Towards a Rational Definition of Osteoporosis

The design of the Farnsworth House is a fine illustration of the famous dictum, "less is more" by Mies van der Rohe. "I have no special talents, I am only passionately curious." summary / samenvatting summary / samenvatting summary / samenvatting summary / samenvatting summary Summary Summary Summary Summary Osteoporosis is a skeletal disorder that is fairly common among the elderly and particularly elderly women. Although osteoporosis is officially defined by a reduced bone density, the increased susceptibility to bone fracture is where its real problem lies. As a fracture is a mechanical phenomenon (the bone is not strong enough or the load on the bone is too high), the main topic of this thesis deals with the mechanical differences between osteoporotic bone and healthy bone, differences that could explain the increased susceptibility to fracture. Both the strength and stiffness of cancellous bone are determined by the bone density, the trabecular architecture, and the tissue mechanical properties. Osteoporotic bone is known to have a lower bone density, which causes a decreased bone strength and stiffness. Osteoporotic bone is also known to have a more anisotropic trabecular architecture. The effects of these changes on the strength and stiffness of osteoporotic cancellous bone have not been determined (Chapter 2). The effects of osteoporosis on the tissue mechanical properties are also unknown (Chapter 2). Neither the strength nor the stiffness of the bone tissue was changed in osteoporotic bone compared to healthy bone. In osteoporotic bone the changed trabecular architecture resulted in an increased mechanical orientation toward the main loading direction. Per amount of bone the osteoporotic architecture provides more stiffness and strength in the main loading direction than the healthy architecture does. So much so that it compensates the reduced bone mass: in our study the stiffness in the main loading direction was as high in the osteoporotic bone as in the healthy bone. This may then even allow normal stress transfer under normal daily loads. However, the enhanced mechanical orientation does make the bone susceptible to unusual loads, such as those that result from a fall. Falls lead to higher than usual loads and more importantly to loads in unusual directions, which the changed osteoporotic architecture is far less able to cope with. This effect may well be the explanation for the fact that 90% of all hip-fractures result from a fall. Every bone fracture starts at some microscopic location in …