Effects of Er:YAG Laser Pretreatment with Different Energy Levels on Bond Strength of Repairing Composite Materials

Abstract

Objective: The purpose of this study was to investigate the effects of Erbium-doped: yttrium-aluminum-garnet (Er:YAG) laser pretreatment with different energy levels on the shear bond strength (SBS) of repairing composite materials. Background data: After long-term usage of composite resins in the mouth, they can need some repair. Repairing composite bonding so it attaches to the old restoration is important for clinical success. Methods: Sixty composite resin materials were used in this study. The aging procedure was performed with 6000 thermocycles in water from 5°C to 55°C. Different surface pretreatment methods [control (no surface treatment), sandblasting, laser treatment (energy level 75, 100, 200, and 300 mJ)] were employed, for a total of six groups. Composite resins of the same kind as their substrates were adhered onto the conditioned substrates. The SBS test was used to assess the durability of all groups. Scanning electron microscopy (SEM) analysis was performed on a representative specimen in each group. Results: Among six surface-treated groups the lowest bond strength was observed in Group C (9.41 MPa) (p<0.001). Group S exhibited the highest bond strength among all groups and showed significant difference from all other groups (33.41 MPa) (p<0.001). In the Er:YAG test groups, bonding values showed varying results from 15.27±0.58 (Group L300) to 25.98±1.08 (Group L75) and showed significant increase when compared with control group (p<0.05). Conclusions: Altough the sandblasted group exhibited the highest test values, Er:YAG laser treatment on repair composite bonding to old composite may become an alternative to other surface treatment methods.

Introduction

Composite resins are used widely in restorative dentistry and continue to evolve with the development of more wear-resistant filler particles, better resin bonding systems, curing refinements, and improved sealing systems.¹ Although the properties of composites have been improved, factors such as wear, discoloration, polymerization shrinkage, and microleakage still limit the longevity of composite restorations.^2
–4

When a composite restoration fails as a result of discoloration, secondary caries formation, ditching at the margins, delamination, or simply fracture, the restoration needs to be repaired or replaced.^5
–7 Total replacement of the restoration is the most common procedure performed in routine clinical practice.⁷ However, when large portions of the restoration are removed completely, significant loss of sound dental tissues occurs because it is often difficult to remove a tooth-colored adhesive restoration without removing an integral part of the tooth.^8,9 Minimal intervention dentistry aimed at limiting the unnecessary removal of healthy tooth structure and repairing the tooth structure is one strategy to deal with this.¹⁰ Complete replacement of a deficient dental restoration results in further extension of the preparation.¹¹ Therefore, repair by partial replacement of the restoration or local extension adjacent to the existing restoration is recommended.^12,13 The preference for restoration replacement might be the result of a complex interplay between the lack of clear standards for replacing restorations and the lack of reimbursement for these treatments. That same study reported that general practitioners would most likely intervene surgically in a defective resin composite restoration, but not in a defective amalgam restoration.

Prospective studies have shown that repaired restorations in permanent teeth have the same or increased longevity as restorations that were replaced completely.¹⁰ In such cases, providing sufficient attachment to the old restoration is important, and can be achieved macromechanically, micromechanically, or chemically.^14,15 Improving the bond strength between new and old composite usually requires increased surface roughness to promote mechanical interlocking and coating old composite with unfilled resin bonding agents to improve surface wetting and chemical bonding. It has been reported that a proper bond between an old composite resin and newly added direct composite resin can be achieved by a combination of mechanical surface treatment and the use of intermediate bonding agents and silanes, which can enhance proper bonding.⁴ Various surface pretreatments have been evaluated for achieving proper repair bonding, including sandblasting, silica coating, silanization, roughening the surface with diamond burs, and etching with phosphoric acid.^16
–18

Lasers are now used widely in medicine and dentistry. Technological advances over the last decade have resulted in the increased use of lasers in dentistry.¹⁹ Because of the improvement in lasers for dentistry, surface treatment with Erbium-doped: yttrium-aluminum-garnet (Er:YAG) lasers is considered to be an alternative to other surface treatments.²⁰ Recently, the use of Er:YAG lasers has been described for surface treatments of filling materials.⁴

This study determined the shear bond strength of a new composite resin bonded to aged composite using different surface treatments. The hypothesis of the study is that the bond strength achieved at the aged/repaired composite interface would be affected by the power setting of the Er:YAG laser.

Materials and Methods

A total of 60 composite resin material (Filtek Z250 Supreme, 3M ESPE, Seefeld, Germany) was used in the study. For the preparation of the specimens, cylindrical acyrilic molds surrounded by a metal ring with an inner diameter of 10 mm and a height of 5 mm were fabricated. Composite resin material was packed in the acyrilic mold with a dental spatula. Excess material was removed by pressing a clear glass plate with a Mylar strip (KerrHawe SA, Bioggio, Switzerland) on the top surface. The composite resin material was cured for 60 sec at a 90 degree angle to the top surface with a LED light-curing unit (Hilux250, Benlioğlu, Ankara, Turkey). Light intensity was >800 mW/cm², as verified by a radiometer after every eight specimens. After the polymerization, the test specimens were wet grounded with 320, 600, 800, and 1000 grit silicon carbide papers with a grinding and polishing machine (Minitech 233, Presi UK Ltd., Oxford, United Kingdom) to achieve standart flat and smooth surfaces. For all composites, the aging procedure was performed with 6000 thermocycles in water from 5°C to 55°C with a dwell time of 30 sec at each temperature, and a transfer time from one water bath to the other of 5 sec (Thermocycler Syndicad GmbH, Munich, Germany). After the aging procedure, the specimens were randomly divided into six groups:

1. Control (Group C); no surface treatment was performed.

2. Sandblasted group (Group S); specimens were sandblasted with 50 μm aluminium oxide (Al₂O₃) powder (Korox 50, Bego, Germany) at 60 psi for 10 sec through a nozzle at a distance of 10 mm and then cleaned with compressed oil free air to remove the remaining powder.

3. Laser etched (Group L); for the remaning four groups, laser applications were performed with Er:YAG laser which was an AT Fidelis (Fotona, Ljubljana, Slovenia) working at 2.940 nm. A dental handpiece angled 90 degrees (R14-C) was used with a cylindirical sapphire (1.3×12 mm) fiberoptic tip. The tip was used at an angle of incidence of 90 under water irrigation. The air and water pressure was set at 2 bar. The application tip was moved from the bottom to the top and maintained in light contact with the composite surface at a 12 mm distance. The laser treatment was performed for all groups under four different power settings (Group L75, energy level 75 mJ–1.1 W; Group L100, energy level 100 mJ–1.5 W; Group L200; energy level 200 mJ–3.0 W; Group L300, energy level 300 mJ–4.5W) with a frequency of 15 Hz, for 45 sec and pulse with 100 μS. The beam spot size was 0.63 mm², the energy density was 47.62 J/cm², and the power density was 476.2 W/cm².

After the surface treatments were completed, composite resins of the same kind as their substrates were adhered onto the conditioned substrates by using translucent polyethylene mold (inner diameter 3.7 mm, height 5 mm). A bonding agent (Adper Single Bond Plus, 3M Espe, Seefeld, Germany) was applied in a thin layer to the aged composite surfaces with an applicator tip (Applicator Tips Longred, Heraeus Kulzer GmbH&Co.KG, Hanau, Germany) and polymerized for 20 sec with a curing light (Hilux250, Benlioğlu, Ankara, Turkey), and the composite resin was placed against the aged composite surface incrementally with a hand instrument and polymerized for 40 sec. After the process was completed, specimens were removed from the polyethylene molds gently. The test specimens were stored in distilled water at 37°C for 24 h.

A universal test machine (Lloyd LRX; Lloyd Instruments PIC., Fareham, Hampshire, England) was used for shear bond strength test at a crosshead speed of 1 mm/m. Each specimen surface was parallel to the direction of the force during the shear strength test. Force was applied to the composite-repair composite interface. The shear bond strength values were calculated in megapascal (MPa) by dividing the failure load (N) to the area of the composite resin (N/πr²). Data were statistically analyzed. The Kolmogorov–Simirnov test showed that the data were of a normal distribution (p>0.05). A homogeneity of variance test was done using Levene's test (F, 0.196; p>0.05). Means and standard deviations of bond strengths were calculated and mean values were compared by one way analysis of variance (ANOVA) (SPSS 12,0; SPSS Inc., Chicago, IL), followed by a multiple comparisons' test performed using a Tukey test (α=0.05).

For illustrating the surface irregularities of the test specimens, one composite specimen from each group was prepared and evaluated. All specimens were coated with gold using a sputter coater (S150B; Edwards, Crawley, United Kingdom) and examined under a field emission scanning electron microscope (SEM) (JSM-6335F; JEOL, Tokyo, Japan) at 15 kV (beam current, <1A; vacuum mode, 5×10⁻⁴ Pa; detector type, secondary electron detector type). The SEM photomicrographs were developed with ×500 magnification for visual inspection (Fig. 1).

FIG. 1.

Scanning electron microscopic (SEM) views of composite test specimen. (A) Group C (no surface treatment). (B) Group S (sandblasted). (C) Group L75 (laser treated with 75 mJ). (D) Group L100 (laser treated with 100 mJ). (E) Group L200 (laser treated with 200 mJ). (F) Group L300 (laser treated with 300 mJ).

Results

One way ANOVA revealed that bond strength values were affected from different surface treatment methods (p<0.001). The mean bond strengths, standard deviations, and group differences for the six different surface treatment groups are shown in Table 1. Among the six surface-treated groups the lowest bond strength was observed in Group C (9.41 MPa) (p<0.001). Group S exhibited the highest bond strength among all groups and showed a significant difference from all other groups (33.41 MPa) (p<0.001). In the Er:YAG test groups, bonding values showed varying results from 15.27±0.58 (Group L300) to 25.98±1.08 (Group L75) depending on applied output power, and all the test values showed a significant increase when compared with the control group (p<0.05). Test specimens with increasing applied energy levels, bonding values were decreased. No significant differences were found between Group L200 and Group L300 (p>0.05) In the SEMs, the surfaces of the laser-irradiated specimens showed pitting, uniformly distributed surface irregularities, projections, droplets, and retentive areas caused by laser pulses and the exhibited irregularities. The surfaces of the L75 group showed some distinctive pits, which were not seen on the other lased surfaces groups. The surfaces of the L200 and L300 groups showed shallow pits and smoother surfaces than group S. The surfaces of the L100 group were rougher than those of the L200 and L300 groups. Group C surfaces were nearly smooth, except for some scratch-like traces and shallows pits (Fig. 1).

Table 1.

Mean Bond Strengths (MPa), SD Values, and Differences Among the Study Groups

Groups	Mean (MPa)
Group C	9.41±0.84^a
Group L200	16.13±0.67^b
Group L300	15.27±0.58^b
Group L100	21.08±0.54^c
Group L75	25.98±1.09^d
Group S	33.41±1.16^e

Different letters indicate significant differences (p<0.05).

Discussion

Within the limitations of this study, test results confirmed the hypothesis that the bond strength achieved at the aged/repaired composite interface is affected by the power setting of the Er:YAG laser. As mentioned, relayering failed composite restorations could be considered a minimally invasive, cost-effective treatment option in an attempt to prolong the service life of aged composite restorations.¹⁸ There have been many similar studies with the same objectives, but they typically evaluated fresh composites.^21
–23 This in vitro study was undertaken in order to compare the effect of conditioning methods on the repair strength of fresh and aged composites. Other in vitro studies aged the test specimens using different methods, including storage in water, citric acid immersion, or subjecting them to thermocycling.²⁴ Water storage is reported to have detrimental effects because the water uptake in the resin matrix causes hydrolysis and the release of filler particles. With thermocycling, water absorption can degrade the structural and physical properties of composite resins. Consequently, the repairing material is also affected by the aging procedures.

There is no consensus on the optimal aging procedure or its effects. According to Özcan et al.¹⁸ and Rinastiti et al.,²⁴ 5000 cycles had more effect on the degradation of the composite tested than other aging methods. Other researchers reported that 5000 thermocycles affected the surface of composite resins and the repair bond strength.²⁴ Gale et al.²⁵ recommended 10,000 cycles for in vitro interface testing. There is no consensus on the dwell time or number of cycles in thermocycling procedures. The number of cycles used ranges from 1 to 1,000,000, with a mean of ∼10,000 and median of 500 cycles.²⁵ Few studies give a reason for the number of cycles selected; however, it was previously reported that 10,000 cycles might represent a service year.²⁵ In this in vitro study, the aim was to represent >6 service months. To simulate the failed composite surface, 6000 thermocycles were used to age the composite resin samples. Determining the effect of the number of cycles on the bond strength of the repairing composite material was not an aim of this study. The thermocycling procedures were performed only to standardize the aging procedure. It is possible that heavier thermostress may cause different results and decrease the bonding test values of lased surfaces. Further investigation is needed to evaluate the effects of heavier thermostress on the bond strength of repairing composites.

Surface roughness, bonding material, repair material, and aging time and conditions are among the variables that affect the bond strength between old and newly added resin-based composites.²¹ It has been reported that sandblasting using alumina particles or silica-modified alumina particles is an effective surface treatment for the repair of composite restorations,^15,17 and etching with 37% phosphoric acid for 20 sec can provide adequate adhesion strength between the aged and fresh composite interface. One study reported that acid etching of the surface does not increase or reduce the bond strength of repairs.²⁶ Another study found no significant morphological changes in the composite surface after acid etching.²⁷ The action of the acid on the resin might be limited to superficial cleaning. Several studies reported that etching with phosphoric acid did not increase the repair bond strength.^28,29 Therefore, etching with phosphoric acid was not added into test protocol.

Murray et al.³⁰ indicated that laser treatment might be a suitable alternative to airborne-particle abrasion or other surface pretreatments for enhancing the bond strength of dental materials. No experimental study has examined the energy level of laser treatment for composite restorative repair materials. As an alternative to other surface treatments, in this study, aged composite surfaces were irradiated with an Er:YAG laser as a surface treatment. Laser irradiation of the aged composite resin before the application of fresh composite resin resulted in higher mean bond strengths than those of the control specimens. Moreover, the output power of the Er:YAG laser affected the strength of the bond between the aged and fresh composite surfaces.

This result is consistent with the results of Kimyai et al.,³¹ who found that laser irradiation is the best surface treatment for repairing laboratory composite resins. Another study examined the reasons for this increased repair bond strength and the effects of laser irradiation on composite resins, and found that a change in the volume of the molten material produced strong expansion forces, which produced projections on the surface of the composite resin structure, and that droplets formed with the removal of molten material.³² SEM showed that Er:YAG laser irradiation of a composite resin caused irregular pitting of the surface, which increased the surface area and resulted in a better stress distribution at the bonding area.³² These studies concluded that when the output power decreased, greater adhesion was obtained.^30,32

In parallel, Tugut et al.³³ investigated on the efficacy of 100, 200 (short and long pulses), 300, and 400 mJ Er:YAG laser irradiation surface treatments and found that 300 mJ resulted in greater adhesion. They reported that the impact of the high-energy pulse caused instant vaporization of water with a massive volumetric expansion, and they declared that this expansion caused the surrounding material to ablate, increasing the surface area. Gökçe et al.³⁴ reported that the shear bond strength was higher after laser treatment at 300 mJ than after treatment at 600–900 mJ. According to the authors, the low bond strengths observed at high power settings might be related to the observed heat-damaged layer. This layer might be poorly attached to the infra-layers of the substrate.

As reported previously, laser irradiation causes ablation at the surface, and the ablated diameter grows with the energy per pulse with a clear tendency to saturation at high energies.³⁴ Increased ablation diameter, depth, and volume might cause a decrease in the retentive area and droplets at the laser-irradiated surface, reducing the micromechanical retention of the repair composite on the old composite. In parallel with other studies, the 75 mJ Er:YAG resulted in the highest bond strength among the laser groups; the Er:YAG 200 and 300 mJ test groups had lower bond strengths. The reason for the significantly higher bond strength with a low laser energy level might be that the low energy level prevents the composite resin surface from deteriorating. Many studies^31
–33 can be found that investigate the effects of different laser output power on the bond strength of repair composites; 200 mJ, 300 mJ, and higher output powers were frequently examined.^33,34 However, studies that examine the effects of low output power on the bond strength of repairing composites were rare; therefore, in the present study, 75 and 100 mJ low energy levels were selected.

The results of this in vitro study are not in accordance with the findings of Burnett et al.,³⁵ who evaluated the effect of Er:YAG laser surface treatments on indirect composites to adhesive resin. They found that 200 mJ Er:YAG application enhanced bonding strength better than air abrasion. Another investigation that compared the effects of three mechanical surface treatments on the repair bond strength of a laboratory composite found no significant difference between the Er:YAG laser and air abrasion.³¹ The reason for these differences might be the composition of composite resins, which can affect the efficacy of mechanical surface treatments. Lizarelli et al.³⁶ concluded that the composite resin composition and filling particle sizes affect the ablation diameter, depth and volume; less material was removed from condensable composite resins compared with microfilled and hybrid composites.³⁶

The present in vitro research did not examine the effects of different kinds of lasers on the bonding strength of repaired composites, and further studies need to evaluate this.

Teixeira et al.³⁷ suggested that the repair bond strength should range from 15 to 25 MPa. In this research, only the control group failed to reach this range. Within the limitations of the present study, although Al₂O₃ powder resulted in higher bonding strengths than the laser groups, further investigations with different laser parameters, energy levels, and composite materials must be performed to develop guidelines for the repair of composite restorations with Er:YAG lasers. Regarding clinical relevance, it can be concluded that sandblasting procedures have limited application in the mouth, whereas laser irradiation is easily performed on composites when repairing them. It should be recommended.

Conclusions

Although sandblasting showed highest test values, Er:YAG laser treatment for repair composite bonding to old composite might become an alternative to other surface treatment methods. Laser output power affects the repair bond strength. Higher laser power levels decrease the repair bond strength.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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