Navicular Syndrome

navhoof

 

Navicular Syndrome can affect any horse; however most cases tend to occur in heavy horses with upright pasterns and small hooves. The poor conformation of these horses means that the heel is taking on too much weight and the force isn’t dispersed throughout the entire hoof as it should be. In fact, anything that adds more weight to the heel and reduces contact between the frog and the ground can cause Navicular. Improper farrier care can easily lead to these issues, as well as disciplines that involve a lot of front leg impact (jumping, barrel racing etc.).

 

Structures

COTH-navicularThe main bone inside the hoof capsule is the Distal Phalanx (P3), commonly known as the coffin bone. The Middle Phalanx (P2) exists partly inside the hoof capsule and also makes up part of the pastern. In the back of the hoof capsule, between P3 and P2, is where the Navicular bone lies. Directly behind the Navicular bone is the Navicular Bursa, which acts as a cushion between the bone and the Deep Digital Flexor Tendon (DDFT). When a horse takes a step, the DDFT slides over the Navicular bone and Bursa, acting like a pulley.

Horse owners tend to use the term Navicular Syndrome to describe any injury involving the bone, but there are actually several different types. Navicular Bursitis refers to inflammation of the Bursa and the adhesions that can develop between the Bursa and the DDFT. True Navicular Disease occurs when the Navicular bone degenerates and is forced to re-model. Along with the above types, Navicular can also refer to arthritis in the hoof and any associated ligament damage. Most of the time when a horse is diagnosed with Navicular Syndrome, there tends to be more than one issue.

 

Biomechanics

heel
Fig. 1

Every time a horse takes a step, he must land heel first (Fig. 1). As the heel spreads and the hoof rolls forward, the frog hits the ground and the digital cushion compresses, which causes proper blood flow through the hoof and into the leg. The last structure involved in a step is the toe as it rolls over and leaves the ground (Fig. 2).

 

breakover
Fig. 2

 

toefirst
Fig. 3

One of the mistakes made with foals is keeping them on soft bedding all the time, preventing the tendons in the heel and the digital cushion from developing correctly. The foal quickly becomes too heavy for its underdeveloped heels and that’s when the pain starts. To avoid pain with each step, he will start landing toe first (Fig. 3) and the development of the heel will grind to a halt. This toe-first landing due to heel pain can easily cause Navicular Syndrome in the future. By landing toe-first, a lot of strain is put on the DDFT, and the fibrocartilage surrounding it becomes damaged. The DDFT itself is quickly compromised and becomes rough and can tear. Each time the horse takes a step, the damaged DDFT rubs against the Navicular bone and starts to wear it down. The bone then begins the re-modeling process to escape the jagged DDFT.

 

Fixing the problem

Proper farrier care is arguably the most important course of treatment for Navicular Syndrome, but not all farriers know how to help a horse with such bad heel pain. It is easy to cover up the problem with bar shoes and pads because of the pressure the bar puts on the heel. It is thought that raising the heel with a bar shoe provides slack to the DDFT and takes pressure off the Navicular bone. This is only the case if a horse is working on a soft surface since the heel will sink far lower than it normally would on a hard surface and needs the extra support. Unfortunately, shoeing a horse with heel pain in a bar shoe means the frog will quickly fall out of function and the blood circulation will become worse. This poor blood circulation will actually lead to a faster degeneration of the Navicular bone and associated tendons.

good angle
Palmar Angle = 4 degrees

In reality, the horse needs the heel to be lowered and needs to have a shorter breakover to keep stress off of the bones and tendons. As the heel drops, the pressure on the frog will increase and the hoof will eventually return to a “correct state”. When the horse walks, the heel will hit first and the toe last, quickly breaking over and raising the hoof off the ground. Bringing the heels down must be done slowly to avoid pain, and the horse must be kept on hard footing to keep the Navicular bone from sinking too low. There is a catch, however. Although the heel must be brought down, it must be done without lowering the Palmar Digital Angle of P3! A correct palmar angle of a front hoof is typically between 3 and 5 degrees (this does vary from horse to horse). By keeping the Palmar Angle positive, there will not be added stress to the Navicular bone and surrounding structures, even though the heel itself is lower.

Proper trimming and shoeing can take some time to ease Navicular pain, and NSAIDs are often given to keep the horse comfortable during that time. Even when the hoof is “correct”, there may still be some irreversible damage and chronic pain associated with it, and the horse might need to remain on an NSAID indefinitely.

As a last resort, with a horse that is still in pain despite proper shoeing and medication, a Palmar Digital Neurectomy may be required. During the surgery, the Palmar Digital nerve is cut to deaden the feeling at the back of the hoof. The horse will no longer be able to feel pain from any issues associated with the Navicular bone. As with any surgery, there are risks that should be thoroughly researched before making a decision.

Once thought of as a career-ending syndrome, or worse, a death sentence, many horses have now been able to return to full/partial work or are comfortably retired. Proper trimming (and shoes if needed) has been shown to restore proper hoof function and has certainly changed the outlook for both horses and riders.

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