Compressive fatigue life of subchondral bone of the metacarpal condyle in thoroughbred racehorses

- We tested equine metacarpal subchondral bone in compressive cyclic loading.
- Fatigue life of equine metacarpal subchondral bone follows a power law function.
- Actual density explains one third of stiffness variation of equine subchondral bone.

In racehorses, fatigue related subchondral bone injury leads to overt fracture or articular surface collapse and subsequent articular cartilage degeneration. We hypothesised that the fatigue behaviour of equine subchondral bone in compression follows a power law function similar to that observed in cortical and trabecular bone. We determined the fatigue life of equine metacarpal subchondral bone in-vitro and investigated the factors influencing initial bone stiffness. Subchondral bone specimens were loaded cyclically in compression [54 MPa (n = 6), 66 MPa (n = 6), 78 MPa (n = 5), and 90 MPa (n = 6)] until failure. The fatigue life curve was determined by linear regression from log transformed number of cycles to failure and load. A general linear model was used to investigate the influence of the following variables on initial Young's Modulus: age (4-8 years), specimen storage time (31-864 days), time in training since most recent rest period (6-32 weeks), limb, actual density (1.6873-1.8684 g/cm3), subchondral bone injury grade (0-3), and cause of death (fatigue injury vs. other). Number of cycles to failure was (median, range) 223,603, 78,316-806,792 at 54 MPa; 69,908, 146-149,855 at 66 MPa; 13204, 614-16,425 at 78 MPa (n = 3); and 4001, 152-11,568 at 90 MPa. The fatigue life curve was σ = 112.2-9.6 log10Nf, (R2 = 0.52, P < 0.001), where Nf is number of cycles to failure and σ is load. Removal of the three horses with the highest SCBI grade resulted in: σ = 134.2-14.1 log10Nf, (R2 = 0.72, P < 0.001). Initial Young's Modulus (mean ± SD) was 2500 ± 494 MPa (n = 22). Actual density (ρ) was the only variable retained in the model to describe initial Young's Modulus (E): E = − 8196.7 + 5880.6ρ, (R2 = 0.34, P = 0.0044). The fatigue behaviour of equine subchondral bone in compression is similar to that of cortical and trabecular bone. These data can be used to model the development of SCBI to optimize training regimes.