In February, the racing world was stunned when retired 1999 Horse of the Year Charismatic sustained a broken pelvis and died from a resulting severed artery at Old Friends. Although the circumstances of Charismatic's accident were especially unusual (since he was both retired and at rest in a stall), pelvic fractures of varying degrees can and do happen in Thoroughbreds, especially those in active training. Traumatic accidents happen, but a horse's pelvis also can fracture from the day-to-day stress of training and racing.
“It's caused by repetitive-cycling stress,” said Dr. Larry Bramlage, lead surgeon at Rood & Riddle Equine Hospital in Lexington. “But the event that causes it to become clinical or painful is often that maximum workload effort that horses put out whenever they push themselves out of the gate. That's why you hear it associated with that very commonly.”
Bramlage explained the stress of training creates microfractures in bone as part of the bone-remodeling process. The horse's body then replaces that stressed bone with new, stronger bone. This constant turnover to replace bone loss with new bone enables the horse's body to repair the damage before more microfractures occur. This is true of all bones in a horse's body, not just the pelvis. When the process works well, the bone stays healthy. But for some horses, the intensity of the demand placed on their bones in training and racing may lead to a catastrophic pelvic fracture.
“Microfractures occur with the structure but they don't disturb the anatomy,” Bramlage said. “When they become a stress fracture, then the actual structure of the bone begins to break down.”
Bramlage said horses that travel efficiently may be less likely to develop a stress fracture of the pelvis, but ironically, those with more ability may end up generating the force sufficient to cause the pelvis to fracture. The massive muscles in the hindquarters are the strongest in the horse's body and deliver the greatest torque.
“It's a combination of how much work they're doing, how efficient they are,” Bramlage said. “Some horses just sort of float over the ground, and they don't seem to have to put out a lot of effort at all, and some horses really pound the ground as they move. So how the horse moves has some effect. It also has to do with the horse's ability because pelvic stress fractures, in particular, are the result of muscle contraction.”
Pelvic fractures are more prevalent in fillies and mares, but research has not uncovered the reason for this.
Horses will tell you they have a problem; you just need to pay attention.
A horse reluctant to train or to come out of the gate could have a condition that makes it painful to perform. When this occurs, have the horse examined by a veterinarian, Bramlage said. Early detection could avoid a catastrophic injury.
Dr. Kevin Haussler, researcher at Colorado State University's Orthopaedic Research Center and a licensed chiropractor, said certain clues might suggest a horse has a developing or existing pelvic problem.
“One of the tests that I do is a compression test,” he said. “I put my fingers and thumbs on both sides of that tuber sacrale [the point at the top of the pelvis, near the spine] and squeeze them together. The normal response is that the horse should flatten the croup a little bit or their back a little bit but not unlock its stifle.”
Haussler said horses with severe pelvic pain literally may fall down when he presses the tuber sacrale together. A horse that is less painful typically will unlock its stifle and flinch away from the pressure.
“The other test I do is to check out the flexibility of the lumbosacral joint or over the sacroiliac joint [where ilium ties into the spine],” he said. “I hook my fingers over the point of the hip, the tuber coxae of the pelvis, then push or pull downward. Normally, the pelvis should kind of rock back and forth — flex and extend. Some of those horses with lots of back pain or lumbosacral pain around the pelvis, they just spasm their back and they don't allow that to move.”
Scintigraphy is the gold standard to diagnose an impending stress fracture. The horse is injected intravenously with a radioactive element that can reveal cracks in the bone and then the horse is scanned. Ultrasound can be useful to detect a more prominent fracture.
When Bramlage finds a stress fracture of the pelvis, he places the horse on stall rest for 60 days. Because of the depth and size of the pelvis, little else can be done but allow it to heal. And this is one injury where hand-walking is not advised.
“When you walk them, you can't keep the gluteal muscles from pulling on that fracture. So you kind of have to protect the horse from itself, and it's one of the few injuries, like lamenesses, that we totally stall them,” he said.
Horses must be cautiously returned to training after lay-up to prevent reinjury.
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