Sinking Heels (Underrun Heels)
The phenomenon of "sinking heels" occurs when the horse's extremities are improperly positioned, especially when the horse's hoof is too pointed. In this case, the hoof-pastern axis is broken. The main cause of this improper position is improper hoof treatment (1)or excessive horn abrasion in the heel area as can be found under iron horseshoes. Excessive in this context means substantial horn abrasion in the rear of the hoof while there is little or no abrasion in the toe area (under iron horseshoes). This uneven strain shifts the load into the heel area of the extremities when supporting the weight. Due to this lack of abrasion, the then overly long horn tubules of the toe wall are forced from their original position (where they cushion a great deal of the pressure from the ground vertically and only yield a fraction of it horizontally and outwards) to a position in which they increasingly give way horizontally to the pressure from the ground. This can be seen in bends of the horn wall, in widening and torn thin layers and in an excessively wide bearing surface in the toe area. In the extreme, the bearing wall can become almost unable to supportloads in the toe area. Since the toe wall then gives way in an outward motion every time the foot is put down, there is that much less abrasion in this area and the heel is subject to more and more strain. Thus the horn tubules of the bearing wall change into an increasingly horizontal position in relation until eventually the toe is "runinng away" from the hoof.
The only way to correct this situation is to eliminate its causes. This means that the almost horizontal horn tubules have to be repositioned physiologically in relation to the ground. The hoof orthopaedist initiates correction by treating the excessively long toe wall tubules in a way that keeps them from acting as a lever. This is primarily done by correctly grating the hoof around the outside and thinning out the excessively wide bearing surface in the toe area. It is important here to grate high up along the hoof wall (avoiding a so called "bull's nose") in order to eliminate any outward leverage. This is called a "reed thatch structure", meaning that the outer horn tubules that were previously the longest and hardest will now become the shortest so that they stabilise the tubules underneath them, which keeps the toe wall from being bent outwards.
This has three positive effects at once: 1) the toe wall regains its bearing function, 2) there can be more abrasion in the toe area because the bearing surface is grated thinner (as described above), and 3) the leverage that causes the horse pain is eliminated, making it easier for the horse to put its foot down. Moreover, the horse will no longer relieve its toes by straining its heels. This means less load upon the heel so that the hoof can "walk itself back" into a balanced situation.
(1) e.g. a constant shortening of the heels - as widely propagated by a well-known school of bare hoof care - often causes this type of positioning.