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July 6, 2007

An MRI Can Shed New Light On Lameness Issues

This technology is changing the way many conditions, including heel pain, fractures and soft tissue injuries, are being diagnosed and treated.

Your horse has been foot sore on and off throughout the show season, and coffin joint injections are less and less helpful.

The cost of treatments for your horse’s vague foot lameness is starting to break the bank. The farrier has tried every shoe under the sun, and your horse still isn’t “quite right.”

Your horse seems better post-joint injections, but you wonder if you should return to full work. Your “navicular horse” is becoming lame enough that nerving may be your only option for soundness, but you wonder if it is the right thing to do.

Cases such as these may benefit from the information garnered from a magnetic resonance imaging study. MRI has become increasingly more frequent as part of a lameness work-up, and veterinary referral centers offering MRI are much more readily available throughout the country, with studies available under general anesthesia or standing sedation.

MRI was first used in human medicine in 1971, and by the 1980s, was widely accepted in the human field for neurologic imaging. Subsequently, MRI became accepted as a vital instrument for musculoskeletal diagnosis, owing to its ability to illustrate changes in multiple tissues such as bone, cartilage, tendons, and ligaments.

MRI in the field of equine medicine is relatively new, given the initial difficulty in trying to image a horse in a human-sized magnet. High-field magnets adapted for equine use and standing low-field magnets built specifi-cally for horses have been used more routinely since the early 2000s. There are a number of instances where MRI is the gold standard for diagnosis. For instance, a grand prix jumper finished a blazing round and was acutely lame afterwards. A nuclear scintigraphy study (“bone scan”) indicated that there was generalized increased remodeling in the bones of the foot. After radiographs were taken and noted to be unremarkable, the horse was sent to MRI. An MRI study was performed on its foot, and severe contusion (bone bruising) of the navicular bone, the coffin bone, and the short pastern bone was diagnosed.

Not only did the MRI enable vets to reach a specific diagnosis, but an appropriate treatment plan was detailed, tailored to this specific injury. Furthermore, an accurate prognosis was attained that allowed the owner to plan the rest of his show season accordingly. Without MRI imaging, the extent and severity of bone injury and cartilage damage cannot be fully appreciated. MRI can also help to avoid potentially catastrophic injury; contused or inflamed bone is much more likely to break if a horse is medicated though such an injury.

A Big Help For Hooves

The foot is a particular region that has benefited immensely from the advent of MRI in horses, as the soft tissues of the foot are subject to the same kinds of damage as the rest of the leg.

Injuries to the collateral ligaments of the coffin joint have been gaining more attention. These are ligaments that run perpendicular to the ground from the short pastern bone to the coffin bone and are imperative for stabilization of the coffin joint. Collateral ligament injury presentation can vary widely in appearance, from mild chronic unsoundness to severe acute lameness. Traditionally, a diagnosis of collateral ligament injury was hindered as only the most proximal extent of the ligament can be visualized with ultrasound where it originates on the short pastern bone.

Nuclear scintigraphy can be helpful (major lesions can be missed) but certainly does not paint a full picture of the degree of injury to this structure, or whether there may be further injury to other foot structures that occurred along with the injury to the collateral ligament (which alters treatment and prognosis). An MRI study could elucidate further degree of damage, as a chronically degenerative collateral ligament that is scarred appears very different than a freshly torn or avulsed ligament.

The location of the injury is also significant. Injuries to the collateral ligaments of the coffin joint are far more difficult to heal and stay healthy if they are located at the attachment to the coffin bone. Consequently, this sort of information helps to determine if it is “worth” trying to fix the injury. Sometimes instead of sinking a lot of time and money into the treatment, it is better to retire the horse. Furthermore, recheck MRI studies can provide useful information when compared to the original study to determine when it is safe to begin rehabilitation.

MRI has redefined diagnosis and management of “navicular syndrome.” Indeed, we rarely refer to it that way anymore. Radio-graphs often “underdiagnose” in this area, as many horses with normal or mildly affected navicular bones on radiographs have significant pathology when viewed with MRI. Radiographs provide a very limited view of the internal structures of the foot, and often by the time you see radiographic changes, the disease process is quite progressed. Pain stemming from the caudal heel region can be due to a number of bony, soft tissue or superficial abnormalities, so the specific pathology must be defined, and defined early.

Many performance horses that are diagnosed with caudal heel pain are afflicted with navicular bone edema (fluid or inflammatory cells in the navicular bone), with very little soft tissue abnormalities. This is a manageable condition, potentially curable with proper hoof balance, shoeing, navicular bursal injections, or biphosphonate drugs if the diagnosis is made at the onset of lameness.

In some horses, however, the lameness is subtle enough that the horse is medicated and continued in work, and some of these horses progress to the changes typical of “navicular disease:” deep digital flexor tendon abnormalities, adhesion formation, surrounding soft tissue and navicular bone degeneration. At that point, the changes are no longer reversible and could be career limiting.

These changes are potentiated as the horse begins to alter his gait to compensate for heel pain. Toe-first landing is a common compensatory mechanism for caudal heel pain, and on MR imaging, can result in further soft tissue damage, such as impar ligament injury and inflammation.

One such case was an upper-level eventer diagnosed with navicular pain. An MRI was performed to determine whether there was something treatable in the foot. Inflammation of the impar ligament was diagnosed, and with corrective shoeing and IRAP injection, the horse returned to full work.

Once these navicular injuries are at an end-stage, the question of “nerving” the horse comes up. With MR imaging, it is easier to determine if the horse is a candidate for this procedure. The concern is a horse that no longer feels pain from his foot will no longer guard it, so the degenerative process occurring in the navicular region will progress faster, with a potentially devastating outcome.

A horse with a degenerative navicular bone and minimal soft tissue involvement is far more likely to have a successful outcome than a horse that is showing degenerative deep flexor tendon changes. Once the heel pain is alleviated and the horse can bear weight normally, the foot can function normally once again. However, any horse that undergoes this procedure and remains in work should have successive MRI studies done to monitor the progression of the disease.

Identifying The Exact Problem

Even fracture diagnosis has significantly improved with the advent of MRI. One particular case involved a sudden onset severe hindlimb lameness, which was attributed to the foot. Diagnostic work-up for a coffin bone fracture or abscess ensued, and nothing could be found. The MRI revealed not only a non-displaced fracture of the wing of the coffin bone, but inflammation of the medial collateral ligament of the  coffin joint where it was adjacent to the fracture margin.

This obviously changed the approach to the management and treatment of the case, as both soft and bony pathology was evident. A follow-up MRI scan was performed to ensure adequate healing of the soft tissue injury. After therapeutic shoeing, extensive rest, and a follow-up MR study, which showed a healed fracture line, the horse slowly returned to work, and is currently sound.

Deep digital flexor tendon injury is a common MRI diagnosis in the foot, and this one structure can have a plethora of etiologies. The deep digital flexor tendon can be injured from degenerative navicular bone processes, or can undergo inflammation or tearing like any other tendon of the leg.

A lot of tendon injuries in human medicine have been shown to be the result of repetitive overload to the tendon, until the tendon eventually gives out. Hence, inflammation, or tendonitis is an early warning sign of potential injury, and these are not necessarily horses that should continue campaigning. Rest until normal tendon architecture is restored is paramount to avoid more significant injury. Unfor-tunately, it is difficult to pick up this subtle warning sign in the foot via ultrasound, due to the hoof itself.

If the tendon is torn, it is important to detail the location, length, and size of the tear; lesions within the tendon (core lesion) carry different prognoses than those found at the margins (dorsal margin tear). A core lesion located further up the foot could potentially be injected with a number of therapeutic agents, whereas dorsal margin tears need a different sort of management to attempt to minimize the adhesion formation that seems to follow this type of injury.

Tears located near the navicular bone often are involved with the navicular bone and can be devastating. There is also a small subset of deep digital flexor tendon abnormalities that show up on MRI, and, depending on the sequence, are deemed to be inactive.

These horses seem to go either way—some go on to eventually have a problem in this region in a variable period of time (months to years), and some horses never seem to be bothered by it.

Acquiring an image that clearly delineates the nature of the deep digital flexor tendon injury is paramount to determining the best course of action for the horse.

Determining The Best Treatment

Moving up the leg, MRI has proven to be a helpful tool. Even though these regions can be imaged with ultrasound and radiographs, the sensitivity of these tools is not always great enough to pick up certain abnormalities.

For example, proximal suspensory desmitis (a “high suspensory”) can be a very difficult diagnosis to make with traditional methods. Often the extent of the injury is difficult to appreciate, especially on hind limbs. There can also be some degree of bony involvement at the origin of the ligament, which can be appreciated on MR scans.

Many proximal suspensory injuries can actually involve the check ligament, which has a surgical option in some cases. If the patient also undergoes an MR study to determine the extent of involvement of the tissues of the suspensory region, adhesions to splints can be seen, and these are often a cause for chronic lameness.

IRAP, stem cell therapies, platelet-rich plasma, shockwave, bone marrow, corticosteroids, biphosphonate drugs—currently, there are many effective (and expensive!) ways to treat musculoskeletal injury, and the best way to choose the appropriate treatment is with specific diagnosis. Is the goal to decrease inflammation of a joint or support regeneration of cartilage? Are we trying to arrest bone remodeling, or support new tendon formation?

MRI has been instrumental in helping veterinarians make such decisions. A torn tendon will be managed differently than one that is inflamed. The aging of the lesion makes a difference as well, as chronic injuries and degenerative processes will carry a different protocol and prognosis than one that is more acute in nature. MR imaging can help stage a lesion as well, which can help paint a more realistic picture for your horse’s future.

One such case was a young warmblood gelding in dressage training. He had initially been consistently lame in the fetlock joint but now had some lameness attributed to the front feet. Many radiographs and ultrasounds had been performed, and no definitive diagnosis had been reached.

The gelding responded initially very well to corticosteroid injections, but they were becoming increasingly less effective. Between lameness exams, radiographs, ultrasounds, and nuclear scintigraphy scans, the owner had spent a sizable amount on the horse and was wondering if she should continue to manage the lameness or change her young mount’s career to pasture ornament.

The owner elected to have an MRI study performed, and cartilage injury was noted on the images, as well as some contusion to the navicular bone. Arthroscopic surgery cleaned up the fetlock joint, and shoeing changes were implemented. The horse returned to full training and is currently competing successfully.

Not all stories end so well, and more often than not, MRI provides the justification to retire or change a horse’s career. This, however, is extremely valuable information, as a horse that needs to be retired young due to progressive navicular disease probably should not be bred.

A recent case was an 8-year-old hunter mare that had a progressive lameness that was exacerbated by jumping (especially in hard footing). The radiographs were unremarkable. The owner referred the horse for MRI prior to joint injections, as she was worried that the injections would mask a more serious underlying problem. The MRI demonstrated significant bony and soft tissue pathology of the navicular region, none of which were fully appreciated with other modalities. Due to the significant changes of her feet, she changed careers, and with proper shoeing, is useful at a lower level.

While MRI is still an expensive procedure (scans typically wind up costing between $800-2,000 dollars, depending on the region and the type of magnet used), the information it can provide is priceless.

If a horse is lame, a specific diagnosis must be reached early, as the longer the lameness goes on, the less likely it is to be manageable. It is much easier to target and treat a problem when it can be seen, outlined, and staged. Furthermore, the costs of trying to manage a lame horse without a definitive diagnosis can sometimes be far more costly, both in time and money, than the MRI itself.

 
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