Advanced arthritis with bone changes and navicular disease are among the most difficult conditions to treat. They often respond poorly to joint injections or joint nutraceuticals. Changes visible on radiographs include osteophyte formation (e.g. the bridging bone in bone spavin) and changes in the subchondral bone, the bone immediately underneath the joint cartilage.
In Europe, these horses have been treated with a drug called Tildren, tiludronate disodium. In the United States, the brand name is Skelid, and it is used to treat osteoporosis and bone changes caused by cancer metastasis to bone. The drug is highly bound to bone because of the presence of two phosphorus-containing complexes. Once in bone, it is taken up by osteoclasts, bone cells that trigger the breakdown of bone mineral.
Since navicular disease involves the loss of mineral density in the navicular bone, this drug’s use makes sense. But it’s less clear with arthritis. It’s been suggested that bisphosphonates protect cartilage from inflammation or ”turn off” inflammatory macrophages, a type of white blood cell. However, lab studies have been unable to confirm this or found the opposite.
Cartilage has no nerve supply, so the pain of arthritis can’t be coming from cartilage lesions anyway. Bone, on the other hand, has nerve endings. Odds are the pain is originating, at least in part, from the bone changes going on in osteophyte formation and inflammation of the subchondral bone.
Although Tildren is approved for use in horses in Europe, there are few formal studies and those involve small numbers of horses and relatively short periods of treatment.
The side effects of this drug in horses are unknown. Among reported human side effects are necrosis (death) of bone in the jaw, often triggered by dental work. Used long term, it may increase bone brittleness and lead to a higher risk of fracture.
Although this drug is not FDA-approved, the FDA is permitting experimental use of Tildren and veterinarians can import the drug on a case-by-case basis. An increasing number of practices are using Tildren.
Strong evidence supporting Tildren use in arthritis is lacking and at this point it would best be reserved for horses that don’t respond to any other forms of treatment and those with extensive disease. There is better evidence regarding the use in navicular disease. A firm diagnosis of navicular bone pathology should be obtained, and full discussion of other treatment options including meticulous attention to trimming and other medication options held before considering this therapy. There is insufficient long term experience with this drug in a large number of horses to determine potential side effects.