Due to the horizontal orientation of the horny tubules associated with underrun heel quarters, excessive weightbearing force is experienced along the side of the tubules rather than along their edge. This, in turn, results in the development of a "stress riser" at the point that normally-oriented tubules meet abnormally-oriented tubules. A considerable amount of torque on the tubules is experienced at this location. It is common, therefore, that horses with underrun heels develop heel quarter bruising, usually on the medial aspect (inside) of the foot where more weightbearing force is exerted.
This problem is generally remedied via mild corrective shoeing. In our opinion, there are two principles that should be implemented to effectively address the problem of underrun heels:
Frog support, which can be applied via a variety of techniques including frog support pads, pour-in pads and heartbar shoes. By implementing frog pressure, we effectively underload the heel quarters. This reduces stress and pain associated with the heel tissue. It also enables new heel to grow more vertically, thereby allowing it to develop more in line with the rest of the hoof wall.
Avoid over-shortening of the toe. In many cases of underrun heels, the farrier attempts to increase heel support by pulling the shoe back on the foot and fitting the heels more full. This often results in "setting" of the toe, whereby the hoof wall along the toe is no longer supported by the shoe underneath it. Setting the shoe can deleteriously affect the internal biomechanics of the foot. By not fitting the toe we have eliminated weightbearing stress along the dorsal (front) of the hoof capsule. Consequently, the hoof wants to "squirt" off of the front of the shoe much like your foot would want to squirt out of the front of high-heel shoes. The foot tries to expand at the toe due to the lack of weightbearing stress coupled with forward movement of the hoof capsule. Concurrently, the heels tend to contract and become more underrun. We therefore recommend fitting the shoe completely to the toe to maintain weightbearing stress along the dorsal aspect (front) of the hoof wall and encourage further heel quarter expansion.