Know Your Anatomy. The term “navicular” refers to problems emanating from the navicular bone or the soft-tissue structures around it, including the navicular bursa that lies in between the bone and the deep digital flexor tendon in the foot. In addition to the bursa, the navicular bone is covered by cartilage (the shock-absorbing layer between all bony junctions in the body).
The navicular bone is held in place by the impar and the navicular suspensory ligaments. Also called the “distal sesamoid” bone, the navicular bone sits deep in the heel region, behind the coffin bone and long pastern bone and overlaid by the deep digital flexor tendon, which stretches around it to attach on the bottom of the coffin bone.
The navicular bursa is a jelly-like shock-absorbing sac that shields the navicular bone from the deep digital flexor tendon as it slides back and forth with the flexion and extension of the limb. The bone and bursa are tightly packed within the hoof capsule.
Navicular Is a “Syndrome.” A syndrome is a number of varying malfunctioning elements in the body that that come together to cause an illness or disorder. A disease, on the other hand, is clear-cut with established, consistent changes occurring in the body.
A relatively small percentage of horses have navicular syndrome, yet virtually all horses have one or more of the conditions that can predispose a horse to it. That’s why we suspect there’s still a lot unknown about navicular syndrome and why it occurs in some horses and not in others. That said, these are established risk factors:
1) Long toes: If a horse has a long toe, he must pull that much harder with his triceps in order to flex the limb and get the heel to leave the ground as he moves forward (also called “breakover point”).
Imagine opening a can of soda: If you use the end of your finger (not the nail), you have to exert a certain amount of force to get the can open. Now imagine having a long finger nail and using it to open the can. You would have to exert a lot more force to duplicate the motion.
So, the longer the horse’s toe, the more force must be exerted to achieve breakover and forward movement. And thus more irritation to the navicular bursa.
2) Low heels: To get an idea why low heels are a bad thing, go stand on the edge of a stair step while holding the rail and stretch your heels down. Move from that position up onto your toes and then back down again, like you’re doing push-ups with your feet. Feel the burn? A similar sensation can be experienced by horses with low heels. Sometimes called “underrun,” “crushed” or “contracted” heels, this condition is most simply noted by the coronet band sloping toward the ground when you view the hoof from the side. The coronet on a horse with normal hooves should run parallel to the ground.
3) Big body, tiny feet: Heavily muscled, stocky horses, like Quarter Horses, sometimes have tiny feet that aren’t built to hold up to the forces from the horse’s body.
4) Obesity: Along the same line, overweight horses increase stress to the navicular bone. If they’re unlucky enough to also have disproportionately small feet, the problem is even more likely to occur.
5) Steep/upright pasterns: The normal around 50-degree pastern is the leg’s “shock absorber.” Horses with a steeper angle are predisposed to navicular because of the increased concussion on the ligaments holding the bone in place.
6) Workload: Jumping, working on steep slopes, or running and then stopping hard are all potential risks for a horse developing navicular syndrome.
Signs of Navicular. Horses with navicular pain almost always have “caudal heel pain,” which is near the heel bulbs. Horses with navicular syndrome classically land “toe-first” and have short, choppy, uneven strides (almost like they’re tip-toeing across broken glass). They stumble frequently and worsen over uneven, hard, rocky ground. They tend to be only mildly lame, but it’s usually bilateral and in the front feet.
In contrast, horses without caudal heel pain land either flat with their hoof on the ground or ever so slightly heel first. They take solid, even steps and move out well. Over time, if the horse changes the way he lands on his foot in an attempt to avoid heel pain, the hoof will change shape. Heels will often become further under run and contracted, and the hooves may take on an oblong shape.
Making the Call. A veterinarian diagnoses navicular syndrome using a combination of clinical observations and diagnostic procedures. The most common test for pain is with hoof testers used by a skilled individual. Virtually all horses, if squeezed too hard, will react to hoof testers (the proper application of hoof testers isn’t as easy as it looks).
The vet will also watch the horse move at the walk and trot on a straight line and in a circle in both directions, on hard and soft ground.
The next step is diagnostic nerve blocks. The veterinarian will strategically inject lidocaine to numb the navicular area and the back of the foot (a palmar digital nerve block). If your horse is experiencing caudal heel pain, he’ll move out differently within minutes of a lidocaine nerve block. Unfortunately, the nerve blocks only last about an hour.
Finally, your veterinarian will take digital radiographs of the feet to look at the navicular bones for signs of degenerative changes, including bone cysts, enlarged vascular channels, asymmetry in the bone, or evidence of bone loss and/or production. (MRI or CT scanning also can be used to definitively diagnose navicular syndrome.)
Getting Through the Woods. Unfortunately, changes to bone can’t be reversed and soft-tissue injuries to the bursa, cartilage and ligaments are difficult to resolve, so navicular can’t be cured. However, it can be managed by focusing on three main cornerstones: corrective hoof trimming and shoeing, pharmaceutical therapy, and care/management practices.
• Farriery. How the hoof is trimmed and shod will make the biggest difference, so you need a farrier trained in the recent approaches to trimming and shoeing navicular horses. Navicular horses move out instantly better after being trimmed and shod effectively. Although no two navicular horses will be shod exactly the same, these are the main principles:
1. Shorten the toe to pull back the breakover point.
2. Avoid over-trimming the heel region or the frog.
3. Create optimal bone alignment and angles in the distal extremity in order to avoid uneven forces and excessive tension on the deep-digital flexor tendon (wedge pads may be necessary).
4. Make sure that the hoof is trimmed level and symmetrical to keep mimimize uneven forces.
5. Use full bar shoes with or without pads for at least the initial shoeing cycles to protect the heel.
• Pharmaceuticals. Non-steroidal anti-inflammatory drugs (NSAIDs) like bute, aspirin, naproxen, banamine and firocoxib (Previcox) are commonly used in navicular horses. Of course, long-term use of these medications can cause ulcers, so they aren’t a solution by any means. Firocoxib is less likely to cause gastric issues.
Isoxsuprine and pentoxifylline cause vasodilation and increase blood flow and were widely used in the past, but their efficacy seems transient. Many vets now use them on a “month-on, month-off” basis.
Perhaps the most significant breakthrough in pharmaceutical therapy for the management of navicular syndrome is Tildren (Tiludronate), a bisphosphonate that decreases bone resorption by inhibiting the activity of osteoclast cells in bone, thus strengthening the bone and reducing further degenerative change.
Tildren is safe and effective for the treatment of navicular syndrome and other bone-related problems in horses, but it isn’t available in the United States. However, the FDA allows veterinarians to import it under personal importation rules.
In addition to systemic therapies, injecting cortisone and hyaluronic acid into the coffin joint and/or the navicular bursa can help. These agents have an anti-inflammatory effect that can provide relief for several months at a time.
• Care And Management. How you manage a navicular horse can make a difference in his soundness:
- Keep weight under control.
- Ride judiciously. Get off on steep downhill sections and avoid rocky/uneven ground.
- Keep shoeing intervals short (every six weeks) to avoid excessive toe growth.
- Keep your horse moving. If he lives in a stall, maximize turn-out and exercise, since movement improves blood flow in the hoof.
Bottom Line. Navicular can’t be cured, but it can be managed. Trimming and shoeing techniques are paramount, so a farrier trained in recent advancements is crucial. No pharmaceutical agent is without drawbacks, but Previcox and Tildren show promise.
Article by Contributing Veterinary Editor Grant Miller, DVM