The term" fragility fractures" refers to fractures that occur in response to the minor kinds of'trauma that are a part of everyday life. Fragility fractures are generally considered to have many contributing causes, both intra-and extraosseous. The extraosseous factors include propensity to fall, impaired postural reflexes during falling, insufficient soft tissue over the point of impact to distribute the force of the fall, and environmental hazards. The prevailing view is that it is the bone itself that is fragile. However, some investigators emphasize instead the mechanics of falling [1] and point out that the force of a fall from standing height, properly applied, is sufficient to break virtually any adult bone, whether or not we might otherwise designate it as" fragile." They conclude that older people are more inclined to break bones such as the hip because, in falling, they fail to adequately protect the hip the way a younger person would.

Whatever may be the ultimate importance of the extraosseous factors, it is intraosseous ones that are the primary focus of this brief overview. Intrinsically, the strength of bones derives from the same basic features that determine the strength of all structures. These are the strength of the structural material itself, its three-dimensional arrangement, and its mass density. The first two are commonly lumped together under the term" bone quality," as contrasted with mass density, which is a reflection of bone quantity. The properties of the structural material itself are not constant, as one might expect, but are subject to change over time. One reason is fatigue damage, which occurs in all structures as a consequence of the inevitable deformation that takes place with loading. Also, irreversible chemical changes occur in all solid materials with the passage of time; concrete's properties are known to change for years after it is laid, and it may be that analogous changes occur in bone, as well. Remodeling with age tends to isolate small volumes of bone, cutting their lines of communication, and thus either killing the osteocytes or changing their metabolism. Also, new bone formation in old individuals is performed by old cells, which might plausibly produce bone matrix of a different quality.