Microdamage in bone: implications for fracture, repair, remodeling, and adaptation.

Fatigue microdamage accumulates in bone as a result of physiological loading. The damage is often manifested as microcracks, which are typically 50-100 mum long. These types of cracks develop in the interstitial bone and frequently abut osteon cement lines. In vitro experimentation has shown that an accumulation of fatigue damage reduces the material properties of bone (e.g., elastic modulus). An accumulation of fatigue damage has been implicated in the etiology of stress fractures and fragility fractures. However, bone has a remarkable ability to detect and repair fatigue microdamage. This article reviews the experimental techniques for identifying and quantifying different types of microdamage in bone, the density of in vivo microcracks at different skeletal locations, the effect of microdamage on bone material properties, the role of microdamage in bone fracture, and the biological mechanisms for the detection and repair of fatigue microdamage.