By Cole Henderson

Navicular disease, now referred to as navicular syndrome, chronic heel lameness, or caudal heel syndrome, was first documented in 1752 by farrier Jeremiah Bridges in his famous book No Foot, No Horse (published some 40 years before the opening of the Royal Veterinary College in London, England). 

What is this mysterious and feared disease that even today may lead to the recommendation that a horse be humanely euthanized? 

If you look at any veterinarian or farrier textbooks, you will be hard pressed to find two experts who agree on the exact science of the cause and effect of navicular disease. It has certainly kept book publishers busy for the last 260 years. The main problem has always been that researchers have been unable to fully reproduce navicular disease in experiments; this has led to endless speculation about what causes horses to experience this condition.

Navicular Anatomy

In order to grasp what the problem is, it is necessary to know something of the anatomy of the horse’s hoof.

Navicular usually occurs in the front feet. The navicular bone is a small bone held in place by ligaments as part of the coffin joint. Here it acts as a fulcrum to increase the leverage on the foot. The navicular bursa is located between the deep digital flexor tendon (DDFT) and the articular cartilage of the navicular bone. Its job is to lubricate the surface of the bone so it can act like a pulley. The DDFT becomes wide and thin as it passes under the bursa and attaches to the coffin bone. It is in this complicated area of bones, tendons, and ligaments that the horse experiences the heel pain we call navicular syndrome.

While navicular disease does not directly shorten the life of a horse, it does cause it to suffer terrible heel pain. Over the years, the farrier world has developed many ways to help relieve this pain and make the horse more comfortable. In order to understand the treatments, you need to know the main theories regarding the causes of navicular disease. As long ago as 1752, navicular disease was attributed to a “lack of exercise resulting in contraction of the foot” by Jeremiah Bridges. Many recent studies have only gone on to prove his theory accurate.

First (and older) Theory

Lack of blood circulation is thought to cause bone degeneration to the navicular bone. The theory is that when a horse is exercised, the blood supply to the foot is good, but when a horse is simply returned to a stable afterwards, the blood supply is decreased. It is these disturbances in the blood circulation that are believed to lead to this degenerative process.

It is also thought that concussive or compressive forces coming through an excessively upright foot could cause this blood supply damage to the navicular bone.

Whatever the cause, it is believed that the damaged navicular bone then causes inflammation to the DDFT and other ligaments surrounding the bone, causing heel pain.

However, when researchers have reduced blood supply in horses’ feet to replicate this damage to the navicular bone, they saw no changes, which puts this theory in doubt. It has also long been known that many horses have degeneration of the navicular bone but do not experience navicular disease, and similarly many horses with navicular disease show no damage to the navicular bone.

Second (more recent) Theory

Dr. Bowker of Michigan State University, a veterinarian at the forefront of hoof studies, has done extensive studies on the back of the equine foot. Dr. Bowker found that in foals, the lateral cartilages that form the foundation of the rear half of the foot are less than 1/16th of an inch thick. These lateral cartilages develop as the foot grows by expansion, flexion, and twisting of the hoof capsule. 

In his comparisons between domestic horses and feral horses, he found that in adult feral horses these lateral cartilages were almost an inch thick and that these horses had a solid floor of cartilage between the frog corium and the digital cushion. In domesticated horses, many foals are kept in small paddocks and on soft terrain; the result is that many adult domestic horses are found to have lateral cartilages as thin as an eighth of an inch. At the same time, the development of the digital cushion in domestic foals falls behind, either through being continually kept on soft terrain or through the tendency to neglect the trimming of foals’ hooves. Many are left to grow long upright heels, reducing frog pressure throughout the growing foal’s life. In adulthood, this results in a back foot too sensitive to be the “landing zone” it was designed to be, so these horses start loading their toes first to avoid discomfort.

Cutaway comparisons of a healthy foot and an unhealthy foot. Note the difference in the size of the digital cushion below the DDFT and navicular bone.

When a horse is said to be sensitive on rocky terrain, people blame the soles. But in fact, almost every time horses shorten their stride, land on their toes, and lean forward, they are avoiding landing on their sensitive frogs and heels. Dr. Bowker found that the more horses try to move this way, the weaker their hoof structures become. When a horse lands on its toes, the weight of the horse then slams into the navicular bone as the heels crash down. This damages the impar ligament, which supplies 80 percent of the blood supply to the navicular bone. 

Michigan State University also blames bone loss on the lack of natural pressure in the heel region from continued toe-first landing. Dr. Bowker is also finding that navicular horses commonly have 40 to 60 percent bone loss in the coffin bone as well.

Placing more doubts on the first theory, a veterinary pathologist called Dr. James Rooney produced a book called The Lame Horse with his findings from studies on thousands of dead horses. By simulating toe-first landings in test machines with dead horse legs, Dr. Rooney found that the order of damage occurring around the navicular bone is to:

1. The fibrocartilage surrounding the navicular bone;
2. The fibrocartilages surrounding the deep flexor tendon;
3. The deep flexor tendon; and, finally,
4. The navicular bone from the rough surface of the damaged deep flexor tendon.

Dr. Rooney’s findings date back to at least 1974.

Although both these doctors have differences in their findings, they do agree that navicular disease is a result of continued toe-first landings by the foot and lack of use of the back of the foot.

Treatment by Shoeing

Over hundreds of years there have been various treatments by farriers to “cure” navicular disease. You will still see many of these used today as they are still taught in many farrier schools.

Most shoes have been designed with the idea of easing the DDFT tendon by easing its work. This has involved high heels, rolled toes, and rocker toes. From experience, I can say that these shoeing treatments (relating to the first theory above) can help a horse be comfortable for months or years, but eventually they do fail and the horse will be even worse than before. At this point these horses are usually euthanized.

Wedges

Horses have been shod with wedge shaped pads between the hoof and the shoe in order to raise the angle of the foot. Previously it was thought to be great practice to raise the back part of the hoof with a wedge or calks so as to relax the flexor tendons and reduce inflammation. In practice, this has long been considered an outdated theory. By trying to raise the hoof in this way, the pressure on the heels is increased and the heels will be completely pushed in or away, resulting in them no longer performing their function. Treating a horse like this only makes the navicular disease worse.

Wedges have long been used as a treatment for navicular disease. However, while they may provide some comfort, they will eventually cause the navicular disease to worsen. Photo courtesy of Cole Henderson

Calks

Shoes with fixed calks are heavy and can cause the horse heel pain. Photo courtesy of Cole Henderson

A second example of how horses have been treated for navicular disease with shoes is with fixed calks. These are horseshoes with a heightened band of iron or steel welded on the branches. Again, the idea is to raise the back of the foot to relax the flexor tendons. Using a shoe with small calks does not work as the calks just sink into the ground.

Because this shoe adds too much weight to the back of the hoof, one of the side effects is that it gives a release of kinetic energy when the hoof moves forward. Because the weighted branches have more kinetic energy than the toe part, when the hoof is set down this creates a shock to the inflamed area which is even more uncomfortable for the horse.

Some horseshoe manufacturers still produce an aluminum version of this as a “navicular shoe,” which does eliminate the weight issue, but it is still putting the same pressure on the heels as wedges do.

Hoof Pads

Shoes with hoof pads and filled with silicone give good support over the whole hoof and engage the frog. Photo courtesy of Cole Henderson

A third example of treatment is a horseshoe with a hoof pad between the hoof and the shoe. This has the effect of evenly distributing the ground pressure over the whole hoof and engaging the frog (which the previous two examples do not do). The space between the pad and the hoof is usually filled with fast setting silicone, which gives a good even support over the hoof. The main disadvantage of a horseshoe with a hoof pad attached is the weight it places on the foot. 

Egg Bar Shoes

There is no proof that egg bar shoes solve or improve navicular disease. Photo courtesy of Cole Henderson

These shoes are, as the name implies, egg shaped and when they are applied the bar should lie in a vertical line with the bar not touching the frogs. When the back of the bar touches the ground first followed by the hoof, it is like walking in ski boots for the horse. These shoes create a large amount of kinetic energy when the horse moves and there are no indications or proof that these shoes either solve or improve the condition.

Bar Shoes

Bar shoes are light and give good support to the heel area, but today there are better alternatives. Photo courtesy of Cole Henderson

Another shoeing treatment is the use of bar shoes. This is a broad, long shoe with a thin metal bar at the back to immobilize the shoe’s movement. This lack of movement helps to equally load the hooves when the feet are turning. They are lighter than egg bar shoes and give good support to protect the sensitive heel area, but nowadays there are better alternatives.

Synthetic Shoes

These are plastic bar horseshoes of material designed to reduce concussion through the hoof and limb. They are much lighter than metal shoes and, importantly for horses that are in a lot of pain, they can be glued into place rather than requiring nailing. They support and protect the heels while engaging the frog and building the digital cushion. Synthetic shoes are by far the most successful form of treatment if trimming alone is not successful.

Treatment by Trimming

Again, we must go back to the theories relating to the cause of heel pain. When heels get long and under-run, as they often are in navicular horses, the weight bearing area of the heels is moved forward until it is directly below the navicular bone rather than behind it as it should be in healthy horses. Contracted heels subject this area to more active pressure than it is designed for. With this unnatural pressure damaging the bone, surely it is not the bone loss that is causing the horse pain, but the pressure creating the bone loss that is causing the pain.

There are very successful ways of dealing with navicular disease by careful trimming. The horse’s sole should be left alone while the heels are gradually trimmed down to increase pressure on the frogs. This isn’t an article about how to trim so I won’t go into details, but trimming has to be done at the correct pace for the horse. Too slow and no difference will be made; too fast and the horse will be sensitive in the back of the foot and continue to walk on his toes. This job is what separates the good hoof trimmers from the hackers.

If the horse is still very uncomfortable following a trim, the use of hoof boots will protect the feet until they are less sensitive. Pads in the boots can make these horses comfortable when nothing else will.

New synthetic shoes are lightweight, support and cushion the whole hoof, and don’t require nailing, making them a good option for horses with navicular if shoes are required. Photo courtesy of Cole Henderson

When the frog begins to be engaged again, it will start to pack into a dense callous and become the weight bearing surface it should be. Only by stopping the toe-first landing and making the horse comfortable enough to place its heel down first can you hope to improve navicular horses.

Properly trimmed, the frogs and digital cushions will toughen up and the horse will get more comfortable. Any damage to the navicular bone will remain, but the horse probably won’t know about it. The horse won’t be the same as a horse that hasn’t gone through this, but it should be sound and usable. In much the same way as we humans collect injuries and pains through our lives, horses do as well. Get them comfortable enough and they will live with the minor aches and pains.

Conclusion

There are no miracle cures for this problem and every horse seems to respond differently to trimming or shoeing. The repair process can be very much trial and error. I have seen some horses go sound from a first trim and others take months to improve.

If Dr. Bowker’s theories are to be believed, the bottom line is that navicular disease is preventable. By allowing foals and young horses to have lots of exercise and time on dry, rough ground at an early stage to develop their frogs and digital cushions, we could be preventing these horses from toe-first landings altogether. Watch that your young horses are landing heel first.

Cole Henderson is an eighth generation journeyman farrier and a contributor to the American Farriers Journal. He operates a farrier service on Vancouver Island, BC.

Main photo: Make sure your horse is trimmed or shod correctly and is landing heel first, especially as a young horse, to prevent navicular disease. Photo credit: ©CanStockPhoto/Kyslynskyy