Chris 0'Sullivan BVSc (Hons), MS, MA Mr, Dipl ACVS Randwick Equine Centre

Chris 0'Sullivan Sydney University Graduate 1995. Associate veterinarian Randwick Equine Centre from 1997-2000. From 2000-2003 worked in the USA at The Ohio State University, returning to Randwick Equine Centre in 2003. Areas of inter­est include fracture repair, stress fractures, lame­ness and upper airway surgery.
http://randwickequine.com.au/contact

Figure 1 Scintigraphy image (bone scan) of a caudal distal humeral stress fracture (arrow). Stress fractures at this site cannot be demonstrated with a radiograph.

WHAT IS A STRESS FRACTURE?

Stress fractures are also often referred to as fatigue or insufficiency fractures. They appear as a localised crack within the bone, and can be likened to a small crack in a glass that with use will gradually elongate causing the glass to eventually break. Stress fractures similarly have the potential to propagate and become a complete fracture if exercise continues. Many complete fractures that result from stress fracture propagation are not amenable to repair and ultimately require humane destruction of the affected horse. Therefore stress fractures are significant injuries and their accurate and early is diagnosis desirable to avoid such a potential event.

Human athletes in a variety of disciplines may suffer from stress fractures, the injuries are not always associated with concussive injury or even high speed. They have been reported in a wide variety of training disciplines including runners, ballerinas, baseball pitchers and military recruits during basic training to name only a few. They are associated with repetitive stresses (forces) on a bone, these forces are not sufficient to break the bone if applied only once. However when applied in a repetitive fashion can result in development of a stress fracture in the bone. Similarly when a wire is bent only once it is not significantly weakened but with repetitive bending the wire gradually develops cracks which propagate and ultimately weakening the wire to the point of failure under a relatively low loading force.

In horses stress fractures are typically the result of the repetitive forces applied while galloping. However, in rare instances they have developed without going any. faster than a trot. So why does bone break under repetitive low stress loads that are generally below the bones breaking strength? Understanding the way that bones adapt to training will help answer this question.

HOW DOES BONE ADAPT TO TRAINING?

Like cardiovascular fitness, bone strength is influenced by training with race trained horses having stronger bones than untrained paddock horses. The foundations for future bone strength are laid down early in the horses' life and training career.

Bone strengthens or weakens in response to the forces applied to it. In other words the bone adapts to cope with the loading environment in which it finds itself. Similar to cardiovascular fitness bone can also detrain and lose strength with inactivity. An interesting anecdote is the example of astronauts in space. Without gravity their bones weaken since the forces are lower than on earth. In space the bones do not need to be strong since little force is placed upon them. Similarly horses that are locked up in a stall for long periods will have lower bone strengths than horses allowed access to exercise. But when theses horses are gradually trained up their bone strength will return. 

The difficult part of training horses to develop optimal bone strength while avoiding injury occurs during the adaptation phase. During this phase the bones are adapting to the forces that high speed work is applying to them. This adaptation process is not instant and is unfortunately relatively slow compared to the rate of cardiovascular fitness gain. Meaning the horses become physically able to work at a level beyond their bone's ability to cope with the work. Bone takes well in excess of 12 weeks to adapt to a given loading scenario and adaptation is fairly specific to the forces applied. In other words trotting makes the bone gain strength for the forces applied to the bone during trotting. As speeds increase the forces applied to the bone change and therefore the bone must adapt to the different forces associated with each speed. Therefore in order to strengthen a bone for high speed work it must be subjected to high speeds for a prolonged period to allow for adaptation.

During the adaptation process the bone is initially resorbed first. This weakens the bone, prior to new bone being laid down that will ultimately make the bone stronger. It is during this weakened state that bones are more Susceptible to developing stress fractures.

Insufficient information is available currently regarding ideal training regimens to avoid bone injury. While veterinarians can give some training advice based on current research to attempt to minimize bone injury the current available knowledge is still relatively limited. It appears short distances of high speed work provided regularly may be optimal for inducing bones adaptive response while minimising the potential damage of longer distances at high speed. Allowing the bone to be subjected to the forces required for it to adapt, without the bone being subject to an excessive number of force cycles that may result in damage.

Further research is required to better identify horses or training regimens with higher likelihood of bone injury. The trainers' task is made more difficult since a small percentage of horses trained with a standard method will develop stress fractures, while the greater percentage of similarly trained individuals will not. Currently identification of these susceptible individuals more likely to suffer stress fractures prior to them commencing training is not possible, research in this area continues.

WHERE DO STRESS FRACTURES COMMONLY OCCUR?

They have been described in a variety of sites in the lower limb. The cannon bone is most often affected and has been described in a variety of sites, the dorsal lateral cannon being the most common site. Other sites affected in the upper limb include the humerus, tibia and scapula. While pelvic and vertebral stress fractures are also seen.

HOW ARE STRESS FRACTURES DIAGNOSED?

While radiographs are the standard for identification of most types of fractures, they are not able to identify the majority of stress fractures when they first occur. Often it can take up to two weeks for a stress fracture to become apparent if at all on a radiograph. A new innovation is the development of Computed Radiography which has significant advantages over plain film radiography. It has aided the radiographic identification of stress fractures particularly of theupper limbs. Scintigraphy (bone scan­ning) is the current gold standard in veterinary science for the diagnosis of stress fractures in racehorses. Scintigraphy is an extremely sensitive imaging modality for identifying stress fractures and is able to show up stress fractures that radiographs cannot demonstrate. It can even show up areas of bone that are weakened and may be likely to develop a stress fracture. Ultrasound has been used to aid identification of pelvic stress fractures, however is not as sensitive or specific as scintigraphy and often lacks the detail required for an accurate diagnosis and prognosis.

Stress fractures generally cause lameness. however horses may not become lame until the crack is big enough that it is destabilizing the bone. Or they may show an intermittent lameness that apparently resolves rapidly with rest. This is the dilemma veterinarians can be faced with particularly in a horse that has been lame and improved with rest. Ideally horses with a possible stress fracture related lameness should be examined sooner than later since many resolve their lameness after short periods of stall rest. Often a scintigraphy may be advised based on history and clinical examination in order to rule out a stress fracture. Since the consequences of continued exercise with a stress fracture, include potential propagation to a complete fracture and loss of the horse.

HOW ARE THEY TREATED?

Stress fractures are typically treated by rest alone. consisting of sequential periods of stall rest followed by small yard and later paddock rest. A total of 12-16 weeks out of work is typically advised and the bone heals well at most stress fracture sites in an uncomplicated manner. In the dorsal cannon bone surgical treatment of typically occurring stress fractures appear to speed healing at this site and minimizes the chance of fracture reoccurrence.

WHAT ARE THE RISKS AND PROGNOSIS Of STRESS FRACTURES?

The major problem with stress fractures is that they can be present with only an intermittent lameness or one that Improves rapidly with rest. So a horse that appeared lame may improve after rest while still having an area of weakened bone with the presence of a small fracture. Occasionally horses will breakdown with a major complete fracture that began as a stress fracture, but never produced an obvious lameness or clinical sign therefore going undetected. Regular monitoring of horses in training for the presence of lameness, and a proactive approach to Investigation will act to maximize the early identification of stress fractures and minimize their potential consequences.