In a paper entitled "The Relationship Between Nuclear Scintigraphy and Standing MRI in 79 Horses With Lameness of the Foot," investigators presented some new information regarding the diagnosis of foot lameness. The author, Mark Martinelli, DVM, MS, PhD, Dipl. ACVS, of California Equine Orthopedics, and Norm Rantanen, DVM, MS, Dipl. ACVR, a consultant radiologist, used nuclear scintigraphy and magnetic resonance imaging (MRI) to investigate difficult lameness cases in 79 sport horses. The findings, along with a brief discussion of each imaging modality, formed the centerpiece of the author's presentation at the 51st annual American Association of Equine Practitioners convention, held Dec. 3-7 in Seattle, Wash.

Traditionally the diagnosis of foot lameness involved regional anesthesia and radiographs. In this study, horses had lameness localized to the foot via regional anesthesia and/or a coffin joint block. Most of the horses had no radiographic changes, or the diagnosis was still in question following radiography. These horses then were imaged with nuclear scintigraphy and magnetic resonance imaging (MRI).

Nuclear scintigraphy proved very helpful in locating the area of injury in these horses, while MRI was useful in defining which structure or structures were involved. Nuclear scintigraphy is considered more sensitive than radiographs for determining specific problems of the foot. While radiographs are useful, they can only show changes in the bone that occur over time. For this reason, they are still very important in the pre-purchase examination. In contrast, scintigraphy can show areas of bone remodeling from recent injury before any appreciable changes are visible on radiographs.

How Scintigraphy and MRI Work Together

Nuclear scintigraphy is a metabolic imaging modality that relies on blood flow and bone remodeling to produce an image. A radioactive compound labeled with a bone-seeking agent is injected intravenously and circulates throughout the horse. The bone-seeking agent localizes in regions of injury or active bone remodeling, taking the radioactivity with it. It is this radioactivity that is measured, producing an image. Areas of injury or increased bone remodeling will appear darker within the image, making it easy to see problem areas.

This technique was first employed in Europe in the late 1970s, and it was investigated more extensively for equine foot lameness by Martinelli and coworkers in the mid-1990s. Today nuclear scintigraphy is considered the best way to investigate lameness in the foot. However, the lack of specificity of the technique (i.e., it tells you where the problem is but not what structure is involved) has led to inaccurate prognoses and generic treatment regimes.

Once a problem area is noted with scintigraphy, MRI can further define any lesions that are present in specific structures. This technology allows clinicians to clearly see all anatomical structures within the foot, including the soft tissue structures. (For more information on how MRI works, see Specificity was especially important in this study because almost 75% of the horses investigated had an injury to one or more soft tissue structures within the foot.

Study Results

The bone scan findings for the 79 study horses were divided up into distinct categories based on previous studies:

  • Focal and intense increased radiopharmaceutical uptake (IRU, also called "hot spots") associated with the navicular bone;
  • Focal IRU associated with the soft tissue structures in front of the navicular bone; and
  • Diffuse IRU associated with the coffin bone.

The investigators differentiated between diffuse IRU associated with the entire coffin bone versus IRU in one or both wings of the coffin bone. The latter pattern was important in this study because it seemed to indicate a problem associated with the collateral ligament insertion, which is a relatively new diagnosis for equine clinicians.

The MRI findings were divided into six different categories:

  1. Navicular disease;
  2. Navicular inflammation;
  3. Impar ligament desmitis;
  4. Deep digital flexor tendon (DDFT) tear;
  5. DDFT insertional tendonitis; and
  6. Collateral ligament desmitis.

There were 55 Warmbloods, 17 Quarter Horses, four Thoroughbreds, and three ponies in the study group. The researchers found that more than half of the Warmbloods had suffered an injury to one or more of the collateral ligaments of the coffin joint. This was considered to be one of the most important findings of the study because collateral ligament desmitis of the foot has not previously been included as a rule-out for horses with lameness originating in the foot. One of the reasons for this, hypothesized the investigators, is that the collateral ligament could not be visualized throughout its entire length prior to the use of MRI.

In contrast, more than half of the Quarter Horses in the study were diagnosed with navicular disease.

What Does it Mean?

Based on the results of this study, Martinelli and Rantanen still recommend that most horses have a bone scan done prior to MRI. This seems to be particularly important for the Warmblood breeds because they seem to have a much higher incidence of soft tissue injuries within the foot; nuclear scintigraphy is the best way to pinpoint the exact location of the injury.

The investigators feel that it is too early to make definitive statements about treatment, but they have made several observations to date. Most importantly, shock wave therapy seems promising for treating some soft tissue injuries of the foot, particularly desmitis of the collateral ligament.

In fact, some horses in this study had been rested prior to the diagnosis of collateral ligament desmitis without any improvement in the lameness. But once collateral ligament desmitis was confirmed via nuclear scintigraphy and MRI, focal treatment with shock wave therapy was recommended. The results were encouraging, with most of the horses returning to soundness. In those horses where shock wave therapy was not curative, another new treatment modality was employed. Interleukin Receptor Antagonist Protein (IRAP) was injected into the coffin joint. IRAP is a new anti-inflammatory therapy that is derived from the horse's own blood.

Take-Home Message

As the old saying goes, knowledge is power. Nuclear scintigraphy and MRI evaluation proved their worth yet again in this study by giving information about the exact nature and location of these horses' injuries, which helped practitioners select the right treatment to return the horses to soundness.

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