Navicular syndrome is one of the most common causes of intermittent forelimb lameness in horses. It is the inflammation or degeneration of the navicular bone and its surrounding structures within the foot. The structures may include the navicular bursa, deep digital flexor tendon, distal impar ligament amongst many others.
Clinical signs include
- Chronic intermittent forelimb lameness affecting one of both feet
- Pointing of the affected foot at rest
- Hoof abnormalities
- Low, under-run heels
- Broken back hoof pastern axis
- Medial lateral (inside to outside) foot imbalance
- One foot may be smaller than the other (usually the leg that is the most lame)
The diagnosis is based on a series of investigations.
- Clinical examination
- Response to nerve blocks
This is an x-ray of a horse’s hoof with a reasonably good foot conformation. The labels point out some of the important structures involved with navicular syndrome.
A. Deep digital flexor tendon (DDFT)
B. Navicular Bursa (fluid sack at back of bone)
C. Navicular bone
D. Pedal (coffin) bone
E. Coffin Joint
This is an example of a horse’s foot with a broken back hoof pastern axis. It refers to the angle of the pastern bones relative to angle of the hoof. The white lines should be one continuous line. In situations like this it puts more pressure on the back of the leg where the deep digital flexor tendon (DDFT) is. This in turn affects the navicular bone and other associated structures. This horse also has a flat pedal bone that is angled back which will also impact on the navicular apparatus and DDFT.
Full clinical examination to include:
- Examining the horse in the stable and stood outside
- Examining the horse’s feet whilst standing and non-weight bearing
- Examining the horse at walk and trot in a straight line
- Flexion tests will also be performed
- Examining the horse on the lunge, preferably on a soft and a hard surface depending on how lame the horse is
Nerve block – a palmar digital nerve block will be performed on the most lame leg. This involves injecting a small volume of local anaesthetic around the nerve to localise the source of the pain. The horse will be re-evaluated within 5-10 minutes
Coffin joint block or navicular bursal block – based on an improvement to the first nerve block, at a minimum of 24 hours later, either one or both of the above blocks can be performed to further localise the site of discomfort
Following on from the nerve blocks, x-rays will always be taken of the horse’s feet. A full foot series will include images from the front and side of the feet as well as special views of the navicular bone. Below are some x-rays of the navicular bone. It is always superimposed on the other structures of the foot because we are only able to x-ray in 2D.
A. The black box roughly defines the outline of the navicular bone. On this particular horse there is a large lolly-pop shaped cyst (black arrows) and a smaller similar shaped one to the right (white arrow). The navicular bone on the other front leg was unremarkable.
B. This x-ray of the same view of the navicular bone as above has less dramatic changes but nonetheless still clinically significant. The black arrows point to little invaginations. A small number can be normal but these are larger and more numerous.
X-rays are a very useful diagnostic aid. However they are not able to demonstrate any concurrent soft tissue injuries and in some circumstances are not always able to detect very subtle bony changes. The gold standard diagnostic tool therefore is an MRI scan. This involves referring the horse to a specialist place that has an MRI scanner. Below are some images with their interpretation of what is going on with the horse.
Above, these images from left to right show a small tear in the DDFT, the second picture shows a synovial mass (thickened lining) in the navicular bursa and the final image demonstrates erosion of the cartilage within the navicular bone
This image demonstrates a mild tendonitis (inflammation) within the DDFT
MRI is a very specialised diagnostic tool and requires experts to study and interpret the images. The result is an accurate description of the pathology within the foot and a resultant treatment plan and prognosis.
Treatment involves several options which hopefully work together to produce a positive outcome. It will depend on various factors as to how many of them are chosen.
Rest and controlled exercise
Rest is important to allow any soft tissue injury to heal or inflammation to subside and to allow the navicular bone to begin the remodelling process. Box rest can vary from as little as 3 weeks to as long as 12 weeks depending on the extent of the pathology.
A controlled exercise programme may involve:
- 10 minutes twice daily in hand walking week 1
- 15 minutes twice daily in hand walking week 2
- 20 minutes twice daily in hand walking week 3 and so on….
Some horses will be turned out in a restricted paddock in this time.
Corrective trimming and shoeing form a large part of the treatment process for navicular disease. This alone may result in the biggest improvement to the lameness. This may include:
- Reducing the toe
- Altering the hoof pastern axis
- Heel support – egg bar shoe, heart bar shoe +/- heel wedge
- The farrier will utilise the x-rays to achieve the best result
NSAIDs – e.g. Phenylbutazone (Bute) for 10-14 days
Corticosteroids – these would be injected into the coffin joint or the navicular bursa provided that there is no concurrent tendon pathology. Only an MRI scan would be able to detect this. Concurrent Hyaluronic acid (HA) is usually injected.
PSGAGs e.g. Cartrophen – intramuscular injections once weekly for 4 weeks
Neutraceuticals – Glucosamine hydrochloride at a minimum maintenance dose of 10,000mg has shown to have some benefit. Chondroitin sulphate and HA are poorly absorbed across the gut so are not beneficial within a supplement.
Equidronate (Tiludronate) – this inhibits the bone eating cells (osteoclasts) and allows the osteoblasts to catch up on the healing.
In some cases surgery should be carried out both from a diagnostic and therapeutic perspective. There are two possible surgeries the second of which is a last resort.
This involves a ‘scope being inserted into the navicular bursa under general anaesthetic
Palmar digital neurectomy
This involves cutting the nerves to the foot to remove the pain sensation. This has welfare implications however.
It can be seen that there have been many medical and surgical advances over the last few years regarding navicular syndrome. It is a condition that if well managed can be controlled and the horse can be in a level of work.