Remodeling and the repair of fatigue damage.

This paper reviews the direct and indirect evidence for and against the idea that bone remodeling repairs fatigue damage. It defines experiments that should be performed to determine whether the accumulation and repair of fatigue damage is relevant to the pathogenesis of osteoporotic fracture. The experimental evidence favors the hypothesis that microdamage evokes local remodeling. The data suggest that the balance between the microdamage burden and bone repair is nearly constant. The indirect evidence comes from clinical observations that show positive relationships between depressed bone formation rate or prolonged remodeling period with bone fracture. More compelling indirect evidence comes from studies in which bone remodeling was pharmaceutically depressed, and fracture incidence rose in direct proportion. Data on microdamage accumulation were not collected in these studies. Conversely, some experimental evidence disputes a direct relationship between fatigue microdamage and repair. In these studies, increased amounts of bone microdamage in hyperadrenocortical dogs, and in irradiated dogs, could not be demonstrated even though bone fragility increased without associated osteopenia. Finally, the indirect evidence that argues that microdamage does not initiate repair is based on inference and does not provide an adequate test of the hypothesis. In balance, the current body of evidence favors the contention that bone remodeling repairs fatigue damage and thereby prevents fracture. Future studies should verify that microdamage accumulates when bone fracture occurs in conjunction with depressed remodeling activation frequency.