Bacterial adhesion to intra‐oral, hard surfaces is firmly influenced by the surface roughness of these structures. Previous studies showed a remarkable higher subgingival bacterial load on rough surfaces when compared to smooth sites. More recently, the additional effect of a further smoothening of intra‐oral hard surfaces on clinical and microbiological parameters was examined in a short‐term experiment. The results indicated that a reduction in surface roughness below Ra=0.2 μm, the so‐called “threshold Ra”, had no further effect on the quantitative/qualitative microbiological adhesion or colonisation, neither supra‐ nor subgingivally. This study aims to examine the long‐term effects of smoothening immoral hard transgingival surfaces. In 6 patients expecting an overdenture in the lower jaw, supported by endosseus titanium implants, 2 different abutments (transmucosal part of the implant): a standard machined titanium (Ra=0.2 μm) and one highly polished and made of a ceramic material (Ra=0.06 μm) were randomly installed. After 3 months of intra‐oral exposure, supra‐ and subgingival plaque samples from both abutments were compared with each other by means of differential phase‐contrast microscopy (DPCM). Clinical periodontal parameters (probing depth, gingival recession, bleeding upon probing and Periotest‐value) were recorded around each abutment. After 12 months. the supra‐ and subgingival samples were additionally cultured in aerobic, CO,‐enriched and anaerobic conditions. The same clinical parameters as at the 3‐month interval were recorded after 12 months. At 3 months, spirochetes and motile organisms were only detected subgingivally around the titanium abutments. After 12 months, however, both abutment‐types harboured equal proportions of spirochetes and motile organisms, both supra‐ and sub‐gingivally. The microbial culturing (month 12) failed to detect large interabutment differences. The differences in number of colony forming units (aerobic and anaerobic) were within one division of a logarithmic scale. The aerobic culture data showed a higher proportion of Gram‐negative organisms in the subgingival flora of the rougher abutments. From the group of potentially “pathogenic” bacteria, only Prevotellu inter‐media and Fusobacterium nucleatum were detected after anaerobic culturing and again the inter‐abutment differences were negligible. Clinically, the smoothest abutment showed a slightly higher increase in probing depth between months 3 and 12, and more bleeding on probing. The present results confirm the findings of our previous short‐term study, indicating that a further reduction of the surface roughness, below a certain “threshold Ra”(0.2 μm), has no major impact on the supra‐ and subgingival microbial composition.
