The present in vitro study aimed to (1) test the effect of selective acid etching of primary enamel prior to the application of universal adhesives on the microtensile bond strength (µTBS) and (2) measure the effect of acid etching time of the primary enamel prior to the application of universal adhesives on the µTBS.
Materials and methods
Approval was granted by the ethical committee of the Faculty of Medicine (Department 11), Justus Liebig University, Gießen (number—AZ: 143/09). A total of 80 caries-free freshly extracted human primary molars were collected. Roots were removed and teeth were mesiodistally sectioned (IsoMet 1000 precision saw, Buehler, ITW Test & Measurement, Leinfelden-Echterdingen, Germany) to increase the sample size. Fifty-four teeth halves were randomized into three groups based on the adhesive type: (1) 3M™ Scotchbond™ Universal adhesive (SU; 3M™ Oral Care, Seefeld, Germany); (2) Clearfil™ Universal Bond Quick (CU; Kuraray Noritake Dental, Okayama, Japan); (3) iBond® Universal adhesive (iBU; Heraeus Kulzer, Hanau, Germany). Aprismatic enamel was removed, and the enamel surface was flattened using a yellow ring stone (Diamant FG878EF, 012G, Busch, Engelskirchen, Germany) followed by sandpaper in two sizes P1200 and P4000 (Buehler, ITW Test & Measurement) respectively. Then, groups were subdivided according to phosphoric acid etching time (DeTrey® Conditioner 36, Dentsply DeTrey, Konstanz, Germany) into three subgroups (SG): SG1 (control group): 0 s; SG2: 15 s; SG3: 30 s. Afterwards, adhesives were applied passively and a composite resin build up using Filtek™ Z250 Universal Restorative System (shade: A3; 3M™ Oral Care) in a height of 7 mm was added. Samples were incubated in distilled water at 37°C for 24 h. Furthermore, samples were dissected (0.7×0.7 mm, IsoMet high speed pro precision cutter, Buehler, ITW Test & Measurement) and each stick was tested separately (µTBS, TC-550, Syndicad, Munich, Germany). The maximum force in which the sticks fractured was recorded in newton (N) and was converted to megapascal (MPa).
Failure patterns were evaluated under light microscope (AZ100M, Nikon, Tokyo, Japan) and intact sticks, not loaded for µTBS testing, were examined under scanning electron microscope (SEM; Amray 1810, Amray, Bedford, MA, USA) to evaluate the enamel-resin interface.
Data were statistically analyzed using software SPSS 26.0 (Statistical Packages for Social Sciences, IBM Statistics, Armonk, NY, USA). Statistical evaluation of µTBS data was performed using linear mixed models (LMM) and the restricted maximum likelihood (REML) method with a level of significance set at p < 0.05. Pairwise comparisons of data were corrected by Sidak adjustment for multiple testing. And, percentages of fracture modes were calculated and were presented descriptively.
Our results showed that phosphoric acid etching enhanced the bond strength of universal adhesives to primary enamel in all the tested groups significantly (p<0.001). µTBS values between subgroups etched for 15 s and 30 s did not show significant differences (p>0.05). Significant differences were only found between the estimated marginal means of groups CU (9.3 MPa; 95% confidence interval [CI] 6.9-11.7) and iBU (16.9 MPa; 95%CI 13.5-20.3) in SG1 (p<0.029) and between the groups SU in SG2 (34.9 MPa; 95%CI 31.3-38.5) and CU in SG3 (25.5 MPa; 95%CI 22.7-28.3; p<0.005; Tab. 1).
Summary of microtensile bond strength (µTBS) values after 24 h storage in distilled water
Zusammenfassung der Mikrozugfestigkeitswerte (µTBS) nach 24-stündiger Lagerung in destilliertem Wasser
Number of sticks
Mean µTBS (MPa)
a,b Different superscript letters indicate significant differences within the same adhesive, c,d Different superscript letters indicate significant differences between the different adhesives. Linear mixed effects model (REML), Sidak (p<0.05)
SU-SG1Scotchbond Universal applied passively in self-etch mode, SU-SG2 Scotchbond Universal applied passively preceded by 15 s phosphoric acid etching, SU-SG3 Scotchbond Universal applied passively preceded by 30 s phosphoric acid etching, CU-SG1 Clearfil Universal Bond Quick applied passively in self-etch mode, CU-SG2 Clearfil Universal Bond Quick applied passively preceded by 15 s phosphoric acid etching, CU-SG3 Clearfil Univeral Bond Quick applied passively preceded by 30 s phosphoric acid etching, iBU-SG1 iBond Universal applied passively in self-etch mode, iBU-SG2 iBond Universal applied passively preceded by 15 s phosphoric acid etching, iBU-SG3 iBond Universal applied passively preceded by 30 s phosphoric acid etching
The most common total fracture mode of all subgroups was the adhesive fracture (74.8 %), followed by mixed fracture in both adhesive and enamel (14.1 %). SEM images showed cracks and voids in the adhesive layer of all adhesives used in the self-etch mode, while groups treated with phosphoric acid showed a clearly visible adhesive layer without noticeable damage, and numerous resin tags (Figs. 1 and 2).
Etching with phosphoric acid remains the gold standard for bonding universal adhesives to primary enamel. Moreover, the applied phosphoric acid etching time (15 vs. 30 s) showed no significant effect on the initial microtensile bond strength (µTBS) of universal adhesives to primary enamel.
Department of Paediatric Dentistry, School of Dentistry, University Medical Centre Gießen and Marburg, Campus Gießen,
Open Access funding enabled and organized by Projekt DEAL.
Compliance with ethical guidelines
Approval was granted by the ethical committee of the Faculty of Medicine (Department 11), Justus Liebig University, Gießen (number —AZ: 143/09).
Conflict of interest
D. Hamdy, S. Amend, S. Lücker and N. Krämer declare that they have no competing interests.
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