Tuesdays with Tony

     Kinda sounds like one of the pieces you make sure you take out of your Thanksgiving turkey before eating it, right? Well, in fact, the navicular is a small bone in your horse’s foot at the back of the coffin joint. It is technically a sesamoid bone, meaning it is not one of the main weight-bearing bones in a joint. Despite this, navicular bones play a huge role in the physiology of how a horse’s foot bears weight.

      Navicular disease syndrome describes a problem with the navicular bone plus or minus any of the structures it connects to. You see, the location of the navicular bone smack in the middle of the hoof makes this tiny bone extremely significant. The navicular bone is in contact with the coffin joint, the navicular bursa, the coffin bone, the short pastern bone, the navicular impar ligament, the deep digital flexor tendon, and the digital cushion. This is why navicular disease can go south so quickly!

      Navicular disease usually manifests itself as forelimb lameness. The lameness may be mild or intermittent at first, and it may be difficult to tell which front foot is lame. The classic feature of navicular disease is that it is almost always bilateral (affecting both front feet). If your horse’s lame foot is blocked (numbed with local anesthesia), and the lameness switches to the other foot, a diagnosis of navicular disease is high on the list.

     X-rays of the front feet are needed to confirm this diagnosis and to get an idea of the severity of your horse’s condition. This is when I get to sit back and watch the docs and techs try to make your horse do circus tricks in order to get some fancy X-ray views. First, they need the horse to put both front feet up on these little wooden blocks. Next, they get the horse to stand on a tunnel with the X-ray plate in it. Then they ask the horse to stand with one leg forward and one leg back while they crouch under the horse’s belly and point the machine at the horse’s heel. Let me tell you, it looks absolutely ridiculous, and is hilarious to watch.

    While X-rays can give the docs a lot of information about the navicular bone itself, MRI is required to fully evaluate the soft tissue structures involved. If it’s in the budget, we would strongly recommend an MRI if your horse is diagnosed with Navicular disease.

     Yes! While navicular disease is a degenerative condition, this diagnosis is certainly not a death sentence for your horse when it is identified in its early stages. Certain breeds, such as Quarter horses and Paints, are more likely to develop the condition than others. If you own one of these horses and notice any forelimb lameness, or if your farrier tells you that your horse is painful across his heels, have him evaluated by a vet ASAP!

     Corrective shoeing is the cornerstone of treatment for navicular disease. Wedge pads will take some pressure off the heels, where the navicular structures are, and redistribute the weight to his toe. Any decent farrier should be able to handle applying wedge pads, but make sure he or she gets a chance to look at your horse’s X-rays to determine what degree of wedge your horse needs. If you have a stellar farrier who is adept at corrective shoeing projects, he may try a therapeutic shoe with a built-in wedge (bars that are thicker at the heel than at the toe), and/or other modifications to suit your horse’s individual needs.

     The corrective farriery is essential to managing a horse with navicular disease, but it’s not always enough by itself. Your vet may prescribe anti-inflammatory medications to give your horse long-term in order to maintain his comfort. Equioxx is commonly employed in these cases because it boasts far fewer negative GI side effects compared to Bute or Banamine. Joint injections are another way to decrease the inflammation associated with navicular disease. Going back to our anatomy lesson from earlier, injections into either the coffin joint or the navicular bursa may help your horse, as the navicular bone borders both of these structures.

    The new kid on the block as far as treating navicular disease goes is a drug called OsPhos. This medication is definitely in vogue right now, especially in the Quarter Horse world, but does it really work? Well, lucky for you, I like to be a well-educated cat, so I did some research on the subject.

     OsPhos has been shown in vitro (that means in a Petri dish in the lab) to reduce osteoclast activity. Osteoclasts are cells that break down bone. One of the primary features of navicular disease is that the navicular bone essentially disintegrates over time (a severely diseased navicular bone will look like a slice of Swiss cheese). Ok, so this sounds like a no-brainer, right? The navicular bone is breaking down, so let’s give a drug that inhibits bone breakdown to stop progression of the disease.

    Well, there are 2 little problems with OsPhos. Remember that whole Petri dish thing? Well, just because we know a drug works in a lab means nada when it comes to how that drug will work when injected into a horse’s body. The medication has been approved by the FDA so we know it is relatively safe for horses, but how well it works at actually treating navicular disease is anybody’s guess.

    The second problem is a recently-recognized side-effect, which we all should have expected when you think about it. It has been suggested (by some very smart veterinarians) that OsPhos inhibits bone healing. This makes sense, because in addition to eating away at diseased navicular bones, osteoclasts actually have an important role in bone healing in a healthy horse. If your horse were to fracture a bone, osteoclasts would get to work right away cleaning up the shattered fragments and making way for osteoblasts (bone-making cells) to lay down new bone. Well, if your horse is in work, and his bones are under stress, they are essentially sustaining millions of micro-fractures all the time. In a normal horse, this would be no big deal. Osteoclasts would swoop in, clean up the damaged bone, and it would be replaced by new bone, which would be even stronger than before. But, in a horse that has been treated with OsPhos, those osteoclasts are inhibited, the damaged bone doesn’t get cleaned up, the new bone can’t be laid down, and the bone is left weaker than before.

     If you’ve followed my cat-splaining this long, good for you! If not, don’t worry, just trust my vets. Here’s their two cents: OsPhos is worth a try in horses with a definitive diagnosis of navicular disease. However, if one treatment doesn’t do the trick, it’s probably not a good idea to give this medication repeatedly. OsPhos is definitely not a lameness cure-all, and it doesn’t make sense to use it in horses to treat anything other than navicular disease.

   Well, I sure hope I still have time to get my nap in! Tomorrow’s another day full of foals, mares, and stallions. I need to be alert to manage my minions as they run around like chickens with their heads cut off.

    See you next week!

           -Tony