Porous metallic/bioceramic composite coatings, produced by plasma arc deposition incorporating highly crystalline hydroxylapatite (HA) in a porous structure of titanium or cobalt-chromium-molybdenum, have been developed for medical and dental device applications. An otherwise brittle and weak HA coating has been anchored by a porous metallic underlayer which is strongly bonded to the substrate. Implants with such coatings have the advantage of mechanical fixation by bony ingrowth into the porous metallic structure coupled with enhanced biocompatibility achieved through the use of HA. This combination produces a more stable bone/implant interfacial region as compared to that achieved by a HA coating applied directly on the implant surface. Once bony ingrowth has taken place, strong bonding has been achieved even if bone resorption of the HA were to take place. Various thicknesses, densities and pore sizes of such composite coatings have been applied to dental and orthopedic devices and have been implanted in the U.S.A., Japan, and Europe showing excellent post insertion clinical results. Physical, mechanical, and chemical properties of selected coatings have been characterized and are reported herein.

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.