When discussing laminitis, it is important to have a consistent vocabulary. All of the following terms are used to describe laminitis in its various forms:
Laminitis is a condition that describes inflammation of the lamellae within the hoof. Note: The terms lamellar and laminar are synonymous. Lamellar and lamellae are the best descriptive terms; in the U.S. laminar is commonly used. Laminitis in the horse is a disease that is induced by multiple, apparently unrelated causes, and results in an acute and/or chronic lameness condition. It is s a complex condition which involves changes of the cardiovascular, endocrine, renal, and gastrointestinal systems, and produces changes in the immunologic status and blood coagulation in addition to the problems in the foot. A laminitic horse shows a characteristic lameness and other signs:
Founder is a term used to describe the stage of laminitis when the hoof begins to fail mechanically.
Sinker: Mechanical failure of all lamellar attachments, which forces the coffin bone downward under the weight of the animal.
Types of Laminitis are also distinguished by various causative agents.
Support Laminitis occurs when one limb bears excessive weight during recovery from injury to the opposite (contralateral) limb.
Traumatic Laminitis (Road Founder) occurs when a horse is overworked on a hard surface.
Metabolic (Biochemical) Laminitis describes laminitis caused by overeating, inactivity, or carbohydrate overload.
Endocrine Laminitis describes laminitis secondary to diseases such as a pituitary adenoma which produces an excess of corticosteroid in the system (i.e. Cushing’s Disease)
Stages of Laminitis
The progression of laminitis can be divided into distinct stages or phases:
Developmental Phase (20-60 hours)
The Developmental stage of Laminitis starts when the horse is exposed to the causative agent, and ends with the first signs of lameness. During this phase, which lasts between 24 and 60 hours, lamellar separation is triggered, but there are no signs of foot pain.
Acute Phase (24-72 hours)
The Acute Phase of Laminitis marks the beginning of clinical changes in the horse’s feet.
The acute phase starts with the first signs of lameness and continues until 72 hours have passed without physical or radiographic evidence of mechanical collapse of the foot. This phase ends abruptly with foot collapse (rotation or sinking of the coffin bone).
Signs include bounding digital pulses, heat, sensitivity to hoof testers, and lameness. A “camped in front” stance is often seen.
Subacute Phase (72 hours to life)
The subacute phase is a mild period with less severe clinical signs and without mechanical failure of the foot. It often ends without permanent lamellar damage, which is the ideal resolution of the acute phase.
Chronic Phase (72 hours to life)
The Chronic phase of Laminitis when the foot begins to fail mechanically. This can occur because of progressive changes to the present in the acute phase or due to mechanical overloading of the foot in the subacute phase of the disease. The term “Founder” is best used to refer to a horse with chronic laminitis.
Because the Chronic phase can last indefinitely, three different subphases are used to describe the condition more specifically:
Early Chronic: Resolves or continues for months
Chronic Active: Recurrence of pain after a period of improvement.
Chronic Stable: Coffin bone is stable. Hoof and sole grow with steady clinical improvement.
This term is used to describe laminitis that either does not or is only minimally responsive to treatment within 7-10 days.
Figure 14. Normal Foot
Understanding laminitis requires a basic knowledge of the normal inner workings of a horse’s foot, and the forces that disrupt them. In the normal horse, the forces of weight bearing are transmitted down through the bones of the leg to the third phalanx (distal phalanx, coffin bone, pedal bone or P3), which is located within the hoof wall. The third phalanx is connected to the hoof wall by the laminae. Laminae run from the top of the foot to the bottom. Laminae projecting outward from the outside of the third phalanx (the sensitive lamina or corium) interlock with laminae projecting inward from the hoof wall (the insensitive lamina). The effect of these interlocking tissues is to greatly increase the surface area which connects the third phalanx to the hoof wall.
When laminitis occurs, there is weakening of the interlocking tissues of the lamina, thought to be a result of lack of blood supply to the affected region. If the inflammation is mild, the problem may go unnoticed and may result in no permanent damage to the foot. In more severe cases, the laminae from the outside of the third phalanx begin to separate from the laminae on the inside of the hoof wall. If enough laminae separate, the weakened laminar junction can no longer support the weight of the horse and the third phalanx begins to move in relation to the hoof wall. This movement can further damage blood supply to the foot and compound the damage.
If the laminae at the front of the foot are affected more severely than those at the heels (the more common situation) the third phalanx begins to rotate due to the strong pull of the deep digital flexor tendon.
The extensor process of the third phalanx rotates dorsally. On radiographs, the distance between the hoof wall and bone is greater distally than proximally.
If the entire laminar junction is affected, Mechanical Failure occurs with displacement of the foot relative to the hoof capsule, increased digital instability, and decreased strength of the lamellar interface causing the third phalanx to begin to move downwards. This condition is called Sinking, Distal Displacement of the Distal Phalanx, or Vertical Displacement of the Coffin Bone.
Lamellar separation is occurring in all parts of the hoof, and the horse’s weight is forcing the bone through the sole. Sinking indicates more severe damage to the lamellae than occurs during rotation. Radiographic evidence includes widening of the space between the hoof wall and the bone.
The vertical distance between the original position of the proximal limit of the extensor process of the third phalanx and its current position (See Figure 17).
The front of the solar margin of the third phalanx begins to compress the blood supply of the sole, especially at the toe of the hoof, and can result in bruising and/or death of tissue.
Solar Prolapse, also known as dropped sole.
The rotated coffin bone causes the sole to bulge downward.
Subsolar/Submural Abscesses: Abscesses appear secondary to the loss of blood supply, death of tissue, and invasion of bacteria. In chronic cases, submural radiolucencies are seen in radiographs.
Coronary Shear Lesions: As rotation of the third phalanx occurs, hoof growth is altered due to the changes in blood supply. The development of new wall is impaired, especially in the toe region. Rings on the hoof wall illustrate the changed growth pattern and are very prominent in chronic founder, often diverging from toe to heel. (See Figure 20)
Flat Feet: Can be evidence of digital collapse or altered hoof growth.
Founder Rings: Horizontal ridges in the hoof wall that indicate previous bouts of chronic laminitis.
Seedy toe is a separation of the horse’s hoof wall from the underlying sensitive laminae at the white line, resulting in a cavity which fills with crumbling dirt, horn and debris and is prone to associated infection. It can occur as a complication of chronic laminitis with rotation and sinking of the third phalanx, followed by secondary infection. Infection with anaerobic bacteria and yeasts, which are common hoof contaminants and an important feature in all forms of seedy toe, can liquefy hoof keratin, separating the sensitive laminae, progressing to rotation of the pedal bone as in laminitis.
Non-Surgical Treatment for Laminitis
Breakover: The moment in a horse’s gait just before the hoof leaves the ground.
Corrective farrier techniques include shortening the toe to modify the breakover point and relieve pressure on the foot as weight shifts. Successful treatment of Laminitis requires an ongoing team effort from the owner, the veterinarian, and the farrier.
Coronary Grooving: Done using a burr to promote dorsal hoof wall growth.
Bar Shoe: Provides wall support.
Sole Protection and Support: Therapeutic shoes evenly distribute pressure on the sole.
Surgical Treatment for Laminitis
Deep Digital Flexor Tenotomy: Performed to relieve the pain of chronic laminitis. Reduces the pull of the deep digital flexor tendon to decrease the tearing forces on the lamellar tissue, and to lower the heels.
Carpal (distal) Check Ligament Desmotomy: Similar goal to DDFT but with less impact; rarely performed.
Dorsal Hoof Wall Resection: Removal of the hoof wall to the depth of the lamellae to expose dead tissue and allow drainage of infected lamellar tissue and body fluids.
—James A. Orsini, DVM