Journal of Indian Society of Pedodontics and Preventive Dentistry
Journal of Indian Society of Pedodontics and Preventive Dentistry
                                                   Official journal of the Indian Society of Pedodontics and Preventive Dentistry                           
Year : 2011  |  Volume : 29  |  Issue : 2  |  Page : 140--143

Comparative evaluation of shear bond strength of composite resin bonded to acid etched or Nd:Yag lased enamel


Mridula Goswami, Asha Singh 
 Department of Pediatric Dentistry, Govt. Dental College and Hospital, Mumbai, India

Correspondence Address:
Mridula Goswami
Department of Pediatric Dentistry, Maulana Azad Institute of Dental Sciences, MAMC Complex, BS Zafar Marg, New Delhi-110 002
India

Abstract

Aims: In vitro evaluation of the shear bond strength of composite resin bonded to enamel which is pretreated using acid etchant or pulsed Nd:YAG. Qualitative morphological changes in enamel surfaces were observed under a scanning electron microscope (SEM). Materials and Methods: 60 extracted human teeth were divided in two groups of 30 each (Groups A and B). In Group A, prepared surface of enamel was etched using 35% phosphoric acid (Scotchbond, 3M). In Group B, enamel was surface treated by a surgical Nd:YAG laser beam (Medilas 4060 Fibertom) at 0.8 W, 10 Hz, for 10 s with 80 mJ/pulse power. Bonding agent (single bond dental adhesive, 3M) was applied over the test areas on 20 samples of Groups A and B each, and light cured. Composite resin (Z 100, 3M) was applied onto the test areas as a 3 mm diameter cylinder, and light cured. The samples were tested for shear bond strength. Remaining 10 samples from each group were observed under SEM for morphological changes. Results: The mean shear bond strength was 20.00 MPa (΁ 1.93) and 13.28 MPa (΁1.97) for Group A and B, respectively. The difference in mean values was statistically significant between Groups A and B (P<0.001). Under SEM, Group A showed typical honeycomb appearance and Group B showed bubble-like cavities. Conclusions: In enamel, acid etch technique showed higher shear bond strength.



How to cite this article:
Goswami M, Singh A. Comparative evaluation of shear bond strength of composite resin bonded to acid etched or Nd:Yag lased enamel.J Indian Soc Pedod Prev Dent 2011;29:140-143


How to cite this URL:
Goswami M, Singh A. Comparative evaluation of shear bond strength of composite resin bonded to acid etched or Nd:Yag lased enamel. J Indian Soc Pedod Prev Dent [serial online] 2011 [cited 2019 Sep 22 ];29:140-143
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Full Text

 Introduction



Pediatric dentistry has seen the use of composite resin in a wide range of applications such as direct anterior and posterior restorations, pit and fissure sealants, bonding of orthodontic brackets. Composite resin restorations enjoy increasing popularity among dentists and patients. The foundation of adhesive restorative dentistry was laid by Michael Buonocore in 1955 [1] by the concept of "Acid Etch Technique." Optimum acid etching results are achieved with 37% phosphoric acid applied for a period of 15 s. [2],[3]

An acid etch technique results in a satisfactory bonding of composite resin with enamel and longer lasting restorations from the standpoint of increased bond strength and decreased microleakage. Disadvantages of enamel acid etching include removal of a surface layer of enamel, variability in etch depth, contamination of the etched surface with water or oil, damage of the etched pattern by overzealous rubbing, and inadequate washing or drying affecting the bond strength adversely. [4]

Dental profession has become extremely enthusiastic about the potentials of LASER in the practice. This lead to the use of laser to pretreat the enamel surface for increasing bond strength between enamel and composite resin. [5] Laser application in dentistry got heightened by the development of Nd:YAG laser since 1987. [6]

The present in vitro study was undertaken to compare the shear bond strength of composite resin bonded to enamel pretreated using acid etchant and pulsed Nd:YAG laser, and to observe qualitative morphological changes in enamel following acid etching and laser treatment by a scanning electron microscope (SEM).

 Materials and Methods



Sixty healthy maxillary first premolar teeth extracted for orthodontic treatment were collected, cleaned, and stored in distilled water. They were grouped into two groups of 30 each, Group A and Group B. A target spot of 3 mm diameter was marked on each tooth. In Group A, the samples were acid etched using 35% phosphoric acid (Scotchbond, 3M) for 15 s and rinsed with distilled water. They were blotted dry with cotton pellets. Out of 30 samples, 10 samples were kept aside for SEM observations. On the remaining 20, bonding agent (Single Bond, 3M) was applied and light cured for 10 s, as per the manufacturer's instructions. A hollow plastic cylinder of 3 mm length and internal diameter of 3 mm was prepared and packed with composite resin restorative material (Z 100, 3M). It was then placed over the marked target spot area and light cured from right, left and top sides for 40 s each side. The plastic cylinder was then cut away. Each tooth was vertically mounted on a block of self cured acrylic resin upto cementoenamel junction [Figure 1]. Samples were stored in distilled water for 24 h before shear bond testing.{Figure 1}

In Group B, after marking the target spot area of 3mm on 30 samples, Black India Ink was applied as a LASER initiator on the target spot. The area was lased with surgical Nd:YAG laser (Medilas 4060 Fibertom), with fiber optic laser tip, which had a Helium-Neon marker for aiming the laser beam [Figure 2]. The area was exposed for 10 s at 10 Hz, with 1.064 μm wave length, 0.8W power, 0.1 s pulse duration and 80 mJ/pulse energy. The ink was then washed away with distilled water and the tooth surface dried with oil free compressed air. On 20 samples for shear bond testing, application of bonding agent and composite resin, mounting and storage was similar as was done for Group A samples.{Figure 2}

Shear bond testing was done on Monsanto's Hounsefield Tensometer, Type "W," with an autographic recorder. Specimens were sheared with knife-edged blade at a cross head speed of 1 mm/min. The bond strength values obtained were in KiloNewtons, which were converted into MegaPascals (MPa). The observations were tabulated and statistically analyzed.

10 samples each from Groups A and B were observed under a SEM (Stereoscan 90, UK).

 Results



The shear bond strength values obtained for Groups A and B samples were tabulated and statistically analyzed using Student's unpaired t-test [Table 1]. The mean shear bond strength of Group A was 20.00 MPa (SD ± 1.93) and that of Group B was 13.28 MPa (SD ± 1.97). The difference was statistically significant (P<0.001).{Table 1}

SEM observation at ΄3500 magnification of Group A samples showed that the acid etched enamel surface had marked porosity and honeycomb like appearance [Figure 3]. Group B samples had irregular appearance of the laser impact site showing areas of indentation and bubble-like cavities with irregular particulate debris [Figure 4].{Figure 3}{Figure 4}

 Discussion



Composite resins gained popularity after Buonocore's work on acid etching of enamel. However, it has certain disadvantages also. [5] Hence, alternative modalities were explored to pretreat the enamel surface to achieve resin bonding. Lasers have shown potential usage in dentistry. It has been experimented to treat the enamel surface to achieve resin bonding. Unlike CO 2 laser, the Nd:YAG laser is focused and the delivery can be restricted to the area of working without causing undesired effects on the adjacent areas.

Human enamel has low absorption of near infrared wavelength lasers. Nd:YAG laser is better absorbed in pigmented tissues, hence, the target area on enamel was coated with chromophore (dark black ink) to increase laser absorption. [4] A continuous wave of a laser can induce undesirable lateral heat in tissues; hence, pulsed mode is preferred as it can deliver enough energy for desirable effects. Very high laser energy can cause charring of superficial enamel. Optimum laser parameters for enamel treatment were suggested way back in 1995. [4],[7] Forces acting on a restoration intraorally are complex in nature but shear bond strength testing is more predictive. [8]

Mean shear bond strength achieved by laser treatment of enamel was statistically significantly lesser than bond strength achieved with acid etching. SEM observations were in accordance with the results of this study. Group A samples showed more porosity and regular honey comb pattern, thus increasing the surface area manifolds and offering better mechanical retention to resin tags. Group B samples showed bubble-like cavities, less porosity, non-uniform roughened pattern which may have been produced by vaporization of small amount of water in the enamel. [4],[5],[9] Such a shallow pattern offers less mechanical retention, hence lower shear bond strength.

Further studies with larger sample size and varying laser parameters are recommended.

 Conclusions



From the present study, it can be concluded that the pulsed ND:YAG laser pretreatment of enamel, with the parameters used in this study, did not result in higher shear bond strength of composite resin to enamel as compared to the acid etching pretreatment. The morphological changes evident from SEM observation suggest that the typical honeycomb-like surface appearance and increased microporosity created in enamel due to acid etching increases the surface area manifolds and hence higher bond strength of composite resin to enamel. Laser pretreatment could only create irregular bubble-like cavities, not adequate to improve bond strength as compared to acid etching.

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