How Wet Should Dentin Be? Comparison of Methods

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S air spray (groups 3 and if), dry cotton pellets' (groups '7 and 8), wet cotton pellets (groups 9 and 10), mi- ... dentin constantly moist.10 Dentinal adhesion is fur-.
How Wet Should Dentin Be? Comparison of Methods to Remove Excess Water During Moist Bonding Gisele Damiana da Silveira Pereiraa/Luis Alexandre Maffei Sartini Paulillob/ Mario Fernando De Goesc/Carlos Tadeu dos Santos Diasd

Purpose: The aim of this study was to evaluate the shear bond strength of two adhesive systems when applied on dentin surfaces with different degree's of wetnesS. Materials and Methods: Two-hundredten dentin specimens were used. After conditioning with 35% phosphoric acid gel and washing, seven,.rnethods'ofClryirig«en,tHi were used: 30 s air spray (groups 1 and 2), 5 " -" !",·/r)"i', t I S air spray (groups 3 and if), dry cotton pellets' (groups '7 and 8), wet cotton pellets (groups 9 and 10), mioJt

,";,

crobrush (groups 11 and 12), absorbent paper (groups 13'and

14). The last group was not dried; the

dentin surfaces were left overwet (grQUPS5 and'6): P.rim~ & Bond 2.1 adhesive was applied on the oddnumbered groups and,Scotchbond Multi ..purpQseon tH,eev~n-numbered groups. Z100 composite cylinders were bonded to tM ',;Idhesive ~l!ld tli~,spedmei:lsw~resubjected toa shear bond test. \oJ: ;-!~.>;,:i;" rl • -. __:~" _ " Results: The Least-sqQa're~'Means testwasuseB't9 compare the following results, where different letters indicate;significantly diff,€rent mElar 'va'lo~;'Group 9 ~(~~)~;':}23:2(a); G3 = 21.3 MPa (ab), G2 = 19.5 MPa (be), G10 = 18.6 MRa (-be),G14 = 16(:{NPa (Cd),:G8'''; ~6.1.'MPa (cd), G4 = 14.6 MPa (de), G13 = 14.0 MPa (de), G11 = 13.9 'MPa (;.F

Model

27 160 187

6100.94146 2771.81846 8872.75992

225.96070 17.32387

13.04

0.0001

Error Total CV

28.4%

DF: degrees of freedom; SQ: sum of squares; MQ: mean square; F: F value; CV: coefficient

to the application of the subsequent treatments. All the specimens were conditioned with 35% phosphoric acid gel (3M Dental Products Division, St Paul, MN, USA) for 15 s and rinsed with distilled water for 20 s. Once this was done, one of the drying methods was applied according to the following experimental groups. Groups 1 and 2: The dentin was dried with an oil- and dust-free air blast for 30 s parallel to the surface at a distance of 1 cm.8 Groups 3 and 4: The dentin was dried with oil- and dust-free air for 5 s parallel to the surface at a distance of 10 cm.8 Groups 5 and 6: The dentin surface remained overwet and no drying method was applied. Groups 7 and 8: The dentin was dried with a dry hydrophilic cotton pellet gently applied to the surface for 10 s. Groups 9 and 10: The dentin was dried with a moist hydrophilic cotton pellet gently applied to the surface for 10 s. Groups 11 and 12: The dentin was dried using a microbrush (Dentsplyj Caulk, Milford, DE, USA) gently applied to the surface for 10 s. Groups 13 and 14: The dentin surface was dried by using small pieces of absorbent filter paper gently applied to the surface for 10 s. Immediately after drying, the adhesive Prime & Bond 2.1 (DentsplyjCaulk) was applied in the odd groups and the adhesive Scotchbond Multi Purpose (3M Dental) was applied in the even groups, according to the respective manufacturer's instructions. A split teflon matrix having a 3-mm-diameter central perforation was positioned over the adhesive area of the samples, and the composite resin Z100 (3M Dental, shade A-2) was applied in incremental layers 2 mm thick. Each layer was photopolymerized for 20 s. The teflon matrix was then carefully separated and a final polymerization was carried

of variation.

cylinder. All the specimens were created under stable temperature conditions (23°C) and 55% humidity. After bonding, the specimens were immersed in distilled water and stored for 7 days at 3rC ± 1°C prior to the shear bond strength test. The tests were performed on a universal testing machine (Emic-DL 500, Ind Bras, Sao Paulo, Brazil), at a crosshead speed of 0.5 mmjmin.3 The shear load was applied by means of a chisel with a 0.5mm-wide blade, positioned near the base of the composite cylinder, as close as possible to the adhesive interface. The load (in MPa) necessary to fracture each specimen was recorded. The means of shear bond strengths were calculated for each group (group = dentin wetness and adhesive system). The shear bond strengths for various dentin wetnesses for each bonding system were compared using one-way ANOVA (Table 1 and 2) and the Least-Squares Means multiple comparison test (Table 3) at a = 0.05. A contrast analysis test was done to compare the bond strengths results of the two adhesive systems (Table 4) at a = 0.05.

Statistical analysis of the data by one-way ANOVA showed a statistically significant F value at p = 0.0001. ANOVA decomposition showed a statistically significant difference for the group bond strengths values, but no effect of blocks (p = 0.217, Table 2). The Least-Squares Means test at a = 0.05 was used to compare the statistical difference among the experimental groups (Table 3 and Fig 1).

Table 2 One-way ANOVA analysis decomposition and group effects Source

OF

Type III SS

Block Group

14 13

5573.82919

to evaluate the block

MQ

313.48226

F Value

PR>F

1.29 24.75

0.2172 0.0001*

22.39159 428.75609

* Mean values were significantly different (p < 0.0001). DF:degreesof freedom; MQ: mean square.

Table 3 groups Group

Least-Squares

Means test for shear bond stregths (MPa) of the

Mean Shear Bond (MPa)

G9

23.2

G3

21.3

G2

19.5

LS Means a*

Prime & Bond 2.1, moist cotton pellet ab

Prime & Bond 2.1, air dried 5 s

SBMP, air dried 30 s

bc

G10

18.6

bc

SBMP, moist cotton pellet

G14

16.3

cd

SBMP, absorbent

G8

16.1

cd

SBMP, dry cotton pellet

G4

14.6

de

G13

14.0

de

Prime & Bond 2.1, absorbent

G11

13.9

de

Prime & Bond 2.1, microbrush

G7

13.5

de

Prime & Bond 2.1, dry cotton pellet

G12

12.1

e

SBMP, air dried

paper 5 s paper

SBMP, microbrush

G1

8.2

f

Prime & Bond 2.1, air dried 30 s

G5

2.7 2.4

g g

Prime & Bond 2.1, overwet

G6

SBMU, overwet

a= 0.05 *Different letters indicate significantly different mean values (p < 0.05).

Table 4 Contrast analysis test for adhesive systems shear bond strengths

SBMP vs P&B 2.1

OF

Contrast

1

6.97487

MQ 6.97487

F Value 0.40

PR>F 0.5266*

* Mean values were not statistically different (p > 0.05).

tistical difference between mean values of Prime & Bond 2.1 and Scotchbond Multi Purpose. The box-plot diagram (Fig 1) is a schematic representation of the distribution of bond strength values. The vertical lines in the box mark the 25th, 50th, and 75th percentiles of the data. The 50th percentile is called the median, which is repre-

sented in the box by a horizontal line, and the 25th and 75th are called quartiles. The '+' sign represents means, and asterisks represent outliers.1,25 For Prime & Bond 2.1, the highest bond strengths were obtained when dentin was dried with moist cotton pellet (23.2 MPa). Interestingly, similar bond strengths were obtained when dentin was dried

Fig 1 Box-plot diagram for mean shear bond strengths (MPa) of the groups. G = groups, P&B = Prime & Bond 2.1, SBMP = Scotchbond Multi Purpose. 30 s = 30-s air blast, 05 s = 5-s air blast, MC = moist cotton, mB = microbrush, AP = absorbent paper, DC = dry cotton, OW = overwet. Vertical lines in the box = 25th, 50th and 75th percentiles. Horizontal line in box = median. Horilontallines outside of the box = quartiles. + = means, * = outliers.

with a 5-s air blast (21.3 MPa). For Scotchbond Multi-Purpose, the highest mean bond strength was obtained when dentin was dried with a 30-s air blast (19.5 MPa). For both adhesive systems, the lowest means were obtained when the dentin surfaces remained overwet (G5: 2.7 MPa; and G6: 2.4 MPa).

In spite of the fact that resin adhesion to dentin is more difficult and less predictable due to its morphology and heterogeneous composition,10 significant progress has been achieved after increasing understanding of the properties the substrate. In order to obtain a strong and long-lasting adhesion of resin to dentin, it is important that a dense, homogeneous, and uniform hybrid layer be formed, regardless of its thickness.3,15 To accomplish this, it is necessary for the demineralized dentin to be completely infiltrated by the adhesive, encasing all

the exposed collagen and the hydroxyapatite.14,15,17 However, several factors can jeopardize monomer diffusion.14 The amount of surface moisture is extremely important for hybridization of dentin. Higher bond strength can be achieved under moist conditions than dry.5-7,21,23The results obtained in the present study support the findings of other studies, in which the drying methods that kept the dentin surface moist and the adhesive systems that rewet the dehydrated substrate were the ones that obtained the highest shear bond strength values.3,4,6 This is substantiated by the bond strength values obtained in Group 9 (23.2 MPa), in which acidetched dentin was dried with moist cotton pellets prior to the application of Prime & Bond 2.1. This drying method left the dentin visibly moist. We speculate that the water occupying the interfibrillar spaces of intertubular dentin was responsible for maintaining the collagen network in an expanded state, thereby preserving the porosity necessary for resin infiltration into intertubular and tubular dentin. The acetone present in the primer interact-

ed with this water.3-5,7,22 This interaction presumably promoted water evaporation and made the primer and the adhesive spread, penetrate, and adapt into the open interfibrillar spaces.16 Despite the fact that the results for this group were the highest values obtained, they did not differ significantly from the values of group 3 (21.3 MPa). In group 3, the dentin was airdried for 5 s at a distance of 10 em from the surface prior to the application of Prime & Bond 2.1. The average bond strength for this group was lower, probably due to slight overdrying, which can cause a collapse of the collagen fibril network,4,5,7 although the degree of wetness in the two groups seemed to be similar to the naked eye. A satisfactory value for adhesive strength was also obtained in group 2 (19.5 MPa), where the conditioned dentin surface was dried by an air blast for 30 s from a distance of 1 em. However, the Scotch bond Multi Purpose system was applied in this group; it contains sufficient water to rewet the dentin, recovering the integrity of the collagen network and its spaces.24 Once dried, collapsed collagen needs water to lower the modulus of elasticity enough to let it re-expand and enable the diffusion of monomers into the collagen fibril network, forming the hybrid layer.5,9,20,21,23,24 The same result was not observed when the Prime & Bond 2.1 was applied to similarly treated dentin (Group 1). Apparently, the acetone present in this system was not capable of expanding the collapsed matrix. Presumably, the monomers could not diffuse into the dried sUbstrate,4,9,20,21,24 resulting in the lower values for adhesion4.9,20,21,24 found in group 1 (8.2 MPa). These were statistically significantly lower than those of the other groups (Table 3, Fig 1). Scotch bond Multi Purpose produced satisfactory results regardless of how dentin was dried7.24 (air dried for 30 s, absorbent paper, or dry cotton pellets, G2 = 19.4 MPa, G14 = 16.3 MPa, and G8 = 16.1 MPa, respectively). We believe that the water content of the primer allowed the collagen fibril network to re-expand to the same degree. These values were not significantly different from those obtained in group 10 (18.6 MPa), where the same adhesive system was applied to a substrate left visibly moist3 after being dried with moist cotton pellets. Apparently, the hydrophilic monomers in the Scotch bond Multi Purpose system can tolerate wide variations in surface moisture.3,6,24 Thus, reliable adhesion can be obtained when this system is employed either in moist or dry conditions.6.9.24

When Prime & Bond 2.1 was applied, the bond strengths obtained with moist substrates (G9 = 23.2 MPa and G3 = 21.3 MPa) were higher (p < 0.05) than with dry substrates (G1 = 8.2 MPa). The application of Prime & Bond 2.1 to relatively dry dentin, as in the groups where the absorbent paper (G13 = 13.9 MPa) or dry cotton pellet was used (G7 = 13.5 MPa), produced significantly (p < 0.05) lower bond strengths (Table 3). This was probably because the composition of this system does not include water, and the drying techniques decreased the surface water concentration of the dentin to a point which allowed the primer's organic solvent to stiffen the collagen network faster than the water could plasticize ip,9 When the microbrush was used as a drying method, a moister surface was observed compared to the previously described conditions. Even though this situation should have favored the Prime & Bond 2.1 system,? it resulted in low values of adhesion for both systems (G11 = 13.9, G12 = 12.1 MPa). This might have occurred because there was excess water on the substrate. Acetone, the volatile polar solvent found in Prime & Bond 2.1, has the ability to displace water, although in this case, it was not capable of evaporating all the excess water. Consequently, much water remained on the surface, which may have caused phase changes and reduced polymerization,21,23 thus reducing the bond strength values, as exemplified by group 11 (13.9 MPa). This group did not differ statistically from the average obtained when this adhesive was applied to the drier substrate of groups 13 (14.0 MPa, absorbent paper) and 7 (13.5 MPa, dry cotton) even though it was visibly wetter. This situation worsened when the water-containing Scotchbond Multi Purpose system was applied to wet dentin that was left "dried" by a microbrush in group 12 (12.1 MPa). The resulting bond strength was probably due to the large amount of residual water that was not evaporated prior to polymerization.21,23,24 The worst results in this study were obtained when no drying method was used and the dentin was left with excess water on its surface (G5 = 2.7, G6 = 2.4, Table 3). According to Tay and others,21-23 the presence of excessive water dilutes the primer of some adhesive systems, and its components separate into different phases. This leads to the formation of water blisters and micelles of resin which are located on the adhesive interface, preventing the penetration of the resinous agent into the dentin and making hybridization of the demineral-

ized matrix difficult. The presence of water-filled blisters may promote the degradation of the bond by hydrolysis of the exposed collagen.21,22 Although both systems had different compositions that resulted in affinity to substrates with different degrees of wetness, the results obtained in the Contrast Analysis Test (Table 4) of these systems did not show significant statistical differences, no matter which drying method was used. Adhesion is highly influenced by the operator, who can commit errors during the different stages of the adhesive procedure, including drying of the dentin. Thus, adhesive systems that are less moisture-sensitive should be employed, because they are less technique sensitive.

The highest shear bond strength was achieved with Prime & Bond 2.1 applied to wet dentin dried with a moist cotton pellet. However, this value was not significantly different from bond strengths obtained when the same adhesive was applied to the dentin dried by a 5-s air blast. The lowest shear bond strengths were obtained when either adhesive system was applied to overwet dentin, and there was no significant difference between their bond strengths to overwet dentin (ca 2 to 3 MPa). Prime & Bond 2.1 showed higher (p < 0.05) shear bond strength values when applied to dentin kept moist after the drying process than when it was applied to the drier dentin. Scotch bond Multi Purpose achieved satisfactory bond strengths (ca 15 to 20 MPa) not only with moist dentin but also with drier dentin surfaces, showing no significant statistical difference between substrates. There was no significant statistical difference between the averages of shear bond strength produced by Prime & Bond 2.1 and Scotch bond Multi Purpose adhesive systems when they were compared with each other, independent of the dentinal drying methods.

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This study was supported

97/04569.

by the FAPESP

Foundation,

grant

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HYBRIDIZATION OF DENTAL HARD TISSUES Nobuo Nakabayashi

and David H. Pashley

The hybridization of dentin-a process that creates a molecular-level mixture of adhesive polymers and dental hard tissues-gives clinicians a versatile new material, useful in a wide array of advanced dental treatments. As the first in-depth exploration of the subject, this book covers the development, present understanding, and future research areas of this multifunctional dental material. A thorough review of the current literature rounds out the text. Valuable for students, researchers, and clinicians seeking a greater understanding of resin hybridization of tooth structure.

Evolution of Dentin-Resin Bonding Properties of Dentin Acid Conditioning and Hybridization of Substrates 4 Characterization of the Hybrid Layer 5 The Quality of the Hybridized Dentin 6 Clinical Applications of Hybrid Layer Formation 129 pp; 80 iI/us (some in color); ISBN 0-87417-575-9

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