RESIDUAL STRENGTH OF EQUINE BONE IS NOT REDUCED BY INTENSE FATIGUE LOADING - IMPLICATIONS FOR STRESS-FRACTURE

Fatigue or stress fractures are an important clinical problem in human s as well as racehorses. An important question in this context is, whe n a bone experiences fatigue damage during extreme use, how much is it weakened compared to its original state? Since there are very limited data on this question and stress fractures are common in racehorses, we sought to determine the effect of fatigue loading on the monotonic strength of equine cortical bone. Beams were machined from the dorsal, medial and lateral cortices of the third metacarpal bones of six thor oughbred racehorses. Beams from left and right bones were assigned to control and fatigue groups, respectively (N = 18 each). The fatigue gr oup was cyclically loaded in three-point bending at 2 Hz for 100,000 c ycles at 0-5000 microstrain while submerged in saline at 37 degrees C. These beams, as well as those in the control group, were then monoton ically loaded to failure in three-point bending. The monotonic load-de flection curves were analyzed for differences using three-factor (fati gue loading, anatomic region, and horse) analysis of variance. The mea n failure load was 3% less in the fatigue group, but this reduction wa s only marginally significant. Neither elastic modulus nor yield stren gth was significantly affected by the fatigue loading. The principal e ffects of Fatigue loading were on post-yield behavior (yield being bas ed on a 0.02% offset criterion). The work done and the load increase b etween yield and failure were both significantly reduced. All the vari ables except post-yield deflection were significantly affected by anat omic region. In summary, loading equivalent to a lifetime of racing do es not significantly weaken equine cortical bone ex vivo. The clinical implication of this may be that the biological repair of fatigue dama ge can actually contribute to stress fracture if pressed too far. Copy right (C) 1996 Elsevier Science Ltd.