OVERVIEW: Navicular syndrome is a commonly diagnosed cause of lameness of the front feet. The term is used to encompass many different injuries to multiple structures within the hoof capsule. The navicular bone is a small boat-shaped bone within the hoof capsule which acts as a gliding surface for the deep digital flexor tendon. This tendon attaches to the bottom of the coffin bone and when it is pulled, causes the foot to flex. There are also a few smaller ligaments within the hoof capsule such as the impar ligament and suspensory ligament of the navicular bone that help to hold the navicular bone in place. While the term navicular disease was first used to describe lameness that was thought to be caused by changes in the navicular bone, more recent use of advanced imaging, such as MRI, has implicated soft tissue damage as another cause of lameness originating from the heel region.
DIAGNOSIS: Diagnosis of navicular syndrome is based primarily on how the horse responds to peri-neural analgesia. Navicular syndrome horses' lameness significantly improves with a palmar digital nerve block, which blocks the heel and sole of the foot. These horse's lameness will commonly switch to the opposite front foot, as the condition is most usually bilateral. Once the region of pain has been localized to the foot, imaging will most likely be pursued. Radiographs of the foot can show changes within the navicular bone. Radiographs may only tell part of the story, so more advanced imaging, likely MRI, is often indicated. MRI allows the soft tissue structures within the foot to be evaluated more closely and may give a more specific cause of lameness. Because soft tissue injuries need to be treated differently than bone injuries, finding the cause of lameness is important.
TREATMENT: Depending on the inciting cause and severity of lameness, multiple treatment options are available. The least invasive is corrective shoeing. Navicular syndrome horses often benefit from a wedge shoe with a pad. The wedge changes the angle of the deep digital flexor tendon running over the navicular bone and may help to ease pain. Corrective shoeing is also indicated in some horses with soft tissue damage, as the wedge will help the deep digital flexor tendon to flex the foot. Other simpler treatments include trying treating the lameness with a course of Phenylbutazone or treatment with Tildren. Tildren is a bisphosphonate that reduces the resorption of bone. It also acts to reduce active inflammation within bone and in turn reduces pain. Injections of both the coffin joint and the navicular bursa are options to help reduce inflammation and pain. The final treatment is neurectomy, which numbs the back of the foot for a prolonged period of time. Neurectomy is a good treatment option for some horses, but significant aftercare is required and it should not be pursued in all cases and most always used as the last alternative.
Stacie Aarsvold, DVM D. Michael Davis, DVM, MS