Interface Mechanics of Porous Titanium Implants

The interfacial shear properties of bone tissue growth into porous coated Ti-6-A1-4V femoral implants have been examined as a function of the pore size of the porous surface. Three particle size range powders (297 microns, 420-500 microns, 595-707 microns) were used to fabricate cylindrical implants which were inserted into the femoral medullary canal of dogs for 6 months. Push-out tests on the removed femurs are reported and reveal: (i) that those implants residing in cortical bone exhibited significantly higher shear properties than the equivalent implants in cancellous bone and (ii) that the interfacial shear strength and stiffness decreased with increasing pore diameter within the range 175-325 microns. The extent of bone ingrowth into the surface of the implants was measured using quantitative optical microscopic techniques. This indicated that the percentage of bone which had grown into the surface was inversely proportional to the square root of the pore size and that further the shear properties of the interface were proportional to the extent of bone ingrowth.