For monitoring of implant health and prevention of infection of the peri-implant tissue, routine maintenance care including probing of the peri-implant tissue has been recommended . Currently, the overall clinical significance of routine clinical probing of the peri-implant tissue has been challenged due to different reasons specifically belonging to the reproducibility and sensitivity to detect peri-implant disease [10, 23]. For example, the determination of the peri-implant pocket depth seems particularly difficult at implants with platform switching and/or expanding emergence profile . On the contrary peri-implant probing seems to be indispensable in monitoring and maintaining peri-implant health . Yet, there exists only limited evidence on the changes of the implant surface associated with routine probing.
Apart from qualitative assessment of ultrastructural changes of the implant surface, various parameters representing the physical roughness are commonly used to quantitatively determine these changes [25, 26]. Together with the surface free energy, particularly the surface roughness has been proposed as the major determining factor on the micrometer scale for the retention of bacteria to the implant surface . Comparing the impact of both determinants, the surface roughness seems more important toward the surface free energy [28, 29].
Mostly, two groups of instruments that are based on tactile or optical determination of the surface texture are used for the measurement of the surface roughness . In addition to the non-contact evaluation of the surface texture, the optical methods have the advantage to analyze not only linear (2D) profiles but also areal (3D) surface parameters. A wide number of methods are available for optical analysis of the surface texture among which the confocal microscopy allows for high vertical and horizontal resolution [30, 31].
Herein, topographical 2D and 3D parameters have been determined using confocal laser scanning microscopy. Considering the various 2D amplitude parameters for untreated implant surfaces, Ra was approximately 0.10 μm on smooth surfaces and 2.0 μm on rough implant surfaces. These values are in line with previous studies on the surface roughness of the same implant type [26, 32], thus confirming the high reproducibility and reliability of the confocal laser scanning microscope for the quantitative determination of ultrastructural changes of the titanium surface receiving experimental probing.
The present results revealed that the motion of the probe tip across the implant body leads to very discrete surface changes only. The metal probe slightly increased the roughness on the smooth surface areas and left a trend for decreased roughness at previously rough parts of the implant. Even smaller changes were observed for surfaces following to the contact with plastic probes. None of these changes reached statistical significance.
So far, there exists a considerable controversy on the importance of the implant surface roughness on bacterial adhesion. Several studies reported a positive correlation between bacterial adhesion and/or the rate of plaque formation with the surface roughness in vitro [33,34,–35] and a shift toward a dysbiotic microflora together with a higher rate of inflammation around implant abutments with rougher surface topography in vivo [17, 36, 37]. On the contrary, the surface roughness did not influence the adhesion of Streptococcus epidermidis and Staphylococcus sanguinis on titanium implants  and had also no influence on the development of plaque and peri-implant inflammation [18, 38]. The lack of consensus on the influence of surface roughness on bacterial adhesion might be attributed to the confinement on two global roughness parameters, i.e., the average roughness (Ra) and the root mean square roughness (Rrms), for the quantitative characterization of the surface topography of dental implants in most of these studies [26, 39]. Since both parameters give no information on the spatial distribution or particular shape of the single features of the ultrastructure, two surfaces might provide the same Ra and Rrms value despite considerable differences of their topography . For a comprehensive analysis of the surface topography, a set of “S parameters” considering not only linear profiles but defined areas of the surface have been proposed instead . Herein, also the 3D analysis revealed only minor and not significant differences of the surface roughness before and after the application of metal probes and even less with plastic probes.
On smooth implant surfaces, one parameter, i.e., Rp, that represents extreme values for the heights and depths of the surface structure was significantly elevated after treatment with metal probes . This observation might indicate that the movement of the probe tip across the implant surface causes rather linear than areal changes, i.e., scratches, at a small number of sites of the surface profile. In fact, the qualitative microscopic analysis confirms linear-shaped scratches caused by the movement of the probe tip.
Different from smooth implant surfaces, the rough surface areas are dedicated to be colonized by the host cells, particularly osteoblasts, in order to integrate the implant body into the osseous tissue. Compared with the smooth parts of the implant surface, the rough surface should be enclosed entirely into the alveolar bone. The rough surface parts of the implant yet might gain contact with the periodontal probe, if peri-implant disease has already caused partial disintegration of the implant . Herein, the rough surface areas showed a tendency for reduced roughness values following experimental probing. As found in the qualitative microscopic analysis, the previous surface structure has been flattened within these scratches. According to recent reports, the adhesion, proliferation, and differentiation of osteoblasts are strongly dependent upon the roughness of titanium surfaces. Rougher surfaces seem to attract osteoblasts more effectively as compared with machined areas or surfaces showing only minor roughness [44, 45]. Hence, the equalization of the surface topography due to routine probing might interfere with the reattachment of osteoblasts following to the successful treatment of peri-implant defects.
Previous studies on periodontal instruments, i.e., ultrasonic scaler and air-powder devices, have found a strong correlation between the roughness of root surfaces and the angulation of the working tip [46, 47]. It is commonly recommended to align the probe as parallel as possible at low angulations related to the implant surface. Considering the lower diameter of implants in comparison with natural teeth together with the mostly protruding emergence profile of the prosthetic restoration, one should realize that probing is commonly performed under higher angulations, i.e., 20°, in daily practice. In order to be able to determine an inherent influence of the application angle on changes of the implant surface, herein, the higher angulation was set at 60° which might be reached clinically only rarely. Comparing the changes of the implant surface topography according to the application angle revealed no differences following to the contact with metal and plastic probes.
From a clinical perspective, yet, it seems questionable if the minor changes of the surface topography as found in this study might, anyhow, impair the peri-implant conditions and/or even increase the risk for the manifestation of a significant bacterial infection ultimately increasing peri-implant inflammation. This study did not determine the influence of the surface changes on the ability of bacteria to adhere to the implant surface.
The present results were found under experimental conditions which might not entirely reflect the real clinical situation during probing of the peri-implant tissue. In this context specifically the individual implant design, i.e., the shape of the implant, might have a considerable impact on the alignment and proper application of the periodontal probe. In addition, in the current study, the application force was adjusted from 0.2 to 0.25 N. Due to the placement into the peri-implant pocket, the probe might be exposed to even higher vectorial application forces in the clinical situation which are primarily dependent on the strength of the peri-implant soft tissue. Moreover, this study considered only one specific type of rough implant surface. The considerable morphological differences between various types of rough implant surfaces might lead to differences in surface changes associated with routine probing including the potential for abrasion of the probe material. Yet, considering the mostly slight but partially stronger ultrastructural changes of the implant surfaces following to contact with metal probes, the use of plastic probes for the clinical evaluation of the peri-implant tissue seems preferable so far.
Taken together this in vitro study has shown that the movement of metal probes and to a lesser extent also of plastic probes over the implant surface caused discrete changes on both smooth and rough titanium surfaces. However, it remains to be elucidated if these changes might gain clinical relevance.