Fatigue fractures in thoroughbred racehorses: Relationships with age, peak bone strain, and training

Abstract

The North American Thoroughbred racehorse was chosen as a model to study the pathogenesis of fatigue failure of bone. This species has a high incidence of spontaneous fatigue failure of bone (bucked shins) during its early training. In vivo strain gauge studies of the third metacarpal bone of four young racehorses running at racing speeds showed high principal compressive strains [-4,841 +/- 572 (SD) microstrain] while two older horses had lower principal compressive strains (-3,317 microstrain measured at racing speed, -3,250 microstrain extrapolated from a slower speed run). Previously reported inertial property measurements of the third metacarpal bone were related to the difference in bone strains seen in young and older horses. The high strains on the surface of the third metacarpal bone associated with young horses in training may lead to high strain, low cycle fatigue. The changing shape of the third metacarpal bone during maturation may be consistent with the lower strains recorded during high speed exercise in the older animals. This phenomenon may allow for the accumulation of additional strain cycles in older animals before failure occurs.

Bone strain gradients and optimization in vertebrate skulls

“…If low strain magnitudes in the postcranium do compensate for the lower remodeling rates in "reptilian" bone (Enlow 1969;de Ricqles 1975), as suggested by Blob and Biewener (1999), then the high strain magnitudes in the Alligator skull must occur very infrequently to avoid fatigue fractures (Lanyon et al 1982;Currey 1984;Burr et al 1985;Nunamaker et al 1990;Davies et al 1993).…”

Distribution of microcrack lengths in bone in vivo and in vitro

“…Results obtained by [20] reported slightly less than 2000 when measuring tibial shafts of soldiers during intensive training regimes. The highest strains registered in vivo are the results obtained by [21], measuring compressive strains in racehorses ranging from 4400 to 5670 . In addition one should remember that in vivo, microdamage is being continuously repaired, whilst in vitro this is not the case, so there could be differences in the data for in vitro loading within the in vivo range, as in the tests of Burr et al 1998 at 2700.…”

Horses damp the spring in their step

“…Musculoskeletal injuries are extremely common in athletic horses, and most of them occur in this spring system, which has been shown to fail through cyclical fatigue damage in as little as 10,000 strides of gallop 20,21 . A statistical correlation between highfrequency components in the ground reaction force and the development of tendonitis in racehorses has also been shown 22 .…”