MY THEORY ON UNDERRUN HEELS
This is a post trim shot; I didn't rasp any of the walls surface, but did round the toe and add a very slight bevel.
My theories on under run heels is that
1) the toe gets long and the horse starts moving toe-first. Toe first movement pulls the toe forward even more and results in a lamellar wedge, and the heel - connected to the toe by the wall - gets pulled forward with the toe.
2) the wall in the toe and heel become compressed at the base at a faster rate than the wall in the quarters with time and travel. Even with shoes on, the heels and toe become compressed - effectively shortened - at ground level and become relatively shorter than the wall in the quarters. The longer quarters begin to flare, and they often flare outward and upward into the coronet band because a shoe is in place as the base keeping them from flaring down. The wall in the quarters presses upwards, distorting the coronet band, and bending the wall around the quarters, folding the heels under.
What we see as a result is growth rings in the foot the stretch downward at the toe and in the heel, and an upward swirl over the nail holes in the quarters.
A long toe goes hand in hand with an under run heel. At the toe of this picture,, I drew an approximation of where this horses toe would be if it hadn't been rasped off so that the hoof looked "normal". It's based on following the angle of the hoof wall at the top 1 inch of hoof wall. The lighter wall at the base of the toe is an indication of where the outer wall has been rasped away to a very thin layer. This weakens the wall considerably, accelerates the rate at which it "compresses" and also accelerates the overall distortion of the foot as the wall begins to bulge outwards slightly.
The line drawn around the base of the foot is a line I use to help clients see length imbalance. The line is drawn relative to the coronet band, following its contour with a rasp. In this case I corrected the line, so its thick, but it still functions as a useful trimming tool in a case like this, where the growth rings have been rasped smooth. This line helps me balance the foot relative to the sole and the coronet band. I use the amount of wall at the sole to determine how short to take the wall, and when the coronet band flares up, I know that the wall has further to drop.
An alternative way of getting this sort of coronet-relative construction line is to use a compass or protractor.
Because the feet were short to begin with, I beveled the toes slightly and added a tiny bevel around the edge of the wall to keep it from chipping. Those small change made a significant difference in how this horse used his feet.