Complete rest may increase the risk of joint collapse in at-risk racehorses – study

Researchers have made important discoveries around the healing of subchondral bone injuries in horses. Photo: Florian Christoph CC BY 2.0 via Wikimedia Commons
Researchers have made important discoveries around the healing of subchondral bone injuries in horses. Photo: Florian Christoph CC BY 2.0 via Wikimedia Commons

A degenerative condition in horses that can result from bone fatigue in the legs may not be improved by rest, research suggests. In fact, drastically reducing exercise may even make it worse.

The scientists, writing in the Equine Veterinary Journal, instead propose that a lower work level might be a better approach to complete rest in dealing with the problem in Thoroughbred racehorses.

Researchers at the University of Melbourne set out to learn more about the causes of joint surface collapse in horses with palmar osteochondral disease, which affects the lower leg bones. It is generally considered a result of bone fatigue (stress fractures), which arise when damage accumulates from repeated loading.

In particular, they wanted to discover if bone resorption  – that’s the process of bone being reabsorbed –  was linked to reduced physical activity and contributed to surface collapse in the affected joints of horses with the disease.

They examined the metacarpal bones from the lower legs of 36 thoroughbred racehorses that had died. Twenty-nine of them were in training at the time of their deaths, while seven were being spelled.

Their racing careers spanned anywhere from 0 to 60 races.

The bones were examined using a high-resolution CT scanner to determine the proportion of the joint surface that had collapsed. A scanning electron microscope was then used to quantify the bone’s porosity and eroded bone surface.

The results were related back to the racing and training histories of the horses.

In 21 cases, inward collapse of the calcified cartilage layer was observed in the electron microscope scans. In six of these cases, it could be seen with the naked eye.

More joint surface collapse was evident when more microfractures were present in the calcified cartilage and on the surface of the subchondral bone (that’s the layer of bone just below the cartilage in a joint).

In the deeper bone, porosity was found to be lower in horses with greater levels of joint surface collapse, whereas on the bone surface there was no association between surface collapse and porosity.

Porosity and surface erosion of the bone were found to be higher in resting horses than those in training. In some resting horses an apparent loss of support for the overlying calcified cartilage layer could be seen.

The study team said the collapse of joint surfaces was common in cases of palmar osteochondral disease and likely resulted from bone fatigue, a known factor in catastrophic racing injuries.

They said subchondral bone resorption appeared to contribute to collapse of the calcified cartilage, which appeared to be increased by a lighter exercise regimen.

Their results showed that bone-eroded surfaces and subchondral bone porosity were not higher in association with lesions, but were greater when horses change to a lower exercise level

“There are no reports of the effect of rest from training on subchondral bone mechanical properties,” they noted.

“We observed pronounced superficial bone loss in resting horses, suggesting that this area is prone to significant degradation of mechanical properties.

“As horses are able to generate large joint surface loads even at modest galloping speeds, weakened subchondral bone is at risk of collapse, with subsequent collapse of the overlying cartilage layers  when a resting horse is returned to exercise or exercises itself if not completely confined.”

They continued: “Withdrawal from training is a logical response to a subchondral bone injury that is the result of excessive cyclic loading of the joint.”

However, their work had shown that this had the potential to hasten subchondral bone loss in horses with palmar osteochondral disease through greater resorption.

They concluded that although collapse of the joint surface may occur while a horse with palmar osteochondral disease continued in race training, the faster bone resorption seen during rest was likely to make the problem worse.

“This has important implications for the management of subchondral bone injuries in horses.”

The study team comprised Chris Whitton, Eleanor Mackie, Ebrahim Bani Hassan, Michiko Mirams and Ali Ghasem-Zadeh.

Role of subchondral bone remodelling in collapse of the articular surface of Thoroughbred racehorses with palmar osteochondral disease.
Bani Hassan E., Mirams M., Ghasem-Zadeh A., Mackie E.J., Whitton R.C.
Equine Vet J. 2016 Mar;48(2):228-33. doi: 10.1111/evj.12415.

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