New insights into bioactivity of ceria-stabilized zirconia: Direct bonding to bone-like hydroxyapatite at nanoscale

Abstract

Osseointegration resulting from biomineralization means tight bone–implant attachment, which is clinically essential for successful dental implant treatment. The osseointegration ability of ceria-stabilized zirconia, a promising implant material, has been questionable and is unclear despite its clinical use due to zirconia's bioinert nature. The purpose of this research was to investigate the osseointegration ability of ceria-stabilized zirconia by clarifying its bioactivity. Here we show that ceria-stabilized zirconia is highly bioactive, contrary to the general consensus. Transmission electron microscopy observation revealed that the zirconia nanocrystals of a ceria-stabilized zirconia substrate directly bonded to osteoblastic cell-precipitated hydroxyapatite crystals at lattice fringe scale. This bonding was achieved without chemical treatment of the substrate surface before use. Hydroxyapatite crystals exhibited a morphology of flexible nanofibers less than 10 nm wide with nanometer-thick plates filling the spaces between nanofibers. Elemental analysis of the hydroxyapatites showed that they contained alkaline metal cations (Na, Mg, and K) as minor elements and that their average Ca/P atomic % ratio was ~1.40, similar to those of bone apatite. High bioactivity of ceria-stabilized zirconia resulted in direct bonding to bone-like hydroxyapatite, suggesting nanoscale direct osseointegration with bone in vivo that contributes to improving the success rate of dental implant treatment.