To understand a disease, one must first understand general anatomy. The navicular bone is located on the palmar or caudle aspect of all four feet in the horse, generally located between the heel bulbs. Let's look at a couple of standard x-rays which demonstrate the location.
The image to the left is what is termed a lateral view of the foot or distal limb. In this image, I have the navicular bone pointed out with a yellow arrow. As you can see, it is generally located between the pastern and coffin bone on the back aspect of the foot. What you cannot see in this x-ray is the placement of the flexor tendons, which are thought to contribute to the pain encountered as well as potential deterioration of the condition. The main tendon that is of interest is the Deep Digital Flexor Tendon (DDFT), which courses down the back of the limb, overtop of the navicular bone and then attaches the bottom of the coffin bone. More on this tendon in a bit and influences upon motion.
Navicular disease is very common in more stocky breeds such as the QH or Appaloosa, in which the horse may be actually carrying more weight on the front limbs. It is also very common in horses with a foot that is too small genetically speaking or trimmed too small in relation to body size. Hoof conformation also plays a major role in the syndrome development and the general conformation of long toe, low heel actually predisposes to this condition more readily. The actual cause of the condition is not completely understood, but we do know that the above influences play a major role. Essentially what is happening is that the navicular bone is degenerating due to perceived decreased blood circulation. Pain is encountered due to this deterioration process. Now, what causes the decreased blood circulation? Excessive strain on the bone due to low heels, too small of a foot in relation to body size as well as possible genetic factors all play a role. The decreased blood circulation leads to obvious inflammation, which then contributes to bone resorption and remodeling. In some cases, the bone can fracture due to weakness.
Clinical Presentation
These horses can present for a wide range of lameness varying from very mild to very obvious. In some cases, the owners have detected a change in gait or overall movement and can't quite pin point down where the issue is originating. In other instances, maybe there is a decreased performance, decreased times in various events or an obvious lameness or reluctance to pick up a gait. Navicular horses are reluctant to put excessive weight on their front heels. When you watch them walk, they are generally stiff, don't reach forward very well having a shortened anterior stride. When they turn, they will shift their weight to the rear limbs, much like a laminitic horse. If you force them to trot or canter, their stride is generally short and choppy.
On physical exam, some of them are quite painful. Often they will have an increased digital pulse to one or both feet. Hoof testers are commonly very positive in the heel region which helps to locate the source of pain. Evaluation of the hoof conformation also is generally a tell tale indicator, as most of them have low heels and overriding the shoes if they are shod.
Diagnostics include not only a physical examination, but also x-rays to determine if there is degeneration present. Nerve blocks are also performed to locate the source of pain. Sometimes, ultrasounds as well as venograms are performed. Here is a radiograph of a low grade navicular horse.
This is the AP or front view of the navicular bone. Again, I have the bone indicated by a yellow arrow. What we are looking for is changes in shape, thickness of the cortical wall as well as any evident degeneration. In this particular horse, I have two red arrows pointing towards the front border of the navicular bone. Here we can appreciate a roughening of the bone border as well as small, black holes that are present within the bone, which indicated bone loss or degeneration. I often refer to these as "lollipop" lesions as their is usually a black "stick" as well as black "head" to the sucker lesion. This particular horse was not that bad in terms of lameness, but there was evident degeneration on x-ray.
Treatment Options
A diagnosis of navicular disease is not an end, but one does have to realize that it is not reversible but can be managed in most instances. First, we have to look at the hoof conformation and correct it when needed. Understand that the DDFT is coursing on the back side of the navicular bone and if the heel is too low, then the tendon is stretched and puts excessive pressure upon the bone, which may compromise blood flow and overall health. The same holds true for a large horse with little feet. In those cases, the heel region is not of adequate size to withstand the weight of the horse. In cases of low heels, we will often times apply wedge heel shoes to take the strain off of the DDFT. In other cases, one may choose an eggbar shoe, which allows the horse to distribute more weight caudally beyond the heel region to the shoe, which relieves pressure.
In other cases, we will use NSAID medications such as bute to help reduce inflammation and pain, but we have to understand that these medications are only addressing a small portion of the inflammatory response as we have discussed before. Some vets will use Isoxsuprine which is intended to increase blood circulation. Others will use Pentoxifylline, which increases the flexibility of the red blood cells and potentially allows them to squeeze into tight spaces delivering oxygen. Both of these medications are used heavily in humans with poor circulation and do a good job, however, personally I have not witnessed tremendous benefit in horses.
In severe cases, navicular bursal injections of corticosteroids may be performed. The intent here is to deliver the medication very close to the bone. Again, we are not solving any problems with this treatment in my opinion and just buying time through mild inflammation reduction. In some cases, cold laser therapy or shock therapy may be employed, often times with little results. The last ditch effort has always been a surgical procedure called a neurectomy, which is a severing of the nerve to the region resulting in a loss of sensation to the heel region. Obviously, there are no therapeutic effects here, but we are just long term numbing the region. There can be some serious side effects to this procedure, sometimes with the horse ending up worse than what they were before the procedure.
Conclusion
Navicular disease is a common and serious condition. If caught early enough, I feel that may horses can do very well with long term changes. Hoof conformation as well as body size of the horse are major players in the development of the disease and one must be conscious of how their horse is trimmed and shod at all times. I agree that there is a blood circulation issue at the root of the problem, but also believe that the inflammatory process plays a major role.
We have used our Cur-OST products, specifically our EQ Plus and EQ Green formulas in many cases of navicular disease with pretty profound results. Curcumin and boswellia are potent inhibitors of inflammation from a much higher level than NSAIDs or even corticosteroids, helping to modulate the pathway from multiple perspectives. The antioxidants and nutritive herbs in the formulas help to combat free radical damage to blood vessels as well as surrounding tissue, plus provide many nutrients that aid in overall health. Interesting enough, curcumin has been shown in several research studies to impact bone remodeling and degeneration by affecting osteoclast activity. Overall, our findings have been that the Cur-OST formulas improve quality of life, overall function and reduce pain for many of these patients.
Here is a video of an Appaloosa mare that has navicular disease and responded well to Cur-OST.
http://www.youtube.com/watch?v=6cmW87xoYEI&list=UUosRnnrpSG6xjimXii3JV4A&index=1
All our best.
Tom Schell, D.V.M.
www.curost.com
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