Hoof Radiographs: More Than Meets the Eye

Something on the X rays looks a bit off—is it significant or of no consequence?

The signs are subtle and they’ve come on slowly, but it’s become quite clear that your mare is lame. She shifts right to left and back again, like ocean waves lapping the shore, making it difficult to know exactly what hurts. Just about the only thing you can grasp from watching the changing tides of her lameness is which foot is affected.

You call your veterinarian, who conducts a complete examination that includes radiographs (X rays) of your mare’s foot. The resulting images are far from textbook, but which structural changes are causing her pain? And which ones are likely little reason for concern?  

“An X ray might be black and white, but interpreting those X rays and deciphering what ‘abnormalities’ are important and which aren’t are definitely filled with shades of gray,” says Tracy Turner, DVM, MS, Dipl. ACVS, ACVSMR, of Anoka Equine Veterinary Services, in Elk River, Minnesota.

In this article we will delve deeper into the radiographic changes that owners and veterinarians might (or might not) find on the X rays of chronically lame, acutely lame, acutely lame due to wounds or trauma, and even nonlame “healthy” horses.  

Considering the Chronically Lame

Onset of chronic lameness is typically slow or on-again-off-again—you might spend months wondering if you actually need to call the vet. Ongoing or recurrent lameness in horses commonly stems from two conditions. One is navicular disease/syndrome, which is forelimb lameness associated with pain in the navicular bone, which lies at the back of the horse’s hoof capsule, and related structures. The other is laminitis, which happens when the tissues that suspend the coffin bone, or third phalanx, inside the hoof fail. Veterinarians can use radiographs to diagnose the disease responsible for a horse’s chronic lameness, as well as follow its progression.

Expected findings

Horses with navicular disease typically have “classic” radiograph findings, making diagnosis fairly straightforward. Veterinarians recommend taking a minimum of five views—X rays captured from different angles—to diagnose or evaluate navicular disease. On those radiographs you might see:  

  • “Lollipop” or “cone” shapes that aren’t as white as the normal surrounding bone at the distal (lower) border of the navicular bone;
  • Calcification, the formation of excess mineral deposits that eventually harden to bone, which causes the proximal (upper) or distal borders of the navicular bone to appear more white than normal;
  • Bone loss or changes along the flexor surface of the navicular bone (the underside, around which the deep digital flexor tendon, or DDFT, runs to meet the coffin bone); and
  • In some cases, “holes” (bone lysis/destruction) or cysts in the navicular bone, which again appear less white than normal.   

For horses with chronic laminitis, common radiographic findings include “sinking” or “rotation” of the coffin bone. You might also see bone loss or remodeling of the coffin bone.

Unexpected findings

Rarely, veterinarians find cystlike lesions in the coffin bone, bi- or tripartite navicular bones (i.e., the bones are not completely calcified and appear to be fractured in two or three pieces, but aren’t), and distal border fragments of the navicular bone. These cause pain in some horses but are inconsequential for others.

“In some Warmbloods, distal border fragments of the navicular bone show up frequently, with or without any associated lameness,” notes Turner.

If your veterinarian notes any of these on radiographs, he or she might recommend additional diagnostic imaging techniques, such as MRI and ultrasonography, or even referral to a specialist to help determine if those uncommon findings are contributing to foot pain.  

Assessing the Acutely Lame Horse

Aside from trauma or injury, a variety of conditions can cause acute, or sudden and severe, lameness. Many soft tissues in the foot, including the DDFT and distal sesamoidean impar ligament, can be damaged. An abscess or bruised sole can cause a rapid-onset lameness so severe it looks like the horse has fractured a bone. It is important to note that soft-tissue injuries, abscesses, and bruises don’t always (or even typically) manifest radiographically because there isn’t enough contrast between those tissues to “distinguish” what is wrong.

Expected findings

“In an acute severe lameness, my differentials are abscess (of the soft tissue structures of the foot) or fracture of either the coffin or navicular bone,” says Turner. “The collateral cartilage (located on either side of the coffin bone, thought to function in hoof expansion/shock absorption) can fracture, but this is rare. More often, people see a calcified collateral that is incompletely calcified (on the X rays), and it looks like a fracture but is not.” This would be diagnosed as sidebone.

It’s important to note that a fracture can take 10 to 14 days to reveal itself because the hoof capsule does not allow obvious bone displacement. Rather, space between fractured pieces of bone is what veterinarians observe on X ray to help diagnose a fracture.

Foot abscesses might be visible on radiographs, but only if gas (produced by bacteria in the abscess) provides contrast between the pus and the hoof tissues and delineates abscess location. Luckily, Turner says, “The new digital radiographic systems are better at finding abscesses because they give you better contrast changes in the soft tissues compared to traditional systems.”  

Unexpected findings

Veterinarians that anticipate a fracture of some kind in an acutely lame horse might also stumble across other “hitchhiking” radiographic lesions. These freeloaders include virtually any conceivable sort of chronic change due to degeneration or osteoarthritis, navicular changes, etc. Although the term “chronic” implies long-standing, be aware that these changes can occur even in young horses that have undergone musculoskeletal stress.  

Although laminitis does not usually occur unilaterally (in one foot only), the practitioner should not rule it out completely in an acutely lame horse. A horse can develop unilateral laminitis if he is sufficiently lame on the complementary limb, resulting in more weight bearing on the uninjured limb. As a result, the once-sound limb is now the primary problem, a condition commonly referred to “supporting-limb laminitis.”

Wounds and Trauma: Imaging the Acutely Injured

Sure, horses might seem lithe and graceful, but they are ridiculously accident-prone, with their feet frequently falling victim to trauma and/or injury. Again, veterinarians are unlikely to note soft-tissue injuries via radiography, but they can use X rays to rule out bone and/or joint involvement in injured horses, such as those with wounds or blunt trauma from a horse kicking a stall wall, for instance.

Expected findings

Veterinarians must look for foreign bodies, fractures, and soft-tissue injuries in injured horses. In addition to “plain” radiographs, practitioners can use a commercial contrast agent that shows up on X rays to help analyze wounds (e.g., whether a wound enters a joint space) along with bursas (fluid-filled structures near joints) and other structures. Turner encourages veterinarians and owners to explore this option, saying, “Contrast agents can give important insight into surrounding soft tissues.”

Unexpected findings

Surprise findings can crop up when radiographing an injured horse with no previous history of lameness.  

“For me, any chronic changes or OCD (osteochondritis dissecans) fragments, etc., are unexpected and must be assessed and explained as to their relation to the lameness,” says Turner.

The Prepurchase Picture

Many seemingly healthy, sound horses undergo routine prepurchase exams before sale. Just to be thorough, most veterinarians recommend taking radiographs of the foot (and frequently other joints, such as the stifles, hocks, fetlocks, or knees, depending on breed). In some cases the prepurchase examination can be an “angst-ridden procedure for the vendor, purchaser, and the examining veterinarian!” says Sarah E. Powell, MA, VetMB, MRCVS, partner at Rossdales Equine Diagnostic Centre, in Newmarket, U.K.

The purpose of the prepurchase examination is to determine if the horse is “fit for purpose,” like in the decision tree below, which can be easier said than done.

“During a prepurchase examination it is not uncommon to identify radiographic abnormalities, especially in a sport horse or racehorse,” she says. “Experience is necessary for determining the relevance (of the radiographic findings) to that particular individual. Not all horses with abnormal X rays will go on to develop lameness. Conversely, a horse which shows lameness during the examination may have normal X rays.”

Expected findings

Powell explains that what you might expect to find in an apparently sound horse’s foot varies, depending on age and how many miles are on the clock. “As a horse ages, the bones and joints, particularly of the lower limbs, accumulate signs of wear and tear due to repetitive loading stresses,” she says.  

Stress-caused damage detectable on radiographs includes:

  1. Increased bone density sclerosis (see sidebar, page 39), which appears whiter on radiographs than normal bone.
  2. Remodeling changes, particularly around joint margins. These can represent either adaptive changes, which are necessary to strengthen bone, or maladaptive changes (harmful ones such as bone bruising). A proportion of horses with maladaptive changes might either be lame at the time of examination or at risk of developing lameness.  

It’s important to recognize that breed differences can affect what you see on foot radiographs, whether the horse is sound or unsound: “Heavier horses and draft breeds, for example, can have quite dramatic bone remodeling surrounding the (high-load-bearing) pastern joint, which should not be confused with osteoarthrosis,” Powell says. “They can also have quite dramatic modeling of the bony attachments of the collateral ligaments of the coffin joint. In a fine-limbed breed that latter change could be a sign of ligament disease, and there would be a greater cause for concern.”

Unexpected findings

“We occasionally get a surprise when taking radiographs of a sound horse with no previous history of lameness, including large subchondral (located just under the cartilage surface within a joint) bone cysts or bone fragments either at the margins of joints or at the site of the major soft-tissue attachments within the foot,” Powell notes.  

In a small number of yearlings undergoing presale radiographs, she and her colleagues have seen bi-partite navicular bones, which she says are congenital (present at birth). “Both of these types of findings may be a real concern for future soundness, particularly when seen in young, unproven horses,” she says.  

Be aware that degenerative changes, such as sidebone or the chips off the navicular bone mentioned earlier, might also appear on X rays.  

“These changes do not just occur in older horses, as they have little to nothing to do with age,” says Turner. “Although ‘arthritic’ changes frequently occur with age, we also see age-related remodeling of both the coffin bone and navicular bones, which are more accurately considered degenerative changes.”  

Again, these degenerative changes might or might not be clinically relevant, and your veterinarian must interpret them based on physical exam findings and the horse’s history.

Which Findings are Relevant?

So now that we know what various expected and unexpected findings might crop up on a foot radiograph, its time to ask: Which ones are causing lameness, or is it something else entirely?

“A thorough clinical examination is extremely important, of course, which gives invaluable information as to the foot shape, conformation, and quality of the hoof horn, etc.,” Powell says. “A small region of bone resorption in the margin of the pedal bone (coffin bone) can indicate a number of things, but if a crack is present in the hoof at this level, and the crack is painful when hoof testers are applied, then this has added relevance.”

In some instances the presentation of the lameness itself gives clues as to the underlying cause or renders certain radiographic findings more significant than others. For example, a veterinarian examining the horse will piece together the radiographic information with observations such as whether the lameness is worse on hard or soft ground or if it’s worse with the affected limb on the inside or outside when circling.

To complicate matters more, poor-quality radiographs and not quite catching the “right” angle can also impact a veterinarian’s ability to interpret X rays. These factors can cause radiographic “artifacts,” or abnormalities due to poor technique, patient movement, etc. These can be misleading during diagnosis.

So, how do you know if your rads are rad or if you are being had?  

Renate Weller, DVM, PhD, MSc, Vet Ed, MRCVS, professor of comparative imaging and biomechanics at the Royal Veterinary College in Hertfordshire, U.K, says it’s quite simple: Can you see everything you are interested in?

One of the best ways to minimize radiographic artifacts is to prepare the horse’s foot properly for the procedure.

“Although there are exceptions to the following ‘rules,’ it is preferable to remove the horse’s shoes, trim the foot with hoof knives to remove loose horn and dirt, pack the foot with Play-Doh, and place a metal marker on the dorsal hoof wall (the front of the hoof) and a pin in the Play-Doh in the front of the frog,” Weller says.  

Yes, movement of either the horse or the person holding the plate during the radiograph can blur the image. What makes the “clinical picture” blurry, however, is when the veterinarian identifies more than one abnormality.  

Always entertain the idea that more than one issue can be the cause of any lameness, says Mathieu Spriet, DVM, MS, Dipl. ACVR, Dipl. ECVDI, associate professor of Clinical Surgical and Radiological Sciences at the UC Davis School of Veterinary Medicine.  

“If we consider that multiple abnormalities could be contributing to the lameness, we will ultimately have more success treating foot pain,” he says. “For example, horses can injure both the deep digital flexor tendon and the distal sesamoidean impar ligament simultaneously. In turn, those injuries can cause resorption or breakdown of the coffin bone and promote the formation of cystlike lesions of the bone.”

Take-Home Message

There’s more to capturing and interpreting foot radiographs than shooting a few views of a horse’s feet, watching him trot, and calling it XYZ condition. Careful preparation of the horse and good-quality X rays will help practitioners obtain as much information as possible. In complex cases, additional diagnostic imaging approaches might be beneficial.