Integrating Imaging in Diagnosing Equine Lameness

Tracy A. Turner, DVM, MS; Dipl.ACVS,

St. Paul, Minnesota 55108

There are several different methods of imaging the equine digit. Imaging is of utmost importance because it will provide pathologic and physiologic information necessary to treat the specific condition. Imaging can be divided into anatomic and physiologic imaging methods. Anatomic imaging modalities include radiology, ultrasonography, computer-aided tomography, and magnetic resonance imaging. Physiologic imaging modalities include scintigraphy and thermography.

Radiologic techniques are the most commonly used to evaluate the horse for lameness. Utilizing plain film radiography, it requires multiple projections to evaluate any area. Essentially the practitioner must attempt to draw conclusions about a three dimensional object utilizing 2 dimensional pictures. Occassionally it becomes necessary to utilize radiographic techniques that provide more information. Contrast radiography is one such technique that provides information amount the articular cartilage and surfaces. It is of particular value in determining whether subchondral cysts commmunicate with the joint or in dilineating a subcutaneous tract. Generally, 5 to 10 ml of contrast injected into the joint is adequate. Pathologic diagnoses are usually made by radiography in conjunction with clinical examination.

Ultrasonographic examination can be used to assess any soft tissue in the horses body. The deeper the tissue that needs to be evaluated the lower wavelength probe needs to be used. The tissues are examined for changes in echogenicity. Changes in echogenicity correspond to changes in the tissue. Ultraonography is most useful in the evaluation of tendons and ligaments but it also can be used to evaluate muscle and cartilage.

Magnetic resonance imaging and computer-aided tomography are both interesting and high detail anatomic imaging tools. However, at this time they are of little value to the practicing veterinarian but may help provide insight to researchers about the pathology.

Physiologic imaging techniques would be those techniques that provide the evaluator with an image that reflects physiologic processes. Unlike anatomical imaging that reflects structure, these images give insight into metabolism or circulation. Thermography and scintigraphy provide the examiner with the opportunity to examine the entire horse. When combined with a thorough clinical examination, these methods are extremely useful in identifying injuries that may have otherwise gone undetected.

Thermography is the pictorial representation of the surface temperature of an object. It is a non-invasive technique that measures emitted heat. A medical thermogram represents the surface temperatures of skin making thermography useful for the detection of inflammation. This ability to non-invasively assess inflammatory change, makes thermography an ideal imaging tool to aid in the diagnosis of certain lameness conditions in the horse. The circulatory pattern and the relative blood flow dictate the thermal pattern which is the basis for thermographic interpretation. The normal thermal pattern of any area can be predicted on the basis of its vascularity and surface contour. Skin overlying muscle is also subject to temperature increase during muscle activity. Injured or diseased tissues will invariably have an altered circulation. One of the cardinal signs of inflammation is heat which is due to increased circulation. Thermographically, the "hot spot" associated with the localized inflammation will generally be seen in the skin directly overlying the injury. However, diseased tissues may in fact have a reduced blood supply either due to swelling, thrombosis of vessels, or infarction of tissues. With such lesions the area of decreased heat is usually surrounded by increased thermal emissions, probably due to shunting of blood.

Scintigraphy utilizes polyphosphonate radiopharmaceuticals administered by intravenous injection and followed by measurement of the distribution of the pharmaceutical by a gamma camera. Concentrations of the pharmaceutical can be detected, as the polyphosphonates bind rapidly to exposed hydroxyapatite crystal. This is generally in areas where bone is actively remodelling. This is the basis of the bone scan but prior to this the distribution of the drug goes through two other phases. It is these phases that can be useful to evaluate soft tissue changes. There are three phases, the vascular phase, the soft tissue phase, and the bone phase. The vascular phase or blood pool phase begins immediately after injection of the pharmaceutical. This phase is dependent on local variations in vascular supply. The most common clinical application for vascular phase scintigraphy is determination of patency of blood vessels. The second phase or soft tissue phase scintigraphy is performed while most of the pharmaceutical is in the extracellular fluid (ECF). This usually begins 1-2 minutes post pharmaceutical injection and lasts until significant uptake of the polyphosphanate by bone, usually 1-2 hours. The distribution of the radiopharmaceutical during this phase is due to local blood flow, capillary density, capillary permeability and regional ECF volume. Because inflammation causes an increase in blood flow, capillary permeability and ECF volume, inflamed tissues accumulate high levels of radiopharmaceutical. This is the basic principle behind evaluation of soft tissue injuries by scintigraphy. The bone phase is the most useful in that the uptake of the radiopharmaceutical always increases around areas of increased remodelling or vascularity. Since injured bone is under going more rapid remodelling, this is the basis for using bone phase to detect injuries. Scintigraphy has been most useful for the detection of lesions in bone and ligaments. Scintigraphy has been particularly useful in the identification of enthesopathy (damage to the insertions of tendons and ligaments on bone).

The purpose of any lameness examination is to be able narrow the problem to a regional diagnosis. Once a regional diagnosis has been made it is possible to assess the area utilizing some type of anatomical imaging modality. Assessment of those anatomical changes serves as the basis for any pathologic diagnosis that may be made, as well as, being important in determining prognosis. For these purposes radiography and ultrasonography are complimentary. Radiography provides information regarding the boney tissues. Radiographs reflect change that has happened. Ultrasonography provides information about boney comntour but moreimportantly provides insight to the soft tiisues that connect bone or provide support. Sonography can give much better insight into the activity of a lesion. That is, is the lesion active or not, do the soft tissues changes reflect an ongoing process or is it a chronic process. In addition, sonography can provide information about joint capsule, collateral ligaments, the consisitncy of joint fluid, and provide insight into the articular cartilage.

However, the pursuit of a regional diagnosis can be difficult. There are 3 instances where this can be frustrating. One, when diagnostic analgesia has failed to eliminate the lameness; two, when the lameness is too subtle to avail itself to diagnostic analgesic techniques, and three, when the patient is not amenable to handling or injection. In these cases, other methods must be used to evaluate the patient. This is where physiologic imaging modalities can be so useful. By providing insight into physiologic changes in the tissues, this can lead the examiner to evaluate those areas utilizing anatomic imaging methods.

Another area in lameness evaluation where imaging can be useful is in preventing injury. This requires the early detection of the physiologic change of injury. Although, the frequent use of an anatomical imaging modality can discover change in one region, physiologic imaging allows the assessment of the entire animal on a routine basis.

The utlization of imaging modalities in the diagnosis and treatment of equine lameness is absolutely necessary. These are the only reliable methods to assess the type and severity of the injury. In addition, the routine use of any method can provide insight into the stresses and strains of the athlete.