Diagnostic Imaging For Lameness

By Christy West

X-rays, ultrasound, MRIs and other diagnostic imaging methods might not often cross a farrier's path. But when they do, they offer a tremendous opportunity to learn about the inner workings of the horse's foot as well as information to help that particular case. At the fourth International Equine Conference on Laminitis and Diseases of the Foot, held Nov. 2-4 in West Palm Beach, Fla., several veterinarians offered discussions of various diagnostic imaging options for foot pain.

Radiographing (X-Raying) The Laminitic Food

"The owner, farrier and veterinarian need solid information to treat the laminitic horse," noted Bruce Lyle, a foot-focused veterinarian from Aubrey, Texas. This solid information is provided by specific radiology techniques, he said.

"The wishy-washy attitude of 'some are going to get better regardless of what you do, and some are going to crater regardless of what you do' (as an excuse for not taking radiographs) is counterproductive to the profession, the industry and most importantly the horse," he warned.

Any diagnostic images must be taken with consistent, disciplined techniques, he noted. He described a radiographic protocol and schedule used by many clinicians who have experienced success in treating founder cases as well as other lamenesses, noting the following points:

* Any horse with significant lameness that is sound with typical foot blocks is a potential laminitis case and should be radiographed. Don't wait until they've got dished feet and diverging growth rings to radiograph them; do it early so you can figure out how to keep them from getting that bad.
* The dorsal wall must be marked with paste, not wire, to allow measurement of the horn-lamellar zone. Wires do not follow hoof contours and can throw you off.
* Both feet must be positioned on blocks, with the hoof at the very edge and touching the cassette to avoid artifact (imaging errors).
* Mark X-rays with names and dates; the time frame is very valuable information.
* He typically takes two or three views of each laminitic foot, including lateral-medial (side) and anterior-posterior (front) views. Oblique or solar margin views can be added.

Once the radiographs are taken, they must be measured and interpreted. Lyle believes rotation by itself is not valuable for prognosis. "I have sound horses in my practice with 45 degrees of rotation and lame ones with 0 degrees," he noted. "So now what?" He looks at the following radiographic measurements on a lateromedial radiograph to diagnose and monitor many foot problems including laminitis.

* Horn-lamellar (HL) zone: This is expressed as two linear measurements taken on a perpendicular line from the coffin bone to the wall marker. The first is just beneath the extensor process, while the second is taken at the bone's tip. The two measurements are identical in healthy feet. Depending on breed, age and use, typical healthy adult feet measure around 15 to 21 mm. "How you treat a (laminitic) horse starting at 20 to 28 mm vs. one at 16 to 18 mm is very different, and it's taken me a lot of years to see that difference," he commented.

* Sole depth: This is the vertical distance between the apex of the coffin bone and the bottom of the sole, not including any cup. Unstable laminitis cases often have a decreasing sole depth and an increasing HL zone, and increased sole depth often correlates to a horse that is healing.

* Coronary band-extensor process distance: This usually refers to the vertical distance from the hairline to the extensor process of the coffin bone, but Lyle prefers measuring this as a direct linear distance rather than a purely vertical one. He believes this distance tends to approximate the HL zone in healthy feet and thus provides a baseline for any pathology present.

* Digital breakover is the horizontal distance from the apex of the coffin bone to the forward-most ground contact point. Reducing this distance often has therapeutic value.

* Palmar/plantar angle denotes the angle the wings of the coffin bone make with the ground and/or the shoe.

These measurements play a role in return to normal activity as well. Lyle allows a return to activity only when:

1. THL zone and sole depth are stable or increasing over at least two consecutive radiographic exams at least 7 days apart.
2. The horse is off pain medication.
3. The horse is sound at the walk with no orthotics.
4. The venogram is normal (if the horse is to return to athletic training after an acute episode with no change in HL zones).

"Disciplined marking and measuring of radiographs allows collection of objective data that, when coupled with a case history, can provide insight into the pros and cons of various shoeing, surgical and medical therapies, as well as guiding a return to training or otherwise normal activity," he concluded.

Interpreting Radiographs

"In order to interpret radiographs or other images, you must look at how form relates to function and lack thereof," Lyle advised. "Think through the sources of pain and learn to look at the influence of foot components on one another and on other components of the lower leg, and put the foot in motion mentally. Add this knowledge to the results of your physical examination and diagnostic anesthesia when necessary, and you can determine the location of pain. Combine this knowledge with a thorough history and radiographic examination, and then you can rationalize how, when and why a horse hurts and create plans to treat the condition.

"Radiographs allow objective analysis of parameters vital to each individual foot. Then surprises are avoided or minimal," he said.

"Radiographic assessment of the digit can provide objective information to enhance communication," he concluded. " 'He's thin-soled' becomes 'There's not but 5 mm between him and Mother Earth.' 'We've got to get some heel under this horse' becomes 'Look at that negative palmar angle and thin mass of sole under P3. By trimming from the third nail hole forward at this angle, we can instantly rotate him forward and let the heels breathe.' Films done following corrective work can then give evidence of what was or wasn't accomplished."

Venograms

"I have had so many laminitis cases where venograms (radiographs taken of the foot after contrast dye has been injected to show the blood vessels) have changed the horses' and their owners' worlds," began Lyle. "In my opinion, this is a life-changing procedure."

Venograms aren't just for laminitis, however. Lyle uses them to gain additional information on many foot problems. "The digital venogram can provide rapid and relatively inexpensive insight into cases of laminitis, abscessation, bruising, scarring, tumors and osteitis," he noted.

In a nutshell, he explained, dye should fill all of the blood vessels. Where it doesn't, damage or compression from imbalance or swelling has compromised blood flow and areas of the foot without blood flow are destined to die (in high-scale laminitis cases) if blood flow isn't restored. But when it's known where blood flow is compromised, the foot's mechanics can be changed to restore it in a window of opportunity provided by the disease and healing processes.

"Venographic studies suggest that laminitis is not an all-or-nothing disease, and that by better defining the problems within each individual foot, specific therapies can be applied to address the problems within that foot," he explained. "Seventy-five percent correctness may equal DVM in vet school, but three-out-of-four viable feet usually equals failure with laminitis patients, and it is the one failed foot that is remembered, not the three that were saved. Venograms can show you what is different in each foot and help you tailor your treatments to increase success rates on all feet."

Ultrasound

Ultrasound can visualize many structures and injuries that block to palmar digital (PD) nerve blocks; it's often underutilized, began Mary Beth Whitcomb, DVM, assistant professor of large animal ultrasound at the University of California, Davis.

She showed several cases to illustrate pastern and foot injuries and structures that can be visualized with ultrasound. Following are several problems she uses ultrasound to identify in cases of apparent foot lameness:

* Deep digital flexor tendonitis.
* Injury of the straight distal sesamoidian ligament.
* Injury/tendonitis of the superficial digital flexor tendon.
* Navicular bursitis (especially easy to see in foals).
* Coffin joint collateral ligament injuries.
* Palmar effusion (distention) of the coffin joint.

In summary, she noted several points:

1. Traditional pastern ultrasound examination is useful for horses blocking to a palmar digital nerve block.
2. Many foot structures can be visualized with ultrasound.
3. Ultrasound examination requires skill and a microconvex transducer (which has a divergent beam, providing an increased field of view to visualize structures within the foot).
4. Although ultrasound cannot replace MRI of the foot, it is less expensive and has many valuable uses.
5. It provides screening before more advanced imaging procedures.
6. Ultrasonographic evidence of injury might provide enough of an answer for many owners.

"Ultrasound of the distal extremity, especially the palmar structures of P2, may provide adequate information to explain the source of foot lameness without the use of advanced imaging procedures," she advised. "Such information can then be used for prognostication and defining treatment options."

Magnetic Resonance Imaging (MRI)

"MRI is one of the most prominent technological advances in the field of equine veterinary medicine," said Mark Silverman DVM, Ms, of San Dieguito Equine Group. "We're in the relative infancy of this technology and because of that, the interpretation of the images is constantly evolving. Along with the great diagnostic power of MRI is the potential for the creation of artifacts.

"Is it a panacea or Pandora's box? MRI allows us to visualize structures that we couldn't appreciate before, giving us a whole new set of things to worry about," he said. "We've added structures to our list of diagnostic possibilities that we couldn't even see before. Historic locations of foot lesions included the navicular bursa, coffin joint, navicular bone (NB), sidebone, coffin bone and collateral ligaments of the coffin joint.

"Now we look for these plus deep digital flexor tendon (DDFT) lesions, impar ligament desmitis, collateral sesamoidean ligament desmitis, navicular bone inflammation, adhesions of the DDFT to the NB, and subchondral bone injury."

He described and displayed examples of several artifacts that can confound interpretation of MRI images, including voids from nail fragments, blood flow, limb movement and averaging artifacts.

"You really have to clean these feet up; even rust in a nail hole can mess you up," he cautioned.

A thorough lameness workup and clinical examination must be done first to focus MRI evaluation, he stressed. "MRI is more like a microscope than a broadsword," he explained.

There are different types of MRI systems: Standing vs. down magnets (where the horse must be put under general anesthesia), and high vs. low-field magnets.

Usually down magnets have higher field strengths, that produce sharper images with less noise and thinner "slices" at a more rapid rate. However, they're more expensive and carry some additional risk to the horse because of the general anesthesia.

"Successful treatment of lameness includes knowledge of biomechanical implications of lesions in different areas," he concluded. "MRI is a powerful yet complex tool that allows imaging of all tissue types and has the potential to image physiologic function, but there are many potential sources of artifact in MRI."

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