Pre-existing stress fractures in the bones of racehorses rarely cause lameness and may not be detected by radiography, but they appear to be a significant cause of fatal injuries, according to researchers at the University of California, Davis, School of Veterinary Medicine. UC Davis veterinarians will report these and other findings from 31/2 years of studying California racing injuries to members of the horse-racing industry Tuesday, Nov. 2, from 10 a.m. to 11:30 a.m. in the Top of the Stretch Room at Santa Anita Racetrack. The new findings are from an ongoing Post-Mortem Examination Program conducted by the UC Davis-based California Veterinary Diagnostic Laboratory System under contract to the California Horse Racing Board. Other significant findings include: • Identification of the cause of 52 percent of all previously unexplainable "sudden deaths" among racehorses examined. Researchers found the majority of these were related to cardiovascular problems or bleeding in the lungs. They are now investigating the causes of these problems. • Discovery that the thickening of the honeycomb-like matrix that fills the horse's inner bone is responsible for increased density or sclerosis of the bone. This condition, in turn, decreases the bone's ability to absorb normal racing stress and leaves it vulnerable to fractures. This is a particular problem in the horse's knee. • Verification that the vast majority of racehorses have normal, sound joints. Breakdown injuries related to the joints is primarily the result of traumatic injury, not pre-existing flaws in the joints. Many chip fractures of the knee are an acute manifestation of an underlying chronic problem. Additionally, researchers discovered a pattern of injuries in the elbow joint of horses, with the more severe injuries possibly affecting horses' performances. "As is true for elite human athletes, equine athletes also are subject to catastrophic injuries," said Dr. Robert Jack, equine medical director for the California Horse Racing Board and member of the UC Davis School of Veterinary Medicine. "While fatal injuries occur in less than two-tenths of 1 percent of thoroughbred racing starts, we are encouraged that the post-mortem study is providing clues to the factors involved in those injuries and is pointing us toward new preventive strategies. "The California racing industry is unified in this goal and aggressively supports research efforts to improve the safety of its equine athletes." Stress fractures are small incomplete fractures that result from the accumulation of microscopic damage incurred during race training; they only penetrate part of the bone. The bone, in response, begins to manufacture new bone material at the site of the stress fracture. If allowed to thoroughly heal, the callus of new bone will become as strong as the original bone. However, if the weakened bone is exposed to continued pressure during training or racing, it may fail and a complete fracture of the bone will occur. Stress fractures in a horse's lower limb can usually be detected with radiography, a diagnostic procedure similar to X-rays. However, such fractures in the large humerus bone, located between the shoulder and elbow joints, are difficult to radiograph because the bone is next to the chest and is beneath six to eight inches of muscle. Since the humerus is so padded with muscle, any pain, heat or swelling associated with an incomplete fracture also is deeply located and difficult to detect. Incomplete stress fractures can, however, be diagnosed with scintigraphy, bone-scanning techniques that can detect stress fractures early in the course of their development. "Horses with an unexplained lameness or performance lower than expected should be evaluated by bone scan to detect any incomplete stress fractures," recommends Dr. Sue Stover, professor of anatomy in the UC Davis School of Veterinary Medicine. The training of horses with stress fractures can be reduced until the stress fracture heals, at which time the bone has regained strength and the horse can return to normal training and racing with minimal risk for a catastrophic complete fracture, Stover said. The post-mortem study is a collaborative research effort between the California Veterinary Diagnostic Laboratory System and the UC Davis Equine Research Laboratory, which receives funds from the Oak Tree Racing Association, the State of California satellite wagering fund, and private donations.