By Amy Rucker, DVM, with Megan McCracken, DVM, Dipl. ACVS

The navicular bone—also known as the distal ­sesamoid—is located within the foot and acts as a fulcrum, altering the angle of the deep digital flexor tendon (DDFT) just proximal to (above) its insertion on the bottom of the coffin bone (third phalanx). Thirty years ago we attributed any heel pain lameness to “navicular syndrome.” Now, with better imaging techniques, we can identify and treat a multitude of navicular-associated issues that fall under the term podotrochlosis, often before bony changes are visible on radiographs (X rays). Injuries can occur to the DDFT, the navicular bone, or a variety of other soft tissue structures within the hoof capsule. How we treat the lameness depends on the information we obtain with our history, physical exam, and imaging findings.

The horse’s history of lameness might vary depending on his stage in the disease process. Acute lameness might be evident in one foot. If the pathology is advanced, the horse might exhibit a shortened stride with both front feet while trotting straight, with inside-leg lameness evident while turning either direction. My favorite way to observe a possible navicular horse is to have the owner trot the horse in hand in a tear-drop shape on sloped ground. The horse might be sound trotting away, then be lame as it turns on the uneven ground. Often a navicular horse is not positive to flexion of the distal limb (fetlock/pastern/coffin joint).

When I look at a foot, I think about what part of it is carrying the heaviest load or has the most force placed upon it. I evaluate the angle of the foot and pastern, the length of the toe and heel, sole depth, the strength of the frog and heel bulbs, whether the horse’s conformation predisposes crushing of the medial (inside) or lateral (outside) heel. Some veterinarians use hoof testers to evaluate for pain, but I find those unreliable due to variations in hoof mass and dryness. I palpate the foot and note excessive fluid/effusion of the coffin joint at the coronary band or DDFT sheath above the heels or thickening of the tendons and ligaments in the pastern area. After examining a horse and watching it in motion, we “block” the lameness out by temporarily anesthetizing local nerves or joints. Most navicular cases improve with a palmar digital nerve block, anesthetizing the heel and sole region. Some require a low ring block to anesthetize the entire navicular apparatus. When the injured area is no longer painful, the horse moves sound, and we know which area of the leg is causing pain, we move to imaging it with radiographs.

Beau, a Tricky Case With a Positive Outcome

Beau was an 8-year-old Quarter Horse gelding purchased for a novice rider as a proven Western performance horse. He was advertised on the internet and had a successful show career with consistent recent competitions. The trainer and rider traveled across several states to try Beau, fell in love with him, and had a prepurchase examination done, where he was deemed sound on physical exam and radiographs. Some of the X ray views offered limited information due to positioning of the foot and amount of exposure and clarity. However, Beau lived in an area without an option for a second opinion exam, so the buyer completed the purchase. 

Beau arrived sound and stayed sound for months before an intermittent left front lameness developed. Initially, the trainer would notice a subtle lameness, but Beau would be sound by the time I performed an exam the next day.  Eventually, I localized the subtle lameness to the foot via nerve blocks and injected the coffin joint with hyaluronic acid and triamcinolone. (These two drugs have different mechanisms to reduce synovitis, or joint inflammation. We also know the steroid triamcinolone will diffuse from the coffin joint into the navicular bursa—the small fluid-filled structure that pads the navicular bone so the DDFT can move over it smoothly.)

Beau’s lameness resolved after treatment, but only for several weeks. When the lameness returned I referred Beau to a hospital for diagnostic imaging. A palmar digital nerve block resolved his consistent left front lameness, and radiographs revealed bony changes of the navicular bone.

At this point, financial limitations would send most horse owners home with recommendations for shoeing changes and other general treatments and a plan to return to work in a couple of weeks if sound. Sometimes, however, we can look beyond the radiographic navicular bone changes and determine whether the acute lameness is due to a different issue, such as a collateral ligament injury to the coffin joint. In this case we performed an MRI on Beau to further evaluate the foot, which allowed us to tailor treatment to his specific issues.

Magnetic resonance imaging enables us to gain additional valuable information about bone and soft tissue. Beau’s MRI showed some of the navicular bone changes visible on the radiographs in more detail. The radiographs had identified a cystic lesion in the navicular bone, and the MRI revealed the lesion was a full-thickness flexor cortex erosion. The scan showed evidence of fluid or edema within the navicular bone, indicating more active or acute injury. The MRI also enabled us to carefully evaluate the soft tissue structures within the foot. Many of these critical structures cannot be visualized with ultrasound, so MRI is often the only way to identify injury. It is especially useful and critical when radiographs are normal because it allows interventions early in the podotrochlosis case.

In Beau’s case we found a core lesion, or central tear, in the medial lobe of the DDFT, as well as fibrillation, or roughening, of the tendon surface. We also saw evidence of inflammation in the navicular bursa (bursitis). The changes to Beau’s navicular bone appeared chronic and probably predated his recent lameness episode. The soft tissue injuries are the likely source of the more acute lameness. As we gain more information about the navicular region, we realize how complicated and interconnected the bone and soft tissue structures are here. With Beau, the roughening of the flexor surface of the navicular bone caused by the chronic erosion likely contributed to his DDFT injury.

These structures’ complicated interplay highlights the need for careful coordination between veterinarian and farrier. As a result of the MRI, we were able to develop a detailed treatment plan tailored to Beau’s injuries. He was started on a rest and rehabilitation program to enable healing of the DDFT injury. This included stall rest and hand-walking. Initially, the hand-walking was only on firm ground. Gradually, we introduced walking in an arena with 6 inches of noncompacted footing. The program slowly progressed to riding at the walk, small paddock turnout, slow introduction of trot work, and a gradual return to full work and turnout. During the months of recovery, the ground conditions were strictly controlled: Beau was never exercised or turned out in uneven or deep footing.

When treating soft tissue injuries it is important to give them time to heal and to increase work gradually to encourage the greatest strength in healing. Avoiding sudden exaggerated movements that might cause re-injury is critical. In addition to the rest and rehabilitation plan, we treated the inflammation in Beau’s navicular bursas with triamcinolone injections. Beau also received bisphosphonate injections. Bisphosphonates have been shown to increase comfort in horses with osseous changes to their navicular bones by inhibiting bone breakdown and resorption.

We also devised a podiatry plan to reduce breakover and tension on the DDFT. Consistent, careful farrier work is critical to success with these patients. Typically, horses with flexor cortical erosions tend to have poor prognoses for return to work and prolonged soundness. But due to the diligent care of his owners, veterinary team, and farrier, Beau returned to performance. If this plan had not led to Beau returning to soundness or if he re-injured himself, we could have pursued additional treatment options. These include navicular bursoscopy, where we use a small camera and instruments to visualize the navicular bursa and remove proliferative synovium, fibrous tissue, and any adhesions that might be present. Another option would be injecting the DDFT lesion directly with platelet-rich plasma or stem cells—either in conjunction with a navicular bursoscopy or with MRI guidance.

Take-Home Message

As veterinarians we do the best we can with radiographs, but MRI provides a more accurate look at pathology, including bone inflammation (not just the bone remodeling evident on X rays). Beau likely came to us with the navicular cortex lesion in his left front and was able to perform until he developed associated lesions in the DDFT and navicular bursa. (Beau’s right front foot had similar bony changes but no soft tissue lesions, and he remained sound on that foot.) 

Beau’s MRI dictated a six-month rehabilitation program in a controlled environment. Unfortunately, many horses with Beau’s initial exam findings and radiographs have their coffin joints and/or navicular bursas injected with medications and return to work when sound (or are turned out in paddocks or pasture with uneven footing), and activity creates further injury. My goal with horses is to maximize their ability to work for a lifetime. In Beau’s case we have made every effort to heal his acute lesions and returned him to work at a lower level.

Beau is still ridden in performance classes but is no longer doing cattle work because we question his ability to stay sound with sudden changes in speed and direction. “Finished,” show-ready 8-year-old horses that can be ridden by an amateur usually come with some orthopedic baggage. I hope our team approach allows Beau and his baggage to continue their journey with a happy owner for years to come.