Ex vivo calibration and validation of in vivo equine bone strain measures.

Abstract: Data are required to confirm that strain gauges recording high bone strains in Thoroughbred racehorses provide an accurate record of bone strain. Objective: To test the accuracy and reliability of very high in vivo strain recordings made during fast exercise in Thoroughbred racehorses. Methods: Strains were recorded during exercise from rosette gauges implanted onto the mid-shaft dorsal cortex of each third metacarpal bone (MC3) in 6 yearling and 6 mature Thoroughbreds in a previous experiment. Bulk elastic modulus (E(US)) was calculated from ultrasound speed and single photon absorptiometry measures. Each cleaned MC3 with the original gauge in situ and new gauges placed in a region of strain similarity (as shown by photoelastic coating) was loaded in a materials testing system (MTS) and strains recorded during loading. Elastic moduli were calculated from strain measures from new rosette gauges on the medial, dorsal, lateral and palmar surfaces (E(m); E(d); E(El); E(p), and bulk moduli calculated from the displacement of the MTS machine heads during loading (Ebt). Peak loads were increased incrementally to failure. Results: Of 14 original gauges tested against new gauges, 11 recorded strains from 80-115% of the new gauges and 3 showed reduced function (31-40%). Ebt were similar to E(m), and E(US) were similar to E(l) and not significantly different from Ed. Maximum strains at yield were recorded by the medial gauges and ranged from -7500 to -16,000 microepsilon. Conclusions: Similarities between recordings from gauges used in vivo and new gauges confirmed the reliability and likely accuracy (or possible underestimate) of very high strains (exceeding -6000 microepsilon) recorded in exercising Thoroughbred racehorses. The similarity between E(bt) and E(m) confirms that the gauges measured the true distortion of the bone in the MTS. These results confirm that mammalian bone may withstand much greater compressive loads than -4000 microepsilon under some conditions at least.