DECREASE IN CANINE PROXIMAL FEMORAL ULTIMATE STRENGTH AND STIFFNESS DUE TO FATIGUE DAMAGE

Fractures of the proximal femur represent a significant health concern especially in the elderly. Fatigue damage and microfractures have bee n implicated in the etiology of hip fractures; however, the extent to which these factors are sufficient to bring about significant reductio ns in proximal femur strength and stiffness is unknown. This study exa mined the hypothesis that fatigue loading of the proximal femur result s in highly correlated decreases in bone stiffness and strength throug h the accumulation of bone microdamage. One canine femur from each of 10 pairs was monotonically loaded to failure to determine the ultimate strength. The contralateral femur was then cyclically loaded at 50% o f the ultimate load value for either 3600 cycles or until a 40% reduct ion in stiffness was achieved. This femur was then monotonically loade d to failure. For two additional femur pairs, the fatigued femur was h istologically processed to reveal bone microdamage. In support of the hypothesis, the data demonstrated a linear relationship between streng th loss and stiffness loss (Adj. R(2) = 0.79, P < 0.0004) with signifi cant decreases in residual whole bone strength (p < 0.004) following c yclic loading. In addition, damage (microcracks) in the cortical bone and broken trabeculae were observed in the neck and head region of the femur fatigued until its stiffness was reduced by 40% but not fractur ed subsequent to cyclic loading. (C) 1997 Elsevier Science Ltd.