Experimental coating defects in hydroxylapatite-coated implants.

Defects in hydroxyapatite (HA)-coated metallic implant systems, including cracks, flakes, or scratches, may occur at the time of surgery or in time because of in vivo loading. Such defects may affect the bone-implant interface response because of increased local metallic corrosion and ion release. Using a canine transcortical push-out model, the interface mechanics and histology of HA-coated titanium and cobalt-chromium-molybdenum alloy implants with and without coating defects were evaluated. The coating defects extended through the HA material to the underlying metallic substrate. Interface mechanical testing and undecalcified histologic techniques were used to evaluate differences in interface response at three, five, six, ten, 12, and 32 weeks postimplantation. There were no statistically significant differences between the HA-coated implants with and without defects for either interface shear strength or stiffness; however, both HA-coated implant types developed significantly greater interface strength and stiffness when compared to uncoated metallic implants. Histologically, in all areas away from the defect, a progression to nearly complete mineralization of osseous tissue directly onto the HA-coated surface was observed with no interpositional fibrous tissue layer. At early time periods (up to six weeks) in the area of the coating defect, bone apposition and mineralization appeared to stop at the edge of the HA coating. At later time periods (greater than ten weeks), the area of the defect was filled with mineralized osseous tissue in approximately one-half of the specimens. A thin fibrous interpositional layer was observed at the interface of the exposed metal substrate.