The old adage of “no foot, no horse” rings as true today as the day the phrase was coined, and there is perhaps no hoof ailment with quite so many unanswered questions as navicular syndrome. As it is a result of several related conditions that, in turn, affect the structures of the hoof, it is difficult to pinpoint the exact causes. The syndrome may affect the navicular bone, navicular bursa, deep digital flexor tendon, and related soft tissue structures that comprise the structure of the hoof. There is much to learn about navicular syndrome, which was initially thought of as theslow deterioration of the navicular bone. Diagnosis and treatment is anything but straightforward and the anatomy of the foot is the first of many complex factors in what is one of the most common causes of foreleg lameness.
Dr. Kate Alexander, Associate Professor at the University of Montreal Faculty of Veterinary Medicine, has been helping diagnose the causes of navicular syndrome. “Traditionally we thought that navicular syndrome mostly affected the bone itself,” she explained, “but what we now know is that it can affect all of the soft tissue structures – tendons, ligaments and the navicular bursa – that surround the bone. At worst, it involves a degenerative process of the bone, which is usually, unfortunately, irreversible. When it’s not the bone that is affected, but instead the tendons or ligaments, it’s not necessarily irreversible, and can be traumatic, but not degenerative. What we are realizing is that it maybe doesn’t have such a bad prognosis in some cases.”
Small but significant, the navicular bone is located at the posterior of the hoof. Also known as the distal sesamoid bone, it is found at the junction of the short pastern and coffin bones; the area affected by navicular syndrome is connected by a series of ligaments and tendons. Along the posterior of the navicular bone and continuing underneath it is the deep digital flexor tendon, which then attaches to the posterior margin of the coffin bone. The navicular bone and deep digital flexor tendon are protected by the navicular bursa, a synovial sac that rests between the two – compressing and lubricating the deep digital flexor tendon as it slides over the navicular bone.
Designed to suit the specific purpose of cushioning the foot and pumping blood, the hoof has changed over time due to farriery care and the intensity and frequency of work. There is evidence to suggest that conformation does play a role in the development of navicular syndrome, though the hereditary link many associate with the diagnosis is most likely determined by conformational flaws that are hereditary and contribute to wear and tear on the bone and surrounding structures, rather than the disease itself being hereditary. Evidence also shows certain breeds, such as the Quarter Horses and Thoroughbreds, are predisposed to navicular syndrome due to certain characteristic conformation traits.
Dr. Alexander admits that it is difficult to pinpoint the exact causes of navicular syndrome. “It’s a really complex set of factors and is hard to say in one individual what it is that caused it. It can be a combination of things: a degenerative problem in the bone, inflammation in the navicular bursa (navicular bursitis), deep digital flexor tendonitis, or it can be inflammation of the collateral ligaments [located between the pastern and the navicular bone] or impar ligament [located between the back of the coffin bone and the deep digital flexor tendon]. Some have suggested a blood supply issue, and in other cases it’s more that we’ve changed the mechanics of the hoof. Right down in that heel area there are seven or eight individual structures that can be affected in a different way and can cause navicular syndrome. We’ve really only recently discovered that it can be this immense variety of pathologies that look the same when the horse presents as a lameness. What makes it so complex is figuring out exactly which structure or structures are affected.”
Repetitive compression or excessive force exerted on the navicular bone are thought to be in part a result of compressed or underslung heels. Knowing the baseline structure of the individual hoof is the first step to identifying the potential warning signs of navicular syndrome. Research suggests that farriery care – specifically the shape, angles, size of the foot and shoe selection, have an impact on the onset of navicular syndrome. “Magnetic resonance imaging has allowed us to realize that navicular syndrome is a really broad term. We don’t know everything there is to know, and MRIs have taught us just how much we don’t know about navicular syndrome. We are now figuring out that the lameness involved can be quite similar, but a result of many different structures. We are still trying to figure out all of the different entities that look like navicular syndrome, we hadn’t really begun to consider the complexity of the disease until quite recently.”
MRI: Unlocking the Secrets of Navicular Syndrome?
A diagnosis of navicular syndrome is reached using a combination of a physical exam and findings from a radiograph (x-ray) ultrasound, or magnetic resonance imaging (MRI). Horses affected by navicular syndrome are usually diagnosed with a history of intermittent lameness that persists, but improves with rest. The lameness is often more apparent on hard surfaces or tight circles, and is sometimes characterized by a choppy gait as the disease is regularly found in both forelegs. Severe cases present with a “pointed” stance – with the horse standing on outstretched forelegs as it tries to alleviate pain in the heel of the hoof. A standard lameness exam will reveal a positive pain response to a pressure test applied over the frog and heels. A nerve block, or local anesthetic, injected into the navicular bursa or rear of the pastern will eliminate sensitivity to the area of the hoof affected by the pain of navicular syndrome and confirm a positive diagnosis before moving to an x-ray, ultrasound or MRI.
“It can be pretty straightforward to pick up pain in the heel region using a combination of jogging and nerve blocks to the heel area,” described Dr. Alexander of the common field test for the disease. “Following that, there are a number of different approaches. Imaging will allow you to see which structure is affected and will give a better idea of the prognosis, which depends on the horse, athletic ability and intended use.”
Depending on whether or not the horse has been brought into a clinic as a referral or if the vet is out working in the field, the most common and inexpensive diagnostic tool is the radiograph, or x-ray, which will give quite a bit of information about whether or not the bone is affected. Because you cannot determine whether or not soft tissues have been affected on an x-ray, some veterinarians will often proceed to an ultrasound, which is again inexpensive, readily available and can give limited additional information on the deep digital flexor tendon. A relatively new diagnostic tool in the detection of the broad spectrum of navicular syndrome is magnetic resonance imaging (MRI), which gives the most in-depth information about what is going on in the bone and soft tissues at once. It also gives a good indication of whether or not the condition is acute, that is recent, or insidious, where it has been present for several months or years.
Together with her colleagues, Dr. Alexander undertook to research the current methods of diagnosing navicular syndrome in the hopes of shedding some light on the complex disease. The research consisted of a comparison of radiography and MRI for the evaluation of navicular bone abnormalities. In the study, horses with foot lameness were presented and examined by three veterinarians. Dr. Alexander explained that certain abnormalities appeared similarly between radiographs and the MRI, while others, such as navicular bone bruising, which cannot be seen on x-rays, was readily seen on MRI. Dr. Alexander noted that “between the different veterinarians, there was greater agreement on radiographic findings than on MRI findings, indicating that MRI interpretation guidelines are not as broadly recognized as radiographic guidelines. In other words, veterinarians are comfortable interpreting x-rays and require further experience and training when interpreting MRI, which is a much more recent modality.”
It was the promise of MRI technology in the study of navicular syndrome that drew Dr. Alexander to research the field. “MRI has changed how we think, see and treat this disease. It really gives us a complete overview of what’s going on in the foot.” She believes MRI shows promise as an emerging diagnostic tool, but it is one that is still in the process of developing interpretation guidelines. “In our research we initially wanted to see if what we already knew was visible on x-rays would also be present on the MRI. It was determined that the MRI picked up on things not necessarily visible on radiographs. We’re still in a learning curve; we are very comfortable with x-rays and now becoming comfortable diagnosing with MRI. It is valid for most veterinaries to start out with x-rays, and depending on what the owner wants to do with the horse, we may or may not do an MRI.”
Financially, a diagnosis using MRI technology is pricey, costing upwards of $2,000, due in part to the fact that MRIs are more invasive that doing x-rays or ultrasounds. It is a serious investment of time, finances and logistical considerations.
There are two types of MRI available in North America: one in which the horse is sedated and examined in a standing position, and another in which the horse is placed under general anaesthesia and placed on a specialized MRI table. Equine MRIs are still an emerging diagnostic tool and not common equipment, with only a handful available for use at universities and private referral practices across Canada. The University of Montreal and University of Guelph have “high-field” MRI units that require the horse be laying down, and the University of Saskatchewan has a “low-field,” or standing, unit.
A horse referred for a highfield MRI will generally be admitted the day before the appointment, when x-rays must be taken to ensure there are no traces of metal or fragmented nails in the foot. After being anaesthetized the horse is hoisted onto a table, where it will lie for the duration of the procedure. Dr. Alexander described the tool as a being similar to a big doughnut. “The feet are positioned in the magnet, which looks like a doughnut hole,” she said, “and the images are acquired through the use of special radiofrequency coils that create a signal allowing us to see the different structures in the foot. In a lot of cases the problem affects both front feet, in which case we will roll the horse over and repeat the procedure on the other foot.”
She further explained that to take images of both feet usually takes two hours, and a protocol has been implemented to ensure the horse is not asleep any longer than that. At all times the horse is monitored closely by a team of specialists. “At the University of Montreal we have a dedicated anaesthesiologist and specialized animal health technicians monitoring the horse at all times; there are MRI and x-ray technicians, lameness experts, and a veterinary radiologist who look at the images and tie that together with the clinical evaluation. We are then able to come to a consensus on what could explain the horse’s lameness.” Recovery is standard for a procedure where the horse is placed under general anaesthesia. The horse is monitored until it is able to stand on its own and then discharged next day.
The other option available in Canada at the University of Saskatchewan is a standing MRI, where the horse is standing, but heavily sedated. The standing MRI is a “U” shaped structure that goes around the foot, one at a time. The advantage to a standing MRI is that the horse is awake, the disadvantage is that the MRI is not able to get quite as big an area on its imaging. This procedure takes approximately two hours, depending on the number of images being taken.
The use of MRI as a diagnostic tool is growing. Not solely used for the detection of navicular syndrome, it is also used in the diagnosis of foot abscesses, bone bruises, joint infection and coffin joint collateral ligament problems, among others. “Here at the University of Montreal, we will average between 30 and 40 cases per year,” Dr. Alexander estimated. “We’ve had the magnet in place for four years now and its value is now really beginning to be recognized, it’s taking off.”
Detection + Diagnosis = Prognosis: An Equation With a Lot of Variables
The treatment of navicular syndrome must be regarded as a unique set of solutions for each individual case, rather than a pre-determined method of management. As there is a variety of causes, so is there a variety of treatment options. If abnormalities are detected in the navicular bone, it is considered a degenerative condition and while it can be expected that with proper maintenance the horse will become useably sound again, the damage is irreversible. Dr. Alexander said that while treatments vary depending on the area affected, the prognosis is often dependant on how long the underlying condition has been present.
“Getting an exact diagnosis is so important to giving a prognosis, which is where the MRI becomes so exciting,” she explained. “There are different ways that we can manage tendon injury or ligament injury, and with something like tendonitis our approach will tend to revolve around reducing inflammation, rest and a gradual return to exercise. If there is adhesion between the deep digital flexor tendon and navicular bone there may be the option to perform surgery to remove tissue. If there is a problem with the navicular bursa you can inject corticosteroids and hydrochloric acids. Each horse responds differently, and treatments are variable depending on so many factors.”
Corrective shoeing and farriery care is of utmost importance in the rehabilitation of a horse diagnosed with navicular syndrome, and a successful prognosis depends on the ability of vet and farrier to work together.
Dr. Alexander is optimistic that MRI will continue to play a large role in the detection and characterization of navicular syndrome and its specific abnormalities. “It has been revolutionary and eye opening to realize how many things can go wrong and to know that it’s not necessarily a terminal condition. I think that magnetic resonance imaging is on the way to making a big difference; we are still on a learning curve, but we can now give owners a more accurate prognosis and treatment management plan. It allows us to know exactly what’s going on.”