Bone Scintigraphy (AAEP 2005)
Using scintigraphy (also called bone scans), "I have imaged 5,000 horses in my university referral practice over the last 12 years and have enormous respect for this imaging tool," said Michael Ross, DVM, Dipl. ACVS, professor of equine surgery at the University of Pennsylvania's New Bolton Center. He presented a discussion of this imaging modality at the 2005 American Association of Equine Practitioners Convention, held Dec. 3-7 in Seattle, Wash.
Scintigraphy is an imaging modality that reveals "hot spots" of bone and muscle metabolism that can indicate remodeling due to stress, fractures, or other causes. Many practitioners use it as a screening tool to pinpoint the location of a bone problem. "Many questions about clinical lameness that were not answered with radiography are now understandable using scintigraphy," he noted.
After imaging more than 400 horses per year for the last several years, Ross believes scintigraphy is a valuable tool in lameness diagnosis, particularly since it can be used to image the upper and lower limb, unlike magnetic resonance imaging. His clinic is also set up to handle views of the sole using a small camera in the ground.
"There's nothing like a hotspot in scintigraphy to make you refocus on your X rays," he noted. "It prompts you to take special views to find the problem." Seeing problems on scintigraphy with similar clinical signs has caused him to make changes in his radiographic protocols for horses with certain clinical signs.
The modality has also led to new diagnoses. "Using bone scintigraphy, we have described a new clinical syndrome causing lameness in some horses," Ross revealed. "We coined the name enostosis-like lesions (ELLs) to describe these medullary opacities (areas of dense bone within the medullary or marrow cavity)."
He reviewed several cases and their scintigraphic findings, commenting that this imaging modality has helped him to understand subtle hind end lamenesses. He made the following observations based on his use of scintigraphy:
- A truly negative bone scan helps point toward a soft tissue problem.
- Many horses have numerous common sources of lameness and seldom a single source of pain.
- Bone remodeling in response to stress is a normal process (resulting in stronger bone that can better handle that stress), but maladaptive bone remodeling signifies a problem. Such problems can be precursors of fractures or other, more significant problems.
- Scintigraphy is also good for monitoring healing.
- Proper case selection is important; scintigraphy is not useful for every lameness case.
- The most common cause of lameness in all types of racehorse and non-racehorse sport horses involves the foot or digit.
- Scintigraphy is one of the few ways to diagnose abnormalities of the upper limbs and axial skeleton.
- It can be used to see generalized muscle damage of rhabdomyolysis (tying-up) or focal areas of traumatic damage. However, focal damage is rarely the cause of the lameness; he noted that the problem is usually found in the other front or hind limb, and it is likely a compensatory injury.
- The sensitivity of stifle scintigraphy seems to be low, and many conditions of this joint fail to produce marked scintigraphic changes.
- Bone scintigraphy should not be used to diagnose proximal suspensory desmitis.
- Multiple hot spots are common, particularly when imaging any upper-level sport horse or seasoned racehorse. Therefore, false-positive scans are common and are not as important as false-negative scans.
- Most horses plait or ropewalk because they have bilateral lameness in the pelvic area.
- If a horse is more lame at the walk than the trot, it is often an upper limb problem.
- Subchondral (just beneath the joint cartilage) bone pain often results in a short, choppy stride.
- High-speed lameness is an early manifestation of the common causes of lameness in the respective racing breeds.
- Thoroughbreds more often have cortical and subchondral bone injury, and upper limb problems, than Standardbreds.
- The distal phalanx seems prone to the effects of maladaptive or nonadaptive remodeling, and in the forelimbs, distribution between right and left forelimbs can be determined by the effects of counterclockwise racing.
- Multiple views of a hot spot should be taken to pinpoint its location.
- Bone scintigraphy is best used in lame horses in which lameness has been localized, but it has also proven to be useful in comprehensive sports medicine examination of horses with poor performance.
- Scintigraphic findings should always be interpreted in conjunction with historical and clinical information.
- Distance between horse and imaging equipment is a key factor.
- Lesions on the medial (inner) aspect of the limb can easily be missed if only a lateral (side) view is taken.
- The lag phase (time between traumatic bone injury occurrence and bone remodeling) has not been studied in horses.
Factors Affecting Scintigraphy Value
- Scintigraphy is less useful in older horses that get little exercise.
- Cold temperatures affect blood flow to the extremities and can hamper use of scintigraphy. Exercise before evaluation helps.
- If a chronically lame horse has been at rest for some time, chances of seeing bone remodeling with scintigraphy are greatly reduced.
About the Author
Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.