Focused Question

This systematic review was conducted to answer the following question: based on clinical, in vitro or in situ studies, do restorative finishing and/or polishing procedures decrease bacterial adherence to the surface of dental materials?

This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA).¹⁰ The protocol is registered with the Prospective Register of Systematic Reviews (PROSPERO: CRD42016036234).

Search Strategy

Four Internet sources were searched for eligible articles published by November 4, 2016: the MEDLINE-PubMed, EMBASE, Cochrane-CENTRAL, and LILACS databases. The structured search was performed using a combination of controlled vocabulary and key words (Table 1), and a similar search strategy was adapted for the other databases. Searches for relevant ongoing trials from the US clinical trials register (http://www.clinicaltrials.gov) and grey literature (OpenGrey repository) were also performed. Manual searches of all references of the selected studies were performed in an attempt to find further relevant reports.

Table 1 Search Keyword: <Intervention AND Control AND Outcome>

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Screening and Study Selection

Two reviewers (DAMD and GKRP) independently screened the articles. Study selection was performed in two steps: 1) evaluation of title and abstract and 2) full-text analysis. Titles and abstracts were evaluated for the preselection of in vitro, in situ, or clinical studies published in the English language that evaluated bacterial adhesion to the surface of dental restorative materials. The reviewers then performed the full-text analysis of the selected studies using the following inclusion criteria:

• Intervention: Finishing/polishing procedures on the surface of dental restorative materials.

• Comparison: Unmodified or treated surfaces with the same material as the intervention group. Thus, studies that evaluated only finishing/polishing procedures among different restorative materials were not considered eligible.

• Quantitative assessment of bacterial adhesion to the surface of restorations.

• Surface characteristics (eg, roughness, free energy) determined using profilometry, scanning electron microscopy, or atomic force microscopy.

Articles that fulfilled all selection criteria were considered eligible for this investigation and submitted to the data extraction process. The concordance between the reviewers for full-text analysis was statistically assessed showing a 0.9 kappa score. Divergences between the reviewers were discussed and resolved by consensus. If a disagreement persisted, the judgment of a third reviewer (FBZ) was decisive.

Data Collection

Both reviewers independently collected the following data from eligible studies: study identification (authors, year of publication, country in which study was conducted), study design, description of methods (restorative material evaluated and description of finishing/polishing procedures), and type of biofilm formation (eg, type of microorganism). The main results and conclusions of the studies were recorded. For cases in which the article did not provide enough data for inclusion in the analysis, the first or corresponding authors were contacted to determine whether additional data could be provided. If contact with the authors was not achieved after three attempts, the article was excluded.

Risk of Bias (Quality Assessment)

The quality assessment of the selected studies was adapted from previous investigations.^11,12 The evaluation of the risk of bias involved the use of a chart considering the following aspects for each study design: description of sample size calculation, randomization of the sample, untreated control group, materials used according to the manufacturer's instructions, description of finishing/polishing standardization, blinding of the examiner of the outcome, and repetition of biofilm experiment (in vitro). If the authors reported the parameter, the article had a Y (yes) on that specific parameter; if it was not possible to find the information, the article received an N (no). Articles that reported zero to two items were classified as at high risk of bias, three or four as medium risk, and five to seven as low risk.

Data Analysis

Due to the considerable variability in the methodologies used to evaluate the effect of the finishing/polishing of different restorative materials on biofilm formation and the consequent heterogeneity of the results, meta-analysis was not possible. Thus, descriptive analysis was performed considering study design, materials, intervention on the surface of the restoration, surface characteristics (roughness and SFE), and the evaluation protocol for biofilm formation and bacterial adhesion.

Effect of Finishing and Polishing on Surface Roughness

The studies included in the present systematic review tested a large variety of finishing and polishing methods, including diamond finishing burs, abrasive paper discs, silicon carbide (SiC) and silicone points, abrasive-impregnated rubber, felt wheels, and polishing pastes. The effect of each method on the restoration surface is reported to be material dependent, and its effectiveness is mainly system dependent.³¹ Thus, the effect of finishing and polishing systems in each material was explored individually as follows.

Impact of Surface Roughness on Bacterial Adhesion

The data collected in this systematic review showed that finishing and polishing affect the surface roughness and promote a heterogeneous impact to bacterial adhesion considering each material evaluated and the method of evaluation of bacterial adhesion outcome (thickness, covered area, biomass, and colony-forming units).

In general, smoother surfaces are less likely to lead to the formation of biofilm regardless of restorative material and are therefore desirable. Based on the present findings, it may be concluded that 1) finishing procedures when not followed by a polishing system provide greater adhesion and retention of bacteria, 2) some studies showed that polishing successfully reestablished the level of biofilm formation observed on untreated or glazed control groups regardless of whether the same pattern of surface roughness was achieved, and 3) other studies showed significant differences of biofilm formation among polishing groups even when similar patterns of surface roughness were compared.

The impact of finishing and polishing methods to titanium abutments was evaluated in two clinical studies included in this review.^27,28 Quirynen and others⁹ evaluated the influence of the surface smoothing on supra- and subgingival biofilm formation comparing titanium abutments with different surface roughnesses (untreated, machined, and manually polishing protocols) in six partially edentulous patients. The data showed no significant differences on colony-forming unit counts between the control (Ra=0.2 μm) and polished groups (manual, Ra=0.06 μm; machined, Ra=0.11 μm). These results indicated that a reduction in surface roughness (a roughness less than 0.2 μm) had no major effect on the microbiologic composition either supra- or subgingivally. Based on these observations, the authors suggested an existence of a threshold roughness (Ra=0.2 μm) below which no further impact on the bacterial adhesion and/or colonization should be expected. This threshold roughness has been extensively used in the literature. Later, Elter and others²⁸ evaluated supra- and subgingival natural human biofilm formation to finishing and untreated titanium abutment surface. Their results showed that finishing the surfaces (Ra=0.4 μm) retained more supragingival biofilm compared to the control (Ra=0.2 μm) analyzed using scanning electron microscopy, while no differences were observed in the subgingival biofilm. These results corroborated the threshold roughness, especially when supragingival biofilm was considered. The greater impact of roughness on supra- than on subgingival biofilm may be explained because the clinical impact of surface roughness becomes especially important when larger shear forces are active.³²

In agreement with the previous studies, Rimondini and others²⁴ evaluated the surface roughness necessary to reduce early (24 hours) in vivo biofilm colonization on titanium disks assigned to different polishing groups. The results showed no significant differences in bacteria biomass among the polishing groups below the threshold roughness. All other in situ studies included in this systematic review compared finished and polished surfaces with roughness above the threshold roughness.^5,25276 Brentel and others⁵ assessed the in situ biofilm formation on feldspar ceramic (VM7, Vita). The biomass assessment showed greater bacterial adhesion when the ceramic surface was ground (finished) only by diamond burs (Ra=2.0 μm) compared to the glazed group (Ra=0.5 μm). On the other hand, when the feldspar ceramic was polished after being ground (F&P (2) Ra=0.8 μm), it successfully reestablished the bacterial adhesion level to the control groups even with a slightly rougher surface. Controversially, results were related by Haralur and others²⁵ evaluating the percentage of covered area by natural biofilm to porcelain (Vita VMK) ceramics. Their results showed that polished groups (Ra=0.6 and 0.9 μm) failed to achieve a similar percentage of bacteria accumulation compared to the smoother groups (autoglazed: Ra=0.4; overglazed: Ra=0.3 μm).

Perez²⁶ evaluated in situ bacterial adhesion to glass ionomer and resin composite specimens submitted to finishing and polishing protocols. The authors found that the ground surfaces (finishing group) always showed drastically rougher surfaces and presented higher biofilm formation compared to the control group, while polished surfaces presented no differences in the control regarding the biofilm accumulation. Based on the data from these in situ studies, it may be stated that mild differences of roughness are not enough to affect the amount of biofilm accumulation, even when comparing surfaces above the threshold roughness, once polishing surfaces achieved similar results of biofilm accumulation compared to the pretreatment surfaces without presenting the same level of surface roughness.

In addition to the data from clinical and in situ studies, most of the articles included in this systematic review used in vitro experiments. In brief, it was observed that several articles found no differences in biofilm formation when surfaces with Ra values above the threshold of 0.2 μm were compared,^{2,13,14,18,23,30} whereas significant differences in biofilm formation were found in other studies in which only smooth surfaces (Ra values up to 0.2 μm) were evaluated.^15,20 Thus, based on data from these laboratory studies, the threshold roughness of 0.2 μm was not fully corroborated, and it should be used cautiously among the different materials evaluated. This divergence may be explained by the intrinsic limitations of laboratory studies, which do not offer the strongest evidence.³³

Only one in vitro study used a polymicrobial biofilm model formed from human saliva,²³ while all other studies used synthesized biofilm. In addition, even though studies of monospecimens have enhanced knowledge of the mechanisms of bacterial adhesion to surfaces and differentiation into multicellular biofilms, the use of polymicrobial biofilm models should be incentivized once the majority of chronic infections harboring polymicrobial communities. Although in vitro models have been extensively used to study dental biofilm, there are limitations when trying to simulate the oral environment and in vivo conditions. It has to be highlighted that during in vivo chronic infection, there is a complex interplay between host and pathogen, with species not directly mixing but rather residing within their own ecological space, something that is not easily replicated in vitro and that leads to observable differences between in vitro and in vivo “chronic infections.”³³

It is well accepted that hard tissues with rougher surfaces in the oral cavity contribute to microorganism retention since rougher surfaces have a greater area for the development of biofilm as well as topographical irregularities that produce niches in which microorganisms are protected from shear forces and salivary flow. Such factors affect microorganism retention only in clinical and in vivo studies, as these factors are rarely simulated in laboratory studies. Therefore, the impact of topographical irregularities on bacterial retention in in vitro studies appears to be limited, and the amount of biofilm in such studies may be strongly related to other factors linked to the biofilm protocol, such as the type of inoculum (bacterial strain and human saliva) and culture conditions (temperature, pH, nutritional status and nutrient flow, presence of salivary pellicle, and incubation time).

Limitations of the Study

The results of the present review should be interpreted cautiously since most of the included studies were carried out using laboratory studies that do not represent the same evidence as clinical studies. Roberts and others³³ stated that “whilst there is no ‘gold-standard' for the study of in vivo and in vitro biofilm formation, it is crucial to know the limiting factors of selected models so as to not over-extrapolate data, and generate assumptions beyond the capabilities of the model.” For this reason, we discussed the results from each study design individually.

Moreover, it must be mentioned that the assessment of the risk of bias showed that most studies had high risk (61%). It was especially critical for in vitro studies, as nine of the 12 articles had high risk while only one had low risk. This result highlights that in vitro studies had poor control regarding the methodological variables that could influence the results, directly affecting the validity of the studies and explaining in part the resulting heterogeneity.