Bone is a dynamic tissue. It is resorbed and deposited in locations depending on mechanical stresses and many other factors. Some chronic equine lameness and pain-causing conditions involve the chronic (and potentially excessive) breakdown of bone. There are two cell types directly involved in this constant process of dissolution and repair of bone. The osteoblast is the cell that deposits and builds bone. The osteoclast dissolves and removes bone.

Bisphosphonates reduce bone breakdown by preventing the bone removing cells (osteoclasts) from resorbing bone. Based on their function then, bisphosphonates should be especially effective in disease states in which bone resorption is an important feature.

Bisphosphonate therapy has two major theoretical advantages to more traditional local therapy. Given systemically (IM or IV depending on particular drug), a bisphosphonate can treat multiple areas of bone turnover at once. It also targets the process of bone destruction itself rather than providing only symptomatic relief. The medication selectively targets areas of active bone destruction at the time of administration.

It has become clear through use of these drugs that there is a relatively short (several week) period of direct pain relief, unrelated to the main mechanism of interfering with bone resorption. The actual effect on osteoclasts and bone resorption should be visible as reduced lameness at 4-8 weeks and this effect should last up to 6 months.

Two bisphosphonates are currently FDA approved in the USA, for navicular disease only. These drugs are Tildren® (tiludronate) and clodronate, marketed as Osphos®.

Tiludronate is administered to horses by intravenous (IV) infusion. Different protocols exist ranging from a single large dose given over one hour, to smaller doses given over 10 days.

Clodronate is administered by a single 15cc IM injection, usually divided into 3 intramuscular sites. Repeated doses may be given at 3-6 months or if signs of lameness recur.

Bisphophonate therapy are often used alongside traditional treatments (e.g. NSAIDs, steroids, supplements) and supportive therapies (e.g. corrective shoeing, controlled exercise).

YOUR VET'S ROLE

Your vet should make a diagnosis before suggesting the use of these drugs. The drugs should be reserved for cases for which bone resorption is thought to play an active (and undesired) role in the disease process and pain. Your vet also considers the horse's general health and whether these drugs are a good and safe fit.

While the drugs are FDA approved for use in navicular disease only, they may provide benefit for other conditions in which bone resorption plays a role in the disease process and pain. These are extra-label uses of the drugs. Examples are bone spavin (hock arthritis), back pain from intervertebral osteoarthritis (spondylosis), osteochondrosis (OCD) as well as a variety of other conditions. This drug class also may have value in treating chronic osteoarthritis. While cartilage loss is the main hallmark of arthritis, part of the process also involves bone loss and remodeling under the cartilage surface. Inhibition of this bone breakdown may thus slow the progression of joint disease and may provide some direct relief from pain.

Your vet will consider these disease processes and when these drugs might have potential application.

YOUR ROLE

It's important to be aware of this drug class and to understand its basic mechanism and limitations. It's also important to be on the lookout for the potential side effects seen right after injection.

Like any treatment for subtle problems, it is very important for you to be as objective as possible when evaluating the treatment's effectiveness. Try to quantify the degree of lameness before and after administration Keep a record of degree of lameness so that you can better determine the real effect of this treatment on your horse. This way, you can also better recognize when a treated horse is again beginning to show signs and might benefit from another injection. Always communicate with your vet when you have any question about the use of this drug on your horse.