Summary : Stress is the force per unit area acting perpendicular to the area under consideration. Stress is intimately associated with an area of plane. The concept of stress demands a particular plane over which the stress is distributed. The strains induced in the loading conditions are measured directly. Empirical relationships can now be formulated relating the calculated stresses and the measured strains. The behavior of whole bone, like that of any structure under stress, depends on the geometry or shape of the structure and the properties of the material of which it is composed. Cyclic loads have been shown to cause progressive micro-fracture in bone tissue, and if whole bones are cyclically loaded at sufficient intensities in either bending or torsion, gross fatigue fractures will result. The magnitude of the loads necessary to cause fracture is not now clearly defined, either clinically or experimentally. Stress shielding refers to a reduction in the stress level within bone and cannot be perceived per se. Bone resorption is the physical manifestation of stress shielding. In accordance with Wolff’s law, the reduction of stresses relative to the natural situation would cause bone to adapt itself by reducing its mass, either by becoming more porous (internal remodeling) or by getting thinner (external remodeling). It has been theorized that periprosthetic bone remodeling secondary to stress shielding may contribute to increased pain or decreased function, fracture of the femur or the femoral component, loss of fixation of the implant, increased prevalence or severity of osteolysis, and difficulty in performing a revision. The reduction of the stem stiffness to reduce stress shielding and prevent severe bone resorption is an attractive design concept. Indeed, a number of manufacturers are now experimenting with flexible “isoelastic,” materials. In any discussion of stress shielding and bone remodeling, it is helpful to consider several concepts related to stiffness or elasticity. The familiar term “isoelastic,” by definition, means “equal in elasticity,” and the connotation derived from isoelastic is that an implant of the same stiffness as the femur is a desirable design objective. Superficially, this seems reasonable, but from engineering standpoint, it may not be entirely realistic. Animal studies have shown that stress shielding is not a significant consideration with Interpore-200. Stress shielding may not be an important consideration for Interpore-200, because the elastic modulus has been shown to approximate that of human bone. In addition, the intimate bonding of regenerated bone to the implant transfers stresses directly to the bone in and around the implant. This sharing of biofunction is a consequence of the composite nature of Interpore-200 and the bone.