What is Navicular Disease
Navicular disease (also called navicular syndrome or navicular bursitis) is one of the most common causes of chronic forelimb lameness in horses.
The condition is progressive and, although it may be controlled by treatment, as yet it cannot be cured. Although the symptoms may vary from horse to horse, inflammatory reaction in both the navicular bursa and the navicular bone cause pain and dysfunction.
Navicular disease is one of two main problems affecting the navicular bone.
The other is fracture of the bone, although the first condition may lead to the second.
The navicular bone is a small boat shaped bone which can cause tremendous problems to a horse. It lies deep within the hoof and nestles behind the last two bones in the foot. It is a type of bone known as sesamoid and there are a number of these bones in the limbs.
Their function is usually to increase the pull of a tendon, particularly in the early stages of movement when a limb is either flexed or extended.
The tendon involved in this case is the deep digital flexor tendon. To find this, run your hand up from the fetlock behind the front leg
until you can feel the taut, bar like tendons. There are clearly two. The deep digital flexor tendon is the inner one of these.
A small bag of fluid lies between this tendon and the navicular bone. This is called the navicular bursa.
Navicular Disease Symptoms
This is a progressive forelimb lameness, developing over many months. It is a condition with many different early signs, which can be difficult to spot.
Recurrent bruising of the sole on the affected feet may signal a problem. This is because affected horses tend to throw more weight onto their toes to avoid navicular pain. Vague, uncertain, and on and off periods of forelimb discomfort may be an indicator. Although the problem may appear to be in one limb only, it is usually in both forefeet but it is difficult for a horse to demonstrate classical signs of lameness in both forelegs at the same time.
The less severely affected leg will mask the severity of the signs of the other limb. Even in quite advanced disease lameness may seem relatively mild, but the gait is strange. Sudden, severe unilateral lameness may occur when one navicular bone weakens and fractures. The fracture is the last straw and a sequel to an undiagnosed problem.
As navicular disease progresses the feet change shape, again because the animal throws the weight onto the front of the foot to ease the pain. The foot grows smaller, narrower and higher in the heel. It becomes more boxy and the frog gets smaller. Therefore, it is useful to compare the size of the two front feet.
What Causes Navicular Disease
But there are three main ideas on what happens
Inadequate blood supply to the navicular bone causes weakness.
Abnormal stress placed on the bursa and the bone by the overlying tendon causing inflammation and decay.
Degeneration of the bone and surrounding tissue leads to arthritic change.
Navicular disease is seen in horses over five years old. There is usually a history of overwork and too frequent exercise, particularly on hard ground. Poor quality foot trimming can also add to the problem.
This is a rare, but serious condition. Animals show sudden, acute lameness. There may have been
a traumatic incident, but the crisis can occur secondarily to navicular disease. X-rays provide a definitive diagnosis.
Navicular fracture is tricky to treat, as the constant movement of the overlying tendon causes instability and poor healing.
Surgery may resolve the problem in selected cases, while others respond to rest and corrective shoeing. Sadly some cases fail to respond
at all. Healing is easy to judge as soundness returns and X-rays reveal good bony healing.
Nerve blocks and regional local anaesthesia may further progress the diagnosis.
How is Navicular Disease Treated
Treatment is dependent on the stage of the disease. Developing a clinical plan with long term thinking is the key
to helping your horse. Some treatments involve:
Good farriery
Correction of any foot imbalance is important. The goal is to create a wide, long and raised heel and short toe.
Supportive shoeing is also useful. In the early stages of the disease, good farriery and gently increasing exercise may be enough
to stabilise the situation.
Non Steroidal Anti-Inflammatory Drugs
NSAIDS, such as bute, will relieve the inflammation and pain. These can be particularly useful if used a day or so before
any anticipated work. It must be remembered that these drugs are not curative and their effects must not be abused.
Drugs that improve the circulation of blood to the foot can be useful. Warfarin may be an option, but this needs careful
monitoring and is less popular at present.
Chondroproctive agents help repair the cartilage covering the navicular bone and can be helpful.
A number of surgical options exist
Certain ligaments may be surgically cut to change the pressures on the navicular bone, reducing the pain.
This is called navicular suspensory desmotomy. Although this approach has promise, any improvements seen are unlikely to be long term.
Alternatively, nerves supplying the area may be cut to block sensation in the bone (neurectomy). This technique is palliative (it lessens pain but does
not offer a cure) and must be regarded as a last resort.
Unfortunately, the loss of sensation that occurs after a neurectomy may, with overwork, hasten the deterioration of the navicular bone.
Navicular disease may not be the obvious cause of lameness at the beginning, due to the insidious nature of the condition.
Other conditions that initially look similar are:
Puncture of the sole
Fracture of the pedal bone
Pedal ostetis
Ringbone
Even laminitis or early ringbone
The truth of the matter frequently becomes apparent as time passes and the condition progresses.