Comparison between two techniques for modeling interface conditions in a porous coated hip endoprosthesis

The geometric and material non-linearities occurring at implant interfaces require a non-linear finite element analysis to simulate accurately the interface conditions. Reviewing the literature, it was found that mainly three different non-linear interface elements have been developed to simulate the interface bonding conditions. While these different types of interface elements have been used to predict micromotions and interface stresses for different geometrical configurations and under different loading conditions, no study has attempted to compare the performance of these elements under similar conditions. The objective of this study is to compare two non-linear interface modeling techniques using gap elements and joint/interface elements. A simplified three-dimensional geometrical model was developed to compare interface stresses and micromotions for both fully and partially coated models. The results show that both non-linear modeling techniques predict dissimilar results for the interface stresses in bone and prosthesis sides. For the fully coated model, and on the bone side, joint/interface elements predict a gradual decrease in stresses from distal end to proximal end. However, 'gap' elements predict almost constant stresses in the mid-stem region and higher stresses at the distal end. On the prosthesis side, small stress differences occur only at the distal and proximal ends. For the partially coated model, most significant stress dissimilarities occur in the uncoated section. The relative micromotions at the interface were also determined. It was found that micromotion patterns obtained using both techniques were similar with higher magnitudes in the case of the 'gap' elements.