Aging and matrix microdamage accumulation in human compact bone

Bone matrix microdamage in bone matrix, evidenced as microcracks, occurs consequent to cyclic loading. Microdamage caused by in vivo loading has been described in human rib cortex; however, the existence and extent of microcracks in human long bone cortices are largely unknown. Using histomorphometric methods to examine the incidence and localization of microcracks in human femoral compact bone specimens, we found that the amount of microdamage present in femoral compact bone increases dramatically with increasing age. Least squares regression analysis showed that in males, microcrack density (Cr.De., #/mm²) increases exponentially with age (r² = 0.70). In females, Cr.De. also increases as an exponential function of increasing age (r² = 0.79), at a significantly higher rate than in male specimens (p < 0.001). The current studies indicate that with increasing age, bone microdamage accumulates more rapidly than intrinsic processes can effect its repair. A combination of cumulative loading history, focal changes in material properties and alteration in the ability of the tissue to perceive and/or react to microcracks may all play role in this accumulation of bone microdamage with aging. This accumulation of microdamage in bone will contribute to decreased strength and stiffness. In addition, and perhaps most significantly for understanding aging and increased bone fragility, matrix microdamage in composite materials like bone will result in a profoundly reduced resistance to fracture. The importance of this accumulation of matrix microdamage in human bone with increasing age in contributing to the increased fragility of the aging skeleton is discussed.