August 3, 2012 - Scintigraphy: Get It While It's Hot
It is not difficult for a practitioner to diagnose a complete displaced bone fracture in a horse. The animal normally will be lame, and there will be swelling. Radiographs will pinpoint the injury and its severity. However, it is not so easy when dealing with a nondisplaced stress fracture.
"Stress fractures," says M.J. Martinelli, DVM, PhD, Dipl. ACVS, of the University of Illinois, "are disruptions to the cortical bone microscopically without concomitant loss of the supporting architecture. If the bone is allowed to rest, the crack should heal uneventfully, and the bone should regain its normal physiological strength.
"In some cases, however, the bone may fail completely, and a displaced fracture results, often with disastrous consequences to the horse."
The disastrous consequence often is a catastrophic injury that can result in euthanasia of the injured animal. Researchers and practitioners agree that in many cases, the fracture could have been prevented if it had been known that a stress fracture or other damage existed. Removing the horse from training and competition and allowing the bone to rest and strengthen would have been all the therapy required in many cases.
But, how can such microscopic damage be detected? Many times radiographs fail to show even stress fractures.
Enter nuclear scintigraphy. This sophisticated technology uses radioactive material that, combined with a bone-seeking agent, is capable of locating stress fractures and other bone damage.
"Nuclear scintigraphy for imaging of the musculoskeletal system has evolved as a means of evaluating the metabolic activity of the bone and surrounding soft tissues," says Martinelli. "Nuclear scintigraphy is a metabolic imaging modality, much different than the anatomic imaging tools radiology and ultrasound. It involves the intravenous injection of a radioactive compound, such as technetium 99m, which has been converted to a radiopharmaceutical by labelling it with a bone-seeking agent. Nuclear scintigraphy relies on blood flow to soft tissues and on the inherent remodeling of bone to produce an image. Historically, it is considered to be much more sensitive than the anatomic imaging modalities (X rays), but not as specific in conveying diagnosis."
Nuclear scintigraphy first was used in humans to locate and identify cancer. In the equine world, it has been focused almost exclusively on locating stress fractures and bone damage. This is one area, says Martinelli, where human medicine has followed equine medicine. Scintigraphy in humans now is used extensively when stress fractures are suspected.
Scintigraphy was first used on a horse in 1978 in Switzerland. It came into use in this country the following year.
One of the leaders in developing scintigraphy use in the United States is the University of Illinois. In 1992, the College of Veterinary Medicine there began offering international training seminars in the use of scintigraphy. Those seminars are held on an annual basis and draw practitioners from around the world. To date, some 200 persons have attended.
Cost of the procedure varies, says Mar-tinelli, depending on the practitioner and the location and number of sites on the horse's body examined with scintigraphy. Exam costs, he says, can range from $300 to $1,000.
The procedure involves injecting the radioactive compound into the horse's bloodstream, normally via the jugular vein. The radioactive material is low-level and com-pletely safe for the horse, says Martinelli.
After the radiopharmaceutical agent is injected intravenously, the horse is placed in front of a gamma camera to record where the radioactive material tends to congregate.
There are three distinct phases of nuclear scintigraphy imaging that are separated by time as well as by the physiological structure being imaged.
The first is the vascular phase. This occurs within two minutes of intravenous injection. This phase is not commonly used, but it does highlight vascular flow to the tissues.
The second is the soft tissue or pool phase. It is so-named because the radiopharmaceutical agent pools in the extra-cellular spaces and soft tissues. This phase usually lasts 30 minutes post-injection.
The final (and perhaps most useful) is the bone phase. It is performed two to three hours after injection. It usually takes that much time for clearance of the radiopharmaceutical agent from the soft tissues surrounding bone.
Radiopharmaceutical uptake in this final phase is due to MDP (a bone-seeking agent) binding to exposed crystals of remodeling bone. Extensive remodeling will cause more of the radioactive compound to be bound to the exposed crystals, leading to formation of a "hot spot" or greater uptake of the agent. The remodeling in the hot spot indicates that the bone had suffered damage; in many cases a microscopic or stress fracture that cannot be detected by radiography.
"The classic example," says Martinelli, "would be a stress fracture in the long bone of a racehorse. Such a lesion may be difficult to detect in the average case, especially when few clinical signs are present after a race. It is not uncommon that a trainer or groom has noticed just a few bad steps after a race and presents the horse for examination solely for that reason. Detection with scintigraphy will often lead to appropriate rest or surgery prior to catastrophic fracture, thus saving the horse's career, and possibly its life."
Perhaps the greatest advance in the use of equine bone scintigraphy in the last 10 years, Martinelli says, has come in deciding when and how to use this procedure in combination with other diagnostic methods.
When clinicians at the University of Illinois Medical Teaching Hospital are asked to examine a lame horse, they first do a traditional workup. That includes a full history, a physical examination, and a lameness examination that might involve regional anesthetics, radiography, and/or ultrasonography. The full history is obtained from the owner or trainer, including the results of any diagnostics or radiographs from the referring veterinarian.
The physical exam includes an in-depth palpation and manipulation of the body and all four legs and a gait examination. Finally, flexion tests are conducted on specific joints in an attempt to exacerbate subtle lamenesses.
The decision on whether to scan the bone with nuclear scintigraphy, says Martinelli, usually involves one of three general scenarios.
The first scenario might be after the clinical examination. The lameness might be localized to a site that can be radiographed and/or sonographed. Most cases at that point are successfully diagnosed and undergo treatment. However, there are cases where radiographs of the site might be negative, even though the examining team is certain the injury area has been identified. It would be at that juncture nuclear scintigraphy would be employed.
In certain situations, Martinelli says, it is more efficient to go directly to scintigraphic imaging. A case in point might be a horse in which local anesthetics (nerve blocks) are ill-advised due to the severity of lameness.
In some cases, Martinelli said, bone scintigraphy is performed after radiographic confirmation of a localized lesion in order to track the healing process.
The second scenario involves the horse with multiple confirmed sites of lameness following the clinical examination. Scintigraphy can be used to examine each of the sites with one diagnostic test.
The third scenario involves horses which have a lameness that is subtle and difficult to localize, even after a thorough clinical examination. In those cases, Martinelli says, scintigraphy frequently is used as a screening procedure prior to radiography.
"Most of these patients are sport horses," Martinelli says, "and the majority are Standardbred or Thoroughbred racehorses. If the patients are in the midst of training and competing, a screening nuclear bone scan often highlights multiple sites in several different legs that exhibit abnormal uptake of the radiopharmaceutical. Although some of these sites are considered to be related to compensatory lameness, they may be significant to the prognosis and future management of the case. Indeed, human sports medicine physicians report pain-tolerant athletes who perform to the limit are imaged and exhibit multiple areas of uptake from other injuries that were unknown or ignored. The situation with equine athletes is similar."
Another emerging use of scintigraphy, says Martinelli, involves pinpointing lamenesses that originate in the foot. Often, those lamenesses fall under such vague terms as "navicular syndrome" or "caudal heel syndrome."
In one study, Martinelli used scintigraphy to evaluate the forefeet of 50 performance horses presented with lameness. Of the 50 horses, 14 were Thoroughbreds, 15 were Thoroughbred crosses, 12 were Warmbloods, and the other nine were mixed breeds. They ranged in age from four to 15 years. Thirty-four of the horses were involved in jumping activities; 16 were used on the flat. Thirty-four were presented because they had demonstrated definite signs of foot lameness.
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Scintigraphy was used to determine where in the foot there was an increased radiopharmaceutical uptake (hot spot), possibly indicating that area was the source of pain. Four obvious patterns of increased uptake were apparent. They include the coffin bone (third phalanx), distal attachment of the deep digital flexor tendon to the coffin bone, navicular bone, and sidebone (ossification of the lateral cartilages of the coffin bone).
"Nuclear scintigraphy," says Martinelli, "may be one way to convey not only a more specific diagnosis for the cause of foot lameness than conventional investigative techniques, but a more accurate prognosis as well."
The study indicates that more uses will be found for scintigraphy for an expanding list of problems. And while that is going on, scintigraphy will continue to be used to spot stress fractures that might precede a catastrophic injury. It would appear that the judicious use of scintigraphy could cut into that number.
- Les Sellnow, thehorse.com