Hoof cracks are common in horses. While some are minor and can be treated with just trimming, others can cause severe lameness and treatment is more involved.In most cases, the cause of a hoof crack is due to abnormal stress on the hoof wall.Dr. Scott Pleasant indicates in the Practitioner that the hoof capsule adapts and changes shape according to how it is loaded, and the hoof wall grows slower in areas where the majority of the weight is borne. 

When stress and strain become excessive, hoof cracks occur so if you have a horse who has them, it is likely there is an imbalance on the load applied on the hoof and the hoof capsule is unstable.For example, toe cracks are commonly noted in horses with long toes and low heels so if you see toe cracks, it is very likely the toes are too long.The long toe causes excessive leverage on the hoof wall in the toe and this leads to small incomplete cracks.If the problem is neglected, the cracks become larger and deeper, and eventually lead to lameness.Quarter cracks commonly develop because the foot is not balanced from outside to inside, which is called a mediolateral balance.To trim either the long toe or sides of the foot correctly, x-rays are required to see where the bone is inside the hoof so you can determine how short to trim the toe and how to level the hoof from side to side.There are many techniques to treat these cracks, from just trimming correctly to applying plates on the hoof wall with screws to decreasing movement of the crack.Regardless, the important thing to remember is that hoof cracks happen because of hoof instability and this must be treated to prevent further cracks from occurring.