Mechanical Properties and Structure of Ti-6Al-4V with Graded-Porosity Coatings Applied by Plasma Spraying for Use in Orthopedic Implants

Abstract

The object of this work was to determine mechanical properties of implants with graded porous coatings without reference to the properties of the ingrown bone. Optimum strength of a bone/implant interface consisting of porous metal and ingrown bone requires a gradation from base metal to the original bone. The composite metal-bone interface can be obtained by applying a metal coating of graded porosity, varying from near zero at the substrate surface to more than 50% at the outermost layer, on the original implant. Graded porous coatings of titanium or Ti-6Al-4V were obtained by plasma spraying of selected particle size fractions in three layers of successively decreasing density, the top coat being made with 300 to 850 μm powder. Tensile and shear strengths of the coatings were determined by cementing coated samples face to face with an adhesive resin to simulate ingrown bone. Data from these tests are given. Shear strength values ranged from 5.6 to 9.9 MPa (815 to 1430 psi) and tensile strength values were 5.1 to 25.5 MPa (745 to 3700 psi). Failure occurred within the porous coating and not at the interface between the substrate and the coating. Corrosion fatigue tests in Hanks' solution at 37°C (98.6°F) and a pH of 7.4, with a cyclic, fully reversed, peak torsional shear strain of ±0.01, gave lifetimes comparable to or better than those reported for mill-annealed Ti-6Al-4V, except for the samples that had been sintered.