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Year : 2005  |  Volume : 8  |  Issue : 4  |  Page : 30-35
Microtensile bond strength of resin luting cement cured using three different light sources


Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Chennai - 600 077, India

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How to cite this article:
Gowrish S, Kavitha S, Narayanan LL. Microtensile bond strength of resin luting cement cured using three different light sources. J Conserv Dent 2005;8:30-5

How to cite this URL:
Gowrish S, Kavitha S, Narayanan LL. Microtensile bond strength of resin luting cement cured using three different light sources. J Conserv Dent [serial online] 2005 [cited 2020 Aug 11];8:30-5. Available from: http://www.jcd.org.in/text.asp?2005/8/4/30/42595

   Introduction Top


Ceramic veneer restorations are gaining popularity for conservative restoration of unaesthetic anterior teeth. The success of such ceramic veneers is highly influenced by strength and durability of the bond formed between the components between the components of bonded veneer complex, which includes the tooth surface, luting resin and the ceramic veneer.

Resin cements are material of choice for ceramic veneer cementation. They assure high bond strength. Their translucency and minimal film thickness provide optimum esthetics and enhance the clinical survival of ceramic veneer restoration. Dual cure resin cements are used usually for cementation of ceramic veneer restorations which polymerize chemically on mixing the base and catalyst and also when subjected to light from a light curing unit which enables adequate polymerization in deeper areas However, adequate polymerization is a crucial factor in obtaining optimal physical properties and satisfying the clinical performance of the bonded veneer restoration .

Normally curing is initiated by visible light with a wavelength of 400-500nm. This can be done from light emitted from various light-curing unit. These light sources vary considerably in terms of cost, chair side time, degree of conversion, heat generated, polymerization shrinkage etc. Manufacture's of `soft start' light curing units and light emitting diode (L.E.D) claim adequate polymerization and bond strength as compared to conventional continuous mode halogen light curing units.

The present study was done to evaluate the efficacy of different sources to polymerize composite resin through ceramic veneer using microtensile bond strength testing method.


   Materials and Methods Top


15 freshly extracted non-carious human maxillary central incisors stored in saline were used for the study with in 3 months of extraction. The roots of the teeth were embedded in die stone 1 mm short of CEJ with the crowns exposed. Tooth preparation was done for ceramic veneer restoration beginning with self limiting depth cut diamond abrasive in a high speed hand piece with water coolant. This was followed by uniform reduction of enamel along the depth cuts with a round ended tapered diamond abrasive. The cervical preparation was completed with chamfer finish line 1 mm above the CEJ with out sharp margins. Incisal edge was reduced by 1.5mm. The labioincisal line angle was rounded. On the Palatal aspect a butt joint preparation was done. The preparation was completed with out any sharp margins.

Addition silicone impressions were taken and refractory die prepared with 'Norivest' (Japan) material. Veneers were prepared with low fusing 'Noritake (Japan) ceramic material, glazed and tooth fitting surface was etched with 20% hydrofluoric acid gel, followed by rinsing and air drying. The etched surface was silinated and bonding agent Excite DSC was applied and light cured for 10 seconds.

The tooth surface was prepared as per manufacture instructions for application of bonding agent. Excite DSC bonding agent was applied and curing was done with a conventional halogen bulb curing unit with a light intensity of 450 mW/Cm2. Variolink II base and catalyst pastes were mixed as per manufacture recommendations and veneers were luted on to the tooth surface. Excess cement was removed.

Then the curing was carried out using three different light sources i.e. L.E.D. (Continuous mode, 400mW/Cm2 for 40 see, 3M ESPE), conventional halogen light (Continuous mode, 450mW/Cm2 for 40 seconds, QHL 75. Dentsply) and 'soft-start' halogen light (100800 mW/Cm2 for 15 seconds followed by 800mW/Cm2 for 25 seconds, Elipar Trilight, 3M ESPE) which served as group A, group B and group C respectively. Light curing was done as per manufacture instructions.

The teeth with restored veneers were embedded in auto polymerizing acrylic resin and the resin was allowed to set. Sectioning was done using 200 um diamond disc in a hard tissue microtome (Leika, Germany) under water coolant in a horizontal direction along the CEJ. This provided the initial disk shaped sections [Figure 3]. This was followed by sectioning in a labiolingual direction to obtain slab shaped specimen measuring lmmx l mmx4mm in dimension. The specimen were prepared by `non'trimming' version technique to obtain slab shaped specimen with 1+0.02mm2 area t the bonded interface4. The cervical and incisal portions were not included for bond strength testing.

Microtensile bond strength testing was carried out by attaching each half of the specimen in a specially made stainless steel assembly with cyanoacrylate glue in a universal testing machine. After the hardening of the cyanoacrylate glue the specimen stressed in tension at a cross head speed of lmmlmin. Total of 30 specimen, 10 per group were tested and bond strength values were measured in mega Pascal's (Mpa).

The values obtained were tabulated and statistical analysis was done. One-way ANOVA was done to calculate the p-value and Multiple Range Test by Tukey - IISD procedure was employed to identify significant groups at 5% level.


   Results Top


The microtensile bond strength values are given in [Table 1] and statistical analysis is given in [Table 2]. Statistical analysis of the data showed higher bond strength values for the luting resin cured with conventional halogen unit and `soft Start' polymerization unit as compared to L.E.D. unit in this study. Mean microtensile bond strength value in group B (21.3+1.3 MPa) was significantly higher than the mean values in group A (15.1+1.1 MPa) and group C (18.8+1.0 MPa) with P<0.05. Also mean value in group C is statistically higher than the mean value in group A (P<0.05).


   Discussion Top


This study evaluated the polymerization of a dual cure resin luting cement, Variolink II polymerized by three different light sources using microtensile bond strength testing methodology. Adequate polymerization of luting agent is essential to provide hardness and strength to the material, as well as colour stability. This is influenced by energy density and for which a light intensity of 400 mW/cm2 is generally recommended.

Adequate polymerization of composite below ceramic restoration may be problematic . The resinous material however, is not fully polymerized but contains a small of residual, free monomer and the polymer structure contains considerable amount of pendant double bonds. This insufficient curing may cause postoperative sensitivity, microleakage, recurrent caries, gap formation and reduction in the physical properties of tooth restoration complex. When a ceramic veneer is luted with a dual cure or light cure cement, the light should penetrate the restoration to enable adequate polymerization and good bonding characteristics. This light penetration can be affected by power supply of light source, intensity and wave length of light veneer thickness and composition, light guiding wand and filters, curing tip distance from the resin etc.

Dual cure resin cement, Variolink II was selected for this study, since it has relatively weak chemical polymerizing component and relies mainly on its polymerizing capability enabling evaluation of polymerization mainly by the light activation.

Microtensile bond strength testing allows a more favorable stress distribution by reducting the defects or porosities at the interface, thus enabling bond failure to occur much closer to the true ultimate strength of the luting agent used .

The lower bond strengths obtained with L.E.D. unit could be due to reduced depth of cure though the intensity of the light was 400mW/ cm2. This is mainly due to light unit characteristics and not source and mode dependent.

Apart from degree of conversion, the degree of cross linking also can affect the bond strength. Since polymers having different linearity (cross link density) may have similar conversion rates.

Composites cured with L.E.D. lights may be less cross-linked than those cured with conventional halogen lights. Soft start curing with halogen lights though does not affect the degree of conversion, but can reduce the degree of cross linking in composite material.

A soft start polymerization is probably associated with relatively few centers of polymer growth, which may result in a more linear polymer structure with relatively few cross links. This is mainly due to higher light intensity which results in fewer centers of polymer growth resulting in a more linear polymer growth with relatively fewer cross linkage. These linear polymer chains with few cross links result in less number of branched or grafted polymer structure, there by reducing the cross linkage contraction strain rates during polymerization. These would have contributed to the lower bond strength in the present study. Composites with lower crosslink densities may be more prone to hydrolysis and water sorption, leading to reduced optimal material property and clinical longevity

In addition, spectral impurities generated by halogen light are highly absorbed by dental materials inducing further heating of the composites during the curing process. These may be responsible for greater cross link density resulting in higher bond strengths with the conventional light unit.

Though microtensile bond strength testing produces predominantly adhesive failure further investigation are required to analyse the mode of failure in the tested specimen.

Further studies are required in this aspect to investigate the degree of conversion, amount of unreacted double bonds polymerization stresses and marginal adaptation.


   Conclusion Top


With in the limitations of this study, it can be concluded that

  1. Conventional halogen bulb continuous mode produced higher bond strength than the rest.
  2. LED units were less effective in polymerizing resin cements under the ceramic veneers.
  3. The polymerization is light dependent and not source and mode dependent.[Figure 1],[Figure 2]


Top
Correspondence Address:
S Gowrish
Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Chennai - 600 077
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.42595

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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2]



 

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