Fundamental load transfer patterns for press-fit, surface-treated intramedullary fixation stems

To extend our understanding of the concentric cylinder idealization of a cementless hip stem, we used finite element analysis to study the effects of various types and amounts of surface treatments on the mechanical environment of the bone-stem interface for different load cases in the early post-operative situation. All analyses used no-tension interface conditions with various values of the frictional coefficient. We found that the shear stresses along the medial bone-stem interface were most sensitive to the type and amount of surface treatment, while the contact regions were relatively insensitive to the surface treatment. Consequently, the surface treatment had a negligible effect on the transfer of bending loads. By contrast, the axial load (when combined with a bending load) was transferred by shear stresses at the lateral stem tip for fully coated stems, but by shear stresses at the medial coating junction for partially coated stems. These findings therefore indicate that transfer of bending loads from the stem to the diaphysis cannot be controlled in the early post-operative situation by surface treatments, while transfer of axial loads can be controlled with the appropriate choice and distribution of a coating. In addition, the correspondence between the predicted shear stress distributions at the porous coating junction of partially coated devices and observed bone hypertrophy patterns that are apparently biased towards the medial aspect of the junction suggest that bone remodeling at the interface around porous coated hip implants may be initially stimulated by the development of small shear stresses.