Improving the bonding ability of the attachment on fluorotic enamel has great potential in clinical application and alternative orthodontic treatment for patients with dental fluorosis. To investigate the effects of different combination of Er: YAG laser and acid etching, 20 healthy teeth and 90 dental fluorosis were randomly allocated to different groups and separately handled. We evaluated shear bonding strength of each group. The changes of enamel structure and fracture mode of attachments were observed by Scanning Electron Microscope and stereo-microscope, respectively. The shear bonding strength of attachment group was higher than bracket group (P < 0.01). On the attachment, the combination of Er: YAG laser and acid etching improved bonding strength than only acid etching group (P < 0.01). The main fracture modes of attachment in the etching group were adhesive interface fracture and mixed fracture, while those in the combination group were mixed fracture and resin cohesion fracture (P < 0.05). In conclusion, the shear bonding strength of the attachment was higher than that of the bracket under the same conditions, and prolonging the etching time improved the bonding performance of the enamel. The excessive etching led to the dissolution of the enamel column structure and reduced the bonding performance. The combination of Er: YAG laser and acid etching improve the bonding ability of clear aligner attachment on the surface of dental fluorosis.
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Effects of different surface preparations of fluorotic enamel on the bonding strength of attachment in Clear aligners
https://doi.org/10.21203/rs.3.rs-3997108/v1
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Dental fluorosis is a type of enamel developmental disorder caused by the exposure of high concentration fluoride in the stage of dental development. It is pathologically characterized by a disturbance in the structure or arrangement of the enamel crystals 1. Dental fluorosis has become one of the most important diseases to be prevented and controlled in China at the time 2. Recent studies have suggested an increasingly high incidence of dental fluorosis in a global scale, concerning from Mexico, Nigeria, and the United States 3–5. Moreover, it is known that the demand for orthodontic treatment is constantly rising 6.
During the bonding procedure of bracket and attachment, a challenge that how to enhance the bonding properties of the fluorotic enamel needs to be addressed 7,8. The special surface structure of dental fluorosis significantly reduces the bonding strength between enamel and brackets. Previous researchers 9 evaluated the shear bonding strength (SBS) between brackets and fluorotic enamel, finding that fluorosis significantly reduced SBS of orthodontic brackets, which led to the detachment of brackets and difficulty of bonding.
Er: YAG laser is the first laser applied to hard tissues of teeth 10, bringing a promising pathway to raise the bonding capacity of fluorotic enamel. Er: YAG laser was also investigated in the fields of periodontitis, dentin hypersensitivity and dental restoration 11–14. Some researchers found that Er: YAG laser etching increased the bonding ability of fluorotic enamel by improving bonding area and providing mechanical bonding pattern 15. However, it is also considered that Er: YAG laser treatment reduces the bonding strength of teeth with dental fluorosis 16,17. Notably, a systematic review and meta-analysis pointed that Er: YAG laser realizing the re-bonding of detached orthodontic brackets, suggesting that the Er: YAG laser could be a new strategy for enhancing the adhesive strength of brackets 18.There is also another systematic review article indicating that Er: YAG laser can reduce damage to enamel structures and that Er: YAG laser at 2940 nm can be used for bonding of dropped brackets 19.
With the emergence of invisible aligner, an increasing number of dentists are choosing invisible orthodontic technique in their practice. However, the bonding strength between the dental fluorosis and the attachment is still unknown. Dating back to the literatures, there is almost no research has been done on bonding performance between fluorotic enamel and clear aligner attachment.
Er: YAG may improve the reduction of bonding strength, which caused by the poor fluorotic structure. And clear aligner attachment may provide an efficient idea for dental fluorosis, which guarantee a lower ratio of shedding. So in this study, we aimed to investigate the bonding strength of moderate fluorotic enamel on brackets and attachments under conventional acid etching firstly. Then we focused on whether Er; YAG laser could improve the bonding ability between the fluorotic enamel and the attachment. The structure of materials surface were observed under Scanning Electron Microscope (SEM). And SBS were measured by electronic universal testing machine.
Scanning Electron Microscope (SEM)
Under SEM, the surface of healthy enamel appeared smooth and flat, while that of fluorotic enamel was rough with different degrees of pit defects (Fig. 1). There were apparent differences of enamel structure after different treatment of teeth with dental fluorosis: incomplete etching of the enamel surface could be seen after the 30s of phosphoric acid etching (Fig. 2A); uniform depression around the enamel column could be seen after 60s of acid etching, showing irregular honeycomb shape (Fig. 2B); dissolution of enamel column could be seen after 90s of phosphoric acid etching, with smear layer on the enamel surface (Fig. 2C); After the Er: YAG laser and acid etching for 30s, the enamel surface was pitted like and the boundary around the enamel columns were clear (Fig. 2D); after the Er: YAG laser and acid etching for 60s, the enamel surface was uniform fish scale-like and the center of the enamel column was concave, and the edge was evident (Fig. 2E); after the Er: YAG laser and acid etching for 90s, the irregular pit on the enamel surface with enamel dissolution and the boundary between the enamel columns was unclear (Fig. 2F).
Shear Bonding Strength (SBS)
The specific values of SBS for each group have been listed in Table 2. The SBS of group A1 was significantly lower than that of group O1, and the SBS of group B1 was markedly lower than that of group O2. The differences were both statistically significant (P < 0.01) (Fig. 3A). The SBS of group O2 (11.74 ± 1.30Mpa) and group C (11.30 ± 1.40MPa) was the highest, and there was no significant difference between the two groups (P = 0.387) (Fig. 3B). Compared with group B, the SBS of group A was significantly smaller, while that of group C was larger, the differences were statistically significant (P < 0.01) (Fig. 3C).
There are differences in the overall mean of SBS among groups A, B, and C (F values are 6.617, 4.714 and 3.782 respectively); multiple comparisons show that there is no statistical significance in the differences of SBS between groups A2 and A3, B2 and B1, B3, C2 and C3 (P > 0.05) (Fig. 3D) .
Fracture modes of attachments
There were significant differences in the fracture modes of attachments in each group (F = 13.185, P < 0.05). The fracture modes in group B were mainly adhesive interface fracture and mixed fracture, while those in group C were mainly mixed fracture and cohesion fracture in resin (Fig. 4).
Dental fluorosis is generally deemed as one of common dental diseases, which caused by high fluoride exposure at the pediatric stage 20,21. Severity of dental fluorosis would change in the age-related manner and influence the aesthetics of smile 22. As more people with dental fluorosis are tending to seek for orthodontic therapy, the debonding of metal brackets from fluorotic enamel challenged the orthodontist 7. It was reported that use of adhesion promoter could enhance the bonding performance of dental fluorosis during orthodontic therapy 23.
Er: YAG laser has been confirmed to improve the structure and function of dental fluorosis, which may alleviate the poor bonding capability 24,25. Moreover, Er: YAG laser was considered to be used in orthodontic brackets re-bonding, providing a better bonding approach 18,26. And the combination of Er: YAG laser etching and conventional etching enhanced the bonding of composite materials 27, which suggesting that Er: YAG laser would be our optimal choice for bonding attachment on fluorotic enamel.
According to the newly modified Dean index 28, 90 teeth with moderate dental fluorosis in this research were selected to minimize the influence of different enamel surface structure between different degrees of dental fluorosis. One study 29 found that the more serious the fluorosis, the higher the mean fluoride content on the enamel surface. Another result 30 showed that the micro tensile strength of various degrees of dental fluorosis after surface preparation had a significant difference. The etching mode of mild dental fluorosis is similar to that of healthy teeth 23,31, while the bonding performance of severe dental fluorosis is significantly reduced 32.
SEM clearly showed the surface characteristics of moderate fluorotic enamel. Under the microscope, it could be seen that there are apparent differences between the healthy enamel and fluorotic enamel: the healthy enamel surface was uniform, smooth and flat; the fluorotic enamel surfaces had different shapes and growth lines: fluorotic areas had depressed enamel defects with a groove-like appearance of varying depths; whereas non-fluorosis-prone areas resembled the surfaces of healthy teeth. According to SEM observation, the structure of fluorotic enamel was approximately same as described in other literatures 33,34.
In this study, SBS of both brackets and attachments on healthy teeth was significantly higher than fluorosis group, which was consistent with our hypothesis that fluorosis reduces the bonding ability of the enamel surface. One paper viewpoint suggested that fluorosis reduced the bonding performance of enamel 35 and had nothing to do with the properties of bonding materials (metal or resin), which was consistent with our hypothesis.
By comparing SBS of attachments and brackets, our study found that the bonding strength of attachments on both healthy enamel and fluorotic enamel was significantly higher than bracket group, proving our hypothesis that attachments have stronger adhesion than metal brackets on fluorotic tooth surfaces. This may be relevant to the different bonding mechanisms of attachments and brackets on the enamel surface. The attachment was composed of resin, which penetrated into the micropores of the enamel surface, forming a resin protrusion that achieve the micromechanical locking and retention with the enamel surface 36. On the contrary, the bonding of the bracket to the enamel mainly depended on the chemical interaction between the enamel and the base of the bracket 37. A previous study 38 believes that the bonding strength of 6-8Mpa can meet the clinical needs of bonding strength of bracket in the fixed orthodontics, which is close to the results of our study 7.12-9.72Mpa. The adhesion of the attachment on healthy teeth was 9.73-13.23Mpa, which also indicates that the resin attachment has more excellent adhesive properties.
Our study showed that prolonging the etching time increased the bonding strength of the fluorotic surface to the brackets. The effect of prolonging etching time on the bonding properties of dental fluorosis is controversial. One research 39 found that extending etching time can provide proper bonding strength for dental fluorosis and specimens, while some 40 held that prolonging etching time or laser treatment did not significantly upgrade the bonding properties of dental fluorosis. In our study, the fluorotic surface was not completely etched by conventional etching for 30s. The enamel interface formed by etching for 60s was irregular and honeycomb-like in appearance. However, when the etching time was too long, it resulted in partial dissolution of the enamel posts and destruction of the etched interface. We also found that SBS of the attachment increased with the appropriate extension of the acid etching time (60s). With acid etching time further extending (90s), the SBS of attachments no longer increased.
In this experiment, SBS of attachment with only prolonged etching time was significantly lower than the combination of Er: YAG laser and etching. According to our research, SBS of the attachment of group C (11.30 ± 1.40MPa) was similar to healthy teeth (11.74 ± 1.30Mpa), and there was no significant difference between the 60s and 90s of acid etching, indicating that the combination of Er: YAG laser and acid etching have favorably lifted bonding ability. The results of attachment fracture modes suggested that Er: YAG laser etching combined with acid etching can provide more adequate bonding strength, further confirming the reliability of this research. As Wang et.al speculated that Er: YAG laser opened the dentin tubules under less formation of smear layer 41. Some perspective emphasized that Er: YAG laser can replace conventional etching 42, while other researchers found that there is no significant difference in SBS between Er: YAG laser and acid etching respectively 43. On the contrary, a recent research concerned that Er: YAG laser might lead to the damage of resin composite, which probably limited the use of laser irradiation 44.
Due to the limitations of vitro studies, the results of this study provide some ideas for clinical treatment, but further clinical studies are needed to confirm them. In the future, the appropriate parameters, usage methods and procedures of the Er: YAG laser can be studied in depth, and more instructive results are going to obtain through clinical experiments.
Sample collection
The extracted premolars were collected from Gulin and Xuyong Countries in Luzhou City (high incidence area of dental fluorosis) from October 2018 to December 2019. Evaluation of the fluorosis level is performed according to the Thylstrup-Fejerskov Index (TF), which classifies dental fluorosis in terms of its absence (TF 0) through the presence of opaque lesions (TF 3) that blend to overtake the entire surface of the enamel, producing the appearance of white chalk (TF 4). In more advanced stages of fluorosis, there is a gradual loss of enamel and anatomical dental deformities (TF 5–9). In this study, 20 healthy teeth (teeth with a TF index score of 0) and 90 dental fluorosis (teeth with a TF index score of 4) were randomly assigned to the control group (group O) and experimental groups (groups A, B, and C), according to Table 1. Then the selected teeth were cleaned to remove periodontal tissue and stored in 1% chloramine solution at 4℃.
Dental exclusion criteria: incomplete buccal surfaces of teeth, teeth with decay, restorations, chips, cracks and dysplasia.
This study was approved by the Biomedical Ethics Committee of The Affiliated Stomatology Hospital of Southwest Medical University (Lot No.20180129001). And Patients had signed informed consent form. The study has been conducted in compliance with the Declaration of Helsinki.
Sample preparation
Cleaning the dental surface
The teeth surface were cleaned for 5s using a low-speed dental handpiece, rubber cup and fluoride-free polishing paste, then rinsed for 5s with water. The teeth surface were dried with clean air (oil-free and water-free) for 3s.
Er: YAG laser etching
The teeth surface were uniformly etched with the Er: YAG laser according to the mode that targets the hard tissue of the tooth on the basis of to the device's instructions. (Syneron, Israel) (100mJ energy, 30Hz frequency, 6/8 water volume, 1.3*14mm working tip, 2mm distance from the working tip to the dental sur-face, irradiating 10s). Uniform moving irradiation in the same way as stacked tiles, then the etched enamel surfaces were rinsed with water for 5s and airdried for 3s.
Acid etching
Tooth samples in each groups were acid etched using 35% phosphoric acid for 30s (Heraeus Kulze, Germany), then rinsed with running water for 5s and airdried for 3s (the etching time is shown in Table 1).
Bonding bracket
According to the Grengloo adhesive instructions (Ormco, USA), the brackets (floor area 11.80mm2,Victory Series Standard Metal Bracket, 3M ESPE Dental Products, USA) for the maxillary premolar were bonded to the center of the clinical crown by the same orthodontist in order to ensure that the brackets were adhered to the same position of the tooth. Apply very thin coat of Ortho Solo to the prepared tooth, and extrude small amount of grengloo adhesive paste onto the bracket pad. The bracket was placed on the center of the crown and positioned with gentle pressure. After removing of the excess adhesive, each bracket was irradiated for 10s with a light-curing lamp (light curing machine LED. F, Woodpecker, China) at a distance of 1mm from the tooth surface.
Bonding attachment
After tooth preparation (etch, rinse and dry), the adhesive prime\(Nano-technology Dental Adhesive, Dentsply, USA) was evenly applied on enamel surface, removing solvent with clean air-syringe and irradiating with a light-curing lamp for 5s. The attachment, 3mm long vertical rectangular resin (Z250 composite resin, Filtek Z250 restoration, ESPE Dental Products, 3M, USA), was selected and filled intact in the template (Clear aligner, Angelalign, China). Then attachment template was placed under proper pressure on the center of the tooth surface and il-luminated with a light-curing lamp for 10s. The template was removed and the excess resin was taken off (the area at the bottom of the attachment was 6.00 mm2). To clarify, Clear aligners were not particularly different from other invisible aligners used in clinical practice.
Observation of enamel surface structure by Scanning Electron Microscope (SEM)
One untreated healthy tooth, one untreated tooth with dental fluorosis, and six treated teeth with dental fluorosis (The treatment methods are shown in Table 1) were randomly selected to prepare 5*5*2mm specimens. After drying for 24 hours, the buccal surfaces of the sectioned samples were coated with gold-palladium layer to im-prove the conductivity during scanning. The enamel surface morphology of each group was observed under scanning electron microscope (SU1510 Scanning Electron Microscope, JEOL, Japan) with magnification ranges from 3000 to 50,000.
|
Groups |
Interventions |
n |
|
|---|---|---|---|
|
O |
O1 |
Healthy teeth + Acid etching 30s + Bracket |
10 |
|
O2 |
Healthy teeth + Acid etching 30s + Attachment |
10 |
|
|
A |
A1 |
Dental fluorosis + Acid etching 30s + Bracket |
10 |
|
A2 |
Dental fluorosis + Acid etching 60s + Bracket |
10 |
|
|
A3 |
Dental fluorosis + Acid etching 90s + Bracket |
10 |
|
|
B |
B1 |
Dental fluorosis + Acid etching 30s + Attachment |
10 |
|
B2 |
Dental fluorosis + Acid etching 60s + Attachment |
10 |
|
|
B3 |
Dental fluorosis + Acid etching 90s + Attachment |
10 |
|
|
C |
C1 |
Dental fluorosis + Er: YAG laser etching + Acid etching 30s + Attachment |
10 |
|
C2 |
Dental fluorosis + Er: YAG laser etching + Acid etching 60s + Attachment |
10 |
|
|
C3 |
Dental fluorosis + Er: YAG laser etching + Acid etching 90s + Attachment |
10 |
|
Determination of Shear Bond Strength (SBS)
The detached teeth with brackets or attachments were embedded in anhydrite, and fixed on the electronic universal testing machine (WDW-100 Electronic Universal Testing Machine, Jinan HengRuijin, China) after being stored for 24h in water at 37℃. The dislocation apparatus was perpendicular to the dental surface at the bottom of the bracket and attachment, ensuring that the dislocation force is parallel to the adhesive interface. Then the dislocation apparatus was loaded at the speed of 1mm/min until the bracket or attachment were separated from the dental surface. The peak value of anti-shearing force was recorded, and the SBS was calculated (Table 2):
Observation of fracture modes of attachments
The fracture modes of the attachment were observed under the stereomicroscope and classified as: ①interface fracture, ②mixed fracture, ③cohesion fracture in resin ④cohesion fracture in enamel.
Statistical analysis
Statistical analysis was performed using the SPSS 20.0(Chicago, IL, USA) program. All the data was expressed by means ± standard deviation (
). Since the data distribution was normal, the difference between groups was assessed using t-test or one-way ANOVA. The frequency represents the fracture modes, and the difference between groups is compared by chi-square test. All tests were performed at a 95% confidence. Differences were considered significant if the P values were less than 0.05. ( ns p > 0.05, * p < 0.05, ** P < 0.01, *** P < 0.001)
Data availability
All the data are available in the main text.
Acknowledgements
This work was supported by the Applied Basic Project of Southwest Medical University (grant number 2021ZKMS015), and the Applied Basic Youth Project of Southwest Medical University (grant number 2021ZKQN038).
Author contributions statement
Jie Lei was responsible for investigation, methodology, data curation, and writing—original draft preparation. Rui Xia was responsible for investigation, methodology, data curation, and writing—original draft preparation. Maoxuan Luo was responsible for writing—review and editing and supervision. Yao Xiao was responsible for supervision, project administration, and funding acquisition. Jie Lei and Rui Xia contributed equally to this paper as co-first authors. All authors have read and agreed to the published version of the manuscript.
Competing interests
The authors declare no competing interests.
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No competing interests reported.