Effects of racetrack exercise on third metacarpal and carpal bone of New Zealand thoroughbred horses.

The response of equine bone to training has not been quantified in racetrack trained horses, only in treadmill exercised horses. Seven two-year-old thoroughbred fillies were trained on sand and grass at a racetrack, in a typical New Zealand flatrace training regime. The horses were exercised 6 days per week for up to 13 weeks. During the day the horses were confined in 4 x 4m sand yards, and were stalled at night. Another 7 fillies of the same age were allowed free exercise in grass yards. The bones of the animals were available after the 13 week experimental period, and were examined using a Siemens Somatom AR CT scanner. To quantify the response of epiphyseal bone, 3mm thick sagittal plane images of the carpus (through the middle of the medial condyle of distal radius) and the distal third metacarpal bone (Mc3) (immediately lateral and medial to the junction of the condyle and the median sagittal ridge) were studied. Appropriate areas of interest were chosen, and the mean tissue density equivalent (Houndsfield Units) was determined. In the carpus, there was a significant effect of exercise in the dorso-distal aspect of the radius (p<0.01), dorsal aspect of radial and third carpal bones (p<0.01 and p<0.001 respectively). In palmaro-distal subchondral bone of Mc3, there was a significant effect on the medial/lateral site (p<0.01), which differed between right and left legs, probably due to the effect of the horses having been trained in one direction around the training track. The mean tissue density of the Mc3 epiphysis of the exercised group was 36.8% greater than that of the non-exercised group (p<0.001). The study demonstrates that bone response is both rapid and substantial, which should prompt the use of non-invasive diagnostic aids to determine the stage of training in which tissue density changes occur.