During intensive training, young racehorses experience a thickening in the layers of bone under the cartilage of joints. These layers, called subchondral (located nearer the surface) and cancellous, become harder and better able to handle the rigors of training. Unfortunately, the process is painful, especially in the carpus (knee). Therefore, it is fairly common to inject corticosteroids into the knee to reduce pain and resume training. Unfortunately, corticosteroids have been associated with fractures of the knee when used in this manner. The reason for this is unknown.

Researchers from Newmarket in Suffolk, The University of London, and Kansas State University recently collaborated on a study to examine the effects of the corticosteroid methylprednisolone acetate (MPA) on the hardness of carpal bones during intensive race training. In addition, differences in effect between the subchondral and cancellous layers were examined.

Eight sound Quarter Horse mares were used for the study. MPA was injected in one knee, and saline in the other. Injections were repeated at 14-day intervals during a 70-day treadmill exercise regimen. Core samples of the carpal bones were then tested for hardness and compared. Surprisingly, the results indicated no significant effect of MPA injection on overall bone hardness in these short-term exercised horses. However, the cancellous layer was found to be harder than the subchondral layer at all sites tested. This might indicate an imbalance in remodeling that could lead to subchondral fractures.

Further studies with more horses undergoing longer and more traditional training are needed to better understand the risk of carpal fracture associated with these drugs