MRI Reveals Heel Problems

Equine foot problems come in a variety of forms, but they have one thing in common: They all pose a concern for the horse owner because without healthy feet, a horse's ability to do much of anything is severely compromised. The old expression "no foot, no horse" was appropriate when coined, and it remains so today.

Technological advances have greatly aided practitioners in understanding the equine foot. One of the newer modalities that has been extremely helpful in diagnosing specific foot problems, says William Moyer, DVM, head of the large animal medicine and surgery department at Texas A&M University, is magnetic resonance imaging (MRI). With MRI, researchers are able to study the equine foot section by section, something that wasn't possible before.

A word about how MRI works is appropriate before launching into a discussion of the foot and problems that can occur in the heel area, because MRI has opened the door to a better understanding of foot conditions in general. At the same time, it has revealed that some foot problems are much more complicated than they were thought to be, and they can involve more than one foot structure. In other words, it appears that our previous understandings of foot problems were often flawed, and MRI is helping us see these problems in a more accurate light.

A description of the modality was provided at the 2003 American Association of Equine Practioners (AAEP) convention in New Orleans by T.S. Mair, BVSc, PhD, DEIM, DESTS, Dipl. ECEIM, MRCVS, of Kent, England. He began his talk as follows:

"Magnetic resonance imaging uses the body's natural magnetic properties to produce detailed images of the tissues. The part of the limb being imaged is placed within a strong magnetic field and subjected to radio-frequency pulses. A unique radio-frequency signal, based on each tissue's magnetic characteristics, is emitted in response to the pulses. These radio-frequency pulses are collected to form the image.

"Magnetic resonance signal intensity varies widely in different musculoskeletal tissues because of the difference in proton density and the status of the chemically free versus chemically bound molecular water. Most diseases manifest themselves by an increase in water content, and therefore, MRI is a sensitive test for the detection of disease. The high soft tissue contrast afforded by MRI makes it ideal for assessment of articular cartilage, ligaments, tendons, joint capsules, synovium, and bone marrow. MRI has proven to be extremely valuable in the evaluation of the musculoskeletal system in man and is also being increasingly used in small animals. Preliminary observations suggest that it will be just as useful in the horse."

The three benefits most commonly mentioned with MRI include: It provides excellent imaging of all tissue types in the field; it emits no ionizing radiation; and it is multiplanar (allowing views of tissues from different angles).

Lameness Diagnostics

One of the problems in diagnosing heel and other foot problems in the horse, says Moyer, is the fact that X rays and local anesthesia (nerve blocks) often do not produce the information needed for a definitive diagnosis. Local anesthesia is a case in point. It is designed to eliminate pain in a specific area. In other words, if a horse is limping because of a problem or injury, local anesthesia will eliminate the pain. The horse will then appear sound and the practitioner can be reasonably certain that the affected area has been located. That's great theory, but it doesn't always work in practice.

One of the problems with local anesthesia for diagnosing foot problems, says Moyer, is that it's nearly impossible to block a specific location. For example, one can block the heel area, but in the process, other areas in the foot also are blocked because of the nerve network's structure.

MRI is often capable of presenting an isolated and accurate picture of what is going on in a specific area when conventional approaches--including X ray, nuclear scintigraphy, and ultrasonography--fail. The problem with MRI is that it is an expensive procedure and there are only a limited number of units in use in the United States. That, however, is changing as some referral facilities are installing units that can be used when a horse is standing, rather than being prostrate under general anesthesia.

Mair revealed some new MRI findings when he reported at the AAEP convention on a study involving 40 horses of mixed age, breed, and use. They included 35 horses with pain localized to the foot by regional analgesia and other conventional techniques, but no specific diagnosis had been reached using those conventional procedures. When examined by MRI, it was revealed that the horses had the following problems: Deep digital flexor tendon damage, 17; navicular bone/ bursa changes, 11; distal interphalangeal (coffin) joint distension, 8; navicular collateral ligament damage, 5; distal sesamoid (navicular) impar ligament damage, 3; distal interphalangeal joint collateral ligament enlargement, 2; sesamoidean ligament damage, 2; no abnormalities detected, 5. As is obvious from the numbers, some of the horses were suffering from more than one injury or problem.

What this tells the horse owner and practitioner is that MRI will be much more accurate than conventional procedures and will remove most of the guesswork involved in diagnosing foot problems in the horse. Something that might have been diagnosed as a bruise causing heel pain, for example, could show up on MRI to be damage to the deep digital flexor tendon or some other part of the sensitive inner structures of the foot.

MRI and Navicular Disease

Navicular disease is considered by many to be the ultimate problem affecting the rearward portion of the equine foot, but diagnosis and treatment have traditionally been inconsistent at best. Why?

"Equine practitioners have long suspected that there were multiple problems in the foot that caused similar clinical signs," said Robert Schneider, DVM, MS, Dipl. ACVS, equine orthopedic surgeon at Washington State University's Veterinary Teaching Hospital, in a presentation at the 2003 AAEP convention. "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; some allow the practitioner to better visualize fluid in bone and soft tissues, while others have 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 has led him to consider new theories about this disease, and has revealed some interesting trends.

"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've 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."

MRI in Practice

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."

Another area where MRI has been quite useful for Schneider is in 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, he 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 has been improving treatment is injecting hyaluronic acid and cortisone into the navicular bursa. This treatment isn't new, said Schneider, but its success has traditionally been poor or variable. The relative rarity of 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.

By Les Sellnow and Christy West

 


FURTHER READING

For more information on magnetic resonance imaging, see the MRI category under Diagnostic Imaging at www.TheHorse.com.


 TYPES OF MAGNETIC RESONANCE IMAGES

Magnetic resonance imaging (MRI) isn't all created equal; there are three imaging sequences commonly used for the equine foot. Unlike other imaging modalities, MRI has very specific "recipes" or sequences for how to take images, and the different sequences highlight different types of structures and pathology. MRI is a very powerful imaging modality because you can modify so many things, but that also makes it very complex to operate and interpret. Most people doing MRI on horses will have standing units for lower limbs only, so they will focus on T1, T2, and STIR images.

  • T1-weighted images show good anatomical detail; bone marrow and fat show up as bright white while cortical (hard outer) bone and tendon appear black.
  • T2-weighted images are similar to T1s and still show good anatomical detail, but also show fluid as bright white (fat is a little darker than T1). They take a bit longer than T1s to acquire.
  • Short-Tau Inversion Recovery, or STIR, images have poor anatomical detail and take a long time to acquire, but they show inflammation and fluid in both soft tissue and bone very well. Fluid is bright white, while fat and bone marrow are very dark.

ARE WE SETTING HORSES UP TO FAIL?

Many common hoof problems are man-made and frequently caused by under-use, says William Moyer, DVM, head of the large animal medicine and surgery department at Texas A&M University. "We see fewer problems involving the feet in horses that get plenty of outdoor exercise," he says. "The problem with many horses in America is that they have gone from a farm/ranch animal to being an apartment dweller."

This means they spend many hours confined to a stall instead of getting normal exercise in the great outdoors. And when they are exercised, it often is on a yielding surface.

"Bones need to be stressed by activity in order to grow strong. It is the same with the hooves," says Moyer. "Many of today's horses walk and perform only on cushiony surfaces. That does little to strengthen them. The hoof is dependent on forces or challenges. When they don't exist, there is no reason for it to grow strong." Too much pampering in the form of bathing can also bring on hoof problems, Moyer contends.

"What do a lot of stables have in common?" he asks. "A wash rack that gets a lot of use. During the show season, some horses are bathed every day. All of the water that runs off a horse's body flows downward to the feet. A horse's hooves aren't designed to absorb that much water, and soon they become soft and are easily damaged."

Quarter cracks, heel cracks, and contracted heels can haunt weakened and/or unbalanced feet. A contracted heel (where the heels draw closer together than is normal), says Moyer, is more of a description than a condition. "Contracted heels are symptomatic of something," he says, "and you have to ask yourself what the cause might be."

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