The New Navicular Paradigm

Here’s what we know and are learning about defining, diagnosing, and treating this painful foot disease.

If you look back through veterinary texts from the turn of the 20th century, you’ll see a perhaps familiar word: Navicular. Veterinarians used the term to describe a “disease process” within the tendon and/or bone of the navicular structures in the back third of the equine foot. The 1904 Practical Stock Doctor reads, “As a general thing, treatment is very unsatisfactory, usually alleviative only.”  

Indeed, identifying problems within a horse’s navicular structures is nothing new. Thankfully, our understanding and treatment of them have changed and advanced.  

Veterinarians have a profoundly more complex understanding of what structures have sustained injury, thanks to the evolution of diagnostic equipment and procedures. And using current palliative approaches—which range widely from regenerative therapy to bisphosphonate administration—they have found success keeping affected horses in work.

What’s in a Name?

Wally Liberman, DVM, of Panorama Equine Medical and Surgical Center, in Redding, California, focuses on diagnosing and treating lameness—in particular, hoof problems. “The evolution of the concept of ‘navicular’ has changed considerably because of our increased imaging capability through digital radiography, ultrasound, and magnetic resonance imaging (MRI),” he says. “MRI as the gold standard for diagnosis has allowed us to appreciate how often things are going on in the zone that we refer to as navicular that actually involve multiple anatomical locations.”

Today, the descriptor “navicular disease” is misleading, inaccurate, and best discarded. “The term caudal (toward the rear) heel pain is similarly misleading,” says Liberman. “This syndrome is not an external hoof capsule disease, which could be part of caudal heel pain; it centers within structures that make up the podotrochlear apparatus within the hoof.”  

More appropriate names for navicular disease, he says, are podotrochleitis or podotrochlosis, both of which are correct anatomical descriptions that include the following structures within the podotrochlear apparatus (see illustration on the next page), any of which have the potential to be diseased:  

  • Distal interphalangeal (coffin) joint;  
  • Cartilaginous component of the dorsal (top) surface of the navicular bone;
  • Subchondral bone, which the cartilage overlies;
  • Medullary (marrow) cavity/trabecular bone (found at the ends of long bones on the limbs, pelvic bones, ribs, skull, and vertebrae);
  • Flexor cortex/cortical-compact bone (the navicular bone’s dense outer “shell”);
  • Fibrocartilaginous aspect of the palmar (bottom) surface of the navicular bone;
  • Entire circumferential ligamentous system—distal sesamoidean impar ligament, collateral sesamoidean ligaments, and chondrosesamoidean ligaments—basically, all the ligaments, which connect bone to bone, that hold the navicular bone in place;
  • Deep digital flexor tendon; and
  • Navicular bursa (the cushioning fluid-filled sac between the navicular bone and the DDFT.

“It is not uncommon to find not just one but, rather, a multitude of those tissue masses involved in what people have historically referred to as navicular disease or syndrome,“ he says. “MRI has allowed us to understand this and to diagnose which tissues are involved.”

Diagnosis: Easier Said Than Done

While it’s now far easier for veterinarians to diagnose exactly what part of the foot is affected than it was in years past, the process still presents challenges. A typical workup includes a thorough clinical lameness exam, consisting of limb manipulation, hoof testers, flexion testing, wedge testing (placing the horse’s heel on a wedge block while holding up the opposite limb and then having the horse trot off), and diagnostic anesthesia.

“In cases where a palmar digital nerve (PDN) block—anesthesia placed in the nerves that innervate the back third of the hoof, the sole, and sometimes the front of the hoof—gives favorable results, all that says to the practitioner is that the problem lies somewhere in the hoof,” says Liberman, referring to the horse appearing sound after such a block. “In some cases it is more appropriate to start by anesthetizing either the medial or lateral palmar digital nerve first, but not both at the same time.” This helps the veterinarian better pinpoint the pain’s specific location.

Historically, once the veterinarian localized the region of discomfort to the foot, he or she would take radiographs to begin forming a working diagnosis and treatment approach. Alexia McKnight, DVM, Dipl. ACVR, of McKnight Insight, in Chadds Ford, Pennsylvania, specializes in radiology, interpreting MRI images and consulting with referring veterinarians worldwide. “It is always important to obtain good-quality diagnostic images of musculoskeletal structures,” she says. “Radiography is available in most practices, but with regards to the foot may have significant limitations. More practitioners are recognizing that often there aren’t appropriate answers with even the most thorough radiographic examination.” Radiographs typically can’t offer any anatomical information about the limb’s crucial supporting soft tissues, which are frequently the sole source of the problem.  

Ultrasonography poses similar challenges; with it veterinarians are limited in their ability to evaluate collagen matrix quality, says McKnight. “Is the scar tissue that fills in a lesion of good quality or poor collagen matrix?” she asks. This can be difficult or impossible to know, and, further, many tendinous and ligamentous structures deep within the hoof are inaccessible with ultrasound.

“Instead, MRI is an anatomical imaging system that ‘sees’ soft tissues and fluid in addition to bone,” says Liberman. “Following identification of abnormal tissue through MRI, a deeper investigation of the radiographs may reveal information.” McKnight adds that an MRI study might facilitate radiographic or ultrasound follow-up, prompting the practitioner to use a particular angle or technique that best reveals the area of concern.  

MRI Studies of the Foot

Veterinarians conducted the first MRI study of a live horse at Washington State University 20 years ago. While this technique is still advancing, it has shed much light on equine foot pathology (disease).

Today many veterinarians perform low-field MRI exams on standing and sedated horses, as well as low- and high-field exams under general anesthesia. The different strengths of magnetic fields provide different resolutions of the images, with high-field producing superior resolution, but at a higher cost. When used by an experienced operator, these machines can unveil excellent diagnostic information.  

“The MRI is an instrument that needs to be used by a skilled operator who can optimize the scanner’s capabilities,” McKnight says. “If any study on any system is poorly done, incomplete, wrought with artifacts (misleading images), or otherwise improperly interpreted, it can lack the appropriate diagnostic information the study set out to achieve.”

The information veterinarians can glean from MRI studies varies significantly depending on image quality and interpretation. For the best results, seek out practitioners with plenty of MRI experience, and send images to a qualified radiologic interpreter.

“If the interpreter doesn’t see something and doesn’t comment on it, it’s natural to believe that it isn’t there,” says McKnight. Significant lesions might go unnoticed or aren’t visible. “Furthermore, the clinical lameness and the pathology seen on imaging have to match. Usually, a marked lameness correlates with marked pathology, assuming the body part undergoing the MRI examination is the source of the lameness (as isolated by the clinician’s lameness exam). If not, then some relevant issue is likely to have been missed or overlooked.”  

Similarly, if your veterinarian’s suggested treatment following MRI shows no signs of success, you might need to seek a second opinion. He or she might have missed or misinterpreted the lesions.  

“Most every anatomical area of the podotrochlear apparatus is commonly involved on the vast number of MRI studies that I evaluate; however, not all horses have all these lesions,” says McKnight. “Many people are under the impression that they are looking for a single lesion. More commonly than not, there are multiple lesions, as many as six to eight. Often it is difficult to know which is the most significant. The greatest clinical success is achieved when all the lesions are treated rather than guessing which one or two might be most significant. Usually, a foot lameness results from a little bit of a lot of things all added up together.”

Therapeutic Discoveries

Shoeing is an important element of pain resolution for affected horses. Corrective shoeing helps standardize breakover, the hoof-pastern angle (the alignment of the front of the hoof and pastern), the palmar angle (formed by the bottom of the coffin bone and the ground), and medial-lateral hoof balance (the comparative evenness of heel length on each side of the hoof). Determining the best shoeing approach, however, can be challenging. “Some horses need wedge pads to lift the heels to decrease tension in the DDFT; whereas in other cases, wedge pads can be counterproductive by tightening tension on an adhesion when what the horse needs is to lower the heels,” Liberman says. “To understand how shoeing is best implemented, each case should be treated based on thorough diagnostic data.”

Therapeutic shoeing

Many owners are keen to leave their horses barefoot. However, he cautions that for some horses, leaving them barefoot could be the kiss of death and actually worsen the pathology. Consider each case based on the horse’s individual needs.   

Physical rehabilitation and movement are also important. “Turning a horse out in a big area is a form of treatment,” says Liberman. “Many horses need controlled exercise in hand, walking under saddle, or bitted in a surcingle with exercise in large circles. If a horse just stands around doing nothing, then static blood flow in the foot won’t allow healing.”

Other treatment options include intra-bursal or intra-articular corticosteroid injections. Liberman notes, however, that sticking a needle through the tendon and into the bursa to inject medication has the potential to create more damage. “This is a double-edged sword,” he says. “If the MRI reveals navicular bursitis and increased fluid within the bursa, it has been my experience that the body may resolve this fluid on its own.”  

He also points out that coffin joint injections with anti-inflammatory medications, such as corticosteroids or polysulfated glycosaminoglycans, don’t always work. “Medication does diffuse into the navicular bursa from the coffin joint, so this can be useful to treat navicular bursitis unless adhesions are present.” A skilled veterinary surgeon might cut the adhesions using bursoscopy (a fiberoptic approach into the navicular bursa).

Liberman notes that many navicular cases he’s seen involve the DDFT; pathology within the tendon can lead to unresolvable lameness. These cases don’t have many therapeutic options, he explains. Because frayed bits of tendon fibers shoot out as foreign protein, they invoke an inflammatory process that isn’t likely to heal, yet the horse can still function. The objective is to keep it from getting worse.

In 2014 the FDA approved bisphosphonates for use in horses in the United States, and veterinarians now prescribe them to manage podotrochlear apparatus disease. Bisphosphonates inhibit bone resorption by encouraging osteoclasts (cells responsible for bone turnover) to undergo cell death, leading to a decrease in bone breakdown. “Utilization of non-nitrogenous bisphosphonates such as Tildren or Osphos is useful but only when there is bone pathology,” says Liberman, adding that they should not be administered to young, growing horses because they interfere with bone development.  

Bisphosphonates can play an important role in managing navicular bone lesions identified through MRI. In recent studies, researchers saw a 16% overall improvement using bisphosphonate treatment over just corrective shoeing for horses with identifiable bone lesions of the podotrochlear apparatus. By the end of six months, nearly 50% were sound as compared to 12% of horses given a placebo.

Currently, only non-nitrogenous bisphosphonates are available for equine treatment, but researchers are evaluating the nitrogenous form. Nitrogenous bisphosphonates’ (Reclast or Zometa, zoledronate or zoledronic acid, used in humans to treat osteoporosis) potency is much greater than that of non-nitrogenous bisphosphonates. It slows bone resorption, allowing cells time to rebuild normal bone and promoting bone remodeling.

Follow-Up Evaluation

Anywhere from six weeks to two years after their initial diagnosis, horses should undergo a follow-up MRI. “Typically, the follow-up is performed at three to 12 months from the initial MRI and/or at the time before the horse is put back into work or increased work,” says McKnight. She says follow-up MRI findings can reveal various outcomes:

  • Extraordinary improvement. “This is seen especially following regenerative therapy using platelet-rich plasma, interleukin receptor antagonist protein, or stem cells,” notes McKnight.
  • No significant difference in MRI lesions, yet the horse is doing better.
  • Significant deterioration, which correlates with increased lameness.
  • The horse appears sound in hand, but MRI pathology is progressive. “This causes concern that soundness will not persist as work level increases,” says McKnight.

Take-Home Message

MRI is the most critical tool for diagnosing foot pain accurately. “Otherwise, it is not uncommon to see a horse owner spend good money after bad, often in many thousands of dollars, chasing a cure,” says Liberman. “Success in managing pain within the podotrochlear apparatus has everything to do with how quickly a proper diagnosis is obtained. The faster the problem is diagnosed, the more quickly the horse receives appropriate treatment.”

MRI has been instrumental in guiding practitioners toward a diagnosis and a prognosis for a horse’s continued athletic longevity. What we used to think we knew about podotrochlear apparatus pathology has been proven, through MRI studies, to be only a very small part of the overall picture. “The information gathered from MRI studies is a humbling process,” says McKnight. Many times the observation that a horse is experiencing foot lameness is just the tip of the iceberg as to the actual source of the underlying pathology.