Journal of Dental Research

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Tensile Properties of Resin-infiltrated Demineralized Human Dentin H. Sano, T. Takatsu, B. Ciucchi, C.M. Russell and D.H. Pashley J DENT RES 1995 74: 1093 DOI: 10.1177/00220345950740041001 The online version of this article can be found at: http://jdr.sagepub.com/content/74/4/1093

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J Dent Res 74(4):1093-1102, April, 1995

Tensile Properties of Resin-infiltrated Demineralized Human Dentin H. Sano1, T. Takatsu1, B. Ciucchi2, C.M. Russell3, D.H. Pashley4f* lDepartment of Operative Dentistry, Tokyo Medical and Dental University, Tokyo, Japan; 2Department of Restorative Dentistry and Endodontics, University of Geneva, Switzerland; 3Office of Biostatistics and Department of Oral Diagnosis and Patient Services, Medical College of Georgia, Augusta; 4Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, GA 30912-1129; *to whom correspondence and reprint requests should be addressed

Abstract. The ability of adhesive resins to restore the physical properties of demineralized dentin has not been well-documented. The unfilled resins that are used for adhesion have relatively low moduli of elasticity and limited ability to increase dentin stiffness, although they may increase the ultimate tensile strength of dentin. This study tested the hypothesis that resin infiltration of demineralized dentin can restore its tensile properties to those of mineralized dentin. Small (ca. 0.5 mm thick x 0.5 mm wide) specimens of demineralized human dentin were infiltrated with one of five different dentin bonding resins over many hours, to determine how these resins altered the tensile properties of dentin. Tensile stress and strain were measured in these and control (mineralized and demineralized) specimens until their ultimate failure. The results indicate that some adhesive resins, after infiltrating demineralized dentin, can restore and even exceed the ultimate tensile strength of mineralized dentin. These resins increased the modulus of elasticity of resin-infiltrated dentin to values equal to or greater than those of the resins but far below those of mineralized dentin. Although the conditions in this experiment were far removed from the manufacturer's recommendations or clinical practice, the results support the potential of resin infiltration for reinforcing dentin. Key words: dentin, collagen, resins, tensile strength.

Introduction The contribution of collagen fibers to the tensile properties of dentin, although unknown, has been assumed to be relatively small (Lee and Swartz, 1970). Recently, the ultimate tensile strength of demineralized bovine dentin was reported to be 28.0 ± 3.9 MPa (Akimoto, 1991). Using human demineralized dentin, Sano et al. (1994a) obtained similar values for the same type of sample preparation. However, since the cross-sectional area of collagen fibers in demineralized dentin may be only from 35 to 40% of the total (the remainder being water), the actual ultimate tensile strength of demineralized dentin may be from 2.5 to 2.9 times higher than its apparent strength of 28 MPa, i.e., approximately 70 to 80 MPa. Both the apparent and the true ultimate tensile strengths of demineralized dentin are higher than the bond strengths of most currently marketed dentin bonding systems. Akimoto (1991) suggested that when adhesive resins infiltrate demineralized dentin and envelope collagen fibers, the collagen may make significant contributions to the strength of resin-dentin bonds. Although most of the unfilled resins utilized in adhesion have relatively low moduli of elasticity (Van Meerbeek et al., 1993), their ultimate tensile strengths may contribute significantly to resin reinforcement of demineralized dentin, and such infiltrated dentin may achieve strengths close to that of mineralized dentin. The ultimate tensile strength of mineralized dentin has recently been reported to be 104 ± 28 MPa (Sano et al., 1994a). This study tested the hypothesis that resin-infiltrated demineralized dentin is as strong as that of mineralized dentin by measuring the tensile properties of mineralized, demineralized, and resininfiltrated demineralized dentin.

Materials and methods

Received April 6, 1994; Accepted October 27, 1994

Teeth Unerupted human third molars were used within one month of extraction. They were stored in isotonic saline containing a few


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Sano et al.

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Table 1.

B

A - Dentin

5mmI |'75±03 mm Enamel

:i

Dentini bonding systems

All Bond 2 (AB), Bisco, Inc., Itasca, IL 1. All Etch Gel (10(Z, phosplhoric acid) 2. Primer A (2/¼, NTG-GMAa in acetone) 3. Primer B (6i'/, Bl'DM in acetone) 4. Adhesive (Bis-GMA, UDMA, lEMA)

3-4mm

(thickness 0.55+0.05 mm)

Mtulti-l'urpose (MP), 3M Dental Products, St. Paul, MN Scotchboind Multi-l'urpose Etchanlt 2. Scotchbond Multi-IPurpose P'rimer (35'/, IHEMA, 15'7

Scotchbond 1.

polyalkenoic resin, 50'/, water). 3. Scotchbond Multi-fIurpose Adhesive HEMA)

(62.9', Bis-C,MA, 37.5',4

Clearfil Liner Bond 2 (CL), Kurarav Co., Ltd., Osaka, Japai l. Liner Bond Primer (Phenyl-l', NMSA, HEMA, water) 2. Liner Bond Adhesive (Bis-GMA, MDI', HEMA, microfiller)

Superbond C&B (SB), Sun Medical Company, Ltd., Kyoto, Japan 1. 10-3 solution (1t)'% citric acid, 3'Y ferric chloride) 2. Liquid (95% MMA, 5% 4-META) 3. Catalyst (TBBO) 4. Powder (PMMA)

C grips

Clearfil I'hotoboind (P'B), Kuraray Co., Ltd., Osaka, Japan 1. K-etchant gel (37%, phosphoric acid) 2. Clearfil Photobond (MDP, HEMA, ethanol, Bis-GMA) a

Figure 1. Schematic of how specimens were prepared from coronal dentin discs or adhesive resin sheets and how they were gripped for testing.

crystals of thymol at 4°C. Slices of mid-coronal dentin (0.5 mm thick) were made by means of an Isomet saw (Buehler, Ltd., Lake Bluff, IL). Dumbbell-shaped dentin specimens were cut from the dentin discs (Fig. 1) by an ultrafine diamond bur operated in a highspeed handpiece with copious air-water spray. For demineralized dentin specimens, the two ends of the mineralized specimens were coated with two layers of nail varnish. The specimens were then placed in histological tissue cassettes (Fisher Scientific, cat. #15-182-500E, Philadelphia, PA) which were immersed in I L of 0.5 M EDTA (pH 7.4) on a magnetic stirrer for 4 d to demineralize the center sections of the specimens. Demineralization was followed in preliminary experiments by the measurement of radiodensity of the middle regions of the specimens by a low-voltage surgical pathology xray unit (Faxitron, operated at 5 kv, Hewett-Packard, Palo Alto, CA) and by measurement of the bending of the specimens when they were clamped by one of the mineralized ends to form a cantilever which was loaded with known weights. This convenient method facilitates longitudinal; non-destructive measurement of the modulus of elasticity of the specimens (Braden, 1976), which falls as the specimens become demineralized. When the modulus fell to its lowest value, the demineralization was terminated.

Chemical names for abbreviations: Bisphenivl-glycidyl-nmethacrvlate Bis-GMA

BIDM EDTA HEMA 4-META MDP MMA NMSA NTG GMA PMMA TBBO UDMA

Bipheniyl diimethacrylate Ethylene diamine tetraacetic acid 2-hydroxyetlhvl methacrylate 4-methacrvloxyethvl trimellitate 1 0-methacrvloyloxydecyl d ihydrogenplhosplhate methyl methacrylate

N-methacryloyl-5-anminosalicvlic acid N-tolylglycine-glycidyl methacrylate

P'oly (methylmethacrylate) tri-n-butylboranie urethane dimethacrylate

in this study. It must be emphasized that none of the dentinibonding systems was used according to manufactuLrer's instructions.

Dentin pre-treatments The EDTA-demineralized dentin specimens were randomuly assigned (10 p7er group) to one of five experimental groups-AB, MP, CL, SB, and IPB to designate All Bond 2, Scotchbonid Multi-Purpose, Clearfil Liner Bond 2, Superbond C&B, and Clearfil Photobond. The specimens were then treated with the appropriate manufacturer's dentin -coniditioninig treatments (Table 2). The use of 3% ferric chloride in 10'/. citric acid has been shown to be crucial for proper resin infiltration Dentin bonding agents (Nakabayashi ct al., 1991) and polymerizationi of the Superboncd Downloaded from jdr.sagepub.com by guest on July 15, 2011 For personal use only. No other uses without permission. used resins C&B (SB) bonding system (Tmai cft al., 1991). The dentin binds Table 1 lists the commercial sources of the adhesive

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Resin-infiltrated Dentin Strength

j Dent Res 74(4) 1995 Table 2. Treatments made to demineralized dentin

Scotchbond Multi-Purpose

Clearfil Photobond

Clearfil Liner Bond

Time

10% maleic acid 15 s

37% phosphoric acid 40 s

10% phosphoric No acid treatment 410 s

Primer Time

HEMA, polyalkenoate 30 s

1 4-

44-

Dehydration

50% hydroxyethylmethacrylate for 8 h followed by 100% hydroxyethylmethacrylate for 8 h followed by 100% hydroxyethylmethacrylate for another 8 h

Resin infiltration time

4Adhesive 8h

Conditioner

Superbond C&B 10% citric acid 3% ferric chloride 40 s

4 A+B 30ss-

1Adhesive 8h

4Adhesive 8h

All Bond 2

50% acetone, 8 h 100% acetone, 8 h 100% acetone, 8 h

50% ethanol, 8 h 100% ethanol, 8 h 1

4Adhesive 8h

95% MMA, 5% META 8h

I

I

41

Adhesive 4h

Adhesive 4h

Adhesive 4h

Adhesive 4h

I

I-

I-

41

Place between two glass slides; light-cured on each side for 60 s to polymerize resin

495% MMA, 5% 4-META 4h

I 95% MMA, 5% 4-META + TBBO catalyst + PMMA 4h

The specimens were then placed in a 37°C oven for 24 h. All samples were polished on wet 600- and 1000-grit SiC paper back to their original 0.5-mm thickness.

Specimens were trimmed by bur into dumbbell shapes. Cut surfaces were sanded. Tensile testing

the ferric ions which participate in the TBBO-catalyzed conversion of methylmethacrylate to polymethylmethacrylate. The molecular weight of these polymers is higher adjacent to dentin than at some distance from the dentin (Akimoto, 1991). For the sake of consistency of experimental design, the specimens used for the other bonding systems were also pretreated with their recommended acidic conditioning agents, with the exception of CL. All resin-infiltrated demineralized specimens were designated by the two-letter material code, followed by RIDD (for resin-infiltrated demineralized dentin).

HEMA rather than being placed immediately into 100% pure solutions. In these procedures, the dentin was never air-dried or exposed to air for more than a few s. After dehydration, the specimens were placed in adhesive resins for 8 h of infiltration followed by another 4 h of resin infiltration in fresh resin prior to polymerization. All infiltrations were done in darkness. Ten specimens received no demineralization or pre-treatments and served as the mineralized human dentin controls (MHD). Another ten specimens were demineralized in EDTA but received no further treatment and served as the demineralized human dentin (DHD) controls.

Groups MP-RIDD, PB-RIDD, and CL-RIDD were dehydrated by immersion in 50% HEMA in water for 8 h followed by 100% Preparation of resin sheets HEMA for 8 h, then by fresh 100% HEMA for another 8 h. These We made resin sheets by placing the liquid resins between glass bonding systems normally use HEMA in their formulations. slides covered with a thin layer of silicon grease. The slides Group AB-RIDD was dehydrated by immersion in 50% acetone were separated by a pair of silicon spacers 0.6 mm thick. The the is in water for 8 h, followed by 100% acetone. Acetone resins, used without their respective primers, were adhesive SB2 Group All Bond system. primer normal vehicle in the visible-light-cured on both sides for 60 s, removed from the RIDD was dehydrated by 50% ethanol, followed by 100% slides, and then placed in a dry 37°C oven for 96 h. (This was ethanol (Table 2) after preliminary experiments revealed this done for the SB resin even though it was chemically cured also adhesive this solvent to be superior to HEMA or acetone for They were then polished to a thickness of 0.5 mm on TBBO.) by resin-infiltrated the that strongest assumed system. We 600- and 1000-grit SiC abrasive paper and trimmed to the demineralized dentin would be created if all of the loosely dumbbell shape (Fig. 1) by an ultrafine diamond bur in a bound water in that dentin (from ca. 65 to 70% by volume) handpiece. Cut surfaces were then polished with highspeed that demonstrated could be replaced by resin. Pilot experiments SiC paper by hand, for elimination of any and 1000-grit 600were the when best specimens occurred resin infiltration surface roughness. This same treatment was done to of alcohol, acetone, or stepped through graded concentrations Downloaded from jdr.sagepub.com by guest on July 15, 2011 For personal use only. No other uses without permission.

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J Dent Res 74(4) 1995

mineralized and resin-filtrated dentin prior to testing. All preparation was done under dry conditions. All pure resin sheets were designated by the two-letter material code, followed by the letters RS (for resin sheet). Polymethylmethacrylate (PMMA) powder was added to the SB-RIDD after 12 h of immersion in 5% 4-META/95% MMA when the TBBO catalyst was added. In the absence of PMMA powder, the SB-RIDD and SB-RS did not polymerize well. While it is unlikely that the PMMA penetrated the DHD very far, it did improve resin polymerization.

Tensile testing Tensile testing was performed by means of a Bencor Multi-T testing device (Danville Engineering Co., Danville, CA) placed in an Instron testing machine (Model 1011, Instron Corporation, Canton, MA) operated at a cross-head speed of 1.0 mm/min. A miniature LVDT (Model MHR-010, Schaevitz Inc., Pennsauken, NJ) was used to measure the linear displacement of the specimens during tension. The output of the LVDT was fed into a digital voltmeter (Model 8240 A, John Fluke Manufacturing Co., Inc., Everett, WA) and was recorded along with time and force to measure the rate of loading and elongation. The LVDT was directly calibrated on an electronic micrometer (Mitsutoyo Co., Tokyo, Japan) capable of measuring i-pm increments. The LVDT had a sensitivity of 100 mV/pm at 5V excitation. The Instron was operated with a 5-kg strain gauge which was calibrated with standard weights. The dumbbell-shaped specimens were placed in custom grips machined with a 0.6-mm-deep lip that permitted the specimens to fit within a groove made in the lip (Fig. 1) to transmit the load to the specimens. The specimens were stabilized in the grips with a small amount of cyanoacrylate applied to the mineralized portions of the specimens, far from that portion of the specimens that was tested to failure. All specimens were tested in air under dry conditions, with the exception of the mineralized and demineralized dentin control specimens, which were always kept wet with water, even during testing. The ultimate tensile strength (UTS) of each specimen was calculated as the maximum force at the point of catastrophic failure divided by the original crosssectional area. Stress was calculated as force divided by crosssectional area and was expressed in MPa. Strain was calculated as the percent elongation of the specimens. The gauge length of each specimen was always measured with a digital micrometer but was approximately 2.5 mm (Fig. 1B). The elastic modulus was measured as the slope of the linear portion of the stress-strain curve up to 0.5% strain for mineralized specimens, up to 1% in the resin-infiltrated specimens, and between 15 and 20% strain for demineralized specimens. These regions of the stress-strain curves were below the proportional limits of these materials. In the demineralized specimens (DHD), the specimens showed extensive strain with little development of stress until strains of about 15% were reached. Since the testing was done under constant speed but the stiffnesses of the MHD, DHD, resins, and resin-infiltrated demineralized dentin were different, the strain rates were different. Toughness was calculated as the area under the stressstrain curve.

Statistical analysis The response measures of ultimate tensile strength (MPa),

elastic modulus (GPa), percent elongation (%), strain rate (% s-1), and toughness (MN-m m73) were considered individually. There were 12 materials used as nominal variables for one-way analysis of variance: mineralized human dentin (MHD), demineralized human dentin (DHD), and the five materials (AB, MP, CL, SB, and PB) as both resin sheets (RS) and dentininfiltrated demineralized dentin (RIDD). Both Dunnett's and Student-Newman-Keuls (SNK) multiple-comparison tests were used to determine specific differences between groups. Bartlett's test was done to assess the homogeneity of variances across groups. Due to the identification of heterogeneity of variances of most of the outcome variables, the analysis of variance and multiple-comparison tests were also performed by use of the ranks of the outcome variables. Paired t tests were used to test thickness differences in the narrow portions of the dentin specimens before and after resin infiltration of demineralized dentin following curing of the resin and re-polishing to the original dimensions.

Results The results of the tensile testing are summarized in Table 3. Some specimens were inadvertently broken in some groups at various stages in the preparation of the samples for testing. Thus, the sample sizes listed often total less than 10. The tensile properties of mineralized and demineralized dentin controls are listed first. The average apparent ultimate tensile strength (UTS) of dentin in this experiment was 106 ± 16 MPa, compared with a value of 30 ± 6 MPa for demineralized human dentin (DHD). The moduli of elasticity of mineralized and demineralized dentin were 13.7 ± 3.4 and 0.21 ± 0.06 GPa, respectively. If one calculates the UTS and modulus of DHD using a cross-sectioned area that was only 30% as great (see "Discussion") as the actual surface area, then the values are 98.7 MPa and 0.70 GPa, respectively. The percent elongations at the point of tensile failure were 2.3 ± 0.8 and 22 ± 4.9%, respectively, for mineralized and demineralized human dentin. Since the moduli of elasticity for MHD and DHD were very different but the cross-head speed of the testing machine was constant (1 mm/min), the strain rates produced were very different (Table 3). The toughness of DHD was more than twice as great as that of MHD (Table 3). When the demineralized dentin matrix was infiltrated with the adhesive resins, the resulting tensile properties improved over that of demineralized dentin. The ultimate tensile strength of AB-RIDD, MP-RIDD, and SB-RIDD equaled or exceeded that of mineralized dentin (MHD). There was no statistical difference at the 0.05 level between either of the three and MHD, by Dunnett's multiple comparison. The one-way analysis of variance indicated differences among the (12) materials for all response variables (UTS, modulus, % elongation, strain rate, and toughness), with a p-value = 0.0001. Bartlett's test was significant at the 0.05 level for UTS, modulus, and % elongation. For this reason, the one-way ANOVA was repeated using the ranks of the response variables. The p-values for one-way ANOVA with ranks were 0.001 each for UTS, modulus, % elongation, strain rate, and toughness. There were slight differences between the order of the


J Dent Res 74(4) 1995


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Table 3. Tensile properties of dentin, resins, and resin-infiltrated demineralized dentin Material Human Dentin (MHD) Mineralized (DHD) Demineralized

(% sec-1) Strain Ratea

(MN-mm3)

Ultimate Tensile Stressa (MPa)

Modulus of Elasticitya (GPa)

105.6 ± 16.3 (10) 29.6 ± 5.9 (10) 98.7b

0.09 + 0.04 (10) 2.3 + 0.8 (10) 13.7 ± 3.4 (10) 4.2 ± 1.1 (10) 11.3 ± 3.5 (10) 0.62 ± 0.09 (10) 22.3 ± 4.9 (10) 0.21 ± 0.06 (10) 0.70b,-

Resin-infiltrated Demineralized Dentin 121.6 ± 20.3 (6) AB-RIDD 111.5 ± 14.5 (10) MP-RIDD 102.6 ± 3.7 (6) CL-RIDD 117.6 ± 12.2 (6) SB-RIDD 57.6 ± 16.4 (6) PB-RIDD

3.55 ± 3.11 ± 2.25 ± 4.11 ± 2.07 ±

0.81 (6) 0.66 (10) 0.32 (6) 0.78 (6) 0.51 (6)

% Elongationa

6.8 6.9 10.2 8.5 6.9

± ± ± ± ±

1.8 (6) 2.4 (10) 2.0 (6) 2.7 (6) 0.6 (6)

0.24 ± 0.20 ± 0.36 ± 0.23 ± 0.26 ±

0.05 (6) 0.03 (10) 0.05 (6) 0.07 (6) 0.05 (6)

Toughnessa

27.7 ± 26.2 + 34.0 ± 38.4 + 14.6 ±

10.8 (6) 18.3 (10) 9.1 (6) 14.5 (6) 3.9 (6)

Resin Sheets 18.3 ± 14.5 (6) 0.18 ± 0.07 (6) 7.1 ± 2.3 (6) 1.82 ± 0.49 (6) 74.8 ± 23.5 (6) AB-RS 22.8 ± 7.6 (6) 0.23 ± 0.06 (9) 7.6 ± 1.8 (9) 1.64 ± 0.39 (9) 83.1 ± 16.2 (9) MP-RS 13.4 + 8.8 (6) 0.27 ± 0.05 (6) 5.6 ± 2.7 (6) 2.06 ± 0.32 (6) CL-RS 65.1 ± 15.9 (6) 9.4 ± 4.4 (6) 0.21 ± 0.05 (6) 5.7 ± 1.9 (6) 1.44 ± 0.32 (6) 48.8 ± 6.4 (6) SB-RS 8.5 ± 5.4 (6) 0.21 ± 0.05 (6) 7.4 ± 3.0 (6) 0.79 ± 0.10 (6) 30.7 ± 7.1 (6) PB-RS a Mean ± SD (n). b Apparent LTS and E based upon the cross-sectional area of the demineralized specimens. If one assumes that only 30% of that volume was occupied by collagen, the true UTS would be 98.7 MPa, and E would be 0.70 GPa.

means and the order of the mean ranks for % elongation and strain rate. This discrepancy did not change the inferences. The Student-Newman-Keuls groupings of the materials may be seen for UTS, modulus, % elongation, strain rate, and toughness in Tables 4A-B, respectively. The materials are arranged by mean rank. All of the resin-infiltrated dentin samples, regardless of commercial source, were stronger than demineralized dentin, and all resin-infiltrated dentin specimens were stronger than their respective resin sheets, with the exception of PB-HY (Table 4A, SNK or Dunnett's multiple-comparison tests, p = 0.05). The modulus of elasticity of the resin-infiltrated dentin was significantly higher in three of the five systems (SB-RIDD, AB-RIDD, and MP-RIDD) compared with the modulus of the pure adhesive resin specimens (Table 4A). The moduli of mineralized and demineralized dentin were significantly different from each other and from all resin-infiltrated dentin (SNK or Dunnett's multiple-comparison tests, p = 0.05). The percent elongation of resin-infiltrated specimens differed from that of the pure adhesive resin samples in only two out of five cases (SB-RS and CL-RS, Table 4B). The strain rates of the samples were significantly different for mineralized and demineralized dentin and for CL-RIDD, CL-RS, and PB-RIDD. The third part of Table 3 summarizes the tensile properties of the adhesive resins included in the commercial dentin-bonding kits. From 0.5-mm-thick resin sheets, dumbbell-shaped specimens were prepared and tested exactly like the dentin specimens. This information was necessary for us to interpret the tensile properties of the resin-infiltrated dentin. It is clear that some resin polymers were stronger and stiffer than others. The system with the highest ultimate tensile strength was MP-RS, while that with the lowest was PB-RS (Table 3). All resins exhibited tensile

strengths that were less than that of mineralized dentin (by either Dunnett's or SNK multiple-comparison tests) but were equal to or greater than the uncorrected value for demineralized dentin. The modulus of elasticity of these resins was much lower than that of mineralized dentin but higher than that of demineralized dentin matrix. Their percent elongation prior to failure was between those of mineralized and demineralized dentin (Table 3). The toughness of SB-RS, CL-RS, and PB-RS was similar to that of DHD, while that of AB-RS and MP-RS was higher than that of the other resin sheets or DHD (by SNK multiplecomparison test). The toughnesses of the resin-infiltrated demineralized dentin hybrids were all greater than those of their corresponding resin sheets (Tables 3 and 4B), with SB-RIDD giving the highest value. The toughest resin sheets (MP-RS, AB-RS, and CL-RS) exceeded the toughness of mineralized dentin (Table 4B), measured by either Dunnett's or SNK multiple-comparison test. Figs. 2 and 3 show the results graphically. Typical stressstrain curves for MHD, DHD, and resin-dentin hybrids are shown in Fig. 4. Modes of failure When the specimens failed during the determination of their ultimate tensile strength, the various groups failed differently. The MHD specimens did not show any apparent narrowing in width (i.e., necking) prior to failure, although the samples were so small that this may not have been detected. All specimens broke in the middle third of the narrow (0.5 x 0.5 mm) portion of the preparation. The DHD specimens did show necking, since they elongated 22% just prior to breaking. The width of the narrow central portion


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Table 4A. Multiple comparisons of ultimate tensile strength and modulus of elasticity ranks.

Table 4B. Multiple comparisons of V/, elongationi, straia rate, anid toughness ranks

Ultimate Tensile Strength"

(X, Elongationa SNKb Strain RZate8 SNKb Touglhniess3 SNKh SNKb

Elastic Modulus

SNKb

AB-RIDD MHD SB-RIDD SB-RIDD MP-RIDD AB-RIDD MHD MlP-RIDD CL-RIDD CL-RIDD MP-RS PB-RIDD AB-RS CL-RS CL-RS AB-RS PB-RIDD MP-RS SB-RS SB-RS PB-RS PB-RS DHD DHD a Descending order of mean ranks. b Student-Newman-Keuls multiple-comparison test.

decreased from 0.5 mm to 0.3 mm at failure. About onethird of the DHD specimens broke at the junction between the demineralized central region and the wider, mineralized ends. Two-thirds of the specimens broke within the narrow central region. All resin sheets broke within the narrow central region, usually in the middle. The RIDD specimens also generally broke in the middle of the central region. The SB-RS and SB-RIDD specimens were optically semitransparent. When they were stressed to failure, the optical properties of the middle narrow region changed from semitransparent to chalky just before specimen failure. This was due to crack formation which occurred as the resin began to undergo plastic deformation. No necking was observed but may have occurred without being noted because of the smallness of the samples.

Discussion

MP-RS

DUD CL-RIDD CL RS PB-RIDD

DHD CL-RIDD SB-RIDD

[PB-RIDD

AB-RIDD

PB-RS

SB-RIDD

AB-RS MP-RIDD AB-RIDD SB-RS CL-RS MHD

MP-RS SB-MS

a b

PB-RS MP-RIDD AB2-RS MUD

Descending order of mean ranks.

SB-RIDD CL -RIDD AB-RIDD M P-RS M -RIDD

AB-1S PB-RIDD Cl -RS DIiD SB-RS PB-RES MHID

Student-Newman-Keuls multiple-comparison test.

the adhesive monomers diffused in the opposite direction. One qualification of the results is that the specimens were never challenged by being soaked in water. However, even with these limitations, the results demonstrate the potential for resin reinforcement of demineralized dentini. The tensile properties of mineralized and demineralized dentin (Table 3) are similar to those reported previously

t60 (5j

140

120I 100-

When dentin is demineralized during the acid-conditioning step common in many recently developed adhesive systems, (61 >..., it undergoes a dramatic loss of tensile strength and stiffness :: t 60 (Table 3). The advantage of demineralizing dentin surfaces 60 is that it provides access to the collagen fibers of the dentin matrix that may be necessary for retention of adhesive resins 80 (6 40 (lo) l (Nakabayashi ft al., 1991; Pashley ct al., 1993). Under some conditions, however, it may be desirable to increase the stiffness of the bonded dentin. The experimental approach used in this study provided an opportunity to explore the potential for restoring the physical properties of MHD DHD AB MP CL SB PB AB MP CL SB PB demineralized dentin. The long dehydration and resinResin-infitrated Dentin SheetfResin infiltration times were necessary because the specimens were 500 pm thick rather than the usual infiltration depth of Figure 2. Ultimate teinsile strength of miniieralized Ldeitii (Ml{D), from 3 to 5 pm. The selection of these time periods was clemineralized dentiin collagein (DHD), resinl-inifiltrated arbitrary and should not be regarded as optimal. deminieralized dentini (IRIDD), anid pure adlhesive resini specimieins. Presumably, the residual solvents, including HEMA, were The dot above the collagen bar represenits the adjusted vzalues (see text for details). See Table I for kev to produict syimtbols. Thec dashe.1 removed from the specimens by diffusion when the fully horizontal lines in the resin-infiltrated specimniis inidicaite tlle sumii dehydrated specimens were placed in the adhesive resins of the correspondinig resiin strenigtli timies ().7, pluis the for 12 h. This would reverse the diffusion gradients, demineralized dentin times 0.3. Brackets designate pltus onie SD. Downloaded from jdr.sagepub.com by guest on July 15, 2011 For personal use only. No other uses without permission. permitting excess solvents to diffuse out of the specimens as Number in parenitlieses are the numiber of speciimells tested.

Res 74(4) 1995 RD 7sie(-inifiltrated Denltinl StrcnCltli j Deiit

(Sano et al., 1994a). Although the modulus of elasticity of mineralized dentin (13.7 + 3.4 Gl'a, Table 3) is similar to that reported by others (Lehman, 1967; Huang, 1992), the ultimate tensile strength of dentin (105.6 ± 16.3 MPa, Table 3) is more than twice that reported by Bowen and Rodriguez (1962) and by Lehman (1967). We attribute that to the fact that our cross-sectional areas were much smaller. If natural defects or stress-concentrating voids are uniformly but relatively sparsely distributed in dentin, then smaller samples may have more uniform stress distribution and hence produce higher tensile strengths (Sano et al., 1994b). The tensile properties of the adhesive resins are often not available from manufacturers; e.x., the data for Scotchbond MP are not available, although Van Meerbeek et al. (1993) reported a modulus of elasticity of 4.8 + 0.3 GPa using a nano-indentation technique. They also reported Young's moduli of All Bond 2 and Clearfil PB adhesives of 4.8 + 0.3 and 3.4 + 0.1 GPa, respectively, using the same technique. These values are all more than twice those that we measured (Table 3). The Kuraray Company provided elastic moduli of 0.36 + 0.04 and 0.86 + 0.05 GPa for Clearfil PB and CL adhesive resins, respectively. Their Clearfil PB modulus was lower than our results (0.79 + 0.10 GPa, Table 3), and we obtained a much higher modulus for LB. The ultimate tensile strengths for PB and LB were reported by the manufacturers to be 25.7 ± 3.8 and 54.7 ± 13.3 MPa, respectively. These values are not statistically different from those that we measured (Table 3). The stress-strain curves of the resin-infiltrated specimens were between those of mineralized and demineralized dentin, but were more similar to those of mineralized

7()99

dentin. When demineralized denitini was tested, the specimenis had to undergo strains of from 5 to 10%' before reaching a linear portion of the stress-strain relationship (Fig. 4). This large elongationi was probably the result of loose organization of the collagen fibers in demineralized dentin. The specimenis that were infiltrated with resin exhibited stress-strain curves more like those of pure resins (not shown). Apparently, the resin componenit of the resindentin hybrids behaved in an elastic manniier in the early stages of strain development. The modulus of elasticity was measured in the linear portions of these curves up to 1 % strain. At higher degrees of strain, the proportional limits were exceeded, and the materials begani a plastic phase of the stress-strain curve. The areas uinder the curves of the resin-hybrids (Fig. 4) were obviously muclh greater than those of the mineralized dentin, indicatinig an increase in toughness. There are no reports in the literature regarding the modulus of elasticity of resin -infiltrated dentini except for the recent report by Van Meerbeek (1993), in which they used a nano-inidentationi technlilque to measure modulus across a resin-dentin interface. When they used the smallest load (1 nN) to produce the best localized naino-inidentations in the hybrid layer, they obtained values of 5.0 + 0.7 GPa for Scotchbond MP, higher than our value of 3.1 + 0.7 GPa. However, their indenter may have touched mineralized dentin and overestimated the modulus. The contributions of collagen and resin to the overall strength of resin-infiltrated demineralized dentini will depend upon the strength of each componenit and on their relative volumes. If one assumes a linear model (Katz, 1971;

18

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10

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c'S C[6,

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0

MHD

DHD

AB MP 4- Res n

CL

SB

infiltrated Denin

PB

AB 0

CL PB SS Sheets ol R es n--

MP

0

5

10

15

Strain (%)

20

25

30

Figure 3. Modulus of elasticity of mineralized dentini (MHD), demineralized dentin collagen (DHD), resini-infiltrated Figure 4. The stress-strain curves of the materials listed in Table 3. demineralized dentin, and pure adhesive resin specimens. Symbols See Table 1 for product symbol c odes. RIDD refers to resinas in Fig. 2. The dot above the collagen bar indicates the adjusted Downloaded from jdr.sagepub.com by guest on July 15, 2011 For personal use only. No other uses without permission. infiltrated demineralized dentin. value (see text for details).

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Table 5. Theoretical estimates of the tensile properties of resin-infiltrated demineralized dentin (A) Calculated values for UTSmJDDa = (SR VR) + (SC Vd)

AB-RS MP-RS CL-RS SB-RS PB-RS

Shear Strength of Resin SR (from Table 3) 74.8 ± 23.5 83.1 65.1 48.8 30.7

± ± ± ±

16.2

15.9 6.4 7.1

Calculated Ultimate Tensile Strength 82.0 ± 16.6 87.8 ± 11.5 75.2 ± 11.3 63.8 ± 4.9 51.1 ± 5.3

Measured Ultimate Tensile Strength 121.6 ± 20.3 111.5 ± 14.5 102.6 ± 3.7 117.6 ± 12.2 57.6 ± 16.4

R 0.67 0.79 0.73 0.54 0.89

± ± ± ± ±

0.18 0.15 0.11 0.07 0.27

(B) Calculated values for ERIDD 1 /ERIDD = VC/Ec + VR/ER Modulus of Elasticity

ERIDD

ER AB-RIDD MP-RIDD CL-RIDD SB-RIDD PB-RIDD a

Definitions: TTS SR SC

VR Vc R

ERIDD ER EC

1.8 1.6 2.1 1.4 0.8

± ± ± ± ±

Calculated

0.5 0.4 0.3 0.3 0.1

1.2 1.2 1.3 1.1 0.8

± ± ± ± ±

0.2 0.2 0.1 0.1 0.1

Measured

R

ERIDD 3.6 3.1 2.3 4.1 2.1

± ± ± ± ±

0.8 0.7 0.3 0.8 0.3

0.35 0.38 0.58 0.27 0.37

± ± ± ± ±

0.09 0.09 0.09 0.06 0.07

Calculated ultimate tensile strength of resin-infiltrated demineralized dentin. Measured tensile strength of resin sheets from Table 3. Corrected tensile strength of demineralized dentin, 98.7 MPa from Table 3. Assumed volume fraction of resin-infiltrated demineralized dentin, 0.7. Assumed volume fraction of collagen fibers in demineralized dentin, 0.3. Ratio of calculated to measured values. Modulus of elasticity of resin-infiltrated dentin. Measured modulus of elasticity of resin, from Table 3. Measured (corrected) modulus of elasticity of collagen (0.7 GPa), from Table 3.

Marshall, 1993), the total stress borne by resin-infiltrated dentin (UTS) would be the product of the volume occupied by the resin (VR) times the strength of that resin (SR), plus the product of the volume occupied by collagen (Vc) and its ultimate tensile strength (Sc) (Table 5A). These theoretical tensile strengths were represented in Fig. 2 as the horizontal dashed lines in each of the resin-infiltrated dentin specimens. Note that the theoretical values were always lower than the measured values. The lack of agreement between the predicted and observed values could be due to the fact that the linear model is not correct. Alternatively, the interaction of the bonding systems and dentin may have resulted in unexpected conseqences. The predicted contribution of collagen to the overall strength of resin-infiltrated demineralized dentin was relatively small (volume of collagen x UTS strength = 0.3 x 98.7 MPa = 29.6 MPa). However, this prediction assumes that none of the pre-treatments of dentin altered either the volume or strength of collagen. While it is unlikely that the fractional volume of collagen increased, it is quite likely that the strength of collagen may have increased due to the dehydration of demineralized dentin that results from acetone, alcohol, or HEMA pre-treatments. Thus, it is possible that dehydrated collagen becomes much stronger

than wet collagen (Pashley, unpublished observations). All of the commercial dentin-bonding systems increased the tensile properties of demineralized dentin above that predicted as the sum of the tensile properties of the demineralized dentin and the strength of the respective resin (Table 5A). The use of a resin volume fraction of 0.7 or 70% requires explanation. The volume of dentin occupied by mineral, organic matrix, and unbound water is generally reported to be about 50, 30, and 20%, respectively (Mjor, 1983; Berkovitz et al., 1989; LeGeros, 1991). Demineralization of the dentin in EDTA removed all of the mineral phase but left the organic matrix intact. Dehydration removed the unbound water phase from the demineralized dentin. In theory, then, the infiltrating resin could occupy a maximum of 70% (50% previously occupied by mineral plus 20% previously occupied by water) of the resin-infiltrated dentin. Using the approach of Marshall et al. (1993), we multiplied the tensile properties of the pure resin by its assumed fractional volume (70%) to obtain an estimate of the contribution of the resin to the overall strength of resininfiltrated dentin (Table 5A). These assumptions would tend to overestimate the strength of resin-infiltrated demineralized dentin by the amount that we overestimated the degree of resin infiltration.


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The remaining 30% of the demineralized dentin matrix Table 6. Summary of tensile properties of resin-infiltrated dentin was presumably composed of the proteins of the matrix, relative to mineralized human dentin 90% of which are collagen. When we calculated the UTS and AB MP CL SB PB the modulus of elasticity of mineralized and demineralized UTS NS NSa NS NS dentin, we used the same cross-sectional area, since we had Elastic modulus lb c 1. shown (unpublished observations) that the dimensions of % Elongation 1 IC I I I demineralized dentin are the same as that of mineralized T Toughness I I I dentin. However, in demineralized dentin, all of the stress I was borne by collagen fibers, which represented only 30% of a NS indicates that the value was not statistically different from that the matrix volume. Thus, in the footnote to Table 3, we of mineralized dentin (p = 0.05). b I indicates that the value was less than that of mineralized dentin. divided the apparent UTS and E for demineralized dentin c T indicates that the value exceeded that of mineralized human by 0.3 to obtain the corrected values of 98.6 MP and 0.70 dentin. GPa, respectively. This value for the modulus of demineralized dentin is similar to the modulus of 0.8 GPa obtained by Rigby et al. (1959) for rat tail tendon. Table 5 presents data on the theoretical UTS of demineralized dentin infiltrated with the five bonding resins, permit better penetration of the adhesive monomers into the assuming that the UTS and E of demineralized dentin was collagen fibers. Thus, there may be several factors 98.7 MPa and 0.70 GPa, respectively, and that the resins contributing in concert to the high strength of resininfiltrated the demineralized dentin to their theoretical limit infiltrated dentin. (i.e., 70%). The last column in Table 5A is the ratio of the The results of this experiment were obtained under theoretical values to the measured values. Since the ratios for conditions far removed from those of clinical practice. They the UTS never exceeded unity, we suggest that this approach should not be extrapolated to values obtained at the did not overestimate the volume of resin infiltration. interface between dentin and adhesive resins in typical in Calculations were made to estimate the modulus of elasticity vitro dentin-bonding studies. Rather, the results are in the resin-infiltrated dentin by the approach of Soderholm analogous to what might be obtained if one could measure (1985), assuming that the resin-infiltrated hybrid was a the tensile strength of the hybrid layer (Nakabayashi et al., composite of resin with a collagen filler. Using this approach 1991) formed under these conditions. Since these values are (Table 5B), even using the highest modulus of elasticity for generally greater than the reported values for adhesion of collagen (0.7 GPa, Table 3), we found that the calculated resins to dentin, the results suggest that adhesive bonds modulus of the resin-infiltrated demineralized dentin was should fail at the interface between the adhesive resin and always much lower than the measured modulus of the the top of the hybrid layer (Tagami et al., 1993). They hybrid (Table 5B), suggesting that the interaction of collagen provide a theoretical limit for the tensile properties of resinand adhesive resins greatly stiffens the biologic polymer. infiltrated dentin made under the conditions used in this There are several possible mechanisms that might be experiment (Table 6). Further research should be directed at responsible for resin-infiltrated dentin being stronger and optimizing the conditions of resin infiltration of stiffer than expected. It is unlikely that the resin-infiltrated demineralized dentin and evaluating the effects of water on samples could have been larger than the mineralized or this resin-collagen hybrid. demineralized dentin specimens, producing a greater crossAcknowledgments sectional area and hence greater strength, because the specimens were carefully hand-polished to remove excess This work was supported, in part, by USPHS Grant DE 06427 resin and restore the original dimensions of the from the National Institute of Dental Research, National demineralized dentin specimens. The 95% confidence Institutes of Health, Bethesda, MD 20892, and by the Medical interval for the difference in the volume of the College of Georgia Dental Research Center. The authors are demineralized dentin specimens before and after resin grateful to Shirley Johnston for secretarial support. infiltration was from -9% to 6%. These differences were not References statistically significant (p = 0.054, paired t test). Perhaps the adhesive resins pre-stressed (Curray, 1964) the dentin Akimoto T, Kadoma Y, Imai Y (1990). Study on adhesion collagen during polymerization contraction in some mechanism of MMA-TBBO resin to dentin. A model unknown manner that resulted in better adhesion of the experiment using decalcified dentin. Jpn I Soc Dent Mater & resin to the collagen or better polymerization of the resin. Devices 9:320-325. Akimoto et al. (1990) reported PMMA of higher molecular Akimoto T (1991). Study of adhesion of MMA-TBBO resin to weights in demineralized dentin compared with resin dentin. Jpn J Soc Dent Mater & Devices 10:42-54. sheets. Another possibility is that the dehydration of the Berkovitz BKB, Boyd A, Frank RM, Hohling HJ, Moxham BJ, demineralized dentin matrix made it much stronger than the Nalbandian J, et al. (1989). Teeth. New York: Springerwet demineralized dentin that gave a value of 99 MPa (Table Verlag, p. 175. 3). The demineralized dentin matrix may have increased its Bowen RL, Rodriguez MS (1962). Tensile strength and modulus tensile properties when treated with dehydrating solutions of elasticity of tooth structure and several restorative such as alcohol, acetone, or HEMA. The pre-treatment of materials. J Am Dent Assoc 64:378-387. demineralized dentin with alcohol, or HEMA may Braden Monly.(1976). Biophysics of the tooth. In: Frontiers in oral Downloadedacetone, from jdr.sagepub.com by guest on July 15, 2011 For personal use No other uses without permission.

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physiology. Kawamura Y, editor. Vol. 2. Basel: S. Karger AG, pp. 1-37. Curray JD (1964). Three analogies to explain the mechanical properties of bone. Biorheology 2:1-10. Huang T-JG, Schilder H, Nathanson D (1992). Effect of moisture content and endodontic treatment on some mechanical properties of human dentin. J Endodont 18:209-215. Imai Y, Kadoma Y, Kojima K, Akimoto T, Ikakura K, Ohta T (1991). Importance of polymerization initiator systems and interfacial initiation of polymerization in adhesive bonding of resin to dentin. J Dent Res 70:1088-1091. Katz JL (1971). Hard tissue as a composite material. I. Bounds on the elastic behavior. J Biomechan 4:455-473. Lee HL, Swartz ML (1970). Surface preparation and various adhesive resins. In: Adhesion in biological systems. Manly RS, editor. New York: Academic Press, pp. 269-289. LeGeros RZ (1991). Calcium phosphates in oral biology and medicine. Vol. 15. Monographs in oral science. Myers H, editor. New York: Karger, pp. 109-110. Lehman MI (1967). Tensile strength of human dentin. J Dent Res 46:197-201. Marshall GW (1993). Dentin: Microstructure and characterization. Quintessence Int 24:606-617. Mjor IA (1983). Reaction patterns in human teeth. Boca Raton,

FL: CRC Press, Inc., p. 73. Nakabayashi N, Nakamura M, Yasuda N (1991). Hybrid layer as a dentin-bonding mechanism. J Esthet Dent 3:133-138. Pashley DH, Ciucchi B, Sano H, Homer JA (1993). Permeability of dentin to adhesive resins. Quintessence Int 24:618-631. Rigby JB, Hirai H, Spikes JD, Eyring H (1959). The mechanical properties of rat tail tendon. J Gen Physiol 43:265-283. Sano H, Ciucchi B, Matthews WG, Pashley DH (1994a). Tensile properties of mineralized and demineralized human and bovine dentin. J Dent Res 73:1205-1211. Sano H, Shono T, Sonoda H, Takutsu T, Ciucchi B, Carvalho R, et al. (1994b). Relationship between surface area for adhesion and tensile bond strength: Evaluation of a microtensile bond test. Dent Mater 10:236-240. Soderholm K-JM (1985). Filler systems and resin interface, In: Posterior composite resin dental restorative materials. Vanherle G, Smith DC, editors. The Netherlands: Peter Szulc Publishing Co., pp. 139-159. Tagami J, Nakajima M, Shono T, Takatsu T, Hosoda H (1993). Effects of aging on dentin bonding. Am J Dent 6:145-147. Van Meerbeek B, Willems G, Celis JP, Roos JR, Braem M, Lambrechts P, et al. (1993). Assessement by nanoindentation of hardness and elasticity of the resin-dentin bonding area. J Dent Res 72:1434-1442.
