It wasn’t that long ago that X-rays were the only diagnostic tool available to veterinarians for determining the cause of lameness. But X-rays have drawbacks, including the inability to show problems with tissues other than bone and the fact that abnormalities and changes seen on X-rays aren’t necessarily the cause of the horse’s pain.

X-rays show bone clearly, top photo, while on MRIs, lower photo, the very dense tissues like hoof wall and the outer, cortical layer of bone appear black and fluids like blood or joint fluid are the brightest. Tendon is in between. What looks like a B-B pellet in this horses coffin bone is a shoe-nail artifact.

The advent of ultrasound was a major step forward in being able to see injury to soft tissues such as tendons and ligaments and smaller structures inside and alongside joints that function to stabilize them. However, the most important part of the horse’s body when it comes to lameness is the foot, and it can’t be completely evaluated by an ultrasound study because of the hoof wall.

As we stated in July 2005, this is where an MRI—magnetic resonance imaging—can be a help. An MRI produces images of incredible detail and contrast and can be set to focus precisely within the body, such as the foot. In fact, because of MRI we now know of several newly defined and important causes of foot pain including:

Collateral Desmitis Of The Distal Interphalangeal Joint. This is inflammation or tearing of stabilizing ligaments on the inside (medial) or outside (lateral) of the coffin joint. The medial ligament is the most often involved. This injury is particularly common in jumpers. Although this condition has also been described after ultrasound examinations, in one study only 32% of the horses with collateral ligament injuries could be detected by ultrasound.

Impar Ligament Problems. The impar ligament attaches the coffin bone to the bottom of the navicular bone. Tears and inflammation in this tiny ligament can result in significant pain for the horse that improves with the usual nerve blocks done for navicular disease but is invisible on X-rays to check for navicular disease.

Fluid In The Navicular Bone. The traditional theory of navicular disease is that it develops following inflammation of the navicular bursa, which sits between the bone and the deep digital flexor tendon. However, MRI studies have found that many horses with early clinical signs of navicular disease have increased fluid, indicating inflammation, inside the bone itself with no evidence of inflammation in the actual bursa.

This early inflammatory response can be seen only on MRI and occurs long before X-ray changes. In some acute cases, this fluid may in essence be actual evidence of a “bone bruise” to the navicular bone area.

Other Navicular-Related Findings. MRI is also accurate in detecting changes in the navicular bursa, the ligaments supporting the navicular bone, and along the length of and insertion of the deep digital flexor tendon within the foot. Any of these can be sources of pain in horses diagnosed with “navicular syndrome” or “heel-pain syndrome” and may in time lead to navicular disease itself.

Facilities With MRI Technology

An equine MRI diagnostic test costs $700 to $1,500 or more. It’s available at universities and private clinics across the country, including: Virginia Equine Imaging, Middleburg, VA Washington State, Pullman, WA California Equine Orthopedics, San Marcos, CA North Carolina State University, Raleigh, NC Virginia-Maryland College of Veterinary Medicine, Blacksburg, VA Colorado State University, Ft. Collins, CO University of Florida, Gainesville, FL Rood & Riddle Equine Hospital, Lexington, KY Univ. of Pennsylvania Veterinary School, Kennett Square, PA Equine Specialty Hospital, Cleveland, OH Randall Veterinary Hospital, Cleveland, OH Fairfield Equine, Newtown, CT South Shore Equine Clinic, Plympton, MA How Does It Work? MRI is short for magnetic resonance imaging. The “heart” of an MRI machine is an extremely strong magnet. MRI machines large enough to image the entire body could pull a heavy metal object from your hands at a distance of several feet from the machine, pull watches off wrists, make metal objects fly through the air, etc.

The other integral part of the machine is a radio-frequency-wave generator. Inside the magnetic field, most of the electrically charged ions in the body are lined up in a way that they cancel out. A small number will remain unpaired and can be stimulated. The radio waves are set at a frequency that will make hydrogen ions in the body vibrate/resonate (the resonance part of the image). When the radio-wave stimulation is turned off, the hydrogen ions gradually return to their unstimulated state, giving off the extra energy in the process. The MRI machine detects the emitted energy and turns this into extremely detailed images. The machine can be set to focus on precise locations and depths within the body and can also be adjusted to focus on specific types of tissue, from bone to less-dense soft tissue like tendon, even joint fluid or blood. The result is images that have more detail and contrast than even a CAT scan. MRI scanners range from huge machines that can hold the entire horse to smaller “tunnels” useful for the distal part of the leg. These require the horse be under general anesthesia and lying on a table. There are also small units that can be used with the horse standing.

Since problems with the various tissues in the foot come from different causes, have different rates of healing and call for different treatments, being able to distinguish among them is important to determining how the lameness should be handled.

For example, if the only finding is fluid in the navicular bone, attention may be paid to better cushioning for the bottom of the foot, with rest and possibly a course of anti-inflammatory agents until the fluid subsides. Navicular bursitis as an isolated finding that would often be treated by local injection of the bursa, with shoeing and the horse’s activity changed to reduce stress on this area.

Problems in the various tendon and ligament structures may be the most critical to correctly identify, both because these structures are extremely slow to heal and also because inflammation and instability in them can easily result in damage to the bones and joints of the foot.

MRI studies of the feet are helping veterinarians to better interpret the results of local anesthetic blocks. Anesthetics injected into the navicular bursa will improve pain related to navicular-bone pathology and, sometimes, pain coming from deep digital flexor injury in the foot.

Blocks of the digital nerves below the fetlock (“heel” or “low” blocks) alleviate pain coming from the navicular bone itself but do not change or improve pain from other injuries. Injection of anesthetic into the coffin joint itself improves pain from arthritis/synovitis there, pain coming from the navicular bone and pain from deep digital flexor injuries, but not pain coming from inflammation of the collateral ligaments.

While radiographs, bone scans and ultrasounds may all pick up some types of problems, it’s clear that only an MRI scan can reliably detect hoof problems coming from all types of injuries. While an MRI is uniquely suited to the foot, its usefulness doesn’t stop there. Remarkably detailed images can be obtained of any joint, even the horse’s brain.

The photos are courtesy of Virginia Equine Imaging, Middleburg, Va., www.vaequine.com, call 540-687-4663.