Subchondral bone fatigue is an important cause of poor performance, morbidity and mortality in racehorses. Fatigued bone is repaired by remodelling, a process that is attenuated under high cyclical loads, yet the effects of training on remodelling activity in Thoroughbred racehorses is unknown. We hypothesised that remodelling rates are lower in training horses than those that are resting from training.
Sections of the palmar metacarpal condyles from Thoroughbred horses that were in race training (n = 24) or resting from training (n = 24) at the time of death were examined with light microscopy and back scattered electron microscopy (BSEM). Bone area fraction (B.Ar/T.Ar), and eroded bone surface (E.Pm/T.Ar) were measured within two regions of interest; (1) the lateral parasagittal groove (PS); and (2) the lateral condylar subchondral bone (LC). BSEM variables were analysed for the effect of group (independent samples t-test), region (paired t-test) and interaction with time since change in training status (general linear model). Mean ± SE are reported.
For both regions of interest E.Pm/T.Ar was lower in the training horses (PS: 0.39 ± 0.06 mm−1, LC: 0.24 ± 0.04 mm−1) than in the resting group (PS: 0.65 ± 0.07 mm−1, P = 0.010, LC: 0.85 ± 0.10 mm−1, P<0.001). Lower subchondral bone porosity was observed, reflected by higher B.Ar/T.Ar in the LC of training horses (90.8 ± 0.6%) than resting horses (85.3 ± 1.4%, P = 0.0010). E.Pm/T.Ar was higher with increased duration of a training period in the lateral condyle (r2 = 0.26, P = 0.011).
Race training inhibits remodelling in the subchondral bone of highly loaded areas of the distal metacarpus. Therefore periods of less intense loading are important for allowing repair of subchondral bone fatigue in Thoroughbred racehorses.