Effect of adhesive air-drying time on bond strength to dentin: A systematic review and meta-analysis

Author’s Accepted Manuscript Effect of adhesive air-drying time on bond strength to dentin: A systematic review and meta-analysis Mohamed M. Awad, Ali Alrahlah, Jukka P. Matinlinna, Hamdi Hosni Hamama www.elsevier.com/locate/ijadhadh

PII: DOI: Reference:

S0143-7496(19)30043-0 https://doi.org/10.1016/j.ijadhadh.2019.02.006 JAAD2343

To appear in: International Journal of Adhesion and Adhesives Accepted date: 30 January 2019 Cite this article as: Mohamed M. Awad, Ali Alrahlah, Jukka P. Matinlinna and Hamdi Hosni Hamama, Effect of adhesive air-drying time on bond strength to dentin: A systematic review and meta-analysis, International Journal of Adhesion and Adhesives, https://doi.org/10.1016/j.ijadhadh.2019.02.006 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of adhesive air-drying time on bond strength to dentin: A systematic review and metaanalysis Mohamed M. Awad1, Ali Alrahlah2,3, Jukka P. Matinlinna4, Hamdi Hosni Hamama5 1

Department of Conservative Dental Sciences, Prince Sattam Bin Abdulaziz University, College of Dentistry, Alkharj, Saudi Arabia 2

Department of Restorative Dental Sciences, College of Dentistry, King Saud University, Riyadh, Saudi Arabia 3

Engineer Abdullah Bugshan research chair for Dental and Oral Rehabilitation, king Saud University, Riyadh, Saudi Arabia Dental Materials Science, Applied Oral Sciences, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China 5

Department of Operative Dentistry, Faculty of Dentistry, Mansoura University, Mansoura, Egypt and Operative Dentistry, Faculty of Dentistry, The University of Hong Kong, Hong Kong SAR, China [email protected] [email protected] *Corresponding author: Department of Conservative Dental Sciences, College of Dentistry, Prince Sattam Bin Abdul-Aziz University, P.O Box 153, Alkharj 11942, Saudi Arabia.

Abstract: Objective: The objective of this review was to evaluate the effect of air-drying time on the adhesion (bond) strength of adhesives to dentin in previously published studies and to conduct a meta-analysis to quantify the differences in the bond strength obtained after the different air-drying times. Methods: An electronic search was performed using the Medline, Cochrane library, and Scopus databases . The included studies were laboratory studies that investigated the effect of adhesive air-drying time on adhesion (bond) strength of resin-based adhesives to coronal dentin. Studies which evaluated the effect of adhesives air-drying time on physical and mechanical properties of adhesives, interfacial properties, bond

1

strength to root dentin, enamel, or bond strength of indirect composite restoration or orthodontic bracket, were excluded. The methodological quality of included studies was assessed. Meta-analysis was performed using Comprehensive Meta-Analysis software, version 2.0 (Biostat, Englewood, NJ, USA). The results of the meta-analysis were subjected to a further one-way analysis of variance, followed by the Tukey post hoc multiple comparison using R-software, version 3.4.3 (R Foundation for Statistical Computing, Vienna, Austria). Results: Thirteen studies fulfilled the inclusion criteria of this review, while only five studies were included in meta-analysis. The effect of adhesive air-drying time on the bond strength was significant in eight studies (61.5%), material-dependent in four studies (30.8%), and not significant in one study (7.7%). Eight

studies (61.54%) presented a medium-risk of bias, three studies (23.08%) presented a high-risk of bias, and two studies (15.38%) presented a low-risk of bias. The analysis of micro-tensile bond strength (μTBS) of adhesives showed statistically significant difference between different air-drying times (p ≥ 0.05). The highest mean μTBS values were: 52.9 ± 11.38 MPa (obtained after air-drying of adhesives for 30 s), followed by 48.26 ± 9.77 MPa (15 s), and 37.76 ± 1.45 (25 s), while the lowest mean μTBS was 33.98 ± 2.30 MPa and 35.79 ± 6.63 MPa (5 s) obtained after 10 s and 5 s respectively. Conclusion: The air-drying time of dentin adhesives is crucial to the adhesion strength to coronal dentin. Adhesive air-drying for shorter durations (5-10 s) may be insufficient to obtain adequately durable bonding to dentin, instead, Air-drying should be performed for longer durations (15-30 s), considering the pressure and distance of air-drying source.

Key words: Adhesive, Air-drying, Solvent-based, Bond strength, Dentin

1.1 Introduction: Dental adhesives for bonding to dentin mainly consist of synthetic resin monomers that can partially infiltrate the tooth structure demineralized substrate. These adhesives may contain hydrophilic or hydrophobic monomers, photo-initiators, stabilizers, solvents and inorganic fillers 2

[1]. Current adhesives can be used according to two major adhesive strategies based on the mode of action. These are: ‘etch-and-rinse'' (ER), in which acid etching with phosphoric acid, H3PO4, is required; and ‘self-etch’ (SE) in which prior etching is not required [2]. Selective enamel etching is an adhesive strategy that combines both ER and SE, in which acid etchant is applied only to enamel followed by application of multi-mode (MM) adhesive to both enamel and dentin [3, 4]. The so-called MM adhesives are the latest generation of dental adhesives [5-8]. Multimode adhesives are designed to bond to tooth structures via both chemical and micromechanical mechanisms due to the presence of some functional monomers, e.g. 10-methacryloyloxydecyl dihydrogen phosphate (MDP). Solvents play a significant role in adhesion and bonding to dentin [1]. In both ER and SE adhesives, the main function of the solvent is to dilute the viscous monomers and facilitate their penetration into the demineralized dentin [9, 10]. In addition, the solvents allow complete ionization of the acidic monomers in SE adhesives. Several solvents such as water, ethanol and acetone [1, 11, 12] have been used in the currently available dental adhesives. However, it is highly recommended to evaporate solvent before the curing step, because the remaining solvent may adversely affect the polymerization of the adhesive layer [13]. Despite of the type of adhesive, air-drying is an essential step to facilitate removal of remaining solvent [14, 15] and excess water [16, 17] which may adversely affect adhesive’s polymerization [14, 18] as well as its mechanical properties [19]. Consequently, this can impair bonding to dentin. Moreover, gentle air-drying can create a uniform adhesive film with even thickness on the adherend substrate [1, 14]. It has been found that errors in adhesive application [9], particularly during air-drying [20], could deteriorate the bond (adhesion) strengths. Inadequate solvent evaporation and air entrapment

3

within the adhesive are common clinical errors occur during air-drying of the adhesives [21, 22]. Given that, the objective of this review was to evaluate the effect of air-drying times on the bond strength of resin-based adhesives to coronal dentin in previously published studies

2.1 Methods: This systematic review was reported in accordance with the guidelines of the Preferred Reporting Items Systematic Review and Meta-Analysis (PRISMA) statement [23]. The research question with reference to participants (P), interventions (I), comparators (C), and outcomes (O), (PICO) [24] was as follow: “Can air-drying time (C) affect the bond strength (O) of resin-based adhesives (I) to coronal dentin (P)?” 2.2 Information Sources and systematic search One of the authors (A.M.M.) performed an electronic search using the Medline, Cochrane library, and Scopus databases to search studies that evaluated the effect of air-drying on bonding of resin-based adhesives to dentin published between 01/01/1990 to 29/10/2018. For the purpose of this review, all the included studies were required to be published in English. The complete search keywords were “air drying + adhesive,” “air blowing + adhesive,” and “solvent evaporation + adhesive.” In addition, a manual search for relevant studies in the references lists of included studies was also performed. After the identification of articles in the databases, they were imported into Endnote X7.7 software (Thompson Reuters, Philadelphia, PA, USA) to remove duplicates. 2.3 Search strategy After removal of the duplicate items, all articles identified through searches were compiled onto one sheet and printed as four copies and distributed among the four authors. Each author

4

independently checked the eligibility criteria for each study at the title/abstract level and if required, at the full-text level. The included studies should be laboratory studies that investigated the effect of adhesive air-drying time the bond (adhesion) strength of resin-based adhesives to coronal dentin. Moreover, studies in which the secondary outcome was to evaluate the effect of air-drying procedure on adhesive dentin bond strength were also included. Studies which evaluated the bond (adhesion) strength of resin cement, indirect composite restorations, or orthodontic brackets were excluded. Agreement of at least three authors was essential for inclusion/exclusion of a study for the systematic review. 2.4 Data extraction and bias risk assessment The included studies are summarized in table.2. Moreover, the methodological quality of each included study was independently assessed by the four authors using the following parameters as adapted and modified from recent systematic reviews of laboratory studies [25, 26], namely: teeth randomization,[25, 26], teeth free of caries [25, 26], blinding of the examiner [25, 26], samples with similar dimensions [26], evaluation of the failure mode [26], sample size calculation [25, 26], and complete air-drying specification. If the authors reported the presence of some of these parameters, the article was said to have a ‘‘Yes’’ for that specific parameter; if it was not possible to find the information, the article received a ‘‘No’’. In the search of the missing or unclear information in the included studies, the corresponding authors were contacted twice by email. As adapted and modified from previous study [25], articles that reported one or two items were classified as having a high-risk of bias, if they reported one of two of the said parameters, medium-risk of bias if they reported three to five items as a medium-risk of bias, and low-risk of bias of they reported six or seven items as a low-risk of bias.

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2.5 Statistical analysis Studies included in the meta-analysis should at least compare two air-drying times using the micro-tensile bond strength (μTBS ) test at the same duration or aging conditions. The sample size, the mean and the standard deviation μTBS ±SD (MPa) were extracted from the studies and subjected to a meta-analysis using Comprehensive Meta-Analysis software, version 2 (Biostat, Englewood, NJ, USA), at the 95% confidence interval. The meta-analysis of this systematic review followed the statistical model of Borenstein [27], which has been designed for comparing the meta-analysis outcomes of different groups within the same study. The pooled mean μTBS ± standard error (SE) of different adhesives obtained following different adhesive air-drying times was calculated using either fixed effect or random effects models. The decision to select either model was based on results of statistical tests for heterogeneity. Data heterogeneity was assessed using the Cochrane Q homogeneity test with significance set at p < 0.10. That said, the results of the meta-analysis were subjected to a further one-way analysis of variance, followed by the Tukey’s post hoc multiple comparison test using R-software, version 3.4.3 (R Foundation for Statistical Computing, Vienna, Austria).

3.1 Results 3.2 Search results The electronic searches resulted in 710 found published articles. After removal of duplicate items, the titles/abstracts of the 446 search results were independently evaluated by the authors and 169 studies were excluded for one (or more) of the following reasons: non-dental study, study that is not relevant to the research question, review articles, and clinical study.

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Fifty-seven studies were assessed in full-text form for eligibility. Following the inclusion and exclusion criteria, 44 studies were excluded. The excluded studies evaluated the effect of airdrying on: 1) physical and mechanical properties of the adhesives, 2) composite restorations, 3) bond (adhesion) strength to root dentin, 4) hybrid layer and adhesive-dentin interfacial properties, 5) bond (adhesion) strength of indirect composite or orthodontic brackets, and 6) bonding to enamel. Studies in which the effects of air-drying pressure, temperature or distance on the bond strength of adhesives to dentin were evaluated were also excluded. Finally, 13 studies fulfilled the originally set inclusion criteria of this review. The search stages are illustrated in the flowchart (Figure 1). The outcomes of the included studies were summarized in Table 1.

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Figure 1: Search flowchart as adapted from the PRISMA Statement. 3.3 Data extraction Table 1. Data extraction items from full-text selected articles are included in the review Adhesive and Manufacturer

AD time and

Bond strength

(category)

specifications

test and aging

Outcome Reference

1.

Saikaew P. et al., 2018 [76]

Clearfil Universal Bond, Kuraray

5, 15, 30 s

μTBS, 24 h

The effect of adhesives air-drying time was

Noritake Dental Inc., Okayama,

D: not mentioned P: not mentioned

37°C, water

material- and storage time-dependent.

Japan (MM)

storage

8

G-Premio BOND, GC Corp.,

One year water

Tokyo, Japan (MM)

storage

Scotchbond Universal, 3M ESPE, MN, USA (MM) Fu J. et al.,

All Bond Universal, Bisco Inc.,

0, 5, 15, 30 s

μTBS, 24 h

Bond strength of the tested adhesive is

2017 [69]

IL, USA (MM)

D: 1.5 cm.

37°C, water

significantly influenced by air-drying time.

Clearfil Universal Bond, Kuraray

P: 2.5 bar

storage

Noritake Dental Inc., Okayama, Japan (MM) 2.

G-Premio BOND, GC Corp., Tokyo, Japan (MM) OptiBond All-in-one, Kerr Corp., CA, USA (1-SEA) Scotchbond Universal, 3M ESPE, MN, USA (MM)

3.

de Sousa

Stae, SDI, Victoria, Australia

10, 30, 60 s

SBS, 24 h 37°C,

Bond strength of the tested adhesive is not

Junior JA.

(2-ER)

D: 10 cm

water storage

significantly influenced by air-drying time.

et al., 2015

XP Bond, Dentsply De Trey, Konstanz, Germany (2-ER)

P: not

[67]

mentioned Ambar, FGM, Joinville, SC, Brazil (2-ER) All-Bond Universal, Bisco Inc.,

5, 15, 25 s

μTBS, 24 h

Air-drying time significantly affect the bond

Martinez

IL, USA (MM)

D: 1.5 cm

37°C, water

strength of MM adhesives in both SE or ER

IV. et al.,

Prime & Bond Elect, Dentsply

P: 1 bar

storage

modes to dentin.

2014[74]

(MM)

Luque-

4.

Scotchbond Universal Adhesive, 3M ESPE, MN, USA (MM) Daneshmehr

Bond Force, Tokuyama, Tokyo,

0, 5, 10 s

SBS

Air-drying is not affecting adhesives

L. et al.,

Japan (1-SEA)

D: 2 cm

Duration:

immediate bond strength however, its effect

2013[66]

FL-Bond II, Shufo Inc, Kyoto,

P: 0.25 MPa (2.5

Immediate

was material dependent after thermal aging.

Japan (1-SEA)

bar)

24 h 37°C, water

5.

6.

Scotch Bond Multi-Purpose, 3M

storage, Thermal

ESPE, MN, USA (3-ER)

aging

Lee Y. and

G-Bond, GC Corp., Tokyo, Japan

1, 5, 10 s

μTBS, 24 h

Air-drying time significantly affect the bond

Park. JW.,

(1-SEA)

D: not

37°C, water

strength of both adhesives to dentin

2012 [73]

Clearfil S3 Bond, Kuraray

mentioned

storage

9

Noritake Dental Inc., Okayama,

P: not

Japan (1-SEA)

mentioned

Fu J. et al.,

G-Bond plus, GC Corp., Tokyo,

5, 10, 15, 20, 25,

μTBS, 24 h

Adhesive air-drying time significantly affect

2012 [68]

Japan (1-SEA)

25, 30, 35 s

37°C, water

the bond strength of all tested adhesives to

BeautiBond, Shufo Inc, Kyoto,

D: 15 mm

storage

dentin.

Japan (1-SEA)

P: 0.25 MPa (2.5

Both long and short Adhesive air-drying times

Easy Bond, 3M ESPE, MN, USA

bar)

negatively affect the adhesives bond strength

7.

(1-SEA)

to dentin

Furuse AY

Adper Prompt L-Pop, 3M ESPE,

Adhesive

SBS, 7 days

Adhesive air-drying time significantly affect

et al., 2008

MN, USA (1-SEA)

immobilization

37°C, water

the bond strength of 5 adhesives to dentin

[70]

Clearfil S3 Bond, Kuraray

period,

storage

while 2 adhesives are not affected.

Noritake Dental Inc., Okayama,

1

Japan (1-SEA)

immobilization

Futurabond NR, (1-SEA)

period,

G-Bond, GC Corp., Tokyo, Japan

2x adhesive

(1-SEA)

immobilizations

Hybrid Bond, Sun Medical, Shiga, Japan (1-SEA)

period.

8.

iBond, Heraeus Kulzer, Hanau, Germany (1-SEA) Xeno III, Dentsply DeTrey, Konstanz, Germany (1-SEA)

/2 Adhesive

D: 3cm P: 40 psi (2.75 bar)

Sadr A. et

Clearfil SE bond, Kuraray

2, 5, 10 s

μSBS, 24 h

Bond strength of two tested adhesives was

al., 2007

Noritake Dental Inc., Okayama,

P: about 4

37°C, water

significantly affected by air-drying time.

[75]

Japan (2-SEA)

kgf/cm2 (3.92

storage

Clearfil Tri-S bond, Kuraray

bar)

Noritake Dental Inc., Okayama,

Distance: 5 cm

9.

Japan (1-SEA) Excite, Ivoclar Vivadent, Schaan, Liechtenstein. (2-ER) OptiBond Solo Plus, Kerr Corp.,

0, 10 s

μTBS, 24 h

Adhesive air-drying time significantly affect

D: 1 cm

37°C, water

the bond strength of tested adhesives to

2006 [71]

CA, USA (2-ER)

P: Gentle

storage

dentin.

Jacobsen T.

Clearfil Protect Bond, Kuraray

0, 5, 10, 20 s

SBS / 24 h

Bond strength of all adhesives is significantly

et al., 2006

Noritake Dental Inc., Okayama,

D:

37°C, water

affected by air-drying time.

[72]

Japan (2-SEA)

approximately 2

storage

G-Bond, GC Corp., Tokyo, Japan

cm

Hashimoto 10. M. et al.,

11.

10

(1-SEA)

P: not

Hybrid Bond, Sun Medical (1-

mentioned

SEA) iBond NG experimental, Heraeus Kulzer (1-SEA) Clearfil S3 Bond, Kuraray Noritake Dental Inc., Okayama, Japan (1-SEA) Chiba Y. et

Adper Prompt L-Pop, 3M ESPE,

0, 5, 10 s

SBS / 24 h

Adhesive air-drying time can significantly

al., 2006

MN, USA (1-SEA)

D: 5 cm

37°C, water

affect the Bond strength of tested adhesives to

[18]

Clearfil Tri-S Bond, Kuraray

P: 0.2 MPa (2

storage

dentin.

Noritake Dental Inc., Okayama,

bar)

Japan (1-SEA) 12.

Fluoro Bond Shake-One, Shufo Inc, Kyoto, Japan (1-SEA) G-Bond, GC Corp., Tokyo, Japan (1-SEA) One-Up Bond F Plus, Tokuyama Dental, Tokyo, Japan (1-SEA) Miyazaki

Impreva Bond, Shufo Inc, Kyoto,

0, 1, 5, 10, 20,

SBS / 24 h

Adhesive air-drying time can significantly

M. et al.,

Japan. (3-ER)

30 s

37°C, water

affect the Bond strength of tested adhesives to

1996[14]

Scotchbond Multi-Purpose, 3M

D: 10 cm

storage

dentin.

Dental Products, MN, USA. (3-

P: 40 psi (2.75

ER)

bar)

13.

Key: 3-ER = three-step etch-and-rinse, 2-ER = two-step etch-and-rinse, 2-SEA = two-step self-etch adhesive, 1-SEA = one-step self-etch adhesive, MM = multi-mode adhesive D = distance of air-drying source, P = pressure of used air. SBS = shear bond strength, μSBS = micro-shear bond strength

11

3.4 Descriptive Analysis Thirty-one (18 SE, 8 ER, and 5 MM) adhesives were used in a total of 13 studies to evaluate the effect of adhesives air-drying time on their bond (adhesion) strength to dentin. The air-drying times varied between 0-60 s, the distance of air-drying source varied between 1-10 cm, and air pressure varied between 1-3.92 bars (Table 1). Air-drying pressure and/or distance were not clearly mentioned in 6 studies (46.1%) [67, 70-73]. μTBS and SBS tests were equally used each in six studies (46.15%) while μSBS was used in only one study (7.7%). In ten studies (76.9 %), the bond strength was evaluated after 24 h while artificial aging was performed only in the remaining three studies (23.1%). The effect of adhesive air-drying time on the bond strength was significant in eight studies (61.5%) [68, 69, 71-75], material-dependent in four studies (30.8%) [18, 66, 70], and not significant in one study (7.7%) [67]. All tested adhesives showed reduced bond strength values when they were not air-dried (0 s) except of three adhesives (One-Up Bond F Plus, and FL-Bond II and Scotch Bond Multi-Purpose). In contrast, the 10 s air-drying time resulted in marked reduction in the bond strength values of four adhesives (One-Up Bond F Plus, Bond Force, FL-Bond II, and Scotch Bond Multi-Purpose). 3.5 Assessment of bias risk According to the parameters considered in the analysis, eight studies (61.54%) presented a medium-risk of bias, three studies (23.08%) presented a high-risk of bias, and two studies (15.38%) presented a low-risk of bias. The results are described in Table 2. The studies scored particularly poorly on the following items: sample size calculation and blinding of the examiner.

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Table 2. Assessment of risk of bias

Study

1. Saikaew P.

Randomizatio

Carie

Similar

Sample

blinding

Failur

Complete

Risk of

n

s free

dimension

size

of the

e

air-drying

bias

s samples

calculatio

examine

mode

specification

n

r

Yes

Yes

Yes

No

s

No

Yes

No

et al., 2018

Mediu m

[76] 2. Fu J. et al.,

Yes

Yes

Yes

Yes

No

Yes

Yes

Low

Yes

Yes

Yes

No

No

Yes

No

Mediu

2017 [69] 3. de Sousa Júnior JA.

m

et al., 2015 [67] 4. Luque-

Yes

Yes

Yes

No

No

Yes

Yes

Martinez

Mediu m

IV. et al., 2014 [74] 5. Daneshmeh

No

Yes

Yes

No

No

Yes

Yes

r L. et al.,

Mediu m

2013 [66] 6. Lee Y. and

No

Yes

Yes

No

No

No

No

High

Yes

Yes

Yes

Yes

No

Yes

Yes

Low

Yes

Yes

Yes

Yes

No

Yes

No

Mediu

Park JW., 2012 [73] 7. Fu J. et al., 2012 [68] 8. Furuse AY. et al., 2008

m

13

[70] 9. Sadr A et

Yes

Yes

Yes

No

No

No

Yes

Mediu

al., 2007

m

[75] 10.Hashimoto

No

Yes

Yes

No

No

No

No

High

Yes

Yes

Yes

No

Yes

Yes

No

Mediu

M. et al. 2006 [71] 11.Jacobsen T. et al., 2006

m

[72] 12.Chiba Y. et

No

No

Yes

No

No

Yes

Yes

Mediu

al., 2006

m

[18] 13.Miyazaki

No

No

Yes

No

No

Yes

Yes

High

M. et al., 1996 [14]

3.6 Meta-analysis

Due to the different experimental conditions, such as the bond strength test used, and air-drying times evaluated, in the 13 studies included in the systematic review, only five studies fulfilled the eligibility criteria to be included in the meta-analysis. The results of the meta-analysis of the mean μTBS values (MPa) of adhesives subjected to different air-drying durations are shown in Figures 2 through 6, respectively. The highest mean μTBS values were: 52.9 ± 11.38 MPa (obtained after air-drying of adhesives for 30 s), followed by 48.26 ± 9.77 MPa (15 s), and 37.76 ± 1.45 (25 s), while the lowest mean μTBS was 33.98 ± 2.30 MPa and 35.79 ± 6.63 MPa (5 s) obtained after 10 s and 5 s respectively. 14

Figure 2. Meta-analysis results of the mean μTBS after 5 s air-drying.

Figure 3. Meta-analysis results of the mean μTBS after 10 s air-drying.

Figure 4. Meta-analysis results of the mean μTBS after 15 s air-drying.

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Figure 5. Meta-analysis results of the mean μTBS after 25 s air-drying.

Figure 6. Meta-analysis results of the mean μTBS after 30 s air-drying.

Table 3. Comparison of mean μTBS (MPa) after different air-drying times. Air-drying time (s) 5a 10 b 15 c 25 d

Mea n 35.79 33.98 48.26 37.76

varianc e 43.98 5.311 95.49 2.11

No of studies 4 4 3 2

SD

Total sample size 554 310 577 500

Significanc e

6.63 2.30 F= 445 .9 9.77 p<0.001* 1.45 11.3 52.90 129.55 2 189 30 e 8 * Results are based on a one-way analysis of variance (ANOVA) and the Tukey post hoc test of the meta-analysis data following the statistical model of Borenstein and others. ** Groups identified by different superscripts were significantly different at p<0.05. Key: SD = standard deviation.

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4.1 Discussion It is widely accepted that systematic reviews are the epitome of high- quality scientific synthesis [77]. A meta-analysis is a well-established method of combining quantitative evidence from different studies, and of extracting a more objective and quantitative summary of the evidence by statistical analysis than could be obtained from a narrative review [78]. To the authors’ knowledge, the current systematic review is the first one to evaluate the effect of the air-drying time of adhesives on the bond strength to coronal dentin. Different air-drying instructions are recommended by manufacturers for adhesives that belong to the same category. Therefore, this systematic review was designed to answer the question: “Can different adhesive air-drying times affect the bond strength of resin-based adhesives to dentin?” The bonding performance of dental adhesives is predominantly evaluated by shear or tensile bond strength tests [79]. Due to the lack of clinical studies evaluating adhesives bond strength, only laboratory studies were included in the review. Air-drying time was selected to be the variable of this review as it is easy for clinicians to control compared to the air-pressure. Also, the air-drying temperature seems to be impractical to control during the routine daily clinical practice, hence it has not been selected as one the current review evaluation variables. Results of this review revealed that the bond strength of adhesives was significantly affected by air-drying times. One objective of the adhesive air-drying procedure is to allow evaporation of solvents remaining within adhesive before polymerization [14, 15]. The residual solvents may be a source for dilution of the components preventing the collision of reactive components before setting [56]. Moreover, they act as inhibitors for the polymerization of reactive resinous components, and provide possible pathways for nanoleakage [36, 41, 80, 81]. Inadequate air-drying may result in creation of small pores and cracks within the adhesive layer which may negatively affect bond strength [18, 82]. 17

In addition, the bond strength of the adhesives may be influenced by the mechanical properties of the adhesives [18, 83, 84], which may be also correlated to the solvent evaporation rate [19]. As confirmed by the results of the meta-analysis, the bond (adhesion) strength values obtained after extended air-drying periods (15-30s) were statistically the highest among different air-drying This might be explained by more effective solvent evaporation of adhesives over extended airdrying times compared to a shorter air-drying times (5-10 s), and the previously mentioned negative effect of residual solvent within adhesive layer. Interestingly, a few adhesives showed high bond strength values without adhesive air-drying (0 s) [18, 66]. Because the bond strength can be affected by mechanical properties of adhesives (such as stiffness, resilience), the mechanical properties of cured adhesive were not found to depend only on the amount solvent remaining within adhesives. Other factors such as concentration of initiators or reactivity of the monomers, might influence the quality of the polymer as well [72]. Given this, it seems that a more non-sensitive polymerization system might compensate the negative effect of residual solvent(s) on the degree of conversion of dentin adhesives. The residual solvents, such as water in (One-Up Bond F Plus™) might not be an obstacle for polymerization because of the excellent polymerization ability of the dye-sensitized photo-polymerization system employed in its composition [18]. Different air-drying times were recommended for the same adhesive by multiple studies. For one studied adhesive (Clearfil S 3 Bond™), two studies recommended 10 s of air-drying [72, 73] and two studies recommended 5 s [18, 75]. In contrast, in another study it was found that the same adhesive was not affected by different air-drying times (5-20 s) [70]. This may be explained by differences in the bond strength tests used in these studies, risk of bias presented and, the lack of standardization of air-drying specifications within studies. Both air-pressure [22, 62] and the 18

distance of the air-drying source (nozzle) [21, 22] can affect the bond strength of the adhesives. In four studies, air-drying specifications were not completely described, three of them presented a high risk of bias. Based on the significant effects of the variables within the adhesive air-drying procedure, it seems that, air-drying till adhesive immobilization may be more practical than a given air-drying time. That said, Furuse et al. found that, adhesive immobilization period may be the most appropriate duration to obtain higher bond strength of the adhesives [70]. While solvent evaporation may affect the longevity of the resin–dentin bonding [85]. One of the limitations of this review is the lack of artificial aging in 76.9% of selected studies, as bond strength tests were performed after 24 h of water storage, which should be considered as ‘immediate’ bond strength testing [6]. In laboratory studies, artificial aging is essential to assess the durability of bond strength and to predict the clinical effectiveness of adhesives [86]. Thermo-cycling and long-term water storage (at 37 ˚C) are widely used and common methods for artificial aging [79, 87]. Water uptake appears to be one of the main reasons for long-term in vivo tooth-to-resin composite bond degradation [79]. Laboratory bond strength tests should be performed in clinically relevant circumstances [86]. Air-drying efficiency may be affected by the configuration of the cavity [88]. Generally speaking, another limitation of the studies may be that they tested bonding to flat dentin surfaces, a condition that may not be clinically relevant. In addition, the measurement of adhesive thickness after air-drying was not attempted in any of the studies. Further long-term dentin bonding studies in which the effect of adhesive air-drying is evaluated under more clinically-relevant conditions are suggested.

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5.1 Conclusions Air-drying time of solvent-based light-cured adhesives is crucial to the adhesion strength of adhesives to the coronal dentin. Short an adhesive air-drying time (5-10 s) might be insufficient to obtain adequately durable bonding to dentin, instead, Air-drying should be performed for longer periods (15-30s), considering the pressure and distance of air-drying source. Acknowledgement: The authors are grateful to the Deanship of Scientific Research, King Saud University, Vice Deanship of Scientific Research Chairs, Engineer Abdullah Bugshan research chair for Dental and Oral Rehabilitation, and to Dr Iman El Sayed, Assistant Professor, Department of Biomedical Informatics and Medical Statistics, Medical Research institute, Alexandria University, Alexandria, Egypt, for their support in the meta-analysis part. Declaration of interest The authors of this manuscript declare that there is no conflict of interest. Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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