According to a Washington state program, every horse involved in a fatality or a breakdown at a Washington state racetrack
must be examined at the veterinary school. About three years ago, members of the radiology group at Washington State University's
(WSU) College of Veterinary Medicine realized this was a wonderful opportunity. They began to image horses from Emerald Downs,
Auburn, Wash., not only to document the areas of injury, but also to look for preexisting diseases everywhere else within
the lower skeleton.
Funded by the Washington State Horse Council, radiography and MRI were performed on all horses that came to Washington State
University for necropsy. Occasionally, a computed tomography (CT) scan and nuclear scintigraphy were performed as well. The
emphasis of the research, according to Russell Tucker, DVM, Dipl. ACVR, chief of radiology at WSU's College of Veterinary
Medicine, and resident graduate student Tom Wilkinson, was to recognize underlying pathology in which MRI shows changes that
are not seen on radiographs and seldom seen on CT scans. MRI and radiographic studies identified patterns associated with
fractures in these horses. Those with catastrophic fractures in one limb often showed evidence of microfractures in the contralateral
limb as well. "The leg with a condylar fracture would essentially explode," Tucker says. "Often in the contralateral limb
of the same horse, we'd see preexisting small breakdowns—contusions or stress fractures in the osteum."
Damage and repair: All performance horses have this process of damage and repair going on to their bones, says noted equine
veterinary surgeon Larry Bramalage. It's the uneven loading of vulnerable areas that can cause fractures.
According to Tucker, the microfracturing in the contralateral leg was not noticeable when the legs were looked at grossly
unlike other regions common for stress fractures where a periosteal reaction is often visible.
Diagnostics that can be performed on the standing horse are preferred. "Although a standing equine MRI used for horses is
not as sensitive as the higher field-strength units and not of the same high resolution for diagnostics, it might be a good
way to screen for microfractures before the injury advances to a catastrophic fracture," Tucker says.
During this project, nuclear radioscintigraphy was also performed using a radiopharmaceutical that adheres to bone tissue.
This technique showed areas of abnormal bone remodeling and is a sensitive way to pick up stress fractures. Nuclear scintigraphy
can be performed on standing horses by using a sedative and an external camera.
"With the help of MRI, and now with the use of nuclear scintigraphy, it may be possible to screen horses without risking general
anesthesia," Tucker says. "Standing MRI and nuclear scintigraphy can identify an area of general concern. Horses showing a
pattern typical of microfracture and/or trabecular breakdown could be put on a lesser regimen of training or rest."
These methodologies are complementary. Nuclear scintigraphy does not replace MRI, and low-field strength MRI is not the same
as high-field strength MRI, but they are all helpful to offer some diagnostic possibilities.
Tucker says data from this study showed that it was important to conduct a complete examination on each patient and not only
focus on the site of breakdown. Since condylar fractures occur as a result of sclerosis of the distal metacarpus (cannon bone),
Wilkinson did a quantitative study in which, by using MRI, he evaluated the degree of sclerosis, or hardening, of the subchondral
bone. In joints there is articular cartilage and subchondral bone, not periosteum. The bone plate supports the cartilage.
"The subchondral bone is a very delicate substance," Tucker says, "so this is where one sees osteochondrosis, a typical disease
in which the bone underneath the cartilage does not form correctly. The cartilage above the bone breaks down, caves in and
What this study found were specific areas of subchondral bone sclerosis and remodeling, which leads to fissuring. Lateral
condylar fractures have a well-known orientation as to where they occur. So horses are at higher risk for condylar fractures
based on specific areas of subchondral bone sclerosis. During an MRI examination, the trouble areas light up as distinct patterns.
They are red flags. "If a horse with this type of subchondral damage continues to train or race, it is basically running on
dangerous wheels and can have a blow out at any time," Tucker says.
"Unless we do more MRIs and more CT scans as a routine matter, we'll never be able to pick out these horses as potential fracture
cases," Nixon says. "Quite honestly, I doubt that we're going to be able to convince trainers and owners of the need to anesthetize
racehorses in the midst of their campaigns to do a CT or MRI to define an obscure lameness. Standing MRI is probably the answer,
but it is not yet sensitive or fast enough to pick up predisposition to these sorts of fractures. The differences in detail
on low-field strength standing MRI and the high-field strength MRI units requiring general anesthesia is quite considerable.
But the gap is closing. A program of a bone scan followed by a standing MRI or CT may be potentially useful to identify these
horses. It's just a question of whether the sensitivity is good enough to separate those that are likely to fracture versus
those that are not. Right now the technology is still being developed to be able to do that."