An unusual tumor occurs in the bone inside the horse’s hoof, called a coffin bone. It’s called a keratoma. It is a benign tumor that develops from the inner hoof wall and is located between the hoof wall and the bone, and obviously there is not much space in that area.  Consequently, the tumor causes pain, lameness and damage to the bone.  Although these tumors are commonly seen in the toe region, keratomas can occur in the quarter, heel or sole and can even be above the coronary band.  As these keratomas grow, they can distort the hoof wall and cause a bulge on it.  However, just because you do not see a bulge does not mean a keratoma cannot be involved.  Usually there is a slowly progressive lameness, and separation of the white line by the keratoma can allow infection to enter the hoof; the problem can be misdiagnosed as a simple hoof abscess.  The sole will usually be positive to hoof testers similar to an abscess. 

However, if you treat the abscess and it recurs, a keratoma must be considered as the underlying cause.  X-rays of the hoof are required to make the diagnosis but a keratoma can appear to be other diseases on the x-ray, and all keratomas do not show up on an x-ray.  A keratoma is soft tissue so you will not see it unless it causes a defect in the bone, which is common but not always there. In this case, an MRI may be required to make the diagnosis.  Treatment requires surgical removal of the mass. Usually the hoof wall has to be removed in order to get rid of the tumor unless it is located at the sole, and then a hole in the sole is made to remove the tumor.  Generally, these horses heal well but 6-12 months is usually required for complete healing.