Bipartite Navicular Bones

You don't see these everyday!
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You don't see these everyday!

Every once in a while a veterinarian will come across a peculiar navicular bone abnormality when radiographing a horse’s foot. Most of the time this occurs because the horse has presented for front limb lameness. But sometimes, the horse isn’t lame at all - the problem is coincidently identified when radiographs are being taken for a prepurchase exam. The condition is called bipartite navicular bone(s.)

A bipartitie navicular bone with the fibrous junction between the two ossification centers indicated by a red arrow.

A bipartitie navicular bone with the fibrous junction between the two ossification centers indicated by a red arrow.

When forming in utero, all bones begin as cartilaginous structures. Special stem cells in the bone begin to collect minerals and ossify the bones. In the vast majority of bones, the ossification process begins in one location and emanates outward until the complete bone is formed. But in the case of bipartite navicular bones, there are two centers of ossification that form the bone. Unfortunately, the result is essentially two bones where there is only supposed to be one. Bipartite navicular bone is a congenital problem - meaning that it occurs when the fetus is developing in utero and the foal is born with it.

Although congenital suggests that the problem is genetic in origin (faulty DNA from the parents), this is not necessarily the case. Congenital issues can arise from environmental factors in the uterus such as toxins, hormonal influences, biochemical mediators, and others at critical times during development or throughout development. In the case of bipartites, the veterinary community just doesn’t yet know what causes them. There is no breed or sex predilection that has been noted - they just seem to happen randomly. Fortunately, they don’t occur very often.

The bipartite development can occur on one, two, three or all of the navicular bones in the horse’s body. Unfortunately, most horses with bipartite navicular bones will have significant ongoing lameness. Remember - the navicular bone sits the heel region of all four feet inside the hoof capsule. Therefore, we tend to see the “classic” navicular-type symptoms: chronic heel pain in which the horse stumbles, bobs its head, lands “toe-first” and takes small steps.

Management strategy for bipartite navicular bones involves radiography to determine the extent of the issue (sometimes there can even be three centers of ossification in the bone!), protective shoeing, and use of medical therapy to help with caudal heel pain. In general, veterinarians are still using antinflammatories such as bute or Previcox to manage navicular problems, +/- intermittent use of isoxuprine to help with blood flow in the foot. Because both Tildren and OsPhos are labeled for navicular bone disease, they may also be helpful in improving the health of the bone and keeping the horse serviceably sound. Remember - both Tildren and OsPhos are bisphosphonates which increase calcium retention in the cortex of the bone.

Sadly, most horses with this condition progress in severity and become too lame to ride, despite therapies. Not exactly a motivating post to start the week, but there is so little information available about bipartite navicular bones, it seemed useful to share with HJ readers.