Underrun heels are defined by comparing the heel tubule angles to the toe wall tubule angles. They should be parallel. If the tubules converge (in the lateral view) from their starting point at the coronet towards the toe, they are underrun. Harder hoof wall is less prone to being crushed into this geometry so the horse with genetically harder feet is better off to start with (in this respect).

There are specific trimming techniques for preventing heels from becoming underrun, if the horse is left barefoot. In general, the heels would be trimmed frequently (in this case, perhaps once a week), bringing them back to line up with the widest part of the frog. Then, one would also relieve, or float, the quarters very slightly under the heel area to prevent the tubules from being crushed by the weight of the leg and start running forward.

It’s also essential to keep the toes as short as possible to avoid drawing the heels forward as the toes grow forward. Maximum turnout is also helpful to prevent ammonia from weakening the tubules and allowing them to become crushed.

jimlfpbef705lines.jpg                    jimanglessm.jpg

Fig. 1                                               Fig. 2

Photos above illustrate an underrun heel.  The superimposed lines in Fig 2 follow the direction of the tubules. In a healthy hoof the tubules are diverging or at least parallel – here they would converge if the lines were extended beyond the toe.  The angle the heel tubule makes with the ground is more acute than the one the toe tubule makes with the ground, which is incorrect (Fig. 2)

 

4yotbs.jpg   4yotbsol.jpg © Julie Leitl, Victoria, Australia

Fig. 3                            Fig. 4

Another example of underrun heels, with the weightbearing point having grown far forward onto the sole (Fig. 4, yellow arrow). The pink arrow shows where the heel should be, i.e. even with the widest point of the frog.

This foot also looks like it may have a negative plane coffin bone, which seems to occur more frequently on hind feet. 

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