The following five commercially available dental zirconia implants were used in this study (Table 1). Bredent whiteSKY™ implant (I1) is made from unground Brezirkon™, an yttrium oxide (Y₂O₃)-stabilized tetragonal polycrystalline zirconium oxide and is sandblasted. Zirconium oxide is endowed with 3 mol% yttrium oxide to gain a rectangle and room temperature stable structure [17]. Straumann® PURE Ceramic Implant (I2) is generally made from yttrium oxide-stabilized tetragonal polycrystalline zirconium oxide. The surface due to the manufacturer is coated with a special process called ZLA™ which shall be similar to the SLA™ process (Sandblasted, Large-grit, Acid-etched) of titanium implants. Ceramic.implant vitaclinical (I3) is made from zirconium oxide. The Zeramex® implant (I4) is made from zirconium and has a sandblasted and etched surface structure with their so-called ZERAFIL™ technology. Camlog’s Ceralog Monobloc M10 ceramic implant (I5) is also made from yttrium-stabilized zirconium dioxide. Unlike the other ceramic implants, it is produced with ceramic injection molding (CIM) technique. This technique requires no sandblasting or etching. The implants’ geometrical design and the surface structure are already molded via CIM before the sintering and hot isostatic pressing (HIP) process.

For a more detailed illustration of the implant surface topology, the technique of scanning electron microscopy (SEM) was used. A Quanta 200 FEG (FEI Company, Netherlands) field emission SEM equipped with environmental low vacuum mode makes it possible to avoid the typical surface charging-up problems of uncoated highly insulating ceramic implants without the need for sample preparation. Therefore, high-resolution SEM images with magnifications up to 25,000 are possible to demonstrate the micro-structured appearances at different locations. Comparable areas for all implants under investigation are selected by splitting up the cylindrical shape of the implant into sections (Fig. 1). For the comparison of surface structures between the tested implants, two regions of interest were selected: machined and rough area (compare Figs. 1 and 3). Each section was observed under different degrees of magnifications (× 2000, × 10,000, × 25,000) with the same microscope parameters (HV 20 kV, Det LFD, pressure 0.90 mbar). The low vacuum pressure in the sample chamber was reduced until charging levels on the sample surface were reduced to the level at which electron imaging of the sample surface was possible.