Non-thermal plasma treatment to enhance the adhesion between enamel surface and orthodontic bracket

Abstract

BACKGROUND:

Adhesion strength of orthodontic attachments to enamel should be within optimal range to resist occlusal forces and to allow debonding without enamel damage.

OBJECTIVE:

The present study compared the effect of non-thermal plasma (NTP) and conventional surface treatment on the adhesion strength of orthodontic bracket to enamel.

METHODS:

A total of 100 premolar teeth were allocated into 5 groups according to the bonding procedure followed: Group 1 (Etch, prime and composite adhesive); Group 2 (Prime and composite adhesive); Group 3 (Glass ionomer cement); Group 4 (NTP, prime and composite adhesive) and Group 5 (NTP and glass ionomer cement). Ten specimens in each group were subjected to artificial aging and the remaining ten specimens served as baseline specimens. Adhesion strength values were recorded after debonding and bond failure types were scored. Water contact angles of the NTP treated and untreated enamel surface were measured.

RESULTS:

Group 1 specimen demonstrated highest bond strength at baseline (21.5 ± 3.01) and thermocycling (15.8 ± 2.87) and the least values were in Group 2 specimens at baseline (3.9 ± 1.01) and thermocycling (1.6 ± 0.7). Among the experimental (NTP) groups, Group 4 specimens exhibited high adhesion strength at baseline (10.2 ± 1.76) and after thermocycling (9.8 ± 2.15) compared to Group 5 specimens at baseline (10.1 ± 1.05) and thermocycling (6.5 ± 2.19). The water contact angle on untreated enamel surface was 53.1° ± 2.1° as compared to 1.4° ± 0.7° in treated surface.

CONCLUSION:

Non-thermal plasma (NTP) treatment in conjunction with composite adhesives demonstrated clinically acceptable adhesion strength and was well within the optimal range (7–14 MPa) for enamel bonding.

Keywords

Non-thermal plasma orthodontic bonding adhesion strength acid etching

1. Introduction

Esthetics is the major concern for patients undergoing orthodontic therapy [1]. Direct bonding is one of the greatest developments in esthetic orthodontic practice. Historically, orthodontists used pitch-fit or prefabricated bands on all teeth to achieve intended tooth movement. The orthodontic brackets were spot welded on to the bands to deliver orthodontic force through arch wires. This procedure had many drawbacks such as increased chair side time, tooth spacing after debanding, gingival trauma, decalcification under bands [2]. During 1960’s, George Newman and Professor Fujio Miura, pioneered the direct bonding of orthodontic brackets to enamel using acid etching [2,3]. The adhesives used during those days demonstrated decreased adhesion strength over time when exposed to oral fluids and mastication. However, the direct bonding systems remained popular as an alternative to bands considering the advantages they possess over the bonding system. Over the years, lots of research has been done to improve and develop optimum bracket bonding to tooth structure.

The invention of acid etching by Buonocore (1955) revolutionized the field of dentistry. Despite the fact, that acid etching can achieve acceptable orthodontic bonding to enamel surfaces, there is a great need to simplify the procedure and minimize the side effects of acid etching and means of microretention enamel bonding. Debonding of orthodontic brackets bonded to acid etched enamel surfaces has a risk of damage to the enamel in the form of cracks, scratches or tissue loss [4]. Furthermore, enamel surface cannot be restored to its original condition due to persistence of resin tags after debonding [5]. Another drawback is that the enamel surface prepared for micro-retention but not covered by adhesive material harbors dental plaque and increases the risk of white spot lesions [6]. To overcome the disadvantages posed by acid etching of enamel, alternative techniques of surface treatment such as sand blasting or air abrasion [7], laser etching [8] and self-etching primers [9] are employed. However, these techniques have not proved to be an ideal.

Non-thermal plasma (NTP) treatment is gaining significant popularity in recent years. This novel technique has been employed for preparation and surface modification of tooth enamel surface before the application of adhesive materials [10,11]. Plasma is a partially or wholly ionized gas with approximately equal amount of positively and negatively charged particles. Any surface exposed to such plasmas can lead to the insertion of chemical functionalities, with the nature of the functionalities being greatly dependent on the chemical composition of the biomaterial and the gas used [12]. As a result, plasma treatment can help as a platform for further surface modification processes, such as the grafting of biomolecules and other functional structures [13]. Plasma is found to have bactericidal effect on dental plaque and dental caries, and furthermore, they do not cause discomfort to patients as it does not initiate any thermal damage to tissues [14,15]. Plasma treatment has been considered as an efficient technology because majority of the properties of the materials/surfaces do not change after treatment with plasma [16]. Non-thermal plasma (NTP) or cold plasmas have been extensively researched for surface modification or preparation of dental implants, adhesion, caries treatment, endodontic therapy and tooth whitening [17]. It is also demonstrated that NTP could improve adhesive penetration at the interface area [11,18].

Therefore, the aims of the present study were to: (a) evaluate the adhesion strength of orthodontic brackets bonded to NTP treated tooth enamel using various bonding protocols; (b) determine the water contact angle of the NTP surface treated and non-treated enamel surfaces, and (c) examine and assess the bond failure mode after debonding. The working hypothesis was that there was no significant difference between non-thermal plasma (NTP) treated and non-treated enamel surfaces with regard to above mentioned study aims.

2. Methodology

An in-vitro study was conducted on one hundred extracted human premolar teeth to investigate the effect of using non-thermal plasma (NTP) admixed with oxygen as a surface treatment on enamel surface prior to orthodontic bonding.

2.1. The non-thermal plasma jet

The non-thermal plasma (NTP) jet device used in this study (KinPen^TM, INP Greifswald, Germany) has a hand-held unit connected to a high-frequency power supply for the generation of plasma jet at atmospheric pressure (Fig. 1). The operating gas Argon (Argon Corp- Great Neck, NY, USA) with a flow rate of 30 standard cubic centimeters per minute admixed with O₂ at a flow rate of 5 standard liters per minute was used. The plasma plume emerging at the exit nozzle was about 1.5 mm in diameter and extends into the surrounding air for a distance up to 5 mm.

Fig. 1.

NTP plume used in the study.

2.2. Sample collection and preparation

For the present study, hundred de-identified human premolars extracted for the purpose of routine orthodontic therapy were collected. The sample collection was in accordance with World Medical Association Declaration of Helsinki, 1975, and as subsequently revised in 2013. The research project was approved by the Institutional Review Board of the College of Dentistry, King Khalid University (SRC/ETH/2018-19/091). Any teeth with surface irregularities (cracks, caries, demineralization, fluorosis, restoration and enamel defects) were discarded and replaced. The teeth were mechanically cleaned using water to remove any blood or residual tissue and then stored in 0.5% Chloramine T disinfectant. Prior to bonding procedure, the root portion of all teeth was individually embedded in acrylic resin (Dentsply- York, PA, USA). The buccal bonding area of all teeth were meticulously polished with admix of non-fluoridated pumice and water slurry for 10 sec using a rubber prophylactic cup attached to a slow speed hand piece. The polished surface was thoroughly cleaned under running water and dried.

2.3. Bonding procedure

The polished premolar teeth were randomly allocated into any of the five (n = 20) groups according to the bonding procedure followed.

2.3.1. Control groups

Group 1: Etch, prime and composite adhesive. This procedure followed the traditional method of orthodontic bonding. The enamel bonding surface was etched with 37% phosphoric acid for 30 seconds followed by washing and drying for ten seconds using oil free air jet. A thin layer of primer (Transbond XT, 3M Unitek, Monrovia, CA, USA) was applied onto the etched tooth surface with a fine disposable brush and allowed to dry. Premolar brackets (GAC DENTSPLY, Ontario, Canada) with a mean surface area of 12.42 mm² were used for orthodontic bonding. The adhesive paste (Transbond XT, 3M Unitek, Monrovia, CA, USA) was applied onto the bracket base and then positioned at the center of the buccal surface. The bracket was then pressed firmly to extrude any excess resin which was later removed with an explorer. Following bracket placement, the bracket edges were light cured (Elipar Free Light 2, 3M ESPE, Seefeld, Germany) on the proximal sides as per the manufacturer’s instructions.

Group 2: Prime and composite adhesive. Orthodontic brackets were bonded to premolars using primer and adhesive as described with Group 1 specimens except there was no surface treatment (etching) of the enamel.

Group 3: Glass ionomer cement. In this group, orthodontic brackets were bonded on to the enamel surface using light cured Glass ionomer cement (Fuji Ortho^TM LC, GC America, Inc., USA). The cement was applied onto the bracket base and then positioned at the center of the buccal surface. The bracket was then pressed firmly to extrude any excess cement which was later removed with an explorer. Following bracket placement, the bracket edges were light cured on the proximal sides as per the manufacturer’s instructions.

2.3.2. Experimental groups

Group 4: Non-thermal plasma, prime and composite adhesive. Teeth were surface treated with NTP for 30 sec, followed by application of a thin layer of primer onto the tooth surface with a fine disposable brush and allowed to dry. The adhesive paste was applied onto the bracket base and then positioned at the center of the buccal surface. The bracket was then pressed firmly to extrude any excess resin which was later removed with an explorer. Following bracket placement, the bracket edges were light cured as per the manufacturer’s instructions.

Group 5: Non-thermal plasma and glass ionomer cement. Teeth were surface treated with NTP for 30 sec and the orthodontic brackets were bonded using GIC. Following bracket placement, the bracket edges were light cured on the proximal sides as per the manufacturer’s instructions. All the specimens from the study groups were stored in deionized water at 37 °C for 24 h.

2.4. Artificial aging and adhesion strength test

Ten randomly selected specimens from each group were subjected to thermo-cycling (Huber 1100, SD Mechatronik, Feldkirchen-Westerham, Germany) with 5000 cycles (5 °C–55 °C with a dwell time of 20 sec and transfer time of 10 sec). 5000 cycles was performed considering the average duration of orthodontic treatment as 2 years [19]. The remaining ten specimens served as baseline specimens.

For bond strength test, a mounting jig was used to align the enamel surface perpendicular to the bottom of the resin blocks and parallel to the force direction during the test. A blunt end chisel was loaded onto a universal testing machine (Instron 5566, Instron-Norwood, MA, USA) and directed at the interface between bracket and enamel in an occluso-gingival direction at a crosshead speed of 0.5 mm/min. The adhesion strength was calculated and presented in megapascals.

2.5. Assessment of bond failure sites

After debonding, both the bracket base and the enamel surfaces of each tooth from all the groups were visually inspected and assessed to determine the predominant sites of bond failure. The sites were scored according to the Adhesive Remnant Index (ARI) using a light stereomicroscope (Nikon SM2-10, Tokyo, Japan) at ×20 magnification.

2.6. Contact angle measurement

The wettability of the enamel surfaces to water before and after NTP treatment was assessed by contact angle measurements using a camera based optical tensiometer (Theta Lite, Dyne Technology, Staffordshire, UK) (n = 5). For the same purpose, the crown portion of the tooth was separated from the root and further sectioned into buccal and lingual halves along the central groove using precision sectioning saw (Isomet 4000, Lake Bluff, Illinois, USA). The sectioned buccal and lingual half was used for contact angle measurements. In experimental group, the enamel surface were treated with NTP for 30 sec and in control group, the enamel surface received no treatment. The enamel sections were placed on a movable turret of the tensiometer and a 0.5 μl of deionized water droplet was dispensed by a syringe tip towards the surface. After the water droplet made contact with the enamel surface, the contact angle was measured after 30–40 s, when the droplet was seemingly secured on the enamel surface. The illuminated drop was captured by the camera from the opposite side and the droplet arc and contact angle was measured (Fig. 2).

Fig. 2.

Contact angle measurements of the enamel surface treated with or without non-thermal plasma treatment (NTP): (a) without NTP treatment, (b) with NTP treatment.

2.7. Statistical analysis

All the data obtained were analyzed using Statistical Package for Social Sciences (SPSS) v 20.0 (SPSS Inc., Chicago, IL, USA). Analysis of variance (ANOVA) and post hoc Tukey test were used to evaluate the differences in adhesion strength values and contact angle measurements between the study groups.

3. Results

The mean adhesion strength of the study groups at baseline and after thermocycling is presented in Table 1. Group 1 specimens demonstrated highest bond strength at baseline (21.5 ± 3.01) and thermocycling (15.8 ± 2.87) and the least adhesion strength values were found in Group 2 specimens at baseline (3.9 ± 1.01) and following thermocycling (1.6 ± 0.7). All the control group specimens showed significant decrease in adhesion strength values following thermocycling (P < 0.05). Among the experimental groups, Group 4 specimens treated with NTP and bonded using adhesive composite exhibited high adhesion strength at baseline (10.2 ± 1.76) and after thermocycling (9.8 ± 2.15) compared to Group 5 specimens at baseline (10.1 ± 1.05) and thermocycling (6.5 ± 2.19). No significant difference was observed between Groups 4 and 5 specimens at baseline whereas after thermocycling the difference between the Groups 4 and 5 was statistically significant (P < 0.05). Contrary to the other groups, Group 4 exhibited insignificantly decreased adhesion strength values after thermocycling (P > 0.05). The adhesion strength of the specimens bonded using GIC in both control and experimental groups were not significant following thermocycling (P > 0.05).

Table 1
Mean (±SD) adhesion strength of the study groups at baseline and after thermocycling

Adhesion strength of the specimens

Study groups Baseline Thermocycled

Group 1 21.5 ±3.01^A,a 15.8 ±2.87^A,b

Group 2 3.9 ±1.01^B,c 1.6 ±0.7^B,d

Group 3 9.7 ±1.49^C,e 4.9 ±1.04^C,f

Group 4 10.2 ±1.76^C,g 9.8 ±2.15^D,g

Group 5 10.1 ±1.05^C,h 6.5 ±2.19^E,f

	Adhesion strength of the specimens
Group 1	21.5 ±3.01^A,a	15.8 ±2.87^A,b
Group 2	3.9 ±1.01^B,c	1.6 ±0.7^B,d
Group 3	9.7 ±1.49^C,e	4.9 ±1.04^C,f
Group 4	10.2 ±1.76^C,g	9.8 ±2.15^D,g
Group 5	10.1 ±1.05^C,h	6.5 ±2.19^E,f

Different upper case alphabets in a row and different lower case alphabets in a column implies statistically significant difference between the groups (P < 0.05).

The Adhesive Remnant Index (ARI) scores were distributed between 0 to 3 among the control groups and 0 to 2 with the experimental groups both at baseline and after thermocycling. The ARI scores of control specimens revealed more adhesive failures at both baseline and after thermocycling whereas specimens in the experimental group showed more cohesive failures. Two instances of enamel fractures were observed in Group 1 specimen at baseline and thermocycling (Table 2).

Table 2

Adhesive remnant index (ARI) scores of the study groups

Groups (n = 10)	ARI Scores
	Baseline				Thermocycled
	0	1	2	3	0	1	2	3
Group 1	0	1	3	4^∗	1	1	2	4^∗
Group 2	1	4	1	4	2	2	3	3
Group 3	3	3	2	2	2	1	3	4
Group 4	5	4	1	0	4	5	1	0
Group 5	3	5	3	0	3	4	3	0

0 = no adhesive remaining on enamel surface; 1 =< than 50% of the adhesive remaining on enamel surface; 2 => than 50% of the adhesive remaining on enamel surface; 3 = all adhesive remaining on the enamel surface with a distinct impression of the bracket. ^∗One instance of enamel fracture in Group 1 specimen.

The mean water contact angle changes on the enamel surfaces with and without plasma treatment are presented in Fig. 3. The contact angle of water droplet on untreated enamel surface was 53.1° ± 2.1°. After plasma treatment, the water contact angle values decreased to 1.4° ± 0.7° indicating a hydrophilic enamel surface.

Fig. 3.

Mean contact angle values of the enamel surface with or without plasma treatment.

4. Discussion

The present study compared the effect of non-thermal plasma (NTP) and conventional surface treatment on the adhesion strength of orthodontic bracket to enamel. Plasma modifies the surface through a gas substance or a combination of gases. Different gas substances have been used to generate plasma, including argon, helium, oxygen, hexamethyldisiloxane (HMDSO) and ethylene diamine [16]. In the dental field, plasma surface treatment was found to enhance the adhesion of composite to tooth substrate [10,20], fiber posts to resin cements [21] and between composite and polyethylene fiber [22]. Furthermore, it has also been used to disinfect root canals and surface treatment of metals for dental implants [16]. The change in tooth substrate after plasma treatment usually occurs by modifying or etching the surface. Surface modification creates new chemical structures, which is achieved by the low temperature of the plasma, reduced quantity of high energy ions and decreased plasma intensity [23]. Etching removes chemical structures from the surface with a prolonged exposure even at a low temperature [20]. The enamel surfaces in the present study were surface modified by NTP treatment for 30 sec (short duration) at low atmospheric pressure with a mix of argon and oxygen (O₂) gas substances. Argon requires low energy for plasma generation in contrast to O₂ plasma which requires high energy for plasma generation [23].

The bonding ability of orthodontic attachment to enamel depends on surface conditioning, subsequent priming and adhesive application [24]. The traditional method or the gold standard of enamel surface treatment is by acid etching which is influenced by the concentration of acids and duration of etching [25]. Although, acid etchants promote micro mechanical interlocking at the interface, they do have many drawbacks such as affecting adhesion strength of orthodontic brackets to enamel; inducing enamel loss during etching, debonding and finishing procedures and initiation of white spot lesions. The enamel loss can be up to 10 microns and acid penetration into the enamel surface can extend up to 100 microns [26]. The rationale for using NTP in orthodontic bonding in the present study was based on previous outcome by Teixeira et al. [27] where the authors confirmed that NTP increased surface energy, surface wettability and adhesion strength between enamel and sealants potentially serving as an alternate to traditional acid etching procedures.

The optimum adhesion strength of orthodontic bracket in a clinical situation ranges from 7–14 MPa [4,5]. In the control groups, Group 1 specimens bonded using a conventional method that demonstrated high bond strength both at baseline and thermocycling. Higher bond strength does not reflect superiority over other groups. In the clinical context, it should also be noted that the brackets need to be debonded without damaging the enamel. Any values exceeding the optimal values (14 MPa) could lead to enamel damage [28], which was also confirmed by the enamel fracture of Group 1 specimens in the present study. Group 2 specimens bonded without enamel etching demonstrated least adhesion strength values which were not acceptable for clinical situation and also confirms the mandatory use of surface preparation procedures prior to bonding. Group 3 specimens bonded using GIC cement exhibited values which were in the optimal range; however, following thermocycling these values decreased significantly and were below the optimal range. It must be noted that there was no surface preparation or conditioner was used before bracket bonding. Gaworski et al. [29] advocate enamel etching or conditioning before bracket bonding to enhance the bond strength of GIC. However, according to the manufacturer, enamel conditioning is optional when using FujiOrtho LC cement.

In this study, we evaluated NTP with both GIC and composite resins. When NTP used in conjunction with composite resins (Group 4), the bond strength was well within the optimal range (10.2 MPa). Similarly, NTP used in conjunction with GIC (Group 5), the bond strength was comparable to that of Group 4 specimens (10.1 MPa). However, following thermocycling, Group 4 specimens exhibited insignificantly decreased bond strength (9.8 MPa) as compared to Group 5 specimens which demonstrated significantly lower bond strength (6.5 MPa). This outcome is well supported by lack of surface conditioning in GIC specimens. In Group 4 specimens, the positive effect of NTP in conjunction with composite resin and hydrophilic primer could have contributed to the less effect on bond strength following thermocycling compared to other groups. Hydrophilic primers improve wettability and bonding of enamel and NTP improves surface energy and polarity of enamel substrate and hence decreased water contact angles were observed [27,30]. Based on the outcome of the present study, the null hypothesis was rejected. Significant differences were observed between the plasma treated and untreated enamel surfaces.

Metelmann et al. [31] evaluated the bond strength of GIC by increasing the hydrophilicity of dental enamel through conditioning with cold atmospheric pressure plasma. The authors concluded that surface conditioning with CAP could not improve the adhesive properties of GIC. This outcome was in disagreement with the outcome of the present study. The difference could be related to the use of different gas substance in both studies. The previous study had used only argon gas whereas the present study used mixture of argon and O₂ gases. Argon plasma is usually used for physical surface modification as compared to O₂ plasma which is mostly used for chemical surface modification. The ARI scores to assess the failure mode of the adhesives after debonding exhibited more of adhesive failures at both baseline and after thermocycling in control groups whereas specimens in experimental groups showed more of cohesive failures. One instance of enamel fracture was observed in Group 1 specimen at baseline. These values might propose the use of NTP surface treatment in reducing cohesive failures.

Saliva contamination to etched enamel cause bond failure. Developing bonding method that could possibly eliminate etching and allowing better primer infiltration is highly desired [27]. Optimizing enamel surface characteristics by using NTP with hydrophilic primers may lead to enhanced interface between brittle enamel and low modulus adhesives which may play a role in maintaining integrity of tooth structure [32].

5. Conclusion

Non-thermal plasma (NTP) treatment in conjunction with composite adhesives demonstrated clinically acceptable adhesion strength and was well within the optimal range (7–14 MPa) for enamel bonding.

Footnotes

Acknowledgements

The project was financially supported by Vice Presidency of Graduate Studies and Scientific Research, King Khalid University.

Conflict of interest

None to declare.

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	Adhesion strength of the specimens
Study groups	Baseline	Thermocycled
Group 1	21.5 ±3.01^A,a	15.8 ±2.87^A,b
Group 2	3.9 ±1.01^B,c	1.6 ±0.7^B,d
Group 3	9.7 ±1.49^C,e	4.9 ±1.04^C,f
Group 4	10.2 ±1.76^C,g	9.8 ±2.15^D,g
Group 5	10.1 ±1.05^C,h	6.5 ±2.19^E,f