A stress fracture is an incomplete fracture occurring in normal bone, and is a well recognised cause of lameness. It is caused by the repetitive loading, and subsequent failure, of the normal structure of cortical bone (the dense bone that makes up the outside of a long bone).
Stress fractures can occur in all horses that undergo repetitive exercise. They are more prevalent in Thoroughbred racehorses, and are most commonly seen in 2-4 year olds, but can be seen in older horses.
As well as being an important cause of lameness, if left undiagnosed stress fractures can progress to catastrophic complete fractures. These are often career (and indeed life) threatening for the horse.
How do stress fractures develop?
Repetitive mechanical loading causes very small amounts of damage to normal bone. It is the accumulation of this micro damage that leads to stress fractures. As the body attempts to heal this damaged bone, it first has to remove the damaged bone before it can replace it with healthy new bone. Unfortunately there is a lag period between the removal of the old bone and laying down of the new healthy bone. If the rate of bone removal is greater than the rate of new bone formation, a stress fracture can occur.
Clinical signs and diagnosis
Stress fractures typically result in lameness which is associated with an increasing intensity of the horses’ training regime. Lameness is usually exercise related and may be intermittent and of varying severity. Bilateral stress fractures are often presented with a history of a “rough” or “shortened” gait, rather than an obvious lameness
Stress fractures commonly involve the upper limb, and is difficult to localise with nerve blocks. A thorough lameness examination is needed to eliminate the lower limb as the cause of lameness, before progressing to diagnostic imaging.
Radiography is not particularly sensitive at identifying stress fractures, especially in the first few weeks of development. Nuclear scintigraphy is therefore the imaging modality of choice, as this allows us to assess the physiologic bone activity (ie those areas which are undergoing active remodelling). The horse is injected with a radioactive substance which emits a specific type of radiation that is able to be detected on a special camera. Areas of bone that are more active than others absorb more of the substance, and therefore emit more radiation, which we can see via the camera.
Fig. 1. The red arrow points to a stress fracture in the distal humerus. The yellow spot is a lot "hotter" (is emitting much more radiation) than the surrounding green area.
Management and prognosis of stress fractures
As a general rule, most stress fractures heal uneventfully when the stress of exercise is removed. The severity of the bone damage will determine the exact recommendations for length of rest required. The prognosis for return to racing is generally considered good, although may vary in certain individuals.