Enostosis-Like Lesions of the Long Bones in Horses

Enostosis-like lesions, also known as bone islands, are a rare condition characterized by bony growths that form in the medullary cavities of the equine skeletal system. They are more commonly diagnosed in older horses and thoroughbred horses. In most cases, ELLs are asymptomatic and cause no distress for the horse that has developed them, but in some situations the growths that are formed in the long bone of the leg cause temporary lameness that generally recedes with rest and nonsteroidal anti-inflammatory drugs.

Enostosis-like lesions are rare bony growths that form within the cavity of the bone rather than on the surface of the bone like enostosis.

In many cases, enostosis-like lesions found in the long bones of the leg are completely asymptomatic. Lameness was more likely to occur when the long bone that is affected is either the humerus or the femur. Lesions in the humerus tend to cause more intense lameness as well. When an ELL does produce lameness, it is usually transient and intermittent, often resolving with minimal interference. 

• Lameness
• The lameness may be transient
• The lamesness may be intermittent

Types

There are several long bones in the leg that can be affected by enostosis-like lesions. 

• Femur - The femur bone is a long bone of the hind leg that extends from the hip to the patella
• Humerus - The humerus bone is a long bone in the forelimb that extends up from the elbow to the shoulder
• Metacarpal or metatarsal bones - The bone from the ankle to knee is known as the metacarpal bone in the forelimb and the metatarsal bone in the hind limb
• Radius - The bone in the forelimb that extends from the horse’s elbow to its knee
• Tibia - This is a long bone in the hind limb that is positioned between the femur and the metatarsal bones

The mechanism that initiates ELL’s in the long bones of horses is poorly understood however there are factors that may contribute to their formation:

• Age - Older horses are much more likely to develop these lesions than younger horses
• Breed - Thoroughbred horses seem to be somewhat predisposed to developing enostosis-like lesions, while Standardbred horses appear to be resistant to developing this disorder

Many of these lesions are found in the long bone incidentally during a scintigraphy or radiography for other conditions. If the cause of the veterinarian visit is due to the lameness, a physical evaluation will be done to see if there is any swelling or damage that can be seen or palpated. Scintigraphic technology will be used to visualize the growth within the bone. In order to perform this procedure, trace amounts of radioactive molecules are introduced to the equine intravenously, and images are taken to uncover the growth.

Radiographs will be used to confirm the size and placement of the bone island. It is important that ELLs that are exhibiting symptoms be correctly diagnosed and differentiated from stress fractures in the legs as the treatment plan will be very different between the two, and the proper treatment for an ELL is very likely to cause severe damage to a hairline fracture.

Entosis-like lesions that are not causing any discomfort are often left untreated and lameness that is caused by this disorder is usually resolved with exercise restriction and rest. Stall rest and the administration of NSAID medication is usually recommended until the lameness has cleared, which generally takes approximately one to three weeks when it is due to ELL.

Nonsteroidal anti-inflammatory drugs (NSAIDs) such as phenylbutazone and flunixin meglumine are commonly used for horses in these situations and can be very helpful in managing both pain and swelling. Horses that are exhibiting lameness should not be ridden for at least six to eight weeks. Damage to the muscles, tendons, or ligaments and stress fractures may take considerably longer to heal than lameness due to enostosis-like lesions, and starting exercise too early with any of these disorders could potentially hinder the healing process or worsen the problem, so getting a professional diagnosis can be crucial.

The prognosis for horses that are not showing any symptoms is excellent and generally requires no treatment, however, approximately half of all horses who get enostosis-like lesions do experience lameness at some point. Horses that experience lameness generally still have a good chance at full recovery, particularly if only one lesion is found. In one study, racehorses with only one enostosis-like lesion were much more likely to be able to return to racing than racehorses with multiple lesions. The severity of the lameness and the placement of the lesion itself both contribute to the amount of time that will be required before the leg is fully healed.