Stress fractures are a well-recognised injury cause of lameness and poor performance in thoroughbred racehorses.
Stress fractures are most frequently reported in the cannon bone (bucked shins), tibia, humerus, radius, and the pelvis. Tibial stress fractures occur most commonly in the unraced two-year old, with stress fractures of the humerus more common in the threeyear old. Any age of horse can be affected. In the tibia and humerus, incomplete fractures have been implicated in the progression to more catastrophic fractures, making the rapid diagnosis and management important in the young racehorse.
The diagnosis of a tibia stress fracture can be challenging even with the most advanced imaging. Lameness grades observed in horses with incomplete stress fractures of the tibia and humerus is wide ranging, however a majority of horses will typically show minimal, to no observable lameness at the walk. At the trot, lameness is usually consistent. In tibial stress fractures, a classic ‘bunny hop’ gait may be observed. Horses can often be seen with an acute lameness that occurs during training, becoming significantly more comfortable in the following 24 hours. This lameness is typically unresponsive to routine limb flexion and nerve blocks. Radiographs of the tibia and humerus are rarely conclusive for a diagnosis of a stress fracture, however these images are beneficial to monitor change one the location and diagnosis is confirmed.
Initially, radiographs frequently of minimal benefit for a diagnosis. In most cases, radiographs play a role after the location of injury is confirmed, but for a definitive diagnosis nuclear scintigraphy (bone scan) is required. Nuclear scintigraphy involves access to a referral hospital, where the horse is injected with a short acting radioactive pharmaceutical that is attracted to sites of bone turnover. The horse then generates the own radiation while standing in-front of a specialised camera (gamma-camera), to create the images of bone and identify fractures, undetectable with radiographs. A diagnosis can then be made, and the veterinarian can then develop a suitable management and monitoring plan. The radioactive pharmaceutical then leaves the horses blood stream and urine after 48 hours and the horse is again safe to handle. Stress fractures of the tibia can occur equally in left and right hind limbs, with minimal influence of track direction. Stress fractures of the humerus, were reported to occur slightly more frequently in the left forelimb in a study of horses on clockwise direction tracks. Treatment and management of stress fractures involves a rest from training, usually with a month of stall rest horses able to move to a paddock once trotting sound for an additional three months.
By Harry J. Markwell, BVSC, DACVS-LA
Registered Specialist in Equine Surgery, WestVets Animal Hospitals
