2.1

Search strategy

The MEDLINE via PubMed, ISI Web of Science and Scopus electronic databases were searched to identify studies that could be considered. The following search strategies were performed: computer search of database, review of reference lists of all included studies, and contact with authors. The search was undertaken to identify literature up to June 13th, 2016.

The computer database search in MEDLINE via PubMed was performed using specific medical subject headings (MeSH) and keywords as follows: ((bovin*) AND (((dentin* adhesive*) OR bond*) OR adhesive system*)) AND ((((((((((((((((((((((((((((((((dentition permanent[MeSH Terms]) OR Permanent Dentition) OR Dentition; Secondary) OR Secondary Dentition) OR Dentition; Adult) OR Adult Dentition) OR Tooth; Deciduous[MeSH Terms]) OR Deciduous Tooth) OR Dentition; Deciduous) OR Deciduous Dentition) OR Deciduous Dentitions) OR Dentitions; Deciduous) OR Dentition*; Primary) OR Primary Dentition*) OR Milk Tooth) OR Tooth; Milk) OR Primary Teeth) OR Teeth; Deciduous) OR Deciduous Teeth) OR Teeth; Primary) OR Tooth; Primary) OR Milk Teeth) OR Teeth; Milk) OR Baby Teeth) OR Teeth; Baby) OR Baby Tooth) OR Tooth; Baby) OR Primary Tooth) OR dentin*[MeSH Terms]) OR Dental Enamel[MeSH Terms]) OR enamel)). For ISI Web of Science and Scopus the following search terms were used: (bovin*) AND (human) AND (bond strength) OR (adhesive system*). No publication year or language limit was considered.

2.2

Selection, inclusion and exclusion criteria

Firstly, titles and abstracts were reviewed independently by two authors (FZMS and AF) and selected per their consensus for further assessment considering the following inclusion criteria: in vitro studies that comparing bond strength of adhesive systems to bovine and human dental hard tissues, regardless of the methodology of the bond strength test employed. If consensus was not reached, the abstract was set aside for further evaluation.

The references of all selected studies were manually searched for further relevant studies that could fulfill the inclusion criteria.

The full texts of all studies that full-filled the inclusion criteria for eligible papers were then reviewed independently by two authors (FZMS and AF) considering the exclusion criteria: studies that evaluated bond strength to brackets or glass ionomer cements (not combined with adhesive systems); papers that compared different substrates ( i.e. , human primary teeth to bovine permanent teeth; or bovine enamel vs. human dentin) or that did not perform comparisons under the same bond strength test methodology. Any disagreements in the eligibility criteria were solved by discussion and consensus by a third reviewer (ROR). The eligibility of studies between the authors showed substantial agreement, with a kappa score of 0.84.

2.3

Data extraction

Two authors (FZMS and AF) performed the data extraction by means of a standardized sheet in Microsoft Office Excel 2013 (Microsoft Corporation, Redmond, WA, USA). A protocol for data extraction was defined: the authors categorized similar information into groups according to the main outcomes of interest. For each paper, the following data were systematically extracted: publication year; country; type of teeth used; sample size; adhesive system evaluated, time of storage before test; the bond strength test used and the substrates evaluated. Bond strength mean values (in MPa) and standard deviations (SD) were extracted from the data.

For studies that did not report the precise bond strength values and that showed the results in graphs or figures, corresponding authors were contacted 3 times by e-mail, with 2 weeks interval, if any information was missing. If no information was provided, the study was excluded from the systematic review.

2.4

Risk of bias assessment

Two authors (FZMS and AF) independently evaluated the risk of bias of each included study considering a score described in previous systematic reviews of in vitro studies . The description of the following parameters were checked in each study: randomization of the teeth for experimental groups, sample size calculation, teeth free of caries, specimen with similar cross section, failure mode evaluation, material applied following manufactureŕs instructions, adhesive and testing procedures performed by a single operator, specimen tested by a blinded operator. If the parameter was described on the text, the study received a “yes” on that specify parameter, otherwise it had a “no”. The risk of bias was classified according to the sum of “yes” received as follows: 1–3 = high, 4–5 = medium, 6–8 = low risk of bias.

2.5

Data analysis

For the meta-analyses, the pooled effect estimates were obtained by comparing means from human and bovine teeth, irrespective of the substrate (enamel or dentin), as well as considering the subgroups, according to the type of substrate – enamel and dentin – separately. For studies that evaluated several adhesive systems, the values were extracted and one mean for each condition was calculated by a formula according to the Cochrane Statistical Guidelines , to obtain a single sample size, mean and standard deviation values for experimental (bovine) and control (human) groups. In the selected studies, only the data of interest were extracted to be analyzed in the meta-analysis. If the study performed a separate group with storage or aging, only immediate bond strength values were considered for analysis.

The statistical differences between-groups were performed in Review Manager (RevMan version 5.3 software, Cochrane Collaboration, Copenhagen, Denmark, 2014) using a random-effect method. Statistical significance was defined as a P-value ≤0.05 (Z test). The amount of specimens was considered as the amount of experimental units. Forest plots were created to illustrate the meta-analysis.

Statistical heterogeneity among studies was assessed via the Cochran Q test, with a threshold p value of 0.1, and inconsistency (I ² ). Values up to 60% were considered as not important to moderate heterogeneity.

2.1

Search strategy

The MEDLINE via PubMed, ISI Web of Science and Scopus electronic databases were searched to identify studies that could be considered. The following search strategies were performed: computer search of database, review of reference lists of all included studies, and contact with authors. The search was undertaken to identify literature up to June 13th, 2016.

The computer database search in MEDLINE via PubMed was performed using specific medical subject headings (MeSH) and keywords as follows: ((bovin*) AND (((dentin* adhesive*) OR bond*) OR adhesive system*)) AND ((((((((((((((((((((((((((((((((dentition permanent[MeSH Terms]) OR Permanent Dentition) OR Dentition; Secondary) OR Secondary Dentition) OR Dentition; Adult) OR Adult Dentition) OR Tooth; Deciduous[MeSH Terms]) OR Deciduous Tooth) OR Dentition; Deciduous) OR Deciduous Dentition) OR Deciduous Dentitions) OR Dentitions; Deciduous) OR Dentition*; Primary) OR Primary Dentition*) OR Milk Tooth) OR Tooth; Milk) OR Primary Teeth) OR Teeth; Deciduous) OR Deciduous Teeth) OR Teeth; Primary) OR Tooth; Primary) OR Milk Teeth) OR Teeth; Milk) OR Baby Teeth) OR Teeth; Baby) OR Baby Tooth) OR Tooth; Baby) OR Primary Tooth) OR dentin*[MeSH Terms]) OR Dental Enamel[MeSH Terms]) OR enamel)). For ISI Web of Science and Scopus the following search terms were used: (bovin*) AND (human) AND (bond strength) OR (adhesive system*). No publication year or language limit was considered.

2.2

Selection, inclusion and exclusion criteria

Firstly, titles and abstracts were reviewed independently by two authors (FZMS and AF) and selected per their consensus for further assessment considering the following inclusion criteria: in vitro studies that comparing bond strength of adhesive systems to bovine and human dental hard tissues, regardless of the methodology of the bond strength test employed. If consensus was not reached, the abstract was set aside for further evaluation.

The references of all selected studies were manually searched for further relevant studies that could fulfill the inclusion criteria.

The full texts of all studies that full-filled the inclusion criteria for eligible papers were then reviewed independently by two authors (FZMS and AF) considering the exclusion criteria: studies that evaluated bond strength to brackets or glass ionomer cements (not combined with adhesive systems); papers that compared different substrates ( i.e. , human primary teeth to bovine permanent teeth; or bovine enamel vs. human dentin) or that did not perform comparisons under the same bond strength test methodology. Any disagreements in the eligibility criteria were solved by discussion and consensus by a third reviewer (ROR). The eligibility of studies between the authors showed substantial agreement, with a kappa score of 0.84.

2.3

Data extraction

Two authors (FZMS and AF) performed the data extraction by means of a standardized sheet in Microsoft Office Excel 2013 (Microsoft Corporation, Redmond, WA, USA). A protocol for data extraction was defined: the authors categorized similar information into groups according to the main outcomes of interest. For each paper, the following data were systematically extracted: publication year; country; type of teeth used; sample size; adhesive system evaluated, time of storage before test; the bond strength test used and the substrates evaluated. Bond strength mean values (in MPa) and standard deviations (SD) were extracted from the data.

For studies that did not report the precise bond strength values and that showed the results in graphs or figures, corresponding authors were contacted 3 times by e-mail, with 2 weeks interval, if any information was missing. If no information was provided, the study was excluded from the systematic review.

2.4

Risk of bias assessment

Two authors (FZMS and AF) independently evaluated the risk of bias of each included study considering a score described in previous systematic reviews of in vitro studies . The description of the following parameters were checked in each study: randomization of the teeth for experimental groups, sample size calculation, teeth free of caries, specimen with similar cross section, failure mode evaluation, material applied following manufactureŕs instructions, adhesive and testing procedures performed by a single operator, specimen tested by a blinded operator. If the parameter was described on the text, the study received a “yes” on that specify parameter, otherwise it had a “no”. The risk of bias was classified according to the sum of “yes” received as follows: 1–3 = high, 4–5 = medium, 6–8 = low risk of bias.

2.5

Data analysis

For the meta-analyses, the pooled effect estimates were obtained by comparing means from human and bovine teeth, irrespective of the substrate (enamel or dentin), as well as considering the subgroups, according to the type of substrate – enamel and dentin – separately. For studies that evaluated several adhesive systems, the values were extracted and one mean for each condition was calculated by a formula according to the Cochrane Statistical Guidelines , to obtain a single sample size, mean and standard deviation values for experimental (bovine) and control (human) groups. In the selected studies, only the data of interest were extracted to be analyzed in the meta-analysis. If the study performed a separate group with storage or aging, only immediate bond strength values were considered for analysis.

The statistical differences between-groups were performed in Review Manager (RevMan version 5.3 software, Cochrane Collaboration, Copenhagen, Denmark, 2014) using a random-effect method. Statistical significance was defined as a P-value ≤0.05 (Z test). The amount of specimens was considered as the amount of experimental units. Forest plots were created to illustrate the meta-analysis.

Statistical heterogeneity among studies was assessed via the Cochran Q test, with a threshold p value of 0.1, and inconsistency (I ² ). Values up to 60% were considered as not important to moderate heterogeneity.

3.1

Search and selection

Fig. 1 depicts a flowchart summarizing the selection process for studies according to the PRISMA statement . From 1,285 potentially eligible studies by the search strategy in PubMed/MEDLINE, ISI Web of Science and Scopus, 15 were duplicates and 1255 were excluded since they did not meet the eligibility criteria. The first screening resulted in 15 studies remained for full-text reading , from which 5 studies were excluded because: evaluated brackets , compared different comparable substrates (primary human vs . permanent bovine) , used different tests parameter for comparison (different cross-sectional bonded area) and evaluated the bond strength of other materials than adhesive systems . As one study was included from the references , 11 studies were included in the systematic review. However, 2 of them were not included in the meta-analyses due to lack of numerical data , even after three attempts to contact the authors by e-mail.

Flowchart diagram of study selection according to PRISMA statement.

3.2

Descriptive analysis

Table 1 shows descriptive extracted data from the included studies in systematic review. All studies were published between 1990 and 2013, with 6 studies published after 2000 . The majority of the studies were conducted in Brazil (n = 4) and United States (n = 2) . In this collection, 16 adhesive systems were evaluated including every commercially available adhesive classification: 3- and 2-step etch-and-rinse systems, 2- and 1-step self-etch systems. Six studies evaluated several adhesive systems in both substrates and 5 evaluated only one adhesive. Most evaluated system were: 1) Scotchbond Multi-Purpose (a.k.a. Scotchbond), considered in four studies and 2) Scotchbond 2 (a.k.a. Single Bond), evaluated in 2 studies . Six studies compared etch-and-rinse with self-etch systems and 5 evaluated only etch-and-rinse systems .

Table 1

Descriptive data from included studies in systematic review.

Paper	Country	Type of human tooth a	Number of teeth per group	Storage/ageing	Adhesive system	Bond strength test b	Substrates
Fowler et al.	United States	Molars and premolars	40	2 weeks	Scotchbond 2 d	SBS	Permanent enamel and dentin
Galhano et al.	Brazil	Single rooted	10	24 h	All Bond 2	PO	Permanent root dentin
Kaplan et al.	Argentina	Premolars	40	1 week	Scotchbond Bondlite Prisma Universal Bond Gluma	TBS	Permanent dentin
Krifka et al. e	Germany	*	88	24 h	Syntac Assortment Adper Prompt L-pop iBond Gluma Clearfil Protect Bond	SBS	Primary enamel and dentin
Lopes et al.	Brazil	Molars	20	24 h	Scotchbond MP Clearfil Liner Bond 2V	SBS	Permanent enamel and dentin
Muench et al.	Brazil	Molars	30	2 weeks	Prime & Bond 2.1 Single Bond Etch&Prime 3.0	TBS	Permanent dentin
Reis et al.	Brazil	Molars	10	24 h	Single Bond	μTBS	Permanent enamel and dentin
Retief et al.	United States	Molars	25	24 h	Scotchbond 2	SBS	Permanent dentin
Rüttermann et al.	Germany	Molars	80	24 h	Clearfil SE Optibond FL	SBS	Permanent enamel and dentin
Saunders	United Kingdom	Premolars	60	2400 termocycles	Scotchbond Topaz Gluma 3M experimental agent	SIBS	Permanent dentin
Shahabi et al. e	Australia	Central incisors, first premolars and third molars	c	48 h	Scotchbond Multi Purpose	SBS	Permanent enamel

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