Finite Element Analysis Stress Distribution Acetabular Cup Biomaterial FINITE ELEMENT ANALYSIS OF PERI-ACETABULAR STRESS OF CEMENTED, METAL-BACKED, AND POROUS TANTALUM BACKED ACETABULAR COMPONENTS

Introduction: In contemporary THA there are two broad approaches to acetabular cup design and materials. One is an all-polyethylene design with polymethylmethacrylate (PMMA) cement. PMMA possesses a modulus of elasticity of about 2 GPa, which is between that of trabecular and subchondral bone. The second more popular approach, especially for active, younger patients, is the porous metal-backed polyethylene design. The porous coatings are commonly plasma sprayed metal powder, sintered beads, or fiber metal mesh, all applied to a solid metal substrate. The resulting porous metal is about 10 to 100 times stiffer than bone, depending on bone and metal type (Co-Cr versus titanium). An undesirable effect of this mismatch in stiffness is non-physiological stressing of bone, especially at the bone-implant interface, which in turn can result in bone densification and resorption within over and under stressed bone, respectively. A second issue associated with many metal backed cups is third body polyethylene debris emanating from the interface between the liner and metal shell [1]. One solution to this problem is direct compression molding of polyethylene into the porous metal shell, and eliminating the solid metal substrate [2]. The purpose of this study was to utilize a previously employed FEA model [3,4] to characterize the acetabular bony stress induced by a porous shell material with similar stiffness as subchondral bone, and to compare similarly calculated FEA results for three clinically well established designs; namely, titanium and Co-Cr metal backed cups, and a cemented allpolyethylene cup. Methods: The subject acetabular cup was constructed from a three dimensional porous tantalum (Hedrocel, Implex Corp.) with a direct compression molded liner, with the polyethylene infiltrating the porous tantalum to a depth of about 2 mm. A previous study showed the bond between the porous tantalum and polyethylene to be integral, and not subject to micro motion or wear [2]. The porous tantalum is an open, fully interconnected 3-D, engineered porous structure with porosity of about 80%, average pore size of 550 mm, and an elastic modulus (stiffness) of 3 GPa, i.e. similar to subchondral bone (1 to 2 GPa). Figure 1 shows the porous tantalum structure, and a cross-section of the non-modular cup. The three established acetabular cup designs were: cemented all-polyethylene, and non-cemented titanium and cobalt-chromium porous metal backed cups.