March, 2011
Comparative Evaluation of Shear Bond Strength and Site of Bond Failures Between Microfilled and Nanofilled Composite Resins Cured Under LED and Conventional Light Cured Systems
Dr. Jeevan V Shetty . MDS.
Dr.Rajkumar S Alle, MDS. DNB
Dr Edwin Devadoss . MDS.
Dr.Suma.T. MDS.
Corresponding Author-
Dr.Rajkumar S Alle MDS. DNB
Professor and Head Dept. Of Orthodontics
Rajarajeshwari Dental College Bangalore, India
Email. rajkumaralle@gmail.com
ABSTRACT
The purpose of this study was to evaluate and compare the shear bond strength and the sites of bond failure for two different bonding materials under two different curing lights. Sixty, non-carious, extracted human premolar teeth were used and they were divided into four groups according to the type of composite material, and the curing light used. Group 1 – (n=15) halogen light using microfilled composite. Group 2 – (n=15) LED using microfilled composite. Group 3 – (n=15) halogen light using nanofilled composite. Group 4 – (n=15) LED using nanofilled composite. The mean shear bond strength was 19.79MPa for group-1, 19.89MPa for group-2, 19,90MPa for group-3 and 20.08MPa for group-4. A one-way analysis of variance and post hoc Tukey test showed no significant difference (P > .05) between groups1, 2, 3 and 4. A Weibull analysis demonstrated that all four groups provided sufficient bond strength. The results from SEM observation showed that in all four groups, the bracket bases were clear and most of the resin was left on the tooth surfaces.
KEY WORDS: Direct bonding; Bond strength; Light cure;
INTRODUCTION
Bonding of brackets to enamel has been a critical issue in orthodontic research, since the significance of achieving a stable bond between the tooth and its bracket.
Since the first dental composites were developed, many efforts to improve their clinical performance have been undertaken. Such studies are of high importance because the mechanical properties of dental composites depend highly on the concentration and particle size of the filler. Microfilled composites appeared in the markets during mid 1960s. Microfilled composites contain particles that are smaller than 1 micron.
One of the recent advances in the last few years in this field is the application of nanotechnology to resin composites. Nanotechnology is of great interest in resin composite research. Nanotechnology deals with the production and manipulation of materials and structures in the range of about 0.1–100 nanometers by various physical or chemical methods. The size of the filler particles are 8–30μm in hybrid composites and 0.7–3.6μm in microhybrid composites. Recently, new fillers with sizes ranging from around 5–100nanometers have been developed. Although 40nm particles were already present in microfilled composites, these materials could be considered precursors of nanofilled composites1. The smallest nano particles are in a form called a colloidal silica, which is produced by “burning” silica compounds, such as SiCl4, in an oxygen atmosphere to form macromolecular structures which fall into this size range.
With the introduction of photosensitive (light-cured) restorative materials in dentistry, various methods were suggested to enhance their polymerization including layering and the use of more powerful light-curing devices.2
The most popular light-curing unit has been the quartz-tungsten-halogen (QTH)–based unit. In these light curing units (LCUs), halogen bulbs generate light when electric energy heats a small tungsten filament to high temperatures
MATERIALS AND METHODS
Sixty extracted noncarious human premolars, without restorations, were collected and stored in distilled water at room temperature. The samples remained submerged in distilled water at all times except when the brackets were being bonded or debonded.
They were divided into four groups (n=15) according to the type of composite material, and the curing light used.
Premolar brackets of 0.022” slot (Roth prescription, Morelli orthodontia, (10.10.983 series) were bonded on the labial surface of the teeth. The surface area of the bracket base was 10.61 mm2 as specified by the manufacturer.
Preparation of group 1
The 15 stainless steel preadjusted edgewise brackets of 0.022” slot (Roth prescription) were bonded on the tooth surface with microfilled composite (Enlight – Ormco) cured under halogen light (Hilux) for 20 seconds.
Preparation of group 2
The 15 stainless steel preadjusted edgewise brackets of 0.022” slot (Roth prescription) were bonded on the tooth surface with microfilled (Enlight – Ormco) composite cured under LED (Bloo) for 20 seconds.
Preparation of group 3
The 15 stainless steel preadjusted edgewise brackets of 0.022” slot (Roth prescription) were bonded on the tooth surface with nanofilled composite (filtek supreme z350 3M) cured under halogen light (Hilux) for 20 seconds.
Preparation of group 4
The 15 stainless steel preadjusted edgewise brackets of 0.022” slot (Roth prescription) were bonded on the tooth surface with nanofilled composite (filtek supreme z350 3M) cured under LED (Bloo) for 20 seconds.
A Lloyd universal testing machine (LR 50K) was used for the shear bond test at a crosshead speed of 5 mm/min (Figure – 11). The acrylic block was clamped in the lower vice and a chisel shaped rod was placed parallel to the bracket in the upper vice. Force was directly applied to the bracket base using a single ended chisel shaped steel rod. The test samples were stressed for debonding at a crosshead speed of 5mm/min until the bracket debonded.
RESULTS
The mean shear bond strength was 19.79MPa for group-1, 19.89MPa for group-2, 19,90MPa for group-3 and 20.08MPa for group-4. (Table 1) A one-way analysis of variance and post hoc Tukey test showed no significant difference (P > .05) between groups1, 2, 3 and 4. A Weibull analysis demonstrated that all four groups provided sufficient bond strength.
Table 1 : Comparison of shear bond strength in four groups studied
Table 2 Anova
Graph 1: Shear bond strength of four groups
Graph 2: Shear bond strenght (Mpa)
SEM Observations
The results from SEM observation showed that in all four groups, the bracket bases were clear and most of the resin was on the tooth surface (failure was at the bracket base – adhesive interface). This was conformed by the SEM.
Figure 1 : SEM picture of Microfilled composite – conventional halogen light at 5000X
Figure 2 : SEM picture of Microfilled composite – conventional halogen light at 50X magnification
Figure 3: SEM picture of Microfilled composite – LED at 5000X magnification
Figure 4: SEM picture of Microfilled composite – LED at 50X magnification
DISCUSSION
In orthodontics, bond strength will be influenced by many factors such as tooth conditioning, adhesive system used, design of the bracket base and mode of cure.
The shear bond strength test is a simple method used for the laboratory evaluation of adhesive system.
The first light cure unit used ultraviolet light which had the disadvantage that 1 minute was required per millimetre of thickness of the composite material. Because of safety concerns of long-term use of the ultraviolet light, VLC unit introduced around 1980 3
The visible light activated resin system use a diketone absorber to create free radicals that initiate the polymerization process. Most dental photoinitiator systems used camphoroquinones as the diketone absorber with the absorption maximum in the blue region of the VLC system at a wave length of 470 nanometers.
Halogen bulbs generate light when electric energy heats a small tungsten filament to high temperature but only small portion is given off as light since most of the energy is converted into heat. Selective filters screen the wavelength so that only blue light is emitted.
Photo initiator molecules sensitive to blue light are activated, creating free radicals that initiate polymerization.
Mills et al: in 1995 proposed solid state light emitting diode (LED) technology for the polymerisation of the light activated dental materials. This type of light curing unit was developed specifically to overcome the shortcomings of the halogen VLC units. Thus, heat production is less and they have a lifetime of over 10,000 hours.
They require no filters to produce blue light, are resistant to shock and vibration and take little power to operate.
Thus, this study was undertaken to compare shear bond strength of nanofilled composite (Filtek Supreme Z 350) and microfilled composite (Enlight Ormco) to orthodontic attachments cured by both halogen (HILUX) (Fig. 5) and LED (Bloo Accordis) (Fig.6) light cure units. (P value: P less than or equal to 0.05).
Fig. 5: Hilux halogen curing light
Fig. 6: Bloo Accordis LED curing light
In this study, the shear bond strength of nanofilled composite showed no significant difference than microfilled composite. And LED light showed no significant difference than conventional light cure system.
CONCLUSION
The shear bond strength of 2 composites and curing lights used in this study ranges from 19.79 to 20.08 Mpa. And the bond failure was at bracket base – adhesive interface. Thus it can be concluded that all the composite resins and curing lights used in the study can be used for orthodontic bonding with good bonding characteristics.
Referances
1. Sebastien Beun et al. Characterization of nanofilled compared to universal and microfilled composites. 2007 dec ; 23;51-59
2. William J D, A comparision of polymerization by light emitting diode and halogen based light curing units. JADA 2002; 133:3, 335-341.
3. N. Rengarajan Krishnaswamy, Chakravarthi Sunitha, – Light emitting diode vs halogen light curing of orthodontic brackets: A Clinical study of bond failures. AJO 2007; 132:4 518-523
4. Serdar Arikan, Neslihan Arhun, Ayca Arman and Sevi Burcak Cehreli. Microleakage beneath Ceramic and Metal brackets photopolymerized with LED or Conventional curing lights. AO 2006; 76;6, 1035-1040.
