Stress enhancement and fatigue susceptibility of porous coated Ti-6Al-4V implants: an elastic analysis.

An elastic stress analysis of porous-coated implant surfaces was performed using the finite element method. Three-hundred-microns-diameter metal beads sinter bonded onto an implant surface were modeled with sinter neck radii of 5, 10, 20, and 50 microns. Smooth-surface, single-bead, single-layer, and double-layer systems were analyzed. The finite element models were loaded to simulate bone-bead contact forces and lateral hip implant tensile forces. Results showed that, for a single bead sinter-bonded onto an implant surface, concentration of stress occurs either at the base of the sinter neck or within the neck itself, depending on the type of load applied. Under lateral hip implant tensile loads, a maximum stress concentration factor of 1.97 was obtained for a single bead sinter-bonded onto a implant surface. Addition of a single layer of beads onto the implant surface resulted in a significant increase in stress at the most proximal and distal ends of the porous layer, with a maximum stress concentration factor of 4.3. Addition of a second layer of beads did not significantly increase the magnitude of the stress concentration occurring at the ends of the porous layer. The results of this study provide stress concentration factors for porous coatings with sinter necks of known dimensions under loading conditions similar to those present along the lateral surface of a hip prosthesis.