Effect of Design Parameters of an Uncemented Hip Stem on Bone Ingrowth—Finite Element Analyses Integrated With Mechanoregulatory Algorithm and Design of Experiment

The degree of bone ingrowth in the porous implant is influenced by design parameters (pore size, thickness of porous buttresses, and core material). The purpose of this numerical study was to assess how the variation of the implant's design parameters influences bone ingrowth into a porous hip stem, as proposed by the authors in a previous study. Eight macromodels of the implant-bone structure were developed based on the design of experiment approach (full factorial design with three parameters and two levels each). These finite element (FE) macromodels were solved in ANSYS under normal walking and stair climbing loadings and physiological boundary conditions. A 3D submodel corresponding to each macromodel was considered for bone ingrowth simulation. Total displacement at the cut boundaries of the macromodels was mapped to submodels. Bone ingrowth was predicted by integrating a phenomenological mechanoregulatory algorithm with FE analysis of the submodels. The ANOVA and regression analysis were performed between design parameters and responses (bone ingrowth and elastic modulus of newly formed tissues). Within the first month following surgery, the study predicted a significant amount of bone ingrowth (32%-72%) even in proximal regions and a smaller amount of fibrous tissue ingrowth (0.5%-3.5%) in all submodels. At equilibrium iteration, the average Young's modulus of the newly formed bone tissue layer ranged from 380 to 1200 MPa, considering all the submodels. The higher pore size, FGM core and half-porous buttresses provide better bone ingrowth that results in a higher elastic modulus of the newly developed tissue inside the pores of the hip stem. This bone ingrowth is expected to potentially improve long-term fixation and stability of the hip stem, reducing the risk of implant loosening.