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 https://doi.org/10.1111/evj.12948
The full study, offered with free access by the journal, can be read here.