Navicular syndrome, also known as navicular disease, is a chronic and often progressive disease process that affects the horse's hoof. It specifically involves the navicular bone, navicular bursa, deep digital flexor tendon (DDFT), and soft tissue structures making up the navicular apparatus. Navicular is one of the most common causes of chronic intermittent forelimb lameness in horses, accounting for one-third of all chronic lameness cases in one study. There are many different types of treatments, however none have proven to be a consistently effective form of therapy. In fact, in a study conducted in 2006, only 65-75 percent of affected horses improve in performance and only 40-50 percent were able to stay sound for 1 to 2 years.
The primary function of the navicular bone is to provide a constant angle of insertion for the DDFT. The specific cause of the syndrome is unknown, however there are three proposed theories which include vascular alterations, chronic inflammation and repetitive biomechanical forces applied to the hoof.
When moving, horses with navicular syndrome tend to place move weight on the toe of their foot in order to avoid exerting pressure on the heel. This is because the heel is where the navicular bone and bursa are located. This shifting of weight alters the horse's gait to a rough, choppy stride. Horses will have varying degrees of lameness, with a sudden or insidious onset. The lameness starts out mild initially and appears to get better when the horse is exercised. Sometimes the horse is lame after work, but will get better with rest. Over time, the lameness gets worse with exercise and is made worse on hard surfaces. Horses often show increased discomfort when turning or when asked to make tight circles. Navicular horses also trip frequently. When stationary, the horse will often be seen pointing the lame leg, stand camped-out with both front legs, or continuously shifting their weight in their front legs due to pain.
Over time, in cases of chronic lameness the horse's hoof conformation may change and present as a contracted (small and narrow) hoof with a high heel. Upon application of hoof testers, horses with navicular syndrome will often demonstrate a pain response (immediate limb withdrawal) when pressure is applied to the middle third region of the frog.
Of the various treatments available, bisphosphonate medications (tiludronate and clodronate) have been shown to be promising. Researchers at Oklahoma State University (OSU) are currently in the process of conducting a clinical trial to test the effectiveness of bisphosphonate therapy on eligible horses.