, and you can see that the heel is quite far forward in relation to the rest of his hoof and both his fetlock and cannon bone.
Now, compare that to the following picture of Salem's hoof (taken from last year). Notice how much less hoof is behind that line. But more importantly, look where the line goes -- straight up through the fetlock to the cannon bone. The heel is in its proper place at the back of the hoof.
Using the same pictures, also note the length of the heels:
Note how long Laz's heel is in comparison to Salem's heel. As I mentioned, some people will think that an underrun heel is short; but if you find the point where the heel touches the ground and trace it all the way back to the coronet, you can see that it's actually quite long. Such a long heel can put the coffin bone in an unnatural position in the hoof, angling it onto its tip instead of allowing it to rest in the natural near-ground-parallel state.
The above photos are a good visual of the same hoof showing an extremely underrun heel on the left and a much-improved heel on the right.
So, what are the consequences of an underrun heel? Well, consider the internal structures of the hoof: the front of the hoof is supported by P3 (the coffin bone), while the back of the hoof is supported by the digital cushion and lateral cartilages. When the heel is drawn forward, more weight/stress is placed on the coffin bone. As Dr. Richard Mansmann notes in the article Underrun Heels: Not So Innocent, "without protection from the heel and digital cushion, the bony column will receive more concussion. When the laminae inside the hoof wall stretch and tear, the bony column even lowers into the hoof, which thins the sole and puts additional pressure on the blood vessels internally. The wings of the coffin bone can become lower than the coffin bone at the toe, increasing bruising and pain. Then the toe will land first to protect the sore heel." Landing toe first can lead to even more damage to the internal structures of the hoof.
And since underrun heels cause the digital cushion and lateral cartilages in the back of the hoof to become passive, they are also weakened. This can make it more difficult for the hoof to function properly once the underrun heel has been corrected.
Underrun heels can lead to a myriad of problems, including quarter cracks, heel cracks, bruised heels, coffin bone synovitis, injuries of the deep digital flexor tendon, pedal bone osteitis, and degeneration of the navicular bone and bursa. Add to that a long toe, which often accompanies an underrun heel, and the hoof wall will be pulled further away from the coffin bone, causing a stretched white line, abscesses, and White Line Disease.
So, where exactly should the heels be? Of course, each horse's conformation is unique, so there is no cookie-cutter diagram or angle that can be applied to every horse. According to Salem's hoof trimmer Candy, the "heel should be approximately 1/8th inch from the periople curls." This is the trimming method taught by The Oregon School of Natural Hoofcare. According to their website, the "periople tufts [or curls] are periople skin at the heel that lift and curl or will wear down from the heel striking the ground in locomotion," and they are "a clue to the baseline of the hoof."
Luckily, the heels can be gradually worked back to their proper place through correct trimming. And once the heel is back where it should be, the issues associated with underrun heels will diminish. I'm confident that this will be the case with Laz; hopefully sometime in the not-too-distant future, we can compare the above photo with one where Laz is closer to his "ideal hoof" and see the drastic differences in the two.
Now go out to the barn and take a good, long look at your horse's hooves!