Two strategies proposed to reduce bone microdamage in racehorses

Horse racing at Golden Gate Fields, Albany, California. Photo: Noah Salzman CC BY-SA 4.0, via Wikimedia Commons
Horse racing at Golden Gate Fields, Albany, California. Photo: Noah Salzman CC BY-SA 4.0, via Wikimedia Commons

Rest periods might need extending or training intensity reduced as a way of reducing bone microdamage in the joints of racehorses, researchers suggest.

Studies have shown that the accumulation of subchondral bone microdamage in the forelegs of Thoroughbred racehorses can lead to more significant bone damage, including life-threatening fractures.

Researchers from the University of Melbourne set out to quantify subchondral bone microdamage in the third metacarpal bone of Thoroughbred racehorses at different stages of the training cycle.

Subchondral bone is the layer of bone just below the cartilage in a joint. It serves as a shock absorber in weight-bearing joints.

Professor Chris Whitton and his colleagues analysed bone samples from 46 racing Thoroughbred horses undergoing post-mortem.

Twenty-six of the horses were in training at the time of death and 20 were resting from training.

Where the cause of death involved a fetlock injury, the other front limb was chosen. Otherwise, the limb was chosen randomly.

The bone samples were examined using micro computed tomography (microCT) to detect calcified microcracks, and light microscopy was used to quantify bulk-stained microcracks.

Racing and training histories were obtained for comparison.

The study team, writing in the Equine Veterinary Journal, reported that subchondral bone microcracks were prevalent among the horses in the study. Subchondral bone microcracks were observed in all bones using at least one method.

Microdamage grade was greater in older horses, levelling-off for horses aged 5 and older. Microcrack density was higher in older horses, and with higher bone volume fraction in the parasagittal groove in horses in training and lower in horses resting from training.

They noted that obvious damage seen on the joint surface is not always a good indicator of the amount of microdamage in the subchondral bone.

The evidence suggested that this fatigue damage appeared to accumulate throughout the racing careers of the horses sampled.

“Our findings suggest that, averaged over time, the rate of microdamage accumulation was greater than the rate of microdamage removal by bone repair in horses in this study.”

Independent of the age of the horse, microdamage levels were lower in the resting horses, consistent with previous work showing higher levels of subchondral bone remodelling during rest periods.

The study team proposed two strategies that might limit the accumulation of microdamage:

  • Adequate rest periods from training to assist bone repair;
  • Training at a lower intensity to reduce the rate of microdamage accumulation.

“The length of rest period required to adequately reduce the burden of fatigued subchondral bone is unknown and will depend on bone turnover rates and the volume of damaged bone present at the commencement of the rest period.

“Returning horses from rest periods results in the additional risk associated with loading subchondral bone that has become more porous in response to a lower loading environment and therefore less resistant to fatigue.

“Therefore, we recommend reduced intensity and duration of training and racing and/or increased duration of rest periods in order to limit the risk of fatigue injury in racehorses.”

The study had backing from Racing Victoria and the Victorian State Government.

Subchondral bone microdamage accumulation in distal metacarpus of Thoroughbred racehorses
R.C. Whitton B.A. Ayodele P.L. Hitchens E.J. Mackie
16 April 2018

The full study, offered with free access by the journal, can be read here

One thought on “Two strategies proposed to reduce bone microdamage in racehorses

  • April 21, 2019 at 10:21 am

    It is quite telling that research sponsored by race clubs and groups always avoids the glaringly obvious risk factor of the surfaces horses are trained and race on. There seems to be a trend amongst race tracks to reduce the depth of sand on tracks where most racehorses do 80 -90 percent of their training. This is, of course, due to the costs of maintenance increasing as the sand levels get higher. At the major training track where I work, the sand has a high level of clay and, in warm weather, forms a solid crust 3-4 cms into the sand and when saturated by rain, water pools in undulations creating soft spots. Both these situations are highly dangerous to the equine limbs yet the racing administration refuses to recognise this hazard. Another modern trend is the introduction of synthetic tracks which, though soft, have no slide or give whatsoever. Even the Racing Victoria Vet, when questioned about the use of these as a training surface, claimed they were “fine once a horse got used to them”. Perhaps it is more a case of the horses learning to endure them. It would be simple to test heart and respiratory rates of horses worked on synthetic, sand or grass to ascertain how they react and which surface the horses feel best on. It is always terribly sad to see catastrophic injuries on race days and everyone is quick to examine the track surface on that day but surely many, if not all of these injuries are the result of a constant weakening of the bone structures, as suggested in this article, over the life of the horse. I feel the only way we can get truly safe facilities for our racehorses is to have the training facilities overseen by an independent group, not race clubs who are protecting their “bottom line”!


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