To improve the mechanical properties of hydroxyapatite (HA, Ca-10(PO4)(6)(OH)(2)) for orthopedic applications, numerous investigators have proposed combining HA with high strength materials, specifically zirconia. Despite the fact that, compared to pure HA, it is now well-established that zirconia and HA composites have improved mechanical properties, the cytocompatibility properties of this composite remain largely uninvestigated. For these reasons, the objective of the present in vitro study was to synthesize HA and partially stabilized zirconia composites for osteoblast (bone-forming cell) adhesion assays. Various sintering temperatures and amounts of zirconia in HA composites were used in order to ascertain their influence on osteoblast adhesion. Results demonstrated increased interactions between HA and partially stabilized zirconia, when either higher sintering temperatures (between 900 and 1300 degrees C for 1 h) or higher zirconia contents (between 10 and 40 wt %) were used during material synthesis. More importantly, greater osteoblast adhesion was measured on HA-zirconia composites sintered either at lower temperatures (specifically, 900 degrees C) or with lower amounts of zirconia added to HA composites (specifically, 10 wt %). Results further indicated that when sintered at lower temperatures the composites possessed smaller nanometer grain sizes with increased surface roughness and a more stable HA phase. For these reasons, this study suggests that to optimize osteoblast adhesion on HA and partially stabilized zirconia composites for orthopedic applications, low sintering temperatures and low amounts of zirconia should be used. This suggests that a delicate balance must be reached between increasing mechanical properties of HA without decreasing osteoblast cytocompatibility properties through zirconia addition. (c) 2006 Wiley Periodicals, Inc.