Laminitis is a disease affecting the horse's foot and has many different causes. Because it has many different causes, it has many different names! For example, laminitis caused by overeating green grass is often called grass founder and laminitis caused by too much grain is often referred to as grain-binge. Laminitis means inflammation of the laminae, which are sensitive tissues that connect the hoof wall to the coffin bone.
The laminae are arranged in vertical rows beginning at the coronary band and continuing down to the ground. The largest percentage of the laminae are located in the front of the hoof, where they are the longest. They become shorter as they approach the heel. The hoof has sensitive and insensitive laminae. The laminae that are connected to the hoof wall are the insensitive laminae; the laminae that are connected to the coffin bone are the sensitive laminae, which are vascular and innervated.
Laminitis is often confused with acute laminitis, or founder. Founder is a maritime term meaning "to sink." Correctly used, it describes a condition in which the sensitive laminae securing the coffin bone in place begin to tear, stretch or begin to detach. In mild cases of laminitis, founder may not occur, or at least be noticeable. However if laminitis progresses, the laminae become more inflamed and swollen, or edematous.At this point the horse may be noticeably lame. The swelling laminae have no where to expand , as they are caught between the unyielding hoof wall and coffin bone. As the swelling progresses, it cuts off blood supply to the laminae, causing ischemia, or lack of oxygen to living cells. The laminae then begin to die and this condition is acute laminitis, or founder.
If the laminae in the front of the hoof stretch and tear, it allows the front part of the coffin bone to pull away from the hoof wall. This allows the back part of the coffin bone to drop to varying degrees. This process is called rotation, and can be measured in degrees. In severe cases all the laminae die, and the coffin bone then drops through the bottom of the hoof. This is called vertical displacement. We say a horse is foundered when either rotation or vertical displacement has occurred.