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 Table of Contents  
ORIGINAL RESEARCH
Year : 2018  |  Volume : 10  |  Issue : 5  |  Page : 244-249

Influence of chemical conditioning on bonding precoated orthodontic brackets to fluorosed enamel surfaces using self-etch primer


1 Department of Pediatric Dentistry and Orthodontics, College of Dentistry, King Khalid University, Abha, Saudi Arabia
2 Department of Restorative Dental Sciences, College of Dentistry, King Khalid University, Abha, Saudi Arabia
3 Department of Pediatric Dentistry and Orthodontics, College of Dentistry, King Saud University, Riyadh, Saudi Arabia
4 Division of Orthodontics, College of Dentistry, King Khalid University, Abha, Saudi Arabia

Date of Web Publication24-Oct-2018

Correspondence Address:
Dr. Ibrahim Alshahrani
Department of Pediatric Dentistry and Orthodontics, College of Dentistry, King Khalid University, Abha
Saudi Arabia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jioh.jioh_190_18

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  Abstract 

Aim and Objectives: The aim of the study was to evaluate the shear bond strength of precoated orthodontic brackets bonded to chemically conditioned, fluorosed enamel surfaces using self-etch primer. Materials and Methods: Three groups of extracted premolars respectively with no (G1), mild (G2), and moderate (G3) fluorosis were recruited for this study. The buccal surfaces of teeth in each group were conditioned in three subgroups with air-water spray (SG1), 35% phosphoric acid (SG2), and 20% citric acid (SG3) before bonding precoated metal orthodontic brackets by the aid of self-etch primer. The bonded brackets in each subgroup were stressed on a universal testing machine to determine their shear bond strength. The debonded surfaces of each specimen were then assessed to determine the adhesive remnant index (ARI) in each subgroup. The obtained data were analyzed with SPSS software (SPSS version 20.0, SPSS, Chicago, IL, USA). Results: Teeth with mild and moderate fluorosis in G2, G3 provided lower bond strength than normal teeth in G1 (Tukey's, P < 0.05). Enamel surfaces conditioned with phosphoric acid in SG2 offered the highest bond strength in comparison to those conditioned with water and citric acid in SG1 and SG3 (Tukey's, P < 0.05). No statistical difference (Chi-squared, H = 16.48) was detected between the tested subgroups in terms of ARI. Conclusion: The presence of enamel fluorosis adversely affects the shear bond strength of the precoated metal brackets when self-etch primer is considered for bonding. Conditioning the mildly fluorosed enamel surfaces for 60 s with phosphoric acid restores the clinically acceptable brackets bond strength values.

Keywords: Bonding, chemical conditioning, enamel, fluorosis, orthodontic brackets


How to cite this article:
Alshahrani I, Abdelaziz KM, Asiry MA, Alayesh A. Influence of chemical conditioning on bonding precoated orthodontic brackets to fluorosed enamel surfaces using self-etch primer. J Int Oral Health 2018;10:244-9

How to cite this URL:
Alshahrani I, Abdelaziz KM, Asiry MA, Alayesh A. Influence of chemical conditioning on bonding precoated orthodontic brackets to fluorosed enamel surfaces using self-etch primer. J Int Oral Health [serial online] 2018 [cited 2020 Apr 5];10:244-9. Available from: http://www.jioh.org/text.asp?2018/10/5/244/243858


  Introduction Top


Smooth and efficient orthodontic treatment is usually affected by the retention and stability of individual components of the selected appliances.[1] Successful bracket retention to tooth enamel is essentially required during all stages of orthodontic treatment, and a bonding value of 5.9–7.8 MPa was reported satisfactory to achieve that success clinically.[2],[3] Both precoated brackets and self-etch resin primers (SEP) have been suggested to save some of the chairside bonding time.[4],[5],[6] These materials were claimed to provide clinically efficient and durable bracket bonding with no harm on enamel surfaces at the time of appliance removal.[7],[8] However, the lower efficacy of SEP in etching enamel surfaces seems responsible for their lower bond strength to enamel in comparison to the conventional etch-and-rinse adhesive (ERA) ones.[5],[6],[9],[10],[11],[12],[13] Therefore, some primers have been introduced with acidic monomers having the capability to chemically adhere to enamel although their contribution to the bonding strength was not clear.[11] Some in vitro studies[12],[13] also recommended the use of SEP on pre-etched normal enamel surfaces to improve their bond strength values.

On the other hand, orthodontists usually encounter some difficulties in bonding brackets to fluorosed enamel surfaces those commonly seen in particular communities.[14],[15] Dental fluorosis is usually formed in response to excessive fluoride uptake during the developmental stages of tooth tissues.[16],[17],[18] Normally, the fluorosed lesions represent a hypermineralized outer most layer and porous, hypomineralized subsurface layer of tooth enamel.[19] This nature could affect the quality of adhesive bonding to enamel especially when the SEP is used to achieve that purpose.[20],[21],[22] Findings of Isci et al.[21] did not support the routine clinical use of SEP to bond orthodontic brackets to the mildly fluorosed teeth. Accordingly, a combination of surface microabrasion and phosphoric acid etching had been suggested by some investigators[23],[24] to improve the adhesive bonding to fluorosed enamel surfaces. Others[25] reported that a preparation of HEMA and Polyacrylic acid could promote adhesive bonding to the pre-etched fluorosed enamel with no need for microabrasion.

Although Mena-Serrano et al.[26] reported that doubling the etching time can improve the bond strength of self-etch primers to dentin, the same procedure seemed unsuccessful in case of fluorosed enamel when resin adhesive was used together with etch-and-bond primers to bond orthodontic brackets.[22] Based on the aforementioned information, no technique was recommended for routine bonding of orthodontic brackets to fluorosed enamel surfaces. Therefore, this in vitro study aimed to evaluate the influence of chemical conditioning of mildly and moderately fluorosed enamel surfaces on the shear bond strength of precoated orthodontic brackets when the self-etch primer was considered for bonding. Thus, the null hypothesis was that chemical conditioning will not improve the bond strength of the self-etch primer used to fix the precoated orthodontic brackets to the fluorosed enamel surfaces.


  Materials and Methods Top


An in vitro experimental study was carried out on extracted teeth to evaluate the influence of chemical conditioning of mildly and moderately fluorosed enamel surfaces on the shear bond strength of precoated orthodontic brackets when the self-etch primer was considered for bonding.

Before the conduct of study ethical approval was obtained from Scientific Research Committee, King Khalid University, Abha, Saudi Arabia (SRC/ETH/2016-17/063).

Sample size estimation

The sample size was calculated with the help of G * Power (Version 3.1.9.2; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) analysis;[27] with effect size d equal to 0.7487013, α = 0.05, power = 0.80 and 95% confidence interval. The mean ± SD values (4.18 ± 0.15 and 4.09 ± 0.08) were assessed from a pilot study.

A sample size of 29 was estimated using the above values; hence, sample 30 was taken to round it off, for each group.

Three groups of freshly extracted maxillary premolars with no, mild, and moderate fluorosis (Groups 1–3, n = 30 for each group) were collected from 18 to 35 years old patients undergoing orthodontic treatment based on the classifying criteria of Thystrup and Fejerskov,[28] as shown in [Table 1]. All teeth were caries and cracks free and void of any surface abnormalities rather than fluorosis. All teeth were subjected to ultrasonic cleaning to remove all soft and hard deposits before their disinfection in 0.1% thymol solution for 24 h. The buccal surfaces of the collected teeth in each group were brushed with oil-free polishing paste before their conditioning in three subgroups (n = 10 each) using air-water spray (SG1, non-treated), 35% phosphoric acid (SG2), and 20% citric acid (SG3). The utilized acids were kept undisturbed against on the selected enamel surfaces for 60 s and washed up with air-water spray for another 15 s. The conditioned surfaces were meticulously air dried before bonding the precoated metal orthodontic brackets (APCPLUS, 3M/Unitek, Monrovia, CA, USA) using self-etch primer (Adper Prompt L-Pop, 3M ESPE, St. Paul, MN). The mixed resin primer was agitated and left in contact with the conditioned enamel surfaces for 15 s before its light curing with Elipar S10 unit (3M ESPE, Seefeld, Germany) for 15 s. The precoated metal brackets were then adjusted nearly to the same position on the buccal surfaces of all teeth by the aid of height bracket positioning gauge (3M/Unitek, Monrovia, CA, USA). The positioned brackets were then subjected to a constant manual pressure of 300 g using Correxgauge (HAAG-STREIT AG, Koeniz, Switzerland) for 10 s before light curing of the adhesive for 10 s at each side of the bonded bracket.
Table 1: Thylstrup and fejerskov index

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Specimens in all subgroups were stored in water at 37 ± 1°C for 60 days and thermocycled for 1500 cycles with 1 min dwell time before testing their bracket-enamel shear bond strength on a universal testing machine (Model 5565; Instron Ltd, Bucks, England) running at a crosshead speed of 0.5 mm/min. The bonding interface of each specimen was stressed on compression using a chisel-edged rod, and the highest load at failure (N) was recorded and divided by the manufacturer stated bracket's surface area (mm2) to calculate the shear bond strength in MPa. The collected bond strength data were analyzed using two-way ANOVA and Tukey's comparisons α = 0.05 to detect any significant differences between test subgroups. The debonded bracket and enamel faces of each tested specimen were also assessed using a light stereomicroscope (Nikon SM2-10, Tokyo, Japan) at ×20 magnification to determine the adhesive remnant index (ARI) in each subgroup.[29] The amount of adhesive remnants on tooth enamel of each specimen was scored as 0 when a total separation at adhesive-tooth interface was detected and no adhesive remained bonded to tooth enamel; 1 when partial separation at adhesive-tooth interface was detected and <50% of the utilized adhesive remained bonded to tooth enamel; 2 when partial separation at adhesive-tooth interface was detected and more than 50% of the utilized adhesive remained bonded to tooth enamel; and 3 when total separation at adhesive-bracket interface was detected and the whole amount of the utilized adhesive remained bonded to tooth enamel. The recorded adhesive remnants scores were then statistically analyzed using Chi-squared test at α = 0.05 to detect the significance of differences between subgroups.


  Results Top


Shear bond strength values and standard deviations of precoated metal brackets to enamel surfaces of different subgroups are listed in [Table 2]. The two-way ANOVA indicated significant differences between different test groups (enamel surfaces, P < 0.0001) and between different subgroups (enamel conditioning, P < 0.0001) in addition to a significant interaction between both variables (P < 0.0001). Tukey's pair-wise comparisons indicated that enamel surfaces with mild and moderate fluorosis in groups 2 and 3 provided lower bond strength values in comparison to normal enamel surfaces of Group 1 (control) (P < 0.05). Within the same group, enamel surfaces conditioned with phosphoric acid (SG2) showed the highest shear bond strength to self-etch adhesives in comparison to those treated with citric acid (SG3) and nontreated (SG1) surfaces (P < 0.05). Enamel surfaces treated with citric acid (SG3) exhibited the lowest bond strength values among all other test subgroups (P < 0.05). Inspection of the debonded specimens showed varied incidences (%) of adhesive remnant scores within each subgroup, although the calculated ARI reflected a true image of the recorded bond strength data of tested subgroups [Table 3]. The statistical analysis of the recorded ARI indicated no significant differences (Chi-square, H = 16.48) between any of the tested subgroups and the control.
Table 2: Shear bond strength (MPa) of precoated brackets to enamel in different subgroups

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Table 3: Incidences (percentage) of different adhesive remnant scores in each subgroup

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  Discussion Top


Fluorosis is usually considered as an obstacle facing optimum bonding to tooth enamel. Many studies[20],[21],[22] reported lower bond strength of dental primers to enamel surfaces with different degrees of fluorosis in comparison to normal enamel. The efficiency of modern orthodontics is greatly dependent on the successful bonding of brackets to enamel or restorative surfaces (if any).[2] Therefore, many types of dental primers have been developed to achieve that purpose; however, both etch-and-bond and self-etch adhesives usually achieve unsatisfactory and less durable bonding of orthodontic appliances in the presence of dental fluorosis.[21],[22] Accordingly, several approaches, have been considered aiming to improve bonding to fluorosed enamel; however, most of them did not achieve the desired results.[23],[24],[25] Although acid conditioning of the fluorosed enamel surfaces before bonding has been considered with the application of regular etch-and-bond resin primers, it seemed of no value in terms of adhesive-enamel bond strength.[22]

Therefore, this in vitro study utilized the acid conditioning together with the application of self-etch primers in bonding precoated metal orthodontic brackets to enamel surfaces with different degrees of fluorosis. The phosphoric acid (35%–37%) is a widely used etchant in everyday dental practice and its application was previously reported to improve the bond strength of self-etch primers to normal enamel surfaces,[12],[13] while 20% citric acid is a known surface cleansing agent and was documented as a powerful chemical conditioner of tooth surfaces in periodontal procedures.[30] The self-etch primers usually offer a simple application procedure, clinically acceptable bond strength, and can save some of patients' and operators' times.[5],[6] These advantages are also applied on the utilized precoated brackets those documented by some authors[4] to offer higher initial bonding values and stability in comparison to regular brackets. This feature surely would help withstand the forces that result from the early insertion of archwires.[4]

Due to its sensitivity and reliability in assessing teeth with dental fluorosis, the Thylstrup and Fejerskov index[28] was used to select teeth with only mild and moderate fluorosis for this in vitro study. These teeth were particularly used because they usually do not require a restorative intervention as the severely fluorosed ones do and mostly could be managed as normal in many dental procedures.[31] The shear bond strength test was selected to evaluate the outcomes of this study because of its popularity in similar studies and its ability to offer a mimic of the stressing situation the bonded brackets may face in real clinical situations.[21],[22] The null hypothesis of the current in vitro study suggested no improvement in the shear bond strength of precoated orthodontic bracket when bonded with self-etch primers to both phosphoric and citric acid-conditioned fluorosed enamel surfaces. However, statistical analysis of the results showed some significant outcomes those necessitate partial acceptance of that null hypothesis.

The results of the present study showed that enamel surfaces with mild and moderate fluorosis in Groups 2 and 3 provided lower bond strength values in comparison to normal enamel surfaces in Group 1 (P < 0.05). These findings came in agreement with the results of some previous studies[32],[33] that revealed the lower shear bond strength of the fluorosed teeth than nonfluorosed teeth. Authors of those studies related their findings to the hypermineralized nature of fluorosed enamel surfaces that resist acid etching leading to a less efficient micromechanical attachment of the applied resin adhesive.[19],[20],[34] Although the results of another study[22] indicated no difference between the shear bond strength to the mildly fluorosed and the nonfluorosed enamel, the conflict in results could be referred to the use of regular etch-and-bond primers in that study to bond orthodontic brackets.

Findings of the present study also declared that enamel surfaces conditioned with phosphoric acid (SG2) showed the highest bond strength to self-etch adhesives in comparison to those treated with citric acid (SG3) and nontreated (SG1) surfaces (P < 0.05) within the same test group. These findings coincide with the results of a previous study[35] that revealed the ability of phosphoric acid etching to offer variable etching pattern and higher bond strength when compared to 50% citric acid and 50% tannic acid etching. The lower acidic solubility of the fluoroapatite, that is normally present with a high concentration in the outer layer of fluorosed enamel, may also complicate bonding of orthodontic brackets.[35] One study[36] suggested the increased etching time of the fluorosed teeth to overcome the noticed lower enamel solubility. Others[37],[38] specify the etching time to be 15–30 s for teeth with normal enamel, 30 s for teeth with mild-to-moderate fluorosis and 60–90 s for those with severe fluorosis. The views vary among different researchers regarding the optimal etching time of fluorosed teeth. In spite of those recommendations, this approach seemed ineffective with regular etch-and-bond primers.[22] Therefore, the current study suggested an elongated etching/conditioning time of 60 s accompanied with the use of self-etch primer having phosphate-based resin monomer in its formulation. The presence of that acidic monomer may synergistically add to the value of phosphoric acid preconditioning, however, this postulation, based on the results of this study, could not be true in case of citric acid preconditioning.

Although the statistical analysis of the ARI indicated no difference between the tested subgroups, a varied distribution of the adhesive remnant scores was noticed. Score 1 that indicates adhesive bond failure at adhesive-enamel interface seems predominant in specimens with relatively lower bond strength values. At the same time, the incidences (%) of other scores those represent the cohesive failure in the adhesive material itself or adhesive failure at adhesive-bracket interface was noticed to increase in specimens with relatively higher bond strength values. Although the recorded incidences of different failure patterns came in accordance to the findings of previous study[39] that correlated the adhesive type of bond failure to the lower bond strength values and the cohesive and mixed failures to the higher bond strength values, another study,[40] in contrary, reported no association between the mode of bond failure and the shear bond strength. These findings may be related to the difference in the nature of the tested dental tissues and their compositional status in addition to the type of adhesive/primer used for bonding.

Using the scanning electron microscope images of the preconditioned fluorosed enamel surfaces before and after application of the self-etch adhesive could be considered as a shortcoming of the current in vitro study. Therefore, employment of this kind of investigation should be considered in further studies to help understand the possible interaction between the utilized conditioner and the self-etch primer.

The primary outcome of the study shows that the enamel surfaces with mild and moderate fluorosis in Groups 2 and 3 provided lower bond strength values in comparison to normal enamel surfaces of Group 1 (control) (P < 0.05). The secondary outcome measures depict that within the same group, enamel surfaces conditioned with phosphoric acid (SG2) showed the highest shear bond strength to self-etch adhesives in comparison to those treated with citric acid (SG3) and nontreated (SG1) surfaces (P < 0.05). Enamel surfaces treated with citric acid (SG3) exhibited the lowest bond strength values among all other test subgroups (P < 0.05). The statistical analysis of the recorded ARI indicated no significant differences (Chi-square, H = 16.48) between any of the tested subgroups and the control.


  Conclusion Top


The presence of enamel fluorosis adversely affects the shear bond strength of the precoated metal brackets when self-etch primer is considered for bonding. Conditioning the mildly fluorosed enamel surfaces for 60 s with phosphoric acid restores the clinically acceptable brackets bond strength values.

Acknowledgment

The authors extend their appreciation to the Deanship of Scientific Research at King Khalid University for supporting this work through the General Research Project under grant number; (GRP-349-38).

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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