Bone density and bone strength

Osteoporosis is defined as "a skeletal disease characterized by compromised bone strength predisposing a person to an increased risk of fracture. Bone strength primarily reflects the integration of bone density and quality." Bone mineral density (BMD) correlates very well with fracture risk, with the relative risk of fracture approximately doubling (range 1.6–2.6) for every standard deviation decrease in BMD, depending on the skeletal site measured and the type of fracture. Many devices and technologies are available for measuring BMD. Dual-energy X-ray absorptiometry (DXA) is the method used to diagnose osteoporosis according to criteria established by the World Health Organization.

Non-BMD factors that may alter bone strength include bone turnover, architecture (size and shape, or bone geometry), microarchitecture, damage accumulation, matrix properties, mineralization, and mineral properties. These factors and probably many others that have not yet been well-defined are collectively called "bone quality" or "bone qualities." Accumulating knowledge regarding bone qualities offers insight into the pathophysiology of osteoporosis and metabolic bone disease, and helps in understanding the mechanisms of action of bone-active drugs. However, with the exception of bone turnover markers and bone geometry, none of these are these presently have clinical applications.

The adult skeleton is in a constant state of remodeling, a process whereby bone and bone matrix (mostly composed of type 1 collagen) is continually being removed by osteoclasts in discrete packets, or bone remodeling units, followed by osteoblast-mediated bone formation and mineralization. With high bone turnover states, such as occurs with estrogen deficiency in the early postmenopausal years, there are more bone remodeling units resulting in a greater number of "stress-risers," or weakened areas of bone. A stress-riser in bone is analogous to a scored line on a sheet of glass, where the glass is more likely to break than in an area that is not scored. Ultimately, thinning and perforation of trabecular structures may occur, as well as impaired mineralization. High bone turnover, which is detected clinically by the finding of elevated markers of bone resorption, has been shown to be an independent risk factor for fracture .

Small bones, as in individuals with a small frame or in women compared to men, are weaker than large bones. This is consistent with the engineering concept that a tubular structure, such as a long bone, has a greater ability to resist bending forces as the diameter increases. Longer hip axis length (the distance from the lateral surface of the greater trochanter to the inner surface of the pelvis, along the axis of the femoral neck), larger femoral neck-shaft angle (the angle between the axis of the femoral neck and the femoral shaft), and wider femoral neck diameter (the width read more