Effect of immediate and delayed dentin sealing on the fracture strength, failure type and Weilbull characteristics of lithiumdisilicate laminate veneers

Abstract

Objectives

Adhesion on dentin is less reliable than on enamel, which could affect the durability of laminate veneers (LV). Immediate dentin sealing (IDS) is suggested instead of delayed dentin sealing (DDS) to overcome hypersensitivity and prevent debonding from dentin. This study evaluated the effect of IDS and DDS on the durability of Li 2 Si 2 O 5 laminate veneers in vitro.

Methods

Window preparations were made on the labial surfaces of sound maxillary central incisors ( N = 50). They were randomly divided into five groups: Group 1: Enamel only + H 3 PO 4 + Adhesive (control); Group 2: <1/4 dentin + H 3 PO 4 + DDS (2 weeks later); Group 3: Complete dentin + H 3 PO 4 + DDS (2 weeks later); Group 4: <1/4 dentin + H 3 PO 4 + IDS; Group 5: Complete dentin + H 3 PO 4 + IDS. Li 2 Si 2 O 5 laminate veneers (e.max Press) were bonded to the labial surfaces of the teeth with adhesive resin cement (Variolink Veneer). IDS layers were silicacoated (CoJet System) and silanized (ESPE-Sil). The teeth with their bonded laminates were thermocycled (10.000× cycles) and then subjected to static loading (1 mm/min). Failure type and location after debonding were classified. Data were analyzed using ANOVA and Tukey’s post hoc test ( α = 0.05). Two-parameter Weibull distribution values including the Weibull modulus, scale ( m ) and shape ( 0 ), values were calculated.

Results

Mean fracture strength ( N ) per group in descending order was as follows: Group 5 (576 ± 254), Group 4 (478 ± 216), Group 1 (473 ± 159), Group 2 (465 ± 186), and Group 3 (314 ± 137). The presence of complete dentin exposure sealed with DDS after 2 weeks on the bonded surface (Group 3) resulted in significantly lower fracture strength results than those in group 5 with IDS ( p = 0.034). Weibull distribution presented higher shape ( 0 ) for Group 1 (3.67), than those of other groups (2.51–2.89). Failure types were predominantly adhesive failure between the cement and the laminate veneer in Groups 1, 2, 4 whereas Group 3 presented more often complete adhesive failures between the cement and dentin. In Group 5, failures showed some IDS and cement with or without ceramic fracture attached on the tooth.

Significance

When laminate veneers are bonded to a large dentin substrate, application of immediate dentin sealing improves adhesion and thereby, the fracture strength of Li 2 Si 2 O 5 laminate veneers.

Introduction

Laminate veneers in particular entail minimal tooth preparation of only 0.3–0.9 mm, which is highly conservative when compared to their full-coverage crown alternative. Although preparation for laminate veneers could be achieved within the vicinity of enamel, some dentin exposure, especially at the cement–enamel junction or below in the cervical area, is sometimes unavoidable . Freehand preparation of such restorations, without the use of putty indices or guiding grooves of depth may yield to deeper preparations with higher amount of dentin exposure . Preparation depth may in fact have consequences on the final fracture strength of minimal invasive restorations, in that lower fracture strength results were reported for laminate veneers when bonded to dentin compared to enamel . Unfortunately, clinical studies on survival of laminate veneers do not often report whether preparations were solely in enamel or dentin. Yet, available evidence from clinical studies that reported dentin exposure after tooth preparation, also reported higher incidence of failures . Recently, a review on the clinical evaluation of laminate veneers bonded to dentin concluded that the survival rate diminished when such restorations were bonded to dentin .

In order to prevent micro-leakage and hypersensitivity, sealing of the dentin prior to impression taking for the indirect restorations was advocated in early 1990s . In addition, other studies concluded that adhesive strength of restorations was improved when dentin was sealed . Adhesive strength after this so called immediate dentin sealing (IDS) was compared with the conventional adhesive cementation, delayed dentin sealing (DDS), which is a common procedure for cementation of fixed dental prosthesis. In these studies, bond strength results employing DDS varied between 2 and 12 MPa, whereas application of IDS resulted in significantly higher mean bond strength results between 15 and 58 MPa depending on the test method . Apparently, application of the adhesive resin on freshly cut dentin and further polymerization of the adhesive resin over time improved adhesion of bonded restorative materials . Furthermore, it was also postulated that application of IDS results in a smooth surface that also improves the adaptation of the indirect restorations .

Clinical studies on the survival rate of laminate veneers bonded onto teeth with existing resin composite restorations did not show encouraging results, providing that the substrate surfaces were not conditioned . However, in an in vitro study, ceramic laminate veneers bonded to a complete composite surface presented higher fracture strength results than those bonded onto enamel . Similarly, clinical survival rate of laminate veneers bonded onto teeth with existing composite restorations after the latter was tribochemical silicacoated, was not less than those bonded on enamel/dentin up to 40 months of evaluation . Thus, it can be anticipated that the presence of adhesive resin would also not impair the bond strength of laminate veneers on the IDS.

The objectives of this study therefore were to (a) compare the fracture strength of laminate veneers with and without IDS application, (b) evaluate the influence of the size of the exposed dentin and (c) failure types after loading until fracture. The first hypothesis tested was that the presence of IDS would positively contribute to the fracture strength of the laminate veneer compared to conventional adhesive cementation (DDS). The second hypothesis tested was that the size of exposed dentin would not decrease the fracture strength of the laminate veneers.

Material and methods

Specimen preparation

The brands, types, main chemical compositions, manufacturers and batch numbers of the materials used for the experiments are listed in Table 1 . Schematic description of the experimental design is presented in Fig. 1 .

Table 1
The brands, types, chemical compositions, manufacturers and batch numbers of the materials used for the experiments.
Materials Type Chemical composition Manufacturer Batch number
Total-etch Etching agent 37% phosphoric acid Ivoclar Vivadent, Schaan, Liechtenstein P14739
OptiBond FL Adhesive resin Primer: Hydroxyethyl methacrylate, Glycerolphophate dimethacrylate, phathalic acid monoethyl methacrylate, ethanol, water, photo-initiator
Adhesive: Triethylene glycol dimethacrylate, Urethane dimethacrylate, Glycerolphophate dimethacrylate, Hydroxyethyl methacrylate, bis-phenol A glycol dimethacrylate, filler, photo initiator
Kerr, Orange, CA, USA 3661962
ESPE-Sil Silane coupling agent Ethyl alcohol, methacryloxypropyl, trimethoxysilane 3M ESPE, St. Paul, Minnesota, USA 1311011
IPS Empress etching gel Ceramic etching gel <5% Hydrofluoric acid Ivoclar Vivadent P14739
CoJet-Sand One component primer Aluminum trioxide particles coated with silica, particle size: 30 μm 3M ESPE 442859
Monobond Plus Ethanol, 3-trimethoxysilsylpropylmethacrylate, methacrylated phosphoric acid ester Ivoclar Vivadent N37750
Syntac Primer Primer Water, acetone, maleic acid, dimethacrylate Ivoclar Vivadent P17329
Syntac Adhesive Adhesive resin Water, gluteraldehyde, maleic acid, poly-ethyleneglycodi-methacrylate Ivoclar Vivadent P15364
Heliobond Adhesive resin Bis-phenol A glycol dimethacrylate, dimethacrylate, initiators and stabilizers Ivoclar Vivadent P06157
Variolink Veneer Light curing resin cement (Medium Value 0) Urethane dimethacrylate, inorganic fillers, ytterbium trifluoride, initiators, stabilizers, pigments Ivoclar Vivadent N64556

Fig. 1
Flow-chart showing experimental sequence and allocation of groups.

Sound human central incisors ( N = 50) of similar size, free of restorations and root canal treatment were selected from a pool of recently extracted teeth. All teeth were screened on the presence of cracks by blue light and those with cracks were eliminated and replaced with new teeth. Before a laminate veneer preparation was made, impressions were made using a high precision condensation silicone (Provil Novo putty fast set, Heraeus, Hanau, Germany) in order to obtain molds for the provisionals. Window type of tooth preparations without incisal overlap, were made with a depth-cutting bur (801 201SC Swiss Dental Products, Intensiv Grancia, Switzerland), with this preparation type adhesion of the laminate did not rely on the macro-mechanical retention as in the case of overlap preparations. After the depth cuts of 0.3 mm were made, preparation was finalized using a round-ended tapered diamond chamfer bur (Swiss Dental Products, FG-2309). The preparations ended 1 mm above the cement–enamel junction.

The amount of dentin exposure was controlled by etching the prepared surface for 5 s and rinsing with water that resulted in a white, dull enamel surface. Thereafter photos of the teeth were analysed and surface area of exposed dentin measured using a custom-made image program (Plaqeval, BME BioMedical Engineering, University of Groningen). Preparation margins remained in enamel in all groups. Smooth margins were created to prevent stress concentration zones using finishing discs (Sof-Lex Contouring and Polishing Discs, 3M ESPE, St Paul, Minnesota, USA).

Experimental groups, IDS and DDS layers

The teeth were than randomly divided into 5 groups.

  • Group 1:

    Preparation was made only in enamel. This group acted as the control group.

  • Group 2:

    In this group, next to enamel, <1/4 of the cervical dentin surface was exposed. Two weeks later, DDS was created.

  • Group 3:

    In this group, dentin was exposed on the complete surface. DDS was created as in Group 2 after 2 weeks.

  • Group 4:

    In this group, next to enamel, <1/4 of the cervical dentin surface was exposed. The IDS was achieved immediately after tooth preparation. Dentin was etched with 37% H 3 PO 4 (Total-etch, Ivoclar Vivadent, Schaan, Liechtenstein) for 10 s followed by 30 s of rinsing with copious water. Then, primer and adhesive resin (Optibond FL, Kerr, Orange, USA) was applied, air-thinned and photo-polymerized for 10 s using an LED polymerization device (Bluephase, Ivoclar Vivadent) from a distance of 2 mm. The output of the polymerization device was 1000 mW/cm 2 throughout the experiment (Bluephasemeter, Ivoclar Vivadent). After application of glycerine gel, the surface was again photo polymerized for 40 s. IDS layer was controlled on presence of voids and excess adhesive resin was removed under the microscope (Opmipico, Zeiss, Oberkochen, Germany).

  • Group 5:

    In this group, dentin was exposed on the complete surface. The IDS was achieved as in Group 4.

Impressions of the preparations were made using a high precision silicon impression material (Prestige light, Vanini Dental Industry, Grassina, Italy). Then provisional laminates (Protemp 4, 3M ESPE, St Paul, Minnesota, USA) were made and applied using a spot etch technique where etching was performed for 10 s in the middle of the preparation. In Groups 4 and 5 spot etching was performed at the enamel margins and glycerine gel was applied in order to prevent adhesion between de IDS and the provisional restoration. After adjusting the temporary restorations using polishing discs (Sof-Lex Countouring and Polishing Disks, 3M ESPE), specimens were stored in distilled water at 37 °C for 2 weeks.

One dental technician fabricated lithium disilicate (Li 2 Si 2 O 5 ) laminate veneers (IPS e.max Press, Ivoclar Vivadent) according to the instructions of the manufacturer. Veneers were first sintered in a ceramic oven (Programat P3000, Ivoclar Vivadent) and glazed. The total thickness of the laminate veneers was 0.6 mm.

Adhesive cementation

A photo-polymerizing resin cement (Variolink Veneer, Ivoclar Vivadent) was used for cementation of the ceramic laminate veneers. A three-step bonding procedure with separate conditioning of the IDS layer was employed to ensure adhesion. Before cementation, provisional restoration was removed; tooth was cleaned with pumice and the fit of ceramic laminate veneers controlled under optical microscope (Zeiss Supra V50, Carl Zeiss, Oberkochen, Germany) (10×).

Cementation surfaces of the ceramic veneers were conditioned using hydrofluoric acid (Ceramic etching gel <5% hydrofluoric acid, Ivoclar Vivadent) for 20 s, rinsed and ultrasonically cleaned (Emag, Valkenswaard, The Netherlands) in distilled water for 5 min. They were then silanized (Monobond Plus, Ivoclar Vivadent), adhesive resin was applied (Heliobond, Ivoclar Vivadent).

In Groups 1–3, teeth were etched with 37% H 3 PO 4 (Total-etch, Ivoclar Vivadent), where enamel was etched for 30 s and dentin for 10 s followed by rinsing with copious water. Primer (Syntac Primer, Ivoclar Vivadent) was applied on the dentin and adhesive resin on the whole preparation (Syntac Adhesive and Heliobond, Ivoclar Vivadent).

In Groups 4 and 5, IDS layer was silica coated (CoJet, 3M, ESPE) using a chairside air-abrasion device (Dento-PrepTM, RØNVIG A/S, Daugaard, Denmark) from a distance of 10 mm, angle of 45 degrees at 2 bar pressure until the surface became matt. Then enamel was etched with 37% H 3 PO 4 for 30 s and rinsed. Silane (ESPE-Sil, 3M, ESPE) was applied at the silica-coated IDS surfaces, followed by adhesive resin application (Syntact Adhesive and Heliobond, Ivoclar Vivadent) on the whole preparation.

Laminate veneers were cemented using photo-polymerizing cement (Variolink Veneer, Ivoclar Vivadent). Excess cement was removed using microbrushes, glycerine gel was applied at the margins of the laminate veneers and photo-polymerized for 40 s from labial, lingual and incisal (≥1000 mW/cm 2 , Bluephase, Ivoclar Vivadent). Cement interface at the margins was polished using rubber points (Astropol, Ivoclar Vivadent).

Aging and fracture test

All specimens were thermocycled (Willytec, Munich, Germany) for 10.000 times between 5 °C and 55 °C with a dwell time of 30 s in each bath. After aging, digital photos of the specimens were made. The teeth with the cemented laminate veneers were embedded perpendicularly in polymethylmethacrylate (Autoplast, Condular, Wager, Switzerland) up to the cemento-enamel junction in the middle of the plastic rings (PVC, diameter: 2 cm, height: 1 cm).

The fracture test was performed in a Universal Testing Machine (Zwick ROELL Z2.5MA, 18-1-3/7, Zwick, Ulm, Germany). In order to simulate the clinical situation as closely as possible, the specimens were mounted onto a metal base and load was applied at 137° at a crosshead speed of 1 mm/min from the incisal direction to the laminate-tooth interface ( Fig. 2 ). The maximum force to produce fracture was recorded.

Fig. 2
The position of the load cell in relation to the laminate veneer-tooth interface in the universal testing machine where loading was applied until fracture.

Failure analysis

Failure sites were initially observed using an optical microscope (Zeiss Supra V50, Carl Zeiss) and classified as follows: Type I: Cohesive ceramic fracture; Type II: Adhesive failure between the cement and ceramic; Type III: Adhesive failure between the cement and enamel; Type IV: Adhesive failure between the cement/IDS and dentin; Type V: Adhesive failure between the IDS/cement and cement; Type VI: Tooth fracture.

Additionally, in order to observe the structural changes on the dentin or IDS, after cleansing with alcohol, two further specimens from each group were first sputter-coated with a 3 nm thick layer of gold (80%)/palladium (20%) (90 s, 45 mA; Balzers SCD 030, Balzers, Liechtenstein) and analyzed using cold field emission Scanning Electron Microscope (SEM) (LEO 440, Electron Microscopy Ltd, Cambridge, UK). Images were made at 25 kV at a magnification of 500× to 5000×.

Statistical analysis

To test whether or not the data were normally distributed, skewness and kurtosis were investigated, Shapiro–Wilk tests were performed and normal Q-Q plots were produced and analysed for all groups. The data appear a little skewed and kurtotic, but they do not differ significantly from normality in any of the groups ( p > 0.05). Consequently, one-way analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) post hoc tests were anticipated to identify possible differences between the groups, using a standard statistical programme (SPSS, PASW statistics 18.0.3, Quarry Bay, Hongkong, China). Maximum likelihood estimation without a correction factor was used for 2-parameter Weibull distribution, including the Weibull modulus, scale ( m ) and shape ( 0 ), to interpret predictability and reliability of interfacial adhesion after fracture test (Minitab Software V.16, State College, PA, USA). P < 0.05 was considered to be statistically significant in all tests.

Material and methods

Specimen preparation

The brands, types, main chemical compositions, manufacturers and batch numbers of the materials used for the experiments are listed in Table 1 . Schematic description of the experimental design is presented in Fig. 1 .

Table 1
The brands, types, chemical compositions, manufacturers and batch numbers of the materials used for the experiments.
Materials Type Chemical composition Manufacturer Batch number
Total-etch Etching agent 37% phosphoric acid Ivoclar Vivadent, Schaan, Liechtenstein P14739
OptiBond FL Adhesive resin Primer: Hydroxyethyl methacrylate, Glycerolphophate dimethacrylate, phathalic acid monoethyl methacrylate, ethanol, water, photo-initiator
Adhesive: Triethylene glycol dimethacrylate, Urethane dimethacrylate, Glycerolphophate dimethacrylate, Hydroxyethyl methacrylate, bis-phenol A glycol dimethacrylate, filler, photo initiator
Kerr, Orange, CA, USA 3661962
ESPE-Sil Silane coupling agent Ethyl alcohol, methacryloxypropyl, trimethoxysilane 3M ESPE, St. Paul, Minnesota, USA 1311011
IPS Empress etching gel Ceramic etching gel <5% Hydrofluoric acid Ivoclar Vivadent P14739
CoJet-Sand One component primer Aluminum trioxide particles coated with silica, particle size: 30 μm 3M ESPE 442859
Monobond Plus Ethanol, 3-trimethoxysilsylpropylmethacrylate, methacrylated phosphoric acid ester Ivoclar Vivadent N37750
Syntac Primer Primer Water, acetone, maleic acid, dimethacrylate Ivoclar Vivadent P17329
Syntac Adhesive Adhesive resin Water, gluteraldehyde, maleic acid, poly-ethyleneglycodi-methacrylate Ivoclar Vivadent P15364
Heliobond Adhesive resin Bis-phenol A glycol dimethacrylate, dimethacrylate, initiators and stabilizers Ivoclar Vivadent P06157
Variolink Veneer Light curing resin cement (Medium Value 0) Urethane dimethacrylate, inorganic fillers, ytterbium trifluoride, initiators, stabilizers, pigments Ivoclar Vivadent N64556
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Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Effect of immediate and delayed dentin sealing on the fracture strength, failure type and Weilbull characteristics of lithiumdisilicate laminate veneers
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