Effect of Er:YAG Laser Pulse Duration on Shear Bond Strength of Metal Brackets Bonded to a Porcelain Surface

Abstract

Objective: The aim of this study was to compare the effect of different Er:YAG laser pulse durations on the shear bond strength (SBS) of metal brackets bonded to porcelain with two different adhesive systems. Background data: Orthodontic brackets do not bond well to feldspathic porcelain surfaces, using resin cement. Various treatment methods have been suggested for the porcelain surface to improve bond strength. Thus far, no orthodontic study has evaluated the effect of different Er-YAG laser pulse durations on porcelain surfaces with or without sandblasting. Methods: In the present study, 150 porcelain crowns were assigned to 10 groups differing in adhesive system and surface treatment. In five groups, the adhesive system was RelyX^™ U 200 and in the other five, Transbond XT was used. For each adhesive system, the porcelain surfaces were treated in one of five different ways: sandblasted, Er:YAG laser short pulse (SP), Er:YAG laser super short pulse (SSP), sandblasted+SP, or sandblasted+SSP. The sandblasted group with Transbond XT served as the control. SBS test was conducted for each group. Samples were examined by scanning electron microscopy. ANOVA and independent t test were used for statistical analysis. Results: The control group had increased roughness and the highest SBS. Er:YAG laser application to the sandblasted porcelain flattened the roughness, and the effects of SP and SSP were similar. Conclusions: Er:YAG laser application did not allow for elimination of the hydrofluoric acid step. RelyX U 200 is a viable alternative to Transbond XT on sandblasted porcelain.

Introduction

I n recent years, because of the increased standard of orthodontic treatment expected by patients, it has become necessary to place brackets on porcelain crowns without the occurrence of bond failures during the course of treatment. However, porcelain has an inert surface, hindering adherence to other bonding materials. Different methods mainly categorized as mechanical, chemical, or a combination of both, have been suggested to overcome this problem by altering the physical characteristics of the surface of porcelain prior to bonding. Mechanical techniques providing clinically acceptable bond strengths able to withstand orthodontic forces consist of deglazing the porcelain by roughening the surface with air abrasion or sandblasting,^1

–4 diamond burs,^5,6 and use of abrasive disks.⁷ Hydrofluoric acid (HFA) gel (9.6%) is the most widely used chemical agent to roughen the porcelain surface to achieve sufficient bond strength.^3,8,9

Whereas each method has the ability to increase the strength of the bond between the orthodontic bracket and the porcelain surface, they each have disadvantages. These include the risk of hazardous effects of HFA on the soft tissues,^10

–13 increased risks of porcelain fracture,¹⁴ and crack initiation¹⁵ associated with mechanical roughening. Hence, clinicians are hesitant to use these techniques.

Although each procedure has different disadvantages in clinical practice, sandblasting the porcelain surface³ and application of 9.6% HFA¹⁶ are still very common methods among orthodontists to improve the bond strength of orthodontic brackets. This is because no alternative surface treatment methods have proven to be superior to these conventional methods.

More recently, high-output lasers such as the Er:YAG laser have been suggested as an efficient method of roughening porcelain surfaces to obtain increased shear bond strength (SBS).¹⁷ The Er:YAG laser, with a light emission wavelength of 2.94 μm, has various parameters that affect aspects of ablation, including irradiation time, contact and noncontact irradiation,¹⁸ energy and pulse repetition rate,¹⁹ and pulse duration.^20
–22 Pulse duration is one of the most substantial variables affecting ablation capacity and surface configuration.^23
–25 The majority of published research has reported the effects of Er:YAG laser pulse duration on enamel and dentin surfaces.^20,22,23 However, there is little information available regarding the effects of Er-YAG laser pulse duration on the feldspathic porcelain surface.^26
–28 Dilber et al.²⁶ stated that sandblasting the feldspathic porcelain caused more increased irregularities when compared with the Er:YAG laser group. Similarly, Shiu et al.²⁷ obtained the highest bond strength from Al₂O₃ group when compared with nine different surface treatment methods including Er;YAG laser and THE Al₂O₃+Er:YAG laser group. And they concluded that Er;YAG laser application with parameters used is not an effective way to roughen the feldspathic porcelain surface for clinical practice. Furthermore, Poosti et al.²⁸ used Er:YAG laser at 2 and 3 W power settings to roughen the porcelain surface prior to bracket bonding. And they obtained higher bond strengths from 9.6% HFA and Nd:YAG laser application.²⁸ Therefore, the aim of this study was to investigate the effect of different Er:YAG laser pulse durations on the SBS of metal brackets, bonded to feldspathic porcelain with two different adhesive systems. The null hypothesis was that the SBS obtained in the sandblasted group would be similar to those obtained in the Er;YAG laser groups, for both short pulse (SP) and super short pulse (SSP) modes.

Materials and Methods

One hundred and fifty porcelain-fused-to-metal crowns (Solera Classic, manufactured by Wohlwend, Schellenberg, Liechtenstein) were fabricated in the shape of a right upper first premolar tooth and randomly divided into 10 equal groups. The treatments applied to each group were as follows (Table 1).

Table 1.

Adhesive Cements, Surface Treatments, and the Er:YAG Laser Parameters Applied for Each Group

Groups (n=15)	Adhesive cement	Surface treatment	Er:YAG laser parameters
1	RelyX^™ U 200	-	-
2	Transbond XT	9.6% HFA	-
3	RelyX^™ U 200	Er:YAG laser	3 W, 20 Hz, 150 mJ, SP mode (300 μs) for 10 sec
4	Transbond XT	Er:YAG laser	3 W, 20 Hz, 150 mJ, SP mode (300 μs) for 10 sec
5	RelyX^™ U 200	Er:YAG laser	3 W, 20 Hz, 150 mJ, SSP mode (50 μs) for 10 sec
6	Transbond XT	Er:YAG laser	3 W, 20 Hz, 150 mJ, SSP mode (50 μs) for 10 sec
7	RelyX^™ U 200	Sandblasting+Er:YAG laser	3 W, 20 Hz, 150 mJ, SP mode (300 μs) for 10 sec
8	Transbond XT	Sandblasting+Er:YAG laser	3 W, 20 Hz, 150 mJ, SP mode (300 μs) for 10 sec
9	RelyX^™ U 200	Sandblasting+Er:YAG laser	3 W, 20 Hz, 150 mJ, SSP mode (50 μs) for 10 sec
10	Transbond XT	Sandblasting+Er:YAG laser	3 W, 20 Hz, 150 mJ, SSP mode (50 μs) for 10 sec

HFA, hydrofluoric acid; SP, short pulse; SSP, super short pulse.

For group 1, porcelain surfaces were sandblasted. RelyX^™ U 200 (Unite, 3M, Unitek, USA) paste was applied to the base of the bracket, after mixing according to the manufacturer's instructions. The bracket was then pressed firmly onto the buccal surface of porcelain and excess resin was removed with a scaler.

For group 2, after sandblasting, the porcelain surfaces were etched with 9.6% HFA gel for 2 min, rinsed with a water/spray combination for 30 sec, and dried. Transbond XT paste was applied to the base of the bracket. After application of a layer of Transbond XT (3M Unitek) light-cured adhesive primer to the porcelain surface, the bracket was pressed firmly onto the buccal surface of porcelain and excess resin was removed with a scaler. This group was used as the control.

For group 3, porcelain surfaces were irradiated with the Er:YAG laser in SP (300 μs) mode. The brackets were then bonded to the porcelain surface via the same method as that used for group 1.

For group 4, porcelain surfaces were irradiated with the Er:YAG laser in SP mode. The brackets were then bonded to the porcelain surface via the same method as that used for group 2.

For group 5, porcelain surfaces were irradiated with the Er:YAG laser in SSP (50 μs) mode. The brackets were then bonded to the porcelain surface via the same method as that used for group 1.

For group 6, porcelain surfaces were irradiated with the Er:YAG laser in the SSP mode. The brackets were then bonded to the porcelain surface via the same method as that used for group 2.

Groups 7, 8, 9, and 10 were subjected to the same procedures as groups 3, 4, 5, and 6, respectively, but the porcelain surfaces were sandblasted beforehand in groups 7–10.

One hundred and fifty upper right first premolar metal brackets (Master series, American Orthodontics, Sheboygan, WI) were used. The surface area of the bracket base was 10.27 mm². After removal of excess resin resulting from bracket placement onto the porcelain surface, all brackets were light cured for 20 sec using a light emitting diode (LED; Demi, Kerr, Orange, USA).

The surface treatments were applied at the center of the middle third of the buccal surface of the porcelain. For surface treatment procedures with air abrasion (Renfert Basic Classic Dental Sandblaster, FL), porcelain surfaces were sandblasted for 3 sec with 50 μm Al₂O₃ particles at a pressure of four bars and a distance of 4 mm from the buccal surface of the porcelain. After sandblasting, the specimens were cleaned with compressed air to remove any remnant debris. Laser irradiation (Er:YAG laser; 2.94 nm, Fotona Fidelis Plus 3, Slovenia) of the porcelain surfaces was performed at power settings of 3 W, 20 Hz, 150 mJ for 10 sec, with a noncontact handpiece positioned 8 mm from the surface. And Er:YAG laser power density and energy fluence were 5.97 W/cm² and 29,856 J/cm², respectively.

After storage in distilled water at 37°C for 24 h, all samples were thermocycled for 500 cycles of 5°C and 55°C, using a dwell time of 30 sec. The specimens were embedded in self-curing acrylic resin to facilitate debonding using a square silicone putty (c-silicone impression material, Zetaplus, Zhermack Clinical, Rovigo, Italy) mold with the buccal surface and bracket exposed and parallel to the horizontal plane. Bond strength was measured using a Universal Testing Machine (AGS-X, Shimadzu, Japan) at a crosshead speed of 1 mm/min until fracture occurred. The tensile load was applied parallel to the long axis of the buccal surface of the restoration in a gingivo-occlusal direction. The SBS was recorded in newton units and then converted to megapascals by dividing the failure load (N) by the surface area of the bracket base. After shear bond testing, the porcelain surfaces were examined with a light stereomicroscope (M165C; Leica Microsystems, Wetzler, Germany) at a magnification of ×10 to determine the amount of resin remnants and classified into one of four types: 0=no resin cement left on the porcelain surface, 1=< 50% of the resin cement left on the porcelain surface, 2=>50% of the resin cement left on the porcelain surface, 3=all resin cement left on the porcelain surface.

For visual evaluation of the surface conditioning methods, one specimen from each different surface treatment group was examined by scanning electron microscopy (SEM) (EVO LS10, Zeiss, Oberkochen, Germany). The SEM images were acquired at ×500 magnifications.

Statistical analysis

All statistical analyses were performed using SPSS version 14.0 for Windows (SPSS Inc., Chicago, IL). One-way analysis of variance (ANOVA) was used to compare the SBSs of each group. The t test for independent samples was used to compare the surface treated groups with the control group; p<0.05 was considered statistically significant.

Results

During thermocycling, all brackets failed in groups 3, 4, 5, and 6. Nine brackets failed in group 7. Because of these bracket failures, groups 3–7 were not included in the statistical analyses. The results of one way ANOVA showed that there were statistically significant differences among groups (p=0.002). The highest SBSs were observed in group 2 (8.83±3.3 MPa), followed by groups 1, 8, 10, and 9 (in that order) with values of 8.25±3.2, 3.48±1.7, 3.11±0.93, and 1.56±0.86 MPa, respectively (Table 2). The results of the independent samples t test indicated that there were no statistically significant differences between group 1 and the control group (p=0.635). There were no statistically significant differences between groups 8 and 10 (p=0.502).

Table 2.

Shear Bond Strength (SBS) to Porcelain (Mpa)

Groups (n=15)	Mean±SD
Group 1	8.25±3.2^a
Group 2 (control)	8.83±3.3^a
Group 8	3.48±1.7^b
Group 9	1.56±0.86
Group10	3.11±0.93^b

Groups with the same superscript letter indicate no statistically significant difference (p>0.05).

Fracture patterns of groups that were tested by the Universal Testing Machine are presented in Table 3. In group 2, type 1 pattern were observed in 86% of specimens, and fracture pattern of 66% of the specimens in groups 1 and 9 were type 1. Also 60% specimens were in type 1 in group 8. The lowest range for type 1 (33%) was found in group 10.

Table 3.

Bond Failure Modes of Groups

Groups	0	1	2	3
1	1	10	4	-
2	1	13	1	-
8	6	9	-	-
9	4	10	1
10	10	5	-	-

SEM photomicrographs of porcelain surfaces treated using different techniques are shown in Fig. 1. Compared with the sandblasted+laser- irradiated groups, for both SP and SSP modes, the sandblasted group showed increased surface roughness with irregularities on the porcelain surface. The glazed porcelain surfaces of the laser-irradiated groups (SP and SSP modes) exhibited mild and similar alterations to each other.

FIG. 1.

Scanning electron microscopic (SEM) images of specimens (original magnification ×500). (a) Sandblasted specimen. (b) sandblasted+Er:YAG in short pulse (SP) mode. (c) Sandblasted+Er:YAG in super short pulse (SSP) mode. (d) Er:YAG laser in SP mode alone. (e) Er-YAG laser in SSP mode alone.

Discussion

In the present study, the average SBS values for groups 1 and 2 (8.25 and 8.83 MPa, respectively) were>5 MPa, which is suggested to be sufficient to sustain orthodontic forces.^29

–32 Additionally, there are many studies suggesting different values for the minimum SBS of orthodontic brackets, ranging from 6.5 to 10 MPa.^33

–36 Some studies, in accordance with the results of this study, have reported that conditioning of the porcelain surface with sandblasting and HFA is necessary in order to achieve acceptable bond strength.^3,9 However, the remarkably high bond strength observed in group 1 in the present study is not in accordance with the results of recent studies.^37
–39

In previous studies, Rely-X Unicem self-adhesive cement proved to have many inadequacies with regard to bracket bonding onto enamel surfaces,^37
–39 and, therefore, it did not become a widely used adhesive in orthodontics. Accordingly, in the present study, RelyX U 200 was used to bond brackets instead of Rely-X Unicem. The improved ingredients of RelyX U 200 might be responsible for the different findings of the current study as compared with those of otherwise similar recent studies. According to the manufacturer, an additional monomer and a new rheology modifier were added to the improved formulation, to optimize the processing of filler particles. These improvements may enhance the mechanical properties of the formulation, resulting in excellent overall adhesion performance of RelyX U 200, as compared with that of Rely-X Unicem.

The results of this study indicated that the bond strengths obtained from groups exposed to Er:YAG laser irradiation after sandblasting were lower than those in groups exposed to sandblasting alone (Table 2). In other words, the statistically significant difference (p=0.002) between the control and laser irradiated groups suggested that Er:YAG laser application to the porcelain surface after sandblasting had an adverse effect on the strength of bonding to the metal brackets. Therefore, the null hypothesis that SBSs obtained in the sandblasted group would be similar to those obtained in Er:YAG laser groups (in SP or SSP modes) was rejected. Dilber et al.²⁶ have reported lower surface roughness values in a sandblasting+Er:YAG laser group as compared with a sandblasting alone group, for feldspathic ceramic discs. Additionally, Shiu et al.²⁷ have suggested that surface treatment of feldspathic ceramics with sandblasting yielded the highest bond strength when compared with sandblasted+Er:YAG laser treatment and Er:YAG laser treatment alone groups. In the present study, SEM images from the sandblasting+Er:YAG laser groups (in both SP and SSP modes) revealed partially smooth areas (Fig. 1) that were evidently a consequence of laser application, based on comparison with the sandblasted alone group. These smoother areas with decreased surface irregularities may have prevented the wettability of the porcelain and adhesive cement, which could be responsible for the lower SBSs observed in the sandblasted+Er:YAG laser groups.

Microexplosions and vaporization resulting in porcelain ablation occurred during Er:YAG laser application as a consequence of interaction between water on the surface of the porcelain, and the Er:YAG laser beam.^40,41 Heating and cooling phases cause a shift in local temperature throughout laser irradiation.⁴² This process can be destructive to the surface topography of materials because of the internal tensions generated at higher laser power settings.^42,43 In light of these considerations, power settings of 3 W, 20 Hz, and 150 mJ with continuous water cooling during laser application were utilized to evaluate the effects of different pulse durations on surface roughness, in accordance with previous studies.^17,40

One of the most influential parameters with regard to the ablative and thermal effect of lasers is pulse duration.^24,25 Several studies have shown that the effects of Er:YAG laser applications utilizing both long and short pulse durations on different tooth surfaces such as enamel and dentin are highly variable and contradictory.^20,22,44 Even where water cooling was used during laser application, longer pulses evidently caused thermal damage, having an adverse effect on bond strength, and better results with regard to ablation on the enamel surface could be obtained with shorter pulses.^45,46 Firat et al.²⁰ reported similar findings for enamel surface treated with Er:YAG laser using long pulse durations. In addition, Sheth et al.⁴⁴ recommended shorter pulses for cavity preparation in dentin to obtain optimal adhesion. However, Firat et al.²⁰ found no statistically significant differences in the improvement of bond strength with regard to the dentin surface between the use of the laser at different pulse durations. Moreover, Nishimoto et al.²² have reported that when the Er-YAG laser was used with longer pulse durations, this resulted in deeper ablation areas with smaller diameter; further, no statistically significant changes were evident in the volume of ablated dentin between different pulse durations. They suggested longer pulse durations for better control of the ablation area on the dentin surface with regard to clinical practice.²² In the present study, comparisons between the differently surface-treated groups revealed statistically significant differences, with the exception of comparison between groups 8 and 10 (p=0.502). Statistically, no significant difference between groups 8 and 10 was evident; this suggested that the use of SP or SSP modes during Er:YAG laser application to a sandblasted porcelain surface did not have significantly different effects on bond strength. Although data from group 7 were not incorporated into statistical analyses because of failure of nine brackets during thermocycling, the low bond strength and failure of brackets in this group supported the aforementioned result.

In addition to this, statistically significant differences found between group 9 and groups 8 and 10 (p=0.002 and p=0.001, respectively) indicated that Er:YAG laser application to the porcelain surface after sandblasting had a more adverse effect on the bond strength of metal brackets bonded with Rely-XU 200, than on those bonded with Transbond-XT. Although statistically significant differences were observed, the mean SBSs of all three groups were lower than the reported clinically acceptable ranges.^33

–36 Despite the local smoother areas depending on laser application, better penetration of adhesive primer, which depends on the flowable viscosity, into the existing microretentive regions (when compared with Rely XU 200 paste) might be a reason for the decreased negative effect of laser application on the sandblasted porcelain surface in group 9.

The failures that occurred in groups 3–6 during thermocycling indicated that irradiating the glazed porcelain surface using only Er:YAG laser treatment (in SP or SSP mode), that is, without sandblasting, did not yield clinically adequate bond strength. Poosti et al.²⁸ have reported that brackets bonded onto porcelain surfaces prepared with Er:YAG laser application at 2 or 3 W power settings had lower bond strengths than those prepared with 9.6% HFA and an Nd:YAG laser. Similarly, Shiu et al.²⁷ reported the lowest bond strength in the Er:YAG laser group, which is in agreement with the results of the current study. Furthermore, Akyil et al.⁴⁷ have reported that Er:YAG laser application to the feldspathic porcelain surface did not provide adequate bond strength. These findings confirm the necessity of sandblasting to improve the potential bond strength of porcelain; a conclusion that is in keeping with the findings of previous reports.^3,9

Conclusions

Within the limitations of this study, the following conclusions can be drawn.

1. Er:YAG laser application alone or after sandblasting is less effective than conventional techniques in improving the SBS of metal brackets to the porcelain surface for both types of adhesive cement investigated.

2. Er:YAG laser application to the sandblasted porcelain surface resulted in flattening in localized areas, thereby resulting in lower SBS.

3. Er-YAG laser application alone is not an appropriate treatment method for the glazed porcelain surface, with regard to obtaining acceptable SBS.

4. RelyX U 200 is a viable alternative to Transbond XT for bonding brackets onto sandblasted porcelain.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

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