Navicular disease, sometimes referred to as "Navicular Syndrome" or even "Caudal Heel Pain Syndrome", is a chronic, degenerative disease of the navicular bone and associated soft tissue structures.  Due to the location of these structures (deep within the foot, see image at left), the complicated anatomy, and the constant pressure that this area is under as the horse moves, this disease process can be as difficult to manage as it is to understand.  We'll start with an overview of the relevant anatomy, then explain how we diagnose this syndrome, and finally how we treat the various stages of caudal heel pain. 

Anatomy of the Navicular Apparatus

The equine foot is a pretty amazing structure.  While we see a hoof and frog on the outside, there is actually quite a bit going on inside.  Especially in the area of the navicular bone.  Let's go over some big players and some little players (but they can all play a big role!)

Bony structures

The bone we are mainly concerned about is the navicular bone, which lies along the back (heel) of the foot, just behind the coffin bone.  The other two important bones would be the short pastern bone (P2), and the coffin bone (pedal bone or P3), which the navicular bone articulates with.

Synovial structures

The largest joint in the vicinity is the coffin joint.  This is where all of the bones articulate - P2, the coffin bone and the navicular bone.  While the coffin joint is not often negatively affected in navicular syndrome, it can become important when discussing treatment modalities.   

The other structure that is extremely important is the navicular bursa.  The bursa is a synovial structure located between the back aspect of the navicular bone and the deep digital flexor tendon (DDF).  Since the bone acts as a fulcrum to redirect the DDF to the coffin bone, the bursa acts as a cushion to help lubricate the movement of the tendon across the surface of the bone.  This becomes very important in the disease process. 

Soft tissue structures

The soft tissue structures associated with the navicular bone include the deep digital flexor tendon, the suspensory ligaments of the navicular bone, the distal sesamoidean impar ligament (we just call it the impar ligament) and the digital cushion, which is not as important in these cases. 

The deep digital flexor tendon runs over the caudal aspect of the navicular bone.  It attaches to the distal aspect of P3. 

The suspensory ligaments of the navicular bone attach to P2 and help hold the bone in place.  In addition, the impar ligament extends from the bottom of the navicular bone and attaches to the coffin bone. 

Whew!! No wonder so much can go wrong!

Navicular disease often shows up as mild or moderate lameness in one or both front limbs.  If present in both front feet, it can cause a shuffling, short strided gait which can look like generalized soreness.  Since there are many other causes of chronic lameness/soreness, we need to have a way to determine where the pain is coming from in order to make a diagnosis. 

Lameness Exam

We begin our examination with a full palpation and examination of the limbs and hooves, as well as the neck and back.  We'll place hoof testers on the feet - these allow us to place specific pressure on the hooves, which can give us information about where in the hooves pain may be coming from.  Then we watch the horse move - generally at a trot in hand on a straightaway, and then on a lunge line in both directions.  If possible, we'll often try to trot the horse on both a hard surface and a soft surface.  The differences in the way the horse moves on hard vs. soft ground, and turning to the left vs. the right, can give us valuable information about what might be going on.

PD nerve block

Nerve Blocks

A lameness exam is a great start, but it only gives us a "hunch" about where the pain is coming from.  To prove that the pain is coming from a specific area, we have to perform a set of "nerve blocks".  These are very specific injections of local anesthetic to temporarily desensitize the nerves to a part of the limb.  For navicular disease, we perform a "palmar digital" nerve block, often abbreviated to a "PD block".  This procedure desensitizes the back two-thirds of the horse's hoof, it takes about 5 minutes to take effect, and it will last for about an hour or so.   When we trot a horse out after performing this nerve block, and the lameness improves by at least 80% or more in their way of going, we know that the lameness is coming from the back portion of the foot.  We can now call it "caudal heel pain", which can include navicular disease.   So, now what?

Radiographs

In order to begin to diagnose what is going on in the navicular area, we have to start with radiographs of the foot, including the navicular bone.  We are looking for bony changes in the navicular bone, including cysts, roughening of the articular surfaces, bony inflammation (edema), fractures, or bony resorption.  All of these changes have classically been described as "navicular disease", so if we see them, we have a diagnosis.  But remember, a large component of the anatomy in this region is soft tissue (tendon/ligament), which will not show up on radiographs.  Ultrasound is how we prefer to look at tendons/ligaments, but deep within the foot, ultrasound is of limited usefulness.  So what then?

MRI

From thehorse.com

The "gold standard" for full diagnosis of navicular disease is an MRI of the lower limb.  This procedure is expensive, and requires travel to a facility such as Tufts, Fairfield Equine or New England Equine.  Depending on the facility, it can be done under full anesthesia or under standing sedation.  The information gained can be helpful in determining treatment and prognosis, especially if significant soft tissue injuries are present.  If an MRI is not an option due to financial constraints, we often assume that there is a soft tissue component, and factor that into our treatment regimen. 

Heel Support

When we first diagnose caudal heel pain in a horse, perhaps without the evidence of any navicular bony changes, we generally recommend a combination of heel support, anti-inflammatory therapy (such as Bute), and a period of rest.  It's important to point out that there is no "one-size-fits-all" recipe for heel support in these horses.  Some horses just need to be trimmed a bit differently, others require heel support in the form of a wedge or bar shoe.  We prefer to discuss the specifics of each case with the horse's farrier, to come up with a trimming/shoeing plan together. 

Joint/Bursa Injections

If trimming/shoeing and a period of rest and bute are not enough to decrease the inflammation in the navicular area, we often have to add in direct injection of steroid/hyaluronic acid into the foot.  We have two options for these injections.  You'll remember from above that the coffin joint contacts the navicular bone on two surfaces.  The coffin joint is fairly straight-forward to access for direct injection, so we can take advantage of that to inject the coffin joint with steroid/hyaluronic acid, and it often will diffuse to the navicular region.  This can be a powerful way to decrease the inflammation. 

Bursal Injection (U. of Minnesota)

If a coffin joint injection is not enough to decrease the inflammation, we also have the option of directly injecting the navicular bursa.  Due to the location of this bursa, this is a much more technically difficult injection, and requires a long needle placed through several sensitive structures under radiographic guidance to check for proper placement.  This is why we usually opt for a coffin joint injection first, only moving to a navicular bursa injection if we have to.  

We nearly always couple this type of therapy with heel support, a short course of Bute, as well as a period of rest, depending on the level of discomfort. 

Tildren

A newer therapy, Tildren, has been available for several years overseas, and only available in the US through special import permits.  It is about to be released in the US in an FDA approved form.  Tildren is a systemic medication, given intravenously in a slow infusion at one visit.  It is a type of bis-phosphonate, which is a medication class that decreases bony resorption, and can increase bone density.  (These types of drugs are often used in humans with osteoporosis).  Tildren is quite expensive, and can have potential side effects such as colic and kidney enzyme elevations, so it must be used with care - but for some cases of navicular cysts and lytic lesions, it can be very helpful to improve comfort. 

Career Change

Unfortunately, due to the degenerative nature of navicular disease, despite treatment some horses are no longer able to perform at the level which they have been, and require a career change in order to remain comfortable.  This may mean no more jumping or barrels, or it may mean light trail rides instead of 50 mile endurance rides

Neurectomy

As a last-resort salvage option, in horses with chronic pain which is not helped by any of the above treatments, we do have the option of neurectomy (often referred to as "nerving").  This procedure cuts the nerve which runs to the back of the foot - the same one we desensitized to diagnosis this syndrome.  This surgical procedure permanently desensitizes the caudal portion of the foot.  It can make a horse with chronic pain much more comfortable, but this comfort comes at a price.  Since the horse can't feel a significant portion of the foot, it will not show lameness after an injury.  Meticulous care must be taken to avoid stone bruises and thrush, and the feet must be inspected regularly for nails or other injuries.  Finally, since lameness is a protective function to decrease further injury, it is possible that a soft tissue structure which was injured as part of the disease process (such as the deep digital flexor tendon) becomes overloaded after neurectomy, resulting in rupture.  Therefore, the decision to nerve a horse must never be taken lightly.

Navicular syndrome is a progressive, degenerative disorder that can be difficult to manage.  Since there are so many structures involved, it becomes harder to direct treatment at one specific problem.  A multi-modal approach is most beneficial and our goal is to keep the horse comfortable for as long as possible within its intended career.