Dorsal metacarpal cortex ultrasound speed and bone size and shape.

In Thoroughbred racehorses, the dorsal cortex (DC) of the third metacarpal bone (MC3) enlarges when horses are first exposed to fast exercise speeds. It is now possible to measure ultrasound speed through 3 to 5 mm depths of bone around the bone shaft using Sunlight Medical's 'Omnisense' machine. The aim of this experiment was to determine whether the ultrasound speed measured in the surface 3-5 mm of the midshaft dorsal cortex of MC3 could be related directly to the midshaft shape and size of the MC3s of young Thoroughbred racehorses at the same stage of training. Sixteen 2-year-old and sixteen 3 to 6-year-old racehorses, exercising at racing speed at the same training stable, had their ultrasound speed measured and were radiographed to measure their MC3 midshaft size (width of DC) and shape ([DC/palmar cortex] x [[DC+palmar cortex]/medulla widths]). There were significant linear relationships between ultrasound speed and radiographic bone size and shape measurements. The faster ultrasound speeds were associated with bones that were both absolutely and proportionately thinner in the dorsal cortex in both groups of racehorses. The measurement of ultrasound speed with the Sunlight Omnisense machine should allow more detailed and accurate predictions to be made for responses of the bone to exercise in the midshaft of MC3 than is possible using radiographic size and shape measurements alone. This should make it possible to predict, for example, how soon after a change in bone shape a specific horse can race with minimal risk of damage to the newly-forming bone surface.