See hoof condition section for the reason that hooves have more of a tendancy to get sandcracks.
Sandcracks are created when hooves are not trimmed regularly, when they are naturally poorly formed eg after laminitis, when the coronary band is injured or when they have thin hoof walls or very dry brittle horn. Sandcracks can be treated with nail or toe clips either side of the fissure. This will stabilise the area enough for supplements to do their work in growing new horn. If the crack is growing up to the coronary band it needs to be stopped before it damages this area and causes permanent damage. In such cases a bar is inserted at the top of the crack to stop it spreading further.
When a horse has deep sandcracks they may need to be sewn together with steel wire and crack filled in with resin.
A permanent sandcrack is formed when the coronary band is damaged. Thus when the cells that produce the horn are damaged they are unable to produce horn to heal up the crack that has occurred.