FATIGUE BEHAVIOR OF THE EQUINE 3RD METACARPUS - MECHANICAL PROPERTY ANALYSIS

This is the first in a series of experiments to study the fatigue prop erties of equine cannon (third metacarpal) bone specimens from Thoroug hbred racehorses. Monotonic and fatigue tests to failure were performe d in four-point bending on diaphyseal specimens in a 37 degrees C sali ne bath to answer three initial questions. (a) Will a linear variable differential transducer yield the same elastic modulus as strain gauge s' (b) Will fatigue results depend on whether the periosteal or endost eal side of the beam is in tension? (c) Are there regional variations in the monotonic and fatigue properties of the cannon bone midshaft? E ighteen left-right pairs of specimens from six horses were used. One b eam of each pair was fitted with strain gauges. Fatigue tests were con ducted on 24 specimens under load control at 2 Hz; an initial range of 0-10,000 microstrain was used so as to produce failure, in a reasonab le period of time. There were no left-right differences in the fatigue or monotonic properties, and the presence of a gauge had no effect on modulus measured by a linear variable differential transducer. Howeve r, gauge-measured moduli were about 1 GPa less than transducer-measure d values. Fatigue life was independent of which side of the beam was i n tension, and there were significant variations in mechanical propert ies around the cortex. The lateral region was stiffer than the dorsal region but the latter had a longer fatigue life. The fixed cylindrical supports used in this experiment eventually produced slight wear groo ves, causing artifactual stiffening at the end of the load cycle in so me specimens. A second experiment using roller supports confirmed the reason for this stiffening. It also showed that fatigue life was short er when roller supports were used but regional differences were simila r.