Effect of Cariogenic Challenge on Bond Strength of Adhesive Systems to Sound and Demineralized Primary and Permanent Enamel Tamara Kerber Tedescoa / Fabio Zovico Maxnuck Soaresb / Rosa Helena Miranda Grandec / Leonardo Eloy Rodrigues Filhod / Rachel de Oliveira Rochae Purpose: To evaluate the effect of cariogenic challenge on the bond strength of adhesive systems to sound and artificially demineralized enamel of primary and permanent teeth. Materials and Methods: Eighty molars (40 primary, 40 permanent) were randomly assigned to 16 groups (n = 5) according to the type of tooth (primary [PRIM] or permanent [PERM]), enamel condition (sound [S] or demineralized [DEM]), treatment after the restorative procedure (control [C] or cariogenic challenge [pH]), and adhesive system (Adper Single Bond [SB] or Clearfil SE Bond [SE]). Teeth from the DEM group were subjected to cariogenic challenge by pH cycling prior to restorative procedures and pH group specimens were subjected to cariogenic challenge before the microshear test. One of two adhesive systems was applied to the flat enamel surfaces and composite cylinders (0.45 mm2) were built. The microshear bond test was performed. The data (MPa) were subjected to ANOVA and Tukey’s test (_ = 0.05). Results: No statistically significant differences were detected between the adhesive systems. The S groups exhibited higher bond strength values than the DEM groups, as did C groups compared to pH groups. PERM groups also had higher bond strength values than PRIM groups, excluding PRIM S and PERM S, which had similar values. Conclusion: The bond strength to demineralized enamel of primary teeth was lower than to the demineralized enamel of permanent teeth. Cariogenic challenge negatively influenced enamel bond strength, regardless of the type of tooth and adhesive system. Keywords: μSBS, adhesion, enamel caries. J Adhes Dent 2014; 16: 421–428. doi: 10.3290/j.jad.a32662
C
onstant improvements in the science of adhesive dentistry have resulted in the development of new adhesive systems with potential for reducing clinical time requirements and enhancing bonding effectiveness. Although clinical testing is essential, laboratory evidence
a
PhD Student, Department of Orthodontics and Pediatric Dentistry, University of São Paulo, São Paulo, SP, Brazil. Hypothesis, performed the experiments in partial fulfillment of requeriments for a degree, wrote the manuscript.
b
Adjunct Professor, Department of Restorative Dentistry, Federal University of Santa Maria, Santa Maria, RS, Brazil. Proofread the manuscript, contributed substantially to discussion.
c
Associate Professor, Department of Biomaterials and Oral Biology, University of São Paulo, São Paulo, SP, Brazil. Proofread the manuscript.
d
Associate Professor, Department of Biomaterials and Oral Biology, University of São Paulo, São Paulo, SP, Brazil. Proofread the manuscript.
e
Adjunct Professor, Department of Stomatology, Federal University of Santa Maria, Santa Maria, RS, Brazil. Idea, hypothesis, experimental design, proofread the manuscript, contribution substantially to discussion.
Correspondence: Rachel de Oliveira Rocha, Department of Stomatology, Federal University of Santa Maria, Rua Floriano Peixoto, 1184, Santa Maria, RS, 97015-372, Brazil. Tel/Fax: +55-55-3220-9291. e-mail: rachelrocha@ smail.ufsm.br
Vol 16, No 5, 2014
Submitted for publication: 25.07.11; accepted for publication: 10.07.14
of these materials’ effcacy should first be gathered,34 especially under critical conditions that expose restorative materials to situations similar to the challenges posed by the oral environment.21,28,30,33,36,41 pH oscillations are common in the oral environment and can cause mineral loss in dental tissues.10 Caries, a highly prevalent oral disease,2 makes restoration of carious enamel and dentin lesions a common practice in routine dentistry. At the same time, the existence of restorative materials with adhesive properties also permits greater conservation of tooth structures during restorative procedures. In this sense, although it is well known that bonding to enamel is very reliable and predictable, especially when using etch-and-rinse adhesive systems,5 it is necessary to consider the possibility that some demineralized enamel may have been unintentionally left after cavity preparation together with sound enamel. Thus, the restorative materials are placed on substrates different from those for which they were originally designed. As bond strength to demineralized enamel has not been included in most adhesive system studies, the investigation of bonding to this substrate is thus relevant, 421
Tedesco et al
Table 1 Experimental groups according to type of tooth, enamel condition, adhesive system and treatment after restorative procedure Type of tooth
Enamel condition
Adhesive system
Treatment after restorative procedure
Primary (n = 40)
Sound (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Demineralized (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Permanent (n = 40)
Sound (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Demineralized (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
since it is necessary for establishing an adequate seal of the cavity and, consequently, ensuring the longevity of the adhesion.43 In addition, pH fluctuations also pose a challenge to the restorative procedures and may affect the dental substrate around the restoration,30 with a possible negative effect on the bonding of the adhesive system and the dental substrates.21,28,33 Because the effectiveness of adhesive systems is directly related to the durability of the restoration, studies must be conducted to examine the behavior and stability of the adhesive interface in response to cariogenic challenge because there are relatively few reports in the literature on this subject.21,26,28,30,33,35,40,41 Many of these studies evaluated demineralization around restorations,26,30,35,40,41 but fewer evaluated bond strength,21,26,28,33 particularly regarding enamel. Most of the scientific evidence on the performance of adhesive systems is based on results from permanent teeth.1,4,12,15,37 However, the microstructural and morphological differences observed between primary and permanent teeth3 make it important to compare the performance of adhesive systems between these substrates.14,28,38 It is expected that mineral depletion in primary teeth will affect the performance of adhesive systems, especially when subjected to a cariogenic challenge.21 Therefore, the purpose of the present study was to evaluate the effect of cariogenic challenge on the bond strength of adhesive systems to sound and demineralized enamel of primary and permanent teeth. It was hypothesized that the cariogenic challenge does not influence bond strength values to enamel. 422
MATERIALS AND METHODS Selection and Preparation of Teeth The study protocol was approved by the Institutional Ethics Board (process CAAE# 0291.0.243.000-10). Eighty erupted sound teeth, consisting of 40 primary molars and 40 permanent third molars from a collection of extracted teeth stored in 0.5% aqueous chloramine, were selected according to the following inclusion criteria: absence of carious lesions, restorations, cracks, and opacities. The teeth were used after obtaining the patient’s signed informed consent. The patients were from an area with water fluoridated at a concentration 0.7 mg F-/liter. The teeth were then randomly assigned to 16 experimental groups (n = 5) as shown in Table 1. The root portion of each tooth was removed by cutting along the transversal plane approximately 1 mm below the cementoenamel junction. The coronal portion was sectioned parallel to the long axis of the tooth in the mesiodistal direction using a cutting machine with a lowspeed water-cooled diamond saw (Labcut 1010, Extec; Enfield, CT, USA), resulting in 2 sections (buccal and lingual). Then, the smooth surfaces of permanent teeth were ground with a rotary polisher (Arotec PL 4; São Paulo, SP, BRA) and 180-grit SiC paper, always under water cooling, to obtain a flat surface. Flat enamel surfaces were created on primary molars by wet grinding with 600-grit SiC paper. For standardization of the smear layer, the surfaces of both primary and permanent teeth were then ground for 60 s with 600-grit SiC paper. The Journal of Adhesive Dentistry
Tedesco et al
Table 2 Adhesive systems: characteristics, general composition, manufacturers, batch, and manufacturers’ instructions Adper Single Bond
Clearfil SE Bond
Type
Two-step etch-and-rinse adhesive system
Two-step self-etching adhesive system
Composition
35% phosphoric acid, HEMA, ethanol, water, bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and photo-initiator
Primer: 10-MDP, HEMA, hydrophilic dimethacrylate, dl- camphorquinone, aromatic tertamine, water Bond: 10-MDP, bis-GMA, HEMA, hydrophilic dimethacrylate, photo-initiator, aromatic tert-amine, silanated colloidal silica
Manufacturer
3M/ESPE St Paul, MN, USA
Kuraray Medical; Tokyo, Japan
Batch
9UN 2011-12
Primer: 00896A 2011-08 Bond: 01321A 2011-08
Manufacturers’ instructions
Etch tooth surface with acid etchant for 15 s then rinse for 10 s; blot excess water with cotton pellet; apply 2 coats of bond with rubbing motion for 15 s; air thin for 5 s
Apply primer with a brush and leave for 20 s; air blow for 5 s; apply bond with a brush and air thin
Abbreviations: bis-GMA: bisphenol A diglycidyl methacrylate; HEMA: 2-hydroxyethyl methacrylate; 10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate.
Cariogenic Challenge Teeth in the DEM groups were subjected to cariogenic challenge by pH cycling prior to restorative procedures. They were separately immersed to 14 cycles of 8 h in 10 ml of demineralizing solution (2.2 mM CaCl2, 2.2 mM NaH2PO4, 0.05 M acetic acid adjusted to pH 4.8 with 1 M KOH) and 16 h in 10 ml of remineralizing solution (1.5 mM CaCl2, 0.9 mM NaH2PO4 and 0.15 mM KCl adjusted to pH 7.0) at room temperature and without agitation.42 Caries induction was confirmed by observation with a polarized light microscope (Zeiss; Oberkochen, Germany) at 50X magnification, with Leica Qwin software (Leica Microsystems; Heidelberg, Germany). After the restorative procedures, the specimens in the pH groups were submitted to cariogenic challenge as described for the DEM groups. Restorative Procedures and Microshear Bond Strength Testing The manufacturers, general compositions, and batch numbers of the adhesive systems used in the present study are given in Table 2. The adhesive systems were applied on exposed enamel surfaces according to the manufacturers’ instructions (Table 2). Prior to light curing of the adhesive system (800 mW/cm2/10 s) with an LED lamp (Olsen; Palhoça, SC, BRA), polyethylene tubes 1.0 mm in height and with an internal diameter of 0.76 mm (Micro-bore Tygon S-54-HL Medical Tubing, Saint-Gobain Performance Plastics; Akron, OH, USA) were placed on the bonded area. After light curing the adhesive system, a resin composite (Filtek Z250, Vol 16, No 5, 2014
3M ESPE; St Paul, MN, USA) was inserted in the tubes and light cured for 20 s, resulting in 5 cylindrical specimens of resin composite with a cross-sectional area of 0.45 mm for each primary tooth and 6 for each permanent tooth. All bonding and restorative procedures were performed by a single operator at a room temperature of 24°C. The specimens were stored in distilled water at 37°C for 24 h. After this period, the Tygon tubes were removed using a blade. Each specimen was examined under a stereomicroscope (Stereo Discovery V20, Carl Zeiss do Brasil; Rio de Janeiro, RJ, BRA) at 10X magnification, and those with interfacial gaps, bubble inclusion, or other defects were eliminated. The specimens were then attached to a universal testing machine (DL 2000, Emic; São José dos Pinhais, PR, BRA), and a thin wire with a diameter of 0.20 mm was used to make a loop around the projection of the load cell and the cylinder of resin composite, maintaining contact with the surface of the tooth as close as possible to the resin/enamel interface. A shear force was applied at a crosshead speed of 1.0 mm/min until failure occurred. Failure Analysis The specimens were then evaluated under an optical microscope (HMV II, Shimadzu; Kyoto, Japan) at 400X magnification to determine the mode of failure.31 The failures were categorized as mixed/adhesive, cohesive in enamel, or cohesive in resin composite. Mixed failures were those that occurred mainly within the adhesive interfaces, but also included small areas of resin composite or enamel. Only the specimens that exhibited mixed/adhesive failures 423
Tedesco et al
Table 3
Mean (SD) bond strengths in MPa and the number of specimens obtained for each experimental group
Adhesive System
Primary Control
Permanent
Cariogenic challenge
Control
Cariogenic challenge
S
DEM
S
DEM
S
DEM
S
DEM
Adper Single Bond
25.81 (4.77)
15.36 (6.78)
21.04 (4.72)
11.02 (4.05)
26.74 (4.61)
21.64 (8.12)
23.13 (8.30)
17.92 (9.62)
21/22
15/19
18/21
15/18
28/30
31/35
26/30
29/31
Clearfil
26.45 (5.77)
14.09 (4.66)
23.02 (5.79)
13.65 (8.81)
19/21
19/20
18/19
17/19
SE Bond
26.20 (4.65) 19.05 (10.15) 22.22 (6.55) 28/30
24/27
25/29
17.38 (9.22) 28/32
S: sound; DEM: demineralized. SD: standard deviation.
Table 4
Mean (SD) bond strengths inMPa for the factors alone
Factors
Bond strength
p-value
(8.17)a (8.46)b
C
onstant improvements in the science of adhesive dentistry have resulted in the development of new adhesive systems with potential for reducing clinical time requirements and enhancing bonding effectiveness. Although clinical testing is essential, laboratory evidence
a
PhD Student, Department of Orthodontics and Pediatric Dentistry, University of São Paulo, São Paulo, SP, Brazil. Hypothesis, performed the experiments in partial fulfillment of requeriments for a degree, wrote the manuscript.
b
Adjunct Professor, Department of Restorative Dentistry, Federal University of Santa Maria, Santa Maria, RS, Brazil. Proofread the manuscript, contributed substantially to discussion.
c
Associate Professor, Department of Biomaterials and Oral Biology, University of São Paulo, São Paulo, SP, Brazil. Proofread the manuscript.
d
Associate Professor, Department of Biomaterials and Oral Biology, University of São Paulo, São Paulo, SP, Brazil. Proofread the manuscript.
e
Adjunct Professor, Department of Stomatology, Federal University of Santa Maria, Santa Maria, RS, Brazil. Idea, hypothesis, experimental design, proofread the manuscript, contribution substantially to discussion.
Correspondence: Rachel de Oliveira Rocha, Department of Stomatology, Federal University of Santa Maria, Rua Floriano Peixoto, 1184, Santa Maria, RS, 97015-372, Brazil. Tel/Fax: +55-55-3220-9291. e-mail: rachelrocha@ smail.ufsm.br
Vol 16, No 5, 2014
Submitted for publication: 25.07.11; accepted for publication: 10.07.14
of these materials’ effcacy should first be gathered,34 especially under critical conditions that expose restorative materials to situations similar to the challenges posed by the oral environment.21,28,30,33,36,41 pH oscillations are common in the oral environment and can cause mineral loss in dental tissues.10 Caries, a highly prevalent oral disease,2 makes restoration of carious enamel and dentin lesions a common practice in routine dentistry. At the same time, the existence of restorative materials with adhesive properties also permits greater conservation of tooth structures during restorative procedures. In this sense, although it is well known that bonding to enamel is very reliable and predictable, especially when using etch-and-rinse adhesive systems,5 it is necessary to consider the possibility that some demineralized enamel may have been unintentionally left after cavity preparation together with sound enamel. Thus, the restorative materials are placed on substrates different from those for which they were originally designed. As bond strength to demineralized enamel has not been included in most adhesive system studies, the investigation of bonding to this substrate is thus relevant, 421
Tedesco et al
Table 1 Experimental groups according to type of tooth, enamel condition, adhesive system and treatment after restorative procedure Type of tooth
Enamel condition
Adhesive system
Treatment after restorative procedure
Primary (n = 40)
Sound (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Demineralized (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Permanent (n = 40)
Sound (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Demineralized (n = 20)
Adper Single Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
Clearfil SE Bond (n = 10)
Control (n = 5) Cariogenic challenge (n = 5)
since it is necessary for establishing an adequate seal of the cavity and, consequently, ensuring the longevity of the adhesion.43 In addition, pH fluctuations also pose a challenge to the restorative procedures and may affect the dental substrate around the restoration,30 with a possible negative effect on the bonding of the adhesive system and the dental substrates.21,28,33 Because the effectiveness of adhesive systems is directly related to the durability of the restoration, studies must be conducted to examine the behavior and stability of the adhesive interface in response to cariogenic challenge because there are relatively few reports in the literature on this subject.21,26,28,30,33,35,40,41 Many of these studies evaluated demineralization around restorations,26,30,35,40,41 but fewer evaluated bond strength,21,26,28,33 particularly regarding enamel. Most of the scientific evidence on the performance of adhesive systems is based on results from permanent teeth.1,4,12,15,37 However, the microstructural and morphological differences observed between primary and permanent teeth3 make it important to compare the performance of adhesive systems between these substrates.14,28,38 It is expected that mineral depletion in primary teeth will affect the performance of adhesive systems, especially when subjected to a cariogenic challenge.21 Therefore, the purpose of the present study was to evaluate the effect of cariogenic challenge on the bond strength of adhesive systems to sound and demineralized enamel of primary and permanent teeth. It was hypothesized that the cariogenic challenge does not influence bond strength values to enamel. 422
MATERIALS AND METHODS Selection and Preparation of Teeth The study protocol was approved by the Institutional Ethics Board (process CAAE# 0291.0.243.000-10). Eighty erupted sound teeth, consisting of 40 primary molars and 40 permanent third molars from a collection of extracted teeth stored in 0.5% aqueous chloramine, were selected according to the following inclusion criteria: absence of carious lesions, restorations, cracks, and opacities. The teeth were used after obtaining the patient’s signed informed consent. The patients were from an area with water fluoridated at a concentration 0.7 mg F-/liter. The teeth were then randomly assigned to 16 experimental groups (n = 5) as shown in Table 1. The root portion of each tooth was removed by cutting along the transversal plane approximately 1 mm below the cementoenamel junction. The coronal portion was sectioned parallel to the long axis of the tooth in the mesiodistal direction using a cutting machine with a lowspeed water-cooled diamond saw (Labcut 1010, Extec; Enfield, CT, USA), resulting in 2 sections (buccal and lingual). Then, the smooth surfaces of permanent teeth were ground with a rotary polisher (Arotec PL 4; São Paulo, SP, BRA) and 180-grit SiC paper, always under water cooling, to obtain a flat surface. Flat enamel surfaces were created on primary molars by wet grinding with 600-grit SiC paper. For standardization of the smear layer, the surfaces of both primary and permanent teeth were then ground for 60 s with 600-grit SiC paper. The Journal of Adhesive Dentistry
Tedesco et al
Table 2 Adhesive systems: characteristics, general composition, manufacturers, batch, and manufacturers’ instructions Adper Single Bond
Clearfil SE Bond
Type
Two-step etch-and-rinse adhesive system
Two-step self-etching adhesive system
Composition
35% phosphoric acid, HEMA, ethanol, water, bis-GMA, dimethacrylate, amines, methacrylic copolymer of polyacrylic and polyitaconic acids, and photo-initiator
Primer: 10-MDP, HEMA, hydrophilic dimethacrylate, dl- camphorquinone, aromatic tertamine, water Bond: 10-MDP, bis-GMA, HEMA, hydrophilic dimethacrylate, photo-initiator, aromatic tert-amine, silanated colloidal silica
Manufacturer
3M/ESPE St Paul, MN, USA
Kuraray Medical; Tokyo, Japan
Batch
9UN 2011-12
Primer: 00896A 2011-08 Bond: 01321A 2011-08
Manufacturers’ instructions
Etch tooth surface with acid etchant for 15 s then rinse for 10 s; blot excess water with cotton pellet; apply 2 coats of bond with rubbing motion for 15 s; air thin for 5 s
Apply primer with a brush and leave for 20 s; air blow for 5 s; apply bond with a brush and air thin
Abbreviations: bis-GMA: bisphenol A diglycidyl methacrylate; HEMA: 2-hydroxyethyl methacrylate; 10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate.
Cariogenic Challenge Teeth in the DEM groups were subjected to cariogenic challenge by pH cycling prior to restorative procedures. They were separately immersed to 14 cycles of 8 h in 10 ml of demineralizing solution (2.2 mM CaCl2, 2.2 mM NaH2PO4, 0.05 M acetic acid adjusted to pH 4.8 with 1 M KOH) and 16 h in 10 ml of remineralizing solution (1.5 mM CaCl2, 0.9 mM NaH2PO4 and 0.15 mM KCl adjusted to pH 7.0) at room temperature and without agitation.42 Caries induction was confirmed by observation with a polarized light microscope (Zeiss; Oberkochen, Germany) at 50X magnification, with Leica Qwin software (Leica Microsystems; Heidelberg, Germany). After the restorative procedures, the specimens in the pH groups were submitted to cariogenic challenge as described for the DEM groups. Restorative Procedures and Microshear Bond Strength Testing The manufacturers, general compositions, and batch numbers of the adhesive systems used in the present study are given in Table 2. The adhesive systems were applied on exposed enamel surfaces according to the manufacturers’ instructions (Table 2). Prior to light curing of the adhesive system (800 mW/cm2/10 s) with an LED lamp (Olsen; Palhoça, SC, BRA), polyethylene tubes 1.0 mm in height and with an internal diameter of 0.76 mm (Micro-bore Tygon S-54-HL Medical Tubing, Saint-Gobain Performance Plastics; Akron, OH, USA) were placed on the bonded area. After light curing the adhesive system, a resin composite (Filtek Z250, Vol 16, No 5, 2014
3M ESPE; St Paul, MN, USA) was inserted in the tubes and light cured for 20 s, resulting in 5 cylindrical specimens of resin composite with a cross-sectional area of 0.45 mm for each primary tooth and 6 for each permanent tooth. All bonding and restorative procedures were performed by a single operator at a room temperature of 24°C. The specimens were stored in distilled water at 37°C for 24 h. After this period, the Tygon tubes were removed using a blade. Each specimen was examined under a stereomicroscope (Stereo Discovery V20, Carl Zeiss do Brasil; Rio de Janeiro, RJ, BRA) at 10X magnification, and those with interfacial gaps, bubble inclusion, or other defects were eliminated. The specimens were then attached to a universal testing machine (DL 2000, Emic; São José dos Pinhais, PR, BRA), and a thin wire with a diameter of 0.20 mm was used to make a loop around the projection of the load cell and the cylinder of resin composite, maintaining contact with the surface of the tooth as close as possible to the resin/enamel interface. A shear force was applied at a crosshead speed of 1.0 mm/min until failure occurred. Failure Analysis The specimens were then evaluated under an optical microscope (HMV II, Shimadzu; Kyoto, Japan) at 400X magnification to determine the mode of failure.31 The failures were categorized as mixed/adhesive, cohesive in enamel, or cohesive in resin composite. Mixed failures were those that occurred mainly within the adhesive interfaces, but also included small areas of resin composite or enamel. Only the specimens that exhibited mixed/adhesive failures 423
Tedesco et al
Table 3
Mean (SD) bond strengths in MPa and the number of specimens obtained for each experimental group
Adhesive System
Primary Control
Permanent
Cariogenic challenge
Control
Cariogenic challenge
S
DEM
S
DEM
S
DEM
S
DEM
Adper Single Bond
25.81 (4.77)
15.36 (6.78)
21.04 (4.72)
11.02 (4.05)
26.74 (4.61)
21.64 (8.12)
23.13 (8.30)
17.92 (9.62)
21/22
15/19
18/21
15/18
28/30
31/35
26/30
29/31
Clearfil
26.45 (5.77)
14.09 (4.66)
23.02 (5.79)
13.65 (8.81)
19/21
19/20
18/19
17/19
SE Bond
26.20 (4.65) 19.05 (10.15) 22.22 (6.55) 28/30
24/27
25/29
17.38 (9.22) 28/32
S: sound; DEM: demineralized. SD: standard deviation.
Table 4
Mean (SD) bond strengths inMPa for the factors alone
Factors
Bond strength
p-value
(8.17)a (8.46)b
