The navicular bursa is a small fluid-filled structure in the foot that acts as a cushion where the deep digital flexor tendon passes over the bone and changes direction. A bursa has many properties similar to a joint or a tendon sheath. Bursae are lined with a membrane which produces synovial fluid. Other common bursae in the horse include the calcaneal bursa at the point of the hock, the bicipital bursa over the point of the shoulder, and the supraspinous bursa over the top of the withers.
Where is the navicular bursa?
The navicular bursa is a small synovial structure situated between the navicular bone and deep digital flexor tendon (DDFT) within the foot. It measures about two centimeters from top to bottom, and if not filled with excess fluid, is only a couple of millimetres thick.
The navicular bursa cushions the contact between the DDFT and the navicular bone and is also the area where the DDFT changes directions from running down the back of the leg to running in a more horizontal direction towards its insertion in the pedal bone.
The navicular bursa is highlighted in the radiograph (x-ray) above using a special dye to improve visualization of it during radiographic examination.
What does 'bursitis' mean?
Bursitis means inflammation of the bursa; this can be inflammation on its own, or a side effect of an infection which represents a more serious condition for a horse.
Navicular bursitis can be inflammatory and part of navicular syndrome, or it can be infectious, eg after an injection or a penetrating wound in the sole. The latter is also referred to as septic navicular bursitis and is an inherently more serious and life-threatening condition.
How does navicular bursitis occur?
Septic navicular bursitis most commonly results from penetration of a nail or other foreign body through the sole of the foot. The central third of the frog is the most dangerous location for a penetration injury because it is located directly below the navicular bursa. Solar penetration in this area is therefore considered a potentially surgical emergency, so you should call your veterinarian immediately if you suspect this has happened to your horse.
Primary navicular bursitis can occur, but more commonly, inflammation results from damage to surrounding structures. The navicular region contains the navicular bursa, DDFT and navicular bone, but these structures are also in close proximity to the pedal bone, coffin joint, short pastern bone and a number of ligaments that interconnect all of these structures.
Primary damage to the DDFT within the foot is common, and as a consequence, structures adjacent to the damaged tendon can become involved. Similarly, remodelling changes on the back of the navicular bone can result in damage and roughening of the dorsal surface of the DDFT and the navicular bursa that is interposed between these two structures.
How is navicular bursitis diagnosed?
Generally, if you notice that your horse is lame and you have asked your veterinarian to examine your horse, your veterinarian will look for signs which may point to the foot as the cause of the lameness such as heat in the hoof wall, pain on hoof tester examination, and a strong pulse in the blood vessels to the foot. Interestingly, in horses with pain coming from the navicular region, very few show a positive response to hoof testers, so a lack of response to hoof testers does not rule out a foot problem.
If the foot is suspected to be the cause of the lameness, the foot may be desensitized with a nerve block just below the fetlock joint. If the lameness resolves or significantly improves, it localizes the lameness to the foot. Once the lameness has been localized to the foot, more specific nerve blocks can be performed to narrow down the areas involved in the lameness, although of course you have to wait for the first block to wear off before you can place the next one, so it can be a time-consuming process.
Some horses with pain in the navicular region will respond to a nerve block of the coffin joint, this is where local anesthetic is injected into the joint under sterile conditions. It is also possible to nerve block the navicular bursa itself and this is considered to be the most specific nerve block for the bursa. This block is technically challenging and requires passage of a three inch needle into the foot, so it must be done under controlled conditions in a sterile environment. For this reason your veterinarian may recommend that the procedure is carried out in at their veterinary clinic or hospital.
Problems within the hoof capsule can be difficult to capture on diagnostic images because the majority of the structures are localized within the hoof capsule. This largely makes ultrasound examination of these areas impossible. The foot can be radiographed (x-rayed), but this will only detect changes in the bones, whereas the majority of conditions within the foot involve soft tissue structures. With the increased use of Magnetic Resonance Imaging (MRI) in horses, identification of foot conditions in horses has made great advances. It has also enabled veterinarians to more precisely diagnose, localise and treat many soft tissue conditions in the foot and provide the client with a more accurate outlook. MRI examination of both feet (always done for comparison) can be performed in the standing, sedated horse and usually takes a few hours to complete. The acquired images are then reviewed and interpreted by a veterinarian experienced in MRI interpretation.
You should be aware that many insurance companies will only pay for a portion of an MRI examination. If your horse is insured and an MRI examination is recommended, check with your insurance company before going ahead with the examination.
How is navicular bursitis treated?
Treatment depends on the actual cause and type of bursitis. If there are other contributing injuries, such as damage to the DDFT or navicular bone, then these factors will have to be taken into consideration.
If inflammation of the bursa alone is the cause of the lameness, direct (intrathecal) medication of the navicular bursa can be considered. This is performed under the same sterile controlled conditions as a nerve block of the bursa, but instead of local anesthetic, usually a combination of a steroid (usually triamcinolone acetonide) and hyaluronic acid is used. Hyaluronic acid is a component of healthy joint and synovial fluid that becomes depleted when structures are inflamed. It has the benefits of being a good anti-inflammatory drug for the lining of the bursa as well as providing lubrication.
If medication of the bursa does not resolve the lameness or if the MRI examination reveals extensive damage to the DDFT or other soft tissue structures in the area, keyhole surgery (navicular bursoscopy) should be considered. Although the procedure requires a general anesthetic, it has the advantage of direct visualization of the bursa through a tiny incision. It allows for making a more accurate prognosis and direct surgical treatment of some lesions.
Farriery should be used alone or in combination with other treatments to improve foot balance and provide protection to the heel (and therefore navicular) region. Use of an experienced farrier trained in remedial farriery is highly recommended.
A final option is to perform a neurectomy (denerving) surgery. The decision of having your horse undergo this procedure should not be taken lightly, but it can be useful in horses that have failed to respond to the other treatments. However, if your horse is denerved it will no longer be allowed to be used under competition rules. Another major downside after a neurectomy is that the horse can no longer feel that portion of the foot, which means infections to that area of the foot can go unnoticed for prolonged time, unless the feet are frequently examined and scrupulously cared for, because the horse will no longer show any lameness to indicate a potential problem. Nerves do regrow over time and other complications can occur; the result after this procedure is therefore rarely permanent.
If your horse has an infected navicular bursa, endoscopic surgery (navicular bursoscopy) and lavage is the most effective treatment, but the condition still carries a guarded outlook.
What is the outlook for a horse with navicular bursitis?
The outlook for non-septic navicular bursitis depends entirely on the structures involved, and this is unique to each case. Many cases, if caught early, can be managed.
Septic navicular bursitis however carries a guarded outlook.