Dr. Jed McKinlay, McKinlay & Peters Equine Hospital
A Keratoma is an uncommon space occupying mass that lies between the hoof wall and the coffin bone of the horse. In other words, it is an abnormal mass of tissue that develops and grows inside the hoof wall. As it grows and expands it puts pressure on the bone and causes pain, visualized as lameness. To us it would be like having a lump of tissue growing under a toe or fingernail that hurts because it doesn’t belong there, it puts pressure on the sensitive tissues under the nail and it keeps getting bigger. As the Keratoma grows and continually puts pressure on the coffin bone the bone begins to remodel and lose its strength and density. In long standing cases the bone in the specific area of the Keratoma degenerates or dissolves completely.
The name Keratoma implies that it is a tumor, which it really isn’t. The mass is thought to originate at the epidermal producing cells at the coronary band or in the sole. It is an abnormal proliferation of those cells that is called hyperplastic. Hyperplasia means “the enlargement of an organ or tissue (in this case the epidermis) caused by an increase in the reproduction rate of it’s cells, often as an initial stage in the development of cancer.” Keratin is “a fibrous protein forming the main structural constituent of hair, feathers, hooves, claws and horns.” So, a Keratoma is hyperplasia of the keratin producing cells in the horses’ foot. Hope that all makes sense.
The reason Keratomas happen is not known. They may be brought on by chronic irritations, inflammation, infection, trauma or just because the keratin producing cells decide to multiply and proliferate at an accelerated rate. Most of the time they occur in the toe region, but may be found literally anywhere inside the foot or even above the coronary band. They may grow quickly or more insidiously.
They always cause pain. This may be slight at first, more of an irritation, but as the size of the lesion grows so does the pain. As you can imagine, the bigger the Keratoma gets, the more pressure it exerts on the surrounding bone and hoof capsule and the more painful they become. The hoof capsule or wall is very tough and unrelenting in its effort to keep the bone and soft tissues of the foot organized and packed into the space God made for them. The wall is tough enough that the pressure is almost always forced toward the inside of the foot, toward the bone. As mentioned earlier the bone then responds by dissolving away to make room for the abnormal and unwelcomed tissue growth. This is a painful process for the horse to go through.
It’s not always as easy as it may seem to diagnose a Keratoma. Whenever we are faced with a lame horse we begin with a good thorough exam. This includes examining the foot with our eyes, hands and usually hoof testers. Regional anesthesia or nerve blocks are often used to isolate the location of the pain. Radiographs are also used, as well as MRI in some cases to track down the source of the horse’s pain.
Keratomas may present with some hoof wall separation that can be viewed on the sole of the foot at the white line. There may be irregularity, separation or unevenness of the white line. Hoof wall cracks may occur. Pressure with hoof testers over the lesion should elicit a painful response, indicative that there is something inside that specific portion of the horse’s foot that is causing the pain. Nerve blocks can desensitize a certain area or region (regional anesthesia) of the foot or leg, like when the dentist numbs a portion of your mouth prior to working on a sensitive tooth. When we numb a specific area and the lameness or pain goes away we know the source of the pain is somewhere in the area that has been desensitized. X-rays will help us see any changes that have taken place in the bone of the foot. Our digital x-rays these days are so beautiful and can be viewed immediately at the horse’s side. If the cause of the horse’s lameness is still not clear the MRI offers diagnostic information about the bone and soft tissues that can’t be had in any other way. The MRI can also help us triangulate the exact position and size of the Keratoma.
Once the lesion is identified it must be removed to effect a cure. This is done but removing the hoof wall overlying the Keratoma and removing the abnormal growth beneath it. This is most often accomplished with the horse standing, sedated and with local anesthesia in place. There is most often a fair amount of bleeding, so a tourniquet is applied above the foot. Following removal of the hoof wall and Keratoma the foot is bandaged, and the tourniquet is removed. In most cases a shoe is applied to the foot to aid in support and aftercare. It usually takes 6-12 months for the defect to grow out completely, depending on how far up the foot the hoof wall resection had to go. The underlying bone and soft tissue generally heal up without complications. Keratomas do not often reoccur and the prognosis for returning to soundness is very good.
To recap, Keratomas are uncommon space occupying masses that usually originate at the coronary band. As they move down the foot they grow in size and begin to put pressure on the tissues beneath them, primarily the coffin bone. The pressure exerted by their size, where there is no excess space, is painful and causes varying degrees of bone loss. Some can be easy to diagnose; others are more challenging. They should be removed surgically, and usually do not reoccur. Keratoma should always be one of the things we look for when a horse has a lameness that is isolated to the foot.
