Adhesion to mild fluorosed enamel: A comparative study of two etching protocols

Adhesion to mild fluorosed enamel: A comparative study of two etching protocols

Abstract

Introduction
The objective of this study was to evaluate the adhesion quality of a two-step total-etch adhesive system, to mild fluorosed enamel, by two different etching protocols.

Materials and methods
Thirteen human teeth (molars and premolars) showing a mild fluorosis (TFI 1-3) according to Thylstrup and Fejerskov index were used.
On each tooth two amelo-dentinal Class II box-only cavities were prepared.
The distal cavities were etched once for 30 seconds using orthophosphoric acid at 37%. The mesial cavities were etched twice for 30 seconds using orthophosphoric acid at 37%.
Following the obturation with resin composite, thermocyling, infiltration with methylene blue and sectioning in the mesio-distal direction, the teeth were observed with a stereomicroscope. Three samples were observed with a scanning microscope.
Methylene blue infiltration degree was evaluated in the cervical and occlusal enamel of each cavity.
The statistical study was conducted using the Fisher exact test.

Results
No significant differences were observed between the two etching protocols p=0.583
The significant differences were observed in adhesion quality between the cervical enamel and the occlusal enamel for the simple etching protocol p=0.035 and for the double etching protocol p=0.045.

Conclusion
Our study showed that composite adhesion to mild fluorosed enamel is not influenced by the etching time and that adhesion is better in the occlusal enamel than in the cervical enamel.

Introduction

Dental fluorosis is an hypomineralisation of the tooth hard tissues induced by an excessive intake of fluoride occurring during odontogenesis (4). Fluorosis is clinically manifested, depending on individual susceptibility, by opaque white spots, lines following the perikymata direction, or wavy yellowish and brownish striations of the enamel. Later, the opaque surfaces develop and turn into chalky and the enamel develops small cavities. After tooth eruption, the enamel pits and fissures turn into brown or black (11).
A fluorosed tooth requires a particular therapeutics (3). The fluorosed enamel is hard, brittle and acido-resistant. These features represent a real problem for resin composite bonding to this enamel which is little studied.problem for resin composite bonding to this enamel which is little studied.
In Tunisia, dental fluorosis represents a real public health issues as it affects 50% of adults aged from 35 to 45 (7). Despite the caries resistance of fluorosed teeth, the prevalence of caries remains high in this country (74%) (7).The practitioner is therefore confronted in his daily practice with restoring these teeth, hence increasing the interest in addressing the problems raised by bonding.
The objective of our study was to evaluate the adhesion quality of a two-step total-etch adhesive system, to mild fluorosed enamel (TFI 1-3), by two different etching protocols.

Materials and methods

Thirteen recently-extracted, non-carious human teeth (molar and premolar, maxillary and mandibular) were stored in water and in a refrigerator until use after cleaning with a pumice water slurry in order to remove tartar, soft tissues and any other debris.
All the collected teeth presented a mild fluorosis according to the Thylstrup and Fejerskov index (TFI 1-3).

Cavity preparation
Class II box-only cavity preparation were prepared on the mesial and distal surfaces of each tooth. These preparations were accomplished with carbide burs in a high-speed handpiece with water spray. There was no occlusal connection between the preparations.
Each cavity was placed at two millimeters of the cementoenamel junction and presents the following dimensions:
-2 mm in the bucco-lingual axis,
-2 mm in the mesio-distal depth
-Buccal and lingual walls of the preparations were approximately parallel and connected to the gingival wall with rounded line angles.
Prepared teeth were stored in distilled water.

Restoration
Following cavity preparation, each tooth was rinsed with distilled water and dried.
The distal faces:
The cavities situated at the distal faces were etched for 30 seconds using ortho-phosphoric acid gel at 35 % and rinsed for 30 seconds with 5cc of distilled water. The preparations were moderately dried using a cotton pellet. A single-component dental adhesive (Opibond Solo, Kerr®) was applied by friction on the surfaces for 10 seconds, gently air dried and light cured for 20 seconds.
A micro hybrid composite (Prodigy^TM, Kerr^®) was used in the preparation in three successive layers and each layer was light cured for 40 seconds. A transparent strip was tightened and held by finger pression against the gingival margin of the cavity so that the preparation could not de overfilled at the gingival margin.
The mésial faces:
The cavities lying at the mésial faces received a double etching: the use of ortho-phosphoric acid gel at 35 % for 30 seconds, followed by a moderate rinsing and drying according to the previously described protocol. It was later followed by a second application of acid for 30 seconds, rinsing and moderate drying.
The adhesive (Opibond Solo, Kerr^®) and the composite (Prodigy^TM, Kerr^®) were applied and light cured according to the procedure used for the distal faces.
The adhesive and the composite were light cured with a halogen lamp.
All the restorations required a finishing using a silicon capsule. All the samples reconstituted were stored in distilled water at room temperature.

Thermocycling
The apices were sealed with a transparent orthodontic resin. A complementary protection was brought with the application of two coats of transparent finger nail varnish on each crown while leaving 1mm around the restorations.
The teeth were thermally cycled between 6°±2°c and 60°±2°c water baths for 400 cycles with a 30 seconds dwell time and 10 second transfer time.

Infiltration at methylene blue
A second application of two coats finger nail varnish on each crown was performed. The crowns were immersed in a methylene blue dye solution at 0,1% for 48hours at room temperature. After withdrawing the teeth from the dye solution, they were rinsed with water and cleaned with abrasive disks to eliminate the dye traces.

Microtome sectioning
The root of each tooth were inserted in a numerated methacrylate resin blocks.
At the vertical plane, each tooth was sectioned mesiodistally (with a microtome (Isomet® Buehler) across the center of the restorations using thicket 0,4mm diamond saw with continuous water irrigation.
After separating the roots of each tooth from the crown, two sections were obtained: one buccal and the other lingual.
Following this stage, four obturations were eliminated.

Stereomicroscope observation
22 composite resin obturations were analyzed.
Each cut was photographed with a stereomicroscope (Zeiss) under magnification of 1,25 and with a digital camera. (fig1).
The observation was separately performed by two observers. The infiltration degree of the product was noted in function of the following scores:
0: no infiltration
1: infiltration of the external third of the enamel
2: infiltration of the two external thirds of the enamel
3: infiltration of all the enamel thickness
4: infiltration of all the enamel and dentin thickness

Figure 1: stereomicroscope observation of the degree of methylene blue infiltration:
sample n°2 (a), sample n°8 (b), sample n°12 (c).

SEM observation
Two buccal cuts were selected for the observation at SEM: samples n°1 and 2.the selected samples were dehydrated with decreasing concentration of alcoholic solutions.
The cuts were observed with SEM Philips XL30 according to the BSE mode at 917x, 2000x and 5000 x magnificences.

Figure 1: stereomicroscope observation of the degree of methylene blue infiltration:
sample n°2 (a), sample n°8 (b), sample n°12 (c).

Statistical study
The Fisher exact test was used to conduct qualitative analyses of the bonding efficacy between:
-the two operative protocols: simple etching (distal) and double etching (mesial)
– and between the cervical and the occlusal enamel of each cavity.
The absence of infiltration (0) and the presence of infiltration (1) were taken as selection criteria.

Results

Stereomicroscope observation
The score attributed to each sample depending on the infiltration is reported on table 1.
The comparison of bonding between the two etching protocols did not present significant differences p=0,583.
Significant differences were observed for the adhesion quality between the cervical enamel and the occlusal one for the protocol with simple etching p=0,035 and for the protocol with double etching p=0,045.
The adhesion of composite to the occlusal enamel is better than its adhesion to the cervical enamel.

SEM observation
Sample 1 showed an homogenous and perfect adhesive interface between composite resin and the enamel. The adhesive formed a thin film. Some small enamel cracks were visible at a great magnificence. (fig2).
Sample 2 presented an homogenous adhesive interface between composite resin and the enamel. The cervical enamel interface showed a more irregular surface than the occlusal enamel interface. We notice the presence of fractures line both at the occlusal and cervical enamel. (fig3).

Discussion

Fluorosed enamel is acido-resistant and its etching remains a delicate act. Few authors were interested in this subject and the rare studies showed contradictory results.
Most of the authors agree that there is a difference in the fluorosed enamel response compared to normal enamel, in the etching technique. Difference with becomes more evident when fluorosis is more intense. (13) (16) (9). Al Sugair and Akpata (1) found that the etching of fluorosed enamel with Thylstrup and Fejerskov index TFI 1-3 does not present significant differences with the normal enamel. Weerainhe and all (16) have shown that fluorosis severity does not influence the adhesion force to the enamel when we use phosphoric acid etching as well as a self-etching adhesive.
De Goes and all (6) have noticed that ortho-phosphoric acid at 35 % used for 15 to 60 seconds produces the same effect for the normal enamel. Ateyah and Akpata (2) have shown that the etching time affects the shear bond strength of composite for the fluorosed enamel. An etching for 120 seconds is better than an etching for 60 seconds.
Belkhir, El Araby, Ermis (4,8,9) think that renewing the etching gel leads to a linear dissolution in the enamel thickness, while the same product used for long time on the enamel surface is rapidly neutralized and risks as well to act on the width of micro-retentions created in the enamel by decreasing their retentive value.
In our study, we noticed that to mild fluorosed enamel, double etching did not give better results than the simple etching: etching of one time 30 seconds is enough. However, in case of severe fluorosed enamel other investigations remain necessary to find the best way to treat this enamel.
Our SEM observation showed that micro reliefs are not very well visible on the totality of the samples. They are not pronounced as in the literature and resemble more to vacuoles. But the fact that we applied the etching gel for one time during 30 seconds and twice for 30 seconds without finding differences in the statistical study as well in the SEM observation leads us to believe that bonding to the enamel is not only a micro mechanical keying. The enamel etching modifies as well the tissue surface state, both fluorosed and normal, by ameliorating the resin wet ability and in ensuring a physic-chemical link in addition to micromechanical one. For Goldberg (10) the resins are infiltrated in inter- crystalline spaces. This is as well the reason why the fluorosed enamel, despite being brittle and acid resistant, reacts positively to bonding techniques.
The adverts of self-etching adhesives have allowed simplifying the bonding protocol but their efficacy in the enamel is still questionable. Weerainhe and all (16) have shown the superiority of bonding on fluorosed teeth by etching with phosphoric acid compared to the self-etching systems.
Our study showed that bonding on the occlusal enamel is better than the cervical enamel for the two protocols. This can be explained by the difference of prisms orientation. The enamel prisms of permanent teeth have an orientation essentially parallel to the dentino-enamel junction except for the cervical enamel where the prisms are oriented towards the exterior with an apical direction (14). The prisms present a transversal orientation in the occlusal enamel and a longitudinal one in the cervical enamel. The base unit of prisms is the crystallite (10, 14). Dissolution which depends on its orientation in the presence of an acid attack: the crystal dissolves easier at its extremities than on the sides (14, 15). Moreover, in a practical point of view, the application of ortho-phosphoric acid and of the adhesive is much better controlled at the occlusal level than the cervical level. The practitioner should therefore pay a particular attention to the restoration of this area.
Besides, the SEM observation of the samples puts into evidence the extreme fragility of the fluorosed enamel witch known for being hard, brittle and porous (4,5). The presence of several enamel fissures and fractures of enamel masses confirms these characteristics. These cracks and breaks are observed only in the occlusal enamel (for the two samples). Despite the fragility of the cervical enamel and mainly its weak thickness, the disposition of prisms reinforced the tissue at this level. (It is easier to cause micro cracks at the extremity of a prism than on the length of its axis).
Ateyah and Akpata (2) observed breaks of cohesive type generalized on the fluorosed enamel of elderly patients for any time of etching.
The observation of mésial obturation of sample n°2 at the cervical enamel shows decaled enamel prisms but still adherent to the composite resin. It seems to be drowned in the resinous mass, which implies that the enamel fracture has taken place before the bonding, therefore during the cavity preparation.
The presence of an adhesive layer of variable thickness (samples 2) is an iatrogenic origin, due to the excessive intake of the adhesive although it was applied by micro-brush by rubbing it against the different walls and by photo polymerizing it immediately. The literature is silent concerning all these details for fluorosed teeth, we think that the success of a bonded restoration depends as well on the severity of fluorosis, the patient’s age and the adhesive system used. It is interesting to know that drilling greatly ameliorates bonding (9).
Other investigations are required to evaluate the resistance to fracture.

Conclusion

Fluorosed enamel is hard and brittle. These characteristics are relative to the degree of fluorosis. This fragility associated to the acid resistance makes of composite resin bonding a very delicate stage.

Our study shows that composite adhesion to mild fluorosed enamel is not influenced by the etching time and that adhesion is better in the occlusal enamel than in the cervical enamel.
This study may lead us to deduce that the mild fluorosed enamel (TF1 1-3) is likely to react to etching by ortho-phosphoric acid as a normal enamel would.

However the practitioner should be vigilant and should work delicately during cavities preparation to avoid any risk of micro fractures and micro cracks of this enamel. He should as well pays a particular attention to the restoration of the cervical wall which is a difficult access and where the enamel seems to be particularly resistant to the bonding procedures.

Acknowledgments
The authors thank Mister Samir Boukottaya for revision of the manuscript.

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Dr. Chems Belkhir: Associate Professor of Endodontics

Dr. Asma Arous: Private clinician

Dr. Mohammed Semir Belkhir: Professor of Endodontics and chairman