Imaging of musculoskeletal problems in horses

After performing a lameness examination to localise the lameness to a particular joint or region the next step would be to image that area.

Over the next few days we will discuss radiography (x-rays), ultrasound and the advanced imaging modalities we might recommend to help us reach a diagnosis.

Radiographs

The lameness examination is important before we take any radiographs as we need to be able to accurately interpret the x-rays in context with how the horse presents clinically as some bone changes may be incidental findings and not clinically significant. Radiographs are most useful in showing us what changes are happening with bone and are less useful for soft tissue injuries.

 Radiography is also frequently used as part of a pre-purchase examination, either for specific areas of concern identified during the examination or for a routine survey of regions which commonly develop problems. 

 How do x-rays work?

X-rays are a type of electromagnetic radiation (similar to light), however they have a much shorter wavelength. This shorter wavelength allows the x-rays to pass through tissues. Tissues that have a low density such as muscles, tendons, ligaments, skin and the hoof capsule allow x-rays to pass through. Tissues that have a high density such as bone don’t allow x-rays to pass through.

The x-ray machine is made up of an x-ray generator and a plate. X-rays are shot out of the generator, towards the plate, and the plate interacts with the beam. As mentioned before, low density tissues allow the x-rays to pass through and hit the plate, whereas high density tissues stop the x-rays from hitting the plate. When the x-rays touch the plate, it turns black. Where the x-rays don’t hit the plate, it stays white. Therefore, bone will appear white on an x-ray.

It is important to take the correct precautions when taking x-rays. Lead gowns, thyroid protectors and gloves are worn to reduce the exposure to scattered x-ray beams. Regulation on who can take x-rays is important for human and animal health and safety. In Australia, people with a current radiation licence can operate an x-ray machine. This includes veterinarians, nurses and technicians.

How do we take Radiographs?

At Kilmore Equine Clinic we have highly trained vets and nurses who pride themselves in producing the highest quality diagnostic images possible. Your horse may require a mild sedative for the procedure to make the process as safe as possible for everyone involved.

We have a portable, digital x-ray machine. This means that we are able to take images both at the clinic and travel out to your own property with our equipment. Digital x-ray machines take detailed images allowing us to assess and diagnose problems that previously would have been much more difficult to accurately identify. These digital images can also be shared quickly and easily if a specialist or second opinion is required.

Although we all do things a little differently, most of us follow some basic general rules:

  • We take “standard” views. For instance, if we are radiographing a fetlock, it is common to take a front to back picture, two side to side pictures (one with the joint flexed and one with the leg in standing position), and two ‘diagonal’ pictures all at a specific angle through the joint.

  • We prefer to take a complete series when we are looking at a bone or joint instead of trying to make a diagnosis on one or two shots. We cannot make an accurate assessment without a complete series of radiographs.

  • If we see something suspicious, we try to visualize it on more than one view. Radiographs can have misleading “artifacts” on them. Sometimes artifacts can be from mud on the skin or air underneath the frog showing up as a black line through the hoof. If we can see it on several views, we can approximate its location and work out if it is indeed a real lesion.

  • It can sometimes be tempting to read radiographs right there on farm, but glare from the sun can take away a lot of detail from the image and if we are in a rush to get to the next appointment subtle lesions could easily be missed. For this reason, we usually like to take the images back to the office and transfer them onto our computer system to study the images closely before making a diagnosis.

  • If an image is blurry or not quite at the right angle it will simply be erased and retaken. There are many lesions that could be missed on a poor-quality image.

  • With each view, we evaluate the radiograph for bone shape, bone density, angles and any other abnormalities.

What conditions are commonly assessed with radiographs?

  • Osteoarthritis: Osteoarthritis is an inflammation of the joint that involves progressive degeneration of the joint cartilage, enlargement of the bone margins, and changes in the capsule around the joint. It can be seen on radiographs as narrowing of joint spaces, bone spurs or a ‘roughening’ of the bone surface. This condition is more common in older horses and is marked by pain and stiffness associated with activity.

  • OCD: Osteochondritis dissecans (OCD) is a relatively common developmental disease that affects the cartilage and bone in the joints of horses. Cartilage in joints with OCD doesn’t form normally; causing the cartilage and bone underneath it to become irregular in thickness and weak. This can cause the development of cartilage and bone flaps that may remain partially attached to the bone or break off and float around in the joint. These loose flaps and areas of abnormal cartilage and bone are visible on x-ray. They cause inflammation in the joint and over time may lead to the development of arthritis. OCD lesions are found commonly on survey radiographs of thoroughbred yearlings but can occur in any breed of horse.

  • Bone infection: Bone infections occur when bacteria is introduced through the bloodstream, traumatic injuries, or following surgery or joint injections. They are usually classed as osteitis (infection of the outer cortex) or osteomyelitis (infection involving both the cortex and medullary cavity - where the bone marrow is stored) Radiographically, bone infection appears as lucency/lysis (often mottled dark areas on x-ray). However, this can take several days/weeks to become apparent on radiographs.

  • Fractures:  Fractures are radiographically seen as radiolucent (dark) lines through a bone or as chips or slabs of bone. Finding them early and intervening is key. Horses can have catastrophic breaks but thankfully more commonly we see fractures that will resolve with surgery or rest.

  • Foot problems: Radiographs are particularly helpful for foot problems. They assist us with diagnosis of hoof imbalance, laminitis, penetrating injuries, infections and navicular disease.

  • Developmental Abnormalities/ Congenital Deformities: An early ‘premature’, or a full term but ‘dysmature’ foal may be born before it has finished normal development. In this case often the bones in the carpi (“knees”) and hocks are not fully mineralized when they begin to walk around. As a result, once the bones do calcify, they can be deformed. This should be assessed for early and these foals confined to limit movement before the bones mineralize.

With some of these conditions radiographic changes can lag behind the clinical picture of a horse by a few days up to a few weeks, so occasionally we may recommend to re x-ray the horse. Repeat x-rays can also be useful to monitor response to treatment or changes in a joint or bone over time.

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Zoe Meyer