Dental implants are one of the most frequently used treatment options for the replacement of missing teeth. The oral microflora and its dynamic interactions with the implant substrata seem to crucially influence the long-term success or failure of dental implants [1,2,3,4,5,6]. As soon as implant surfaces are exposed to the human oral cavity, they are immediately colonized by microorganisms [7, 8]. The initial bacterial adhesion on implants is the first and essential step in the geneses of complex peri-implant biofilms, which, in turn, may result in peri-implantitis and loss of the supporting bone [3].

The type of implant material and its specific texture and physico-chemical surface properties influence the quantity and quality of microbial colonization [1, 9,10,11,12]. In modern biomaterial research, implant surfaces are mainly modified to increase osseous integration into the alveolar bone; recently however, implant surfaces are also modified to reduce biofilm formation after exposure to the oral cavity. Innovative implant materials or surface modifications with reduced adhesion properties or even with antibacterial properties are of pertinent clinical interest [13, 14]. Up to now, monolithic titanium has been the most frequently used base material and gold standard for the construction of implant systems. Titanium is known for its excellent biocompatibility and outstanding mechanical properties [15]. Zirconia implant materials (ZrO₂) were introduced as an alternative to titanium implants, mainly because of their supposedly reduced potential to adhere microorganisms [1, 16,17,18,19]. Surface roughness, texture, and wettability are regarded as the most significant surface factors influencing microbial accumulation on implants [9, 10, 12, 20]. Increased surface roughness on implant surfaces correlates with faster and firmer integration into the surrounding bone [21]. On the other hand, however, most studies indicate a positive correlation between surface roughness and the amount of adhering bacteria [1, 9,10,11, 19, 20, 22, 23]. For titanium implant surfaces, Bollen et al. found a threshold R _a value of 0.2 μm, and lower values did not further influence the quantity of bacterial adhesion [24]. In almost every corresponding investigation, the arithmetical mean roughness R _a —which is measured by stylus profilometer—is used as a parameter to describe implant surface roughness. Rupp et al. showed that surfaces with very different morphologies may share the same R _a value. Furthermore, R _a values alone may be inadequate to describe “surface roughness” in respect to its potential influence on microbial adhesion [25]. For this reason, we additionally applied atomic force microscopy (AFM) for a three-dimensional assessment of the surface topography of the tested materials. AFM, which was developed to obtain fine details of a surface on a molecular scale, was found to be the most suitable instrument for surface roughness measurements [11, 26]. Furthermore, the crucial influence of surface wettability on bacterial adhesion is widely accepted, but there is still conflicting evidence if substrata with hydrophobic properties reduce or enhance the quantity of adhering microorganisms [9, 10, 27,28,29,30,31]. Although most studies describe surface roughness rather than wettability as the dominant factor for bacterial adhesion, the data on this matter is somewhat ambiguous [9,10,11, 20, 32,33,34,35,36,37]. So far, no study has yet varied surface roughness and hydrophobicity in well-defined patterns to define the crucial surface factor for different bacterial species.