Founder, also known as laminitis, is an inflammation of the laminae or tissues that connects the hoof wall to the coffin bone. Because the laminae are between a rock and a hard place (hoof wall and coffin bone) they have nowhere to expand to accommodate the swelling. This causes pressure on the blood vessels in the laminae, and if it persists, will cause the laminae to die.
The laminae in the front of the hoof, which carry most of the weight, will stretch and tear allowing the front part of the coffin bone to pull away from the hoof wall. This is called "rotation". In severe cases, all laminae die allowing the coffin bone to drop through the bottom of the hoof. This is called vertical displacement or sinking.
Most vets say a horse has "foundered" when either rotation or sinking has occurred. Accurate diagnosis of laminitis is done by a veterinarian exam, and x-rays are helpful in determining the degree and severity of laminitis. Prompt treatment is critical and is aimed at controlling pain and inflammation. Limiting inflammation to the laminae is important as well as stabilizing the foot and coffin bone.
Recovery of laminitis depends on the amount of damage done to the laminae. Severe cases may require corrective trimming and shoeing and/or surgery. Management of a foundered horse is best accomplished through cooperation of the horse owner, vet and farrier. Prevention is dependent on identifying and correcting an underlying cause, as laminitis can be triggered by diverse events including grain overload, retained placenta, colic, or insulin resistance.