Natural factors that affect the shape and strength of the aging human femur.

The human skeleton loses bone mass with increasing age. A wide variety of evidence suggests that the skeleton compensates for this decreased bone mass by increasing the second moment of area in the midshaft of long bones. Strain is the mechanical transducer of bone's adaptation to its mechanical environment. In the absence of strain, bone resorption occurs. Contemporary femoral prostheses are stiffer than the cortical bone that serves as their host and, therefore, induce stress shielding and bone resorption that augments the naturally occurring loss of bone mass with age. With the expectation that total hip replacement can endure 15-25 years, one must be aware that a new limit to its longevity may be the biologic failure of the host bone. Particulate debris, stress shielding, and the natural consequences of aging are conspiring to make the proximal femur a diminishing site. The design strategy and materials of fabrication of future total hip prostheses should seek to maximize the preservation of bone.