Quarter and heel cracks in the hoof, in most cases are the result of the medial-lateral imbalances of the hoof and limb. These cracks are coming from the perioplic ring down and can reach up to the lower edge of the hoof. Imbalance occurs due to poor limbs conformation, resulting weight horses cover hoof not evenly.  

Related reasons:

1. The inability of the farrier to recognize of medial-lateral imbalance and take measures to reduce it.
2. The poor quality of the horn.
3. Very low or collapsed heels.
4. The hard ground.
5. Short horseshoe.
6. Discrepancy of the size of the hooves of the weight of the horse.

A healthy hoof relies on the heel, but expands in the rear quarter. Bar corners I also call heel posts, shift forward and straight under the weight of the horse. And when the hoof lifts off the ground, the heel posts shift back.

A hoof with a sloping, long pastern, and a small amount of horns heel will experience the strongest stress from blows during fastest paces on a solid track.

Before treating the crack, it is necessary to identify the cause of its occurrence. Sometimes it helps to just eliminate the cause. But in any case it is necessary to achieve by trimming and shoeing of a uniform weight distribution of the horse on the hoof. For shoeing, my preference is to use a bar shoe.

References

1. Stephen O’Grady, DVM.  Managing Quarter Cracks. American Farriers Journal July/August 2008, p.74-77.
2. Simon Curtis. Corrective Farriery Volume II, 2006 Great Britain, p.454-473.