MRI in Navicular Horses
"Magnetic resonance imaging (MRI) has added to our knowledge of navicular disease, in large part because for the first time we can see into the horse's foot," said Robert Schneider, DVM, MS, Dipl. ACVS, equine orthopedic surgeon at Washington State University's Veterinary Teaching Hospital. His presentation at the 2003 convention of the American Association of Equine Practitioners focused on what he has learned about using MRI to diagnose and evaluate treatments for navicular disease and nearby pathologies since 1997.
"MRI has proven to be a valuable tool for making specific diagnoses in horses with performance-limiting lameness problems," he stated. "Navicular disease is one of the most common causes of performance-limiting lameness in many types of athletic horses. Despite the high incidence of the disease, our understanding of the problem is relatively limited.
"Equine practitioners have long suspected that there were multiple problems in the foot that caused similar clinical signs," he continued. "Because of the limitations of radiographs, many causes of bilateral (in both front feet or both hind feet) foot pain were recognized and diagnosed as navicular disease. MRI has allowed us to begin to differentiate some of these problems from navicular disease."
He described the different types of MRI images that can be obtained; some allow the practitioner to better visualize fluid in bone and soft tissues, while others yield better detail for imaging anatomical structures.
"Gaining knowledge is a slow process, and we are still on the steep phase of the learning curve with MRI of the horse's foot," he noted. Washington State's experience with MRI in horses with acute, chronic, and suspected navicular disease have led Schneider to consider new theories about this disease, and have revealed some interesting trends.
Questioning Old Theories
"The old theory that (navicular) disease starts as a bursitis and progresses into the bone does not seem to be supported by MRI observations," Schneider said. Bursitis is inflammation of a bursa, or fluid-filled sac that cushions a bone against another structure; in this case, bursitis of the navicular bursa that cushions the navicular bone against the pressure of the deep digital flexor tendon or DDFT. He noted that they have seen many horses with problems in the navicular bone that did not have problems in the navicular bursa.
What they have found in several horses was excess fluid in the medullary (marrow) cavity of the navicular bone. "Excessive fluid is not present in the navicular bone of normal horses," he explained. "The amount of fluid signal (seen on MRI) does seem to correlate with the clinical signs. The most fluid is usually found in the bone of the lamest leg. It seems to be one of the early detectable signs that an inflammatory process has started in the navicular bone."
If that inflammation increases in the bone and "breaks out the back," it can cause adhesions of the bone to the DDFT, he said. This is considered to be the "end stage" of navicular disease, with significant impact on the horse's movement, and everything a practitioner can do for a horse aims to prevent this.
"The DDFT runs over the navicular bone like a rope over a pulley," Schneider explained. "When the rope sticks to the pulley, you can imagine what that means for the horse."
"The real power of MRI is being able to go through the many images in order, going up the leg to follow the size of the lesions," Schneider added. He has found a host of pathologies in the foot that might or might not correlate with navicular disease. These include:
- Desmitis (ligament inflammation) of the supporting soft tissues of the navicular bone--the impar ligament (which attaches the navicular bone to the coffin bone, or P3) and the proximal suspensory ligament of the navicular bone (which anchors this bone to the second phalanx).
- Navicular bursitis. "Bursitis is not a common finding in many horses, which suggests that it is not the initiating cause of navicular disease in the majority of cases," Schneider said.
- Abnormal tissue in the proximal (upper) bursa (possibly fibrous scar tissue from chronic inflammation).
- Tendinitis of the DDFT proximal to (above) the navicular bone, extending above the middle of the proximal phalanx (P1). Schneider noted that in some horses this was the only pathology, pointing to hyperextension of the distal interphalangeal joint (DIPJ, or coffin joint) as the cause of tendon fiber tearing rather than a problem in the navicular region. "Tendinitis of the DDFT should be a separate diagnosis in horses with little or no involvement of the navicular bone," he said. "Blocking the digital flexor tendon sheath may be a technique that can be used to diagnose deep flexor tendinitis without MRI. However, more clinical cases where MRI has confirmed the presence of tendinitis need to be evaluated before we can be confident of this diagnostic technique. This technique is not specific for the DDFT."
- Many horses had more than one problem visible on MRI. "(Clinical signs of) navicular disease may result from more than one cause and more than one focus of initial inflammation," Schneider noted. "It also suggests that some of the abnormalities may have a common etiology (cause) or are inter-related." He listed possible causes of "navicular" clinical signs as sustained pressure between the DDFT and navicular bone, an interruption in blood supply to the navicular bone, chronic navicular bursitis, and chronic or acute strain on the supporting soft tissues of the navicular bone. "Based on the variation observed, many more horses with navicular disease will have to be evaluated with MRI before patterns and combinations that are of clinical importance can be established," he added.
- "The attachment of the impar ligament to the navicular bone might be more susceptible to injury based on the frequent observation of fluid in the distal one-third of the bone," Schneider said. "Strain loading of these structures when a horse is pushing off the front foot may explain why horses with navicular disease are short-strided. It also may be why shortening the toe to ease breakover is beneficial for some horses." He added that the navicular bone can be displaced relative to P2 and P3 with some stresses; chronic repetitive strain of these ligaments could explain some lameness from this area.
- Hemorrhage in the medullary cavity of the navicular bone from trauma to the frog. "This possibility should be considered in horses with an acute history and MRI findings of fluid in the medullary cavity of the navicular bone with little or no soft tissue involvement."
- Arthritis in the DIPJ. "Diagnosing arthritis of the DIPJ will be improved with MRI, but it will still be difficult to separate it from navicular disease in some horses," Schneider stated. "In some horses, it is very likely to be part of the same problem."
Schneider noted that correlating clinical signs with MRI findings provides valuable information regarding interpretation of findings and treatments; the necessary next step in research is correlating these findings with histopathological observations (postmortem tissue examination).
Acute vs. Chronic Cases
Clinical signs seen in 30 horses with acute-onset navicular disease (of less than six months duration) included bilateral forelimb lameness, sensitivity to hoof testers over the middle third of the frog, and positive response to a palmar digital nerve block of both front feet. MRI findings included:
- Fluid in the distal area of the navicular bone;
- Fluid, irregularity, and thickening of the impar ligament;
- Thickened proximal suspensory ligament of the navicular bone (NSL);
- Thickening, irregularity, and fluid in the DDFT above the navicular bone;
- Diffuse fluid in the bone's medullary cavity;
- Increased fluid in the navicular bursa; and
- Fluid at the insertion of the impar ligament on the navicular bone.
In 50 horses with chronic navicular disease, the following MRI findings were observed more frequently:
- DDFT tendinitis;
- Thickening of the NSL; and
- Scar tissue within the navicular bursa.
Using MRI in Clinical Cases
"The following problems were diagnosed as navicular disease in different horses prior to MRI: Chronic laminitis with mild rotation of P3, early bilateral osteoarthritis of the proximal interphalangeal joint, palmar (caudal or rearward) heel laminar pain, and bilateral desmitis of the straight sesamoidean ligament proximal to (above) its insertion on P2," Schneider said. "These horses were diagnosed with navicular disease because they had bilateral forelimb lameness that switched to the opposite leg after a palmar digital nerve block."
Hoof testers are frequently used to identify the location of pain in foot-sore horses, but they are not necessarily specific for any particular problem. "It has been stated that all horses with navicular disease are responsive to hoof tester pressure over the middle one-third of the frog," Schneider said, noting that their experience with navicular horses and MRI supported this theory. "However, so were horses with DDF tendinitis, navicular bursitis, and impar ligament desmitis. Not all horses with sensitivity to hoof testers over the frog have navicular disease. There are some horses sensitive to hoof testers who are not lame or have lameness problems that are located more proximal (higher up) in the limb.
"Previously, the diagnosis of navicular disease has been based for the most part on clinical signs because of poor correlation with observations from radiographs," he continued. "MRI now allows us to make this diagnosis with more confidence and differentiate multiple causes of similar clinical signs. As this technology becomes more available and our experience with it increases, it will become the preferred imaging technique for horses with a clinical diagnosis of navicular disease."
MRI for Guiding Treatment Decisions
Differentiating horses with true navicular syndrome from those with nearby problems has improved treatment because veterinarians can use it to prescribe more targeted treatment for a specific problem, Schneider said. For example, he noted that horses with tendinitis require different treatment (injection of anti-inflammatory medication into the tendon sheath) than those with navicular bone pathology, but those tendon treatments wouldn't be recommended if the practitioner thought the horse had a bone problem. Also, rest for these horses--which is usually not recommended for navicular disease--might be beneficial for healing in horses with soft tissue damage.
Schneider noted that injecting the tendon sheath with hyaluronic acid and methyl prednisone acetate (Depo-Medrol) has "improved the lameness in every horse in which it has been used (at the university). Some horses have continued in performance for two years without a second treatment. Some horses have improved, but lameness has persisted and these horses have not been able to return to performance."
Another example of how MRI is improving treatment is injecting hyaluronic acid and cortisone into the navicular bursa. This treatment is not new, said Schneider, but its success has traditionally been poor or variable. The relative rarity of actual navicular bursitis (as diagnosed with MRI) in horses with clinical signs of navicular disease could be the reason--if bursitis isn't the problem, then treating the bursa won't help the horse.
"Being able to select horses that are candidates for this treatment may improve the success of this procedure," Schneider added. In horses with true navicular bursitis, he noted that bursa injection has helped some horses quite a bit, with some returning to performance.
"As MRI increases, our knowledge about navicular disease, early diagnosis, new treatments, and/or better case selection for current treatments will hopefully follow," he concluded.
About the Author
Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.
POLL: University Equine Hospitals